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Short title: PROHIBITIN 3 controls stress-induced SA synthesis 1
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TITLE 8
PROHIBITIN 3 forms complexes with ISOCHORISMATE SYNTHASE 9
1 to regulate stress-induced salicylic acid biosynthesis in Arabidopsis 10
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Authors: 15
Aldo Seguel1, Joanna Jelenska2, Ariel Herrera-Vásquez1, Sharon K. Marr3, Michael B. 16
Joyce2, Kelsey R. Gagesch2, Nadia Shakoor2, Shang-Chuan Jiang2, Alejandro 17
Fonseca1, Mary C. Wildermuth3, Jean T. Greenberg2,* and Loreto Holuigue1,*. 18
1Departamento de Genética Molecular y Microbiología, Facultad de Ciencias Biológicas, 19
Pontificia Universidad Católica de Chile. 20
2Department of Molecular Genetics and Cell Biology, The University of Chicago. 21
3Department of Plant and Microbial Pathology, University of California, Berkeley. 22
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* Corresponding authors 24
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One sentence summary: A PROHIBITIN-family protein promotes salicylic acid 28
production via the isochorismate pathway in response to stress. 29
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FOOTNOTES 31
Plant Physiology Preview. Published on February 1, 2018, as DOI:10.1104/pp.17.00941
Copyright 2018 by the American Society of Plant Biologists
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List of author contributions: 33
J.J., J.T.G., A.S., and LH designed the research; J.J., J.T.G. and L.H. supervised the 34
experiments; A.S., J.J, A.H.-V., A.F., M.B.J., K.R.G., N.S. and S.C.J. performed the 35
experiments; A.S. and J.J. analyzed the data; S.K.M. and M.C.W. generated and tested the 36
anti-ICS1 antibody; AS, LH and JTG wrote article with contributions from other authors. 37
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Funding Information: 39
This work was supported by: the National Commission for Science and Technology 40
CONICYT (FONDECYT grant N° 1141202 to L.H.), the Millennium Science Initiative 41
(Nucleus for Plant Synthetic and Systems Biology, grant NC130030 to Rodrigo Gutiérrez 42
and L.H.), the National Institute of Health (R01 GM54292 to J.T.G), the National Science 43
Foundation (NSF IOS- 1456904 to J.T.G., NSF2010 0822393 to J.T.G. and Richard W. 44
Michelmore, and NSF IOS-1449100 to M.C.W.). A.S. was supported by a PhD fellowship 45
from CONICYT, K.R.G was supported by NSF training grant 1062713 and N.S. by MCB 46
training grant T32 GM 007183. 47
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Corresponding authors emails: 52
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ABSTRACT 56
Salicylic acid (SA) is a major defense signal in plants. In Arabidopsis (Arabidopsis 57
thaliana), the chloroplast-localized isochorismate pathway is the main source of SA 58
biosynthesis during abiotic stress or pathogen infections. In the first step of the pathway, 59
the enzyme ISOCHORISMATE SYNTHASE 1 (ICS1) converts chorismate to 60
isochorismate. An unknown enzyme subsequently converts isochorismate to SA. Here, we 61
show that ICS1 protein levels increase during UV-C stress. To identify proteins that may 62
play roles in SA production by regulating ICS1, we analyzed proteins that co-63
immunoprecipitated with ICS1 via mass spectrometry. The ICS1 complexes contained a 64
large number of peptides from the PROHIBITIN (PHB) protein family, with PHB3 the 65
most abundant. PHB proteins have diverse biological functions that include acting as 66
scaffolds for protein complex formation and stabilization. PHB3 was previously reported to 67
localize to mitochondria. Using fractionation, protease protection, and live imaging, we 68
show that PHB3 also localizes to chloroplasts, where ICS1 resides. Notably, loss of PHB3 69
function led to decreased ICS1 protein levels in response to UV-C stress. However, ICS1 70
transcript levels remain unchanged, indicating that ICS1 is regulated post-transcriptionally. 71
The phb3 mutant displayed reduced levels of SA, the SA-regulated protein PR1, and 72
hypersensitive cell death in response to UV-C and avirulent strains of Pseudomonas 73
syringae, and correspondingly supported increased growth of P. syringae. Expression of a 74
PHB3 transgene in the phb3 mutant complemented all of these phenotypes. We suggest a 75
model in which the formation of PHB3-ICS1 complexes stabilizes ICS1 to promote SA 76
production in response to stress. 77
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INTRODUCTION 79
The plant hormone salicylic acid (SA) plays diverse roles in development and stress 80
defense responses (Vlot et al., 2009; Dempsey et al., 2011). Its developmental roles include 81
the regulation of flowering time (Martínez et al., 2004), leaf longevity (slow cell death; 82
Morris et al., 2000; Zhang et al., 2013), and biomass (Abreu et al., 2009) in Arabidopsis 83
(Arabidopsis thaliana) as well as thermogenesis in Arum lily inflorescences (Raskin et al., 84
1987). Endogenous SA promotes seed germination under high salinity conditions (Lee et 85
al., 2010), basal thermotolerance (Clarke et al., 2004; Clarke et al., 2009), and growth and 86
development in the presence of the heavy metal cadmium (Guo et al., 2016). During biotic 87
interactions with certain pathogens, SA is needed for the activation of plant defense 88
responses that suppress pathogen growth (Vlot et al., 2009). Such responses can include the 89
transcriptional upregulation of a large number of pathogenesis-related (PR) genes (Uknes et 90
al., 1992; Cao et al., 1997), rapid programmed cell death (the hypersensitive response, Coll 91
et al., 2011; Doorn et al., 2011), defense priming (Beckers et al., 2009; Tateda et al., 2014; 92
Zhang et al., 2014), and systemic immunity to new infections (Fu and Dong, 2013). 93
A major mechanism of SA deployment is through its increased synthesis in 94
response to pathogens and abiotic stresses, such as treatments with UV-C light, high light 95
radiation, and ozone (Yalpani et al., 1994; Nawrath et al., 2002; Ogawa et al., 2005; Mateo 96
et al., 2006; Catinot et al., 2008; Garcion et al., 2008). Interestingly, the Enhanced Disease 97
Susceptibility 5 (EDS5) SA transporter in the chloroplast envelope of Arabidopsis is 98
necessary for stress-induced SA accumulation, possibly due to a regulatory coupling of SA 99
transport and synthesis (Serrano et al., 2013; Yamasaki et al., 2013). Recently, a catabolic 100
enzyme that converts SA to 2,3 dihydroxybenzoic acid in Arabidopsis was described 101
(Zhang et al., 2013). Other enzymes that impact SA levels have been found or inferred from 102
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the presence of metabolites related to SA (Mauch et al., 2001; Nugroho et al., 2001; Vlot et 103
al., 2009; Simoh et al., 2010; Dempsey et al., 2011; Zhang et al., 2017). 104
In plants, there is evidence for two biosynthetic routes for SA: the isochorismate 105
(IC) and the phenylalanine ammonia-lyase (PAL) pathways. As a precursor, both pathways 106
use chorismate, which is the end product of the shikimate pathway that operates in plastids 107
(Poulsen and Verpoorte, 1991; Schmid and Amrhein, 1995). The IC pathway has two steps: 108
one is the reversible conversion of chorismate to isochorismate catalyzed by 109
ISOCHORISMATE SYNTHASE 1 (ICS1), a plastid-localized enzyme (Strawn et al., 110
2007). The second step is the conversion of IC to SA by a mechanism that is still unknown 111
in plants (Strawn et al., 2007). 112
Regulation of SA production under stress conditions through ICS activity differs in 113
different plant species. In Arabidopsis, most of the basal SA is produced via the PAL 114
pathway (Huang et al., 2010), whereas the IC pathway is responsible for approximately 115
90% of SA production induced by pathogens or UV-C light (Wildermuth et al., 2001; 116
Garcion et al., 2008). However, it has been shown that unlike Arabidopsis, the PAL and IC 117
pathways are equally important for pathogen-induced SA biosynthesis in soybean (Glycine 118
max; Shine et al., 2016). Both Arabidopsis and soybean have two ICS genes. In other land 119
plants, such as poplar (Populus trichocarpa), rice (Oryza sativa), castor bean (Ricinus 120
communis), grapeveine (Vitis vinifera), and alfalfa (Medicago truncatula), alternative 121
splicing of a single ICS gene works as a mechanism to modulate ICS function (Macaulay et 122
al., 2017). Therefore, differential ICS regulation may have evolved to accommodate the 123
chemical defense strategies of different plants (Yuan et al., 2009). 124
The level of SA in different tissues after stress treatment was suggested to be critical 125
for defining the type of defense reaction that is established (Fu et al., 2012). Therefore, 126
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knowledge of how SA levels are controlled is a central issue for understanding plant 127
defense responses and stress physiology. A number of proteins are needed for normal SA 128
accumulation due to their regulatory and signaling roles in defense (e.g. Cui et al., 2017; 129
Ding et al., 2015; Lu et al., 2003; Nomura et al., 2012; Okrent et al., 2010; Song et al., 130
2004; Wang et al., 2014). Transcriptional regulators of the ICS1 gene have also been 131
studied (Chen et al., 2009; Zhang et al., 2010; Dempsey et al., 2011; van Verk et al., 2011; 132
Zheng et al., 2012; Wang et al., 2015; Zheng et al., 2015). However, proteins implicated in 133
the direct regulation of ICS1 protein and/or activity levels have yet to be documented. In 134
this study, proteins found to co-immunoprecipitate with ICS1 were identified, and the role 135
of one of them, PROHIBITIN 3 (PHB3), is characterized. PHBs play important and diverse 136
biological functions, such as acting as scaffold proteins for complex formation and 137
stabilization (Steglich et al., 1999; Nijtmans et al., 2000; Van Aken et al., 2007). Many 138
molecular studies in diverse organisms such as yeast, mammals, and plants, have focused 139
on the roles of PHBs in mitochondria (Nijtmans et al., 2000; Piechota et al., 2010; Steglich 140
et al., 1999). The current view is that the principal function of PHBs in mitochondria is to 141
support the structural organization of the inner mitochondrial membrane (Piechota et al., 142
2015). In plants, PHBs play a positive role in the development of new tissues and organs, 143
possibly by maintaining optimal mitochondrial activity (Ahn et al., 2006; Van Aken et al., 144
2007). We show here that PHB3 impacts both ICS1 protein and SA accumulation and is 145
found in chloroplasts, making it a potentially direct regulator of this important pathway. 146
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RESULTS 147
Identification of PROHIBITINS as proteins that form complexes with ICS1 148
We took advantage of previously constructed sid2-2 plants that contain a 149
complementing ICS1-V5 transgene (Strawn et al., 2007) to identify proteins with which 150
ICS1 forms complexes using affinity purification and proteomics. V5 antibody-conjugated 151
beads were used to immunoprecipitate (IP) proteins from extracts of untreated and UV-C-152
treated sid2-2/ICS1-V5 or wild-type (WT) plants. Liquid chromatography-tandem mass 153
spectrometry (LC-MS/MS) analysis showed that together with ICS1 peptides, a large 154
number of peptides from a family of proteins called PROHIBITINs (PHBs, Table 1, 155
Supplemental Table S1) were found in IPs from the ICS1-V5 plant extracts. Supplemental 156
Fig. S1 shows the enrichment of PHBs using spectral counting and Supplemental Table S1 157
shows the top proteins from which peptides were found to be specifically enriched, 158
including PHB3, PHB2, PHB4 and several proteins in the superfamily to which PHBs 159
belong (called Band 7). Immunoblotting using an antibody that recognizes PHB3 and 160
PHB4 (Snedden and Fromm, 1997; Van Aken et al., 2007) confirmed that PHB3/4 proteins 161
were present in the ICS1-containing complexes (Fig. 1A). It was previously shown that the 162
majority of the immunoblot signal found with this antibody comes from PHB3 (Van Aken 163
et al., 2007). The amount of ICS1 and PHB3/4 recovered after IP of ICS1-V5 was 164
unaffected by UV-C light 24 hours post-treatment (Fig. 1B, 1C). These results indicate that 165
the ICS1 protein forms complexes with PHBs. 166
167
ICS1 protein levels are UV-C inducible 168
ICS1 transcript and SA levels are induced by UV-C stress in Arabidopsis (Garcion 169
et al., 2008; Martínez et al., 2004; Nawrath et al., 2002; Serrano et al., 2013; Shapiro and 170
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Zhang, 2001). As expected, under our conditions, UV-C induced ICS1 transcript levels in 171
WT plants (Fig. 2A, left panel). Immunoblot analysis using an antibody that specifically 172
recognizes ICS1 showed that UV-C also induced ICS1 protein accumulation in WT, but not 173
in the negative control (sid2-2 mutant plants; Fig. 2B). 174
These results contrasted with what was observed in sid2-2/ICS1-V5 plants. In 175
particular, sid2-2/ICS1-V5 plants had high levels of both ICS1-V5 transcript and protein 176
even without stress (Fig. 2A, C, D, see also Fig. 1). ICS1 protein levels were so high in the 177
ICS1-V5 plants that the immunoblots were completely overexposed under conditions 178
where ICS1 protein could be detected with the ICS1 antibody in WT (Fig. 2C). Therefore, 179
the blot in Fig. 2C was stripped and re-probed with V5 antibody. In sid2-2/ICS1-V5 plants, 180
basal levels of PR1 and SA were elevated (Fig. 2D, E). Although significant differences in 181
ICS1-V5 transcript or protein levels in response to UV-C were not detected at the time 182
points tested (Fig. 2A, D), SA and PR1 were elevated (Fig 2D, F). Basal levels of SA in 183
sid2-2/ICS1-V5 were affected by the growth conditions; soil-grown plants (Fig. 2E) showed 184
higher levels of SA than in vitro-grown plants (Fig. 2F). These results indicate that the 185
ICS1 protein accumulates upon UV-C stress exposure in WT plants. 186
187
PHB3 localizes to chloroplasts and mitochondria 188
Since ICS1 forms complexes with PHB3/4 and other PHBs, a pool of PHBs may be 189
present in chloroplasts where ICS1 localizes (Strawn et al., 2007). Previously, PHB3 was 190
described as a mitochondrial protein (Ahn et al., 2006; Van Aken et al., 2007). To evaluate 191
the subcellular localization of PHB3, we used confocal microscopy for live imaging of 192
PHB3-GFP and organelle markers transiently expressed in N. benthamiana, and PHB3-193
GFP constitutively expressed in Arabidopsis. The PHB3-GFP fusion was functional, as it 194
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could rescue the known small plant phenotype of an Arabidopsis phb3-3 loss-of-function 195
mutant (Wang et al., 2010) (Fig. 3A). This mutant was confirmed to have reduced levels of 196
PHB3 (Supplemental Fig. S2). The control, untransformed N. benthamiana leaves show 197
very little background signal under our imaging conditions (Fig. 3B; Supplemental Fig. S3). 198
PHB3-GFP localized in chloroplasts and small structures resembling mitochondria in both 199
species (Fig. 3C-E). A portion of the PHB3-GFP signal colocalized with chlorophyll 200
autofluorescence (Fig. 3C; Supplemental Fig. S3; Supplemental Video S1). Additionally, 201
PHB3-GFP colocalized with OEP7-RFP, a marker for the envelope membrane of 202
chloroplasts, around N. benthamiana chloroplasts (Lee et al., 2011; Fig. 3C and 203
Supplemental Fig. S3). Stable Arabidopsis transformants also showed colocalization of a 204
portion of the PHB3-GFP signal with chlorophyll autofluorescence in epidermal 205
chloroplasts (Fig. 3D and Supplemental Fig. S3). 206
A portion of the PHB3-GFP signal outside chloroplasts colocalized in small 207
structures with the mitochondrial markers COX4-mCherry (Nelson et al., 2007) and COX4-208
dsRed (Li et al., 2017) in N. benthamiana (Fig. 3E and Supplemental Fig. S3). PHB3-GFP 209
was also visible in similar structures in Arabidopsis (Fig. 3D). Videos of transiently-210
transformed N. benthamiana show characteristic mobility of the mitochondria labeled with 211
PHB3-GFP and COX4-mCherry (Supplemental Videos S1 and S2). 212
To confirm the chloroplast localization of PHB3 and co-fractionation with ICS1, we 213
isolated chloroplasts from sid2-2/ICS1-V5 plants treated or not with UV-C and performed 214
immunoblot analysis. Chloroplasts from non-treated WT plants were also assessed for 215
PHB3/4 localization. PHB3/4 was detected in chloroplast fractions of untreated WT plants, 216
as well as in untreated and UV-C-treated sid2-2/ICS1-V5 plants (Fig. 4A). As expected, 217
ICS1-V5 was detected in the chloroplasts of untreated and treated sid2-2/ICS1-V5 plants. 218
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We did not observe changes of PHB3/4 or ICS1 localization upon UV treatment. The 219
immunodetection of protein markers for different subcellular fractions confirmed the purity 220
of the chloroplast fractions. Importantly, chloroplasts were not contaminated with the 221
mitochondrial marker Prx IIF (Fig. 4A). 222
PHB is associated with membranes in mitochondria (Piechota et al., 2010; Tatsuta 223
et al., 2005). To test the localization of PHB in chloroplasts, we partitioned isolated 224
chloroplasts into membrane and soluble fractions (Fig. 4B). PHB was detected with the 225
PHB3/4 antibody in the membrane, but not in the soluble fractions of Arabidopsis, 226
spinach and N. benthamiana chloroplasts. The purity of the fractions was confirmed by 227
immunodetection of inner envelope Tic110 and stromal cpHsp70 proteins (Fig. 4B). 228
To test if PHB is a peripheral or internal chloroplast protein, we treated isolated N. 229
benthamiana and Arabidopsis chloroplasts with thermolysin to remove surface proteins 230
(Fig. 4C,D). Transiently produced AtPHB3-GFP in N. benthamiana as well as the native 231
PHBs in N. benthamiana (Fig. 4C) and Arabidopsis (Fig. 4D) chloroplasts were mostly 232
protected from thermolysin. We observed partial proteolysis of the chloroplast inner 233
envelope protein Tic110 (Jackson et al., 1998; Inaba et al., 2003) in both species (Fig. 4C-234
D), as previously reported (Hardré et al., 2014). The outer envelope protein SFR2 from 235
Arabidopsis, detected with an antibody that did not recognize the N. benthamiana protein 236
(Roston et al., 2014), was digested by thermolysin (Fig. 4D). In contrast, the thylakoid 237
membrane-associated proteins AtpB and LHCII, as well as the stromal cpHSP70, were 238
fully protected from proteolysis (Fig. 4C-D). Together, these data demonstrate that a pool 239
of PHB3 localizes to chloroplasts, most likely to the inner envelope. This result is 240
consistent with the finding that this protein forms complexes with ICS1. 241
242
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SA and PR1 levels in response to UV-C treatment and bacterial infections are reduced 243
in the phb3-3 mutant 244
Since ICS1 forms complexes with PHB3, we evaluated whether the phb3-3 mutant 245
is compromised for SA biosynthesis and/or SA-dependent responses after UV-C stress. 246
After UV-C treatment for 24 h, phb3-3 mutant plants showed reduced SA levels compared 247
to WT. However, SA levels in phb3-3 were not as low as those in the SA biosynthesis 248
mutant sid2-2 (Fig. 5A). The SA response marker PR1 also showed lower levels in phb3-3 249
than in WT (Fig. 5B). UV-C failed to induce PR1 in sid2-2 (Fig. 5C), confirming that this 250
marker is SA-dependent under these conditions. Importantly, PR1 was induced in the phb3-251
3 mutant upon SA treatment, indicating that this mutant still responds to SA (Fig. 5D). 252
We also evaluated whether the phb3-3 mutant was compromised for SA 253
accumulation and responses after infection with a pathogen that induces SA. For this 254
purpose, WT and phb3-3 plants were inoculated by flooding (Ishiga et al., 2011) with an 255
avirulent strain of Pseudomonas syringae pv. tomato DC3000 carrying AvrRpm1 256
(Pst/AvrRpm1). Compared to WT, phb3-3 plants showed lower free SA levels 12 h after 257
infection (Fig. 6A), a phenotype that we previously found strongly correlates with 258
hypersusceptibility (e.g. see (Lu et al., 2003; Song et al., 2004; Lee et al., 2007)). 259
Additionally, glycosylated SA levels were lower in phb3-3 mutant plants compared to WT 260
throughout the time course (Fig. 6A). We also detected a lower accumulation of PR1 261
protein in phb3-3 mutant plants compared to WT plants after infection (Fig. 6B). 262
As predicted by the reduced SA accumulation, phb3-3 mutant plants showed greater 263
growth of Pst/AvrRpm1 compared to WT, similar to the sid2-2 mutant phenotype (Fig. 7A). 264
The phb3-3 mutant was also more susceptible to the avirulent strain Pst/AvrRpt2, and 265
showed similar levels of bacterial growth as sid2-2 (Fig. 7B). This result was expected, 266
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since Pst/AvrRpt2 normally triggers SA-dependent disease resistance (Nawrath and 267
Métraux, 1999). In WT plants, these two pathogen isolates induce hypersensitive cell death 268
that can be detected at the individual cell level (Kim et al., 2005). To monitor this, we 269
performed light microscopy of leaves stained with trypan blue within 10 h of infection. 270
Both phb3-3 and sid2-2 showed reduced trypan blue staining relative to WT after 271
inoculation with Pst/AvrRpm1 and Pst/AvrRpt2 strains (Fig. 7C-D). Similar phenotypic 272
effects of sid2 mutations on single cell death events were reported previously (Brodersen et 273
al., 2005; Ichimura et al., 2006; Yao and Greenberg, 2006; Lu, 2009). 274
These experiments show that PHB3 promotes SA production during stress and 275
infection and is needed for SA-dependent processes such as suppression of pathogen 276
growth and the hypersensitive response at the single cell level. 277
278
Expression of PHB3 complements phb3-3 mutant plants in abiotic and biotic stress 279
responses 280
To determine whether the main phenotypes observed in phb3-3 mutant plants were 281
due to the lack of PHB3 function, we transformed the mutant with a pUBQ10:PHB3-V5 282
rescue construct. The small plant phenotype of phb3-3 was complemented in transgenic 283
lines PHB3-V5 #15 and PHB3-V5 #17 (Fig. 8A). The production of PHB3-V5 protein in 284
both lines was verified by immunodetection of the V5 epitope (Fig. 8B). We noticed that in 285
phb3-3 leaf cross sections the tissue morphology appeared less organized than in WT, a 286
phenotype that was also complemented by the PHB3 transgene (Fig. 8C). Importantly, the 287
complemented lines were restored for the UV-C-induced SA and PR1 as well as pathogen 288
resistance phenotypes (Fig. 8D-F). These results confirm that the phenotypes we observed 289
are due to the phb3-3 mutation. 290
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291
PHB3 regulates ICS1 protein levels 292
Since SA levels were decreased in UV-C-stressed phb3-3 plants, we evaluated the 293
effect of the phb3-3 mutation on ICS1 levels. ICS1 transcript levels in WT, phb3-3 and 294
phb3-3/PHB3-V5 complemented plants treated with UV-C light similarly increased with a 295
peak at 2 h after irradiation (Fig. 9A). These data suggest that the regulation of ICS1 by 296
PHB3 may occur at a post-transcriptional level. A decrease in the ICS1 protein levels was 297
detected in phb3-3 plants after the UV-C treatment compared to WT (Fig. 9B). Furthermore, 298
we detected an increase in ICS1 protein levels in the PHB3-V5 complemented lines (Fig. 299
9B) relative to WT levels, in agreement with the increase in SA levels detected in these 300
lines (Fig. 8D). 301
In contrast to ICS1, the levels of chloroplast proteins Tic110, NOA1, AtpB, and 302
LOX2 were not significantly altered in the phb3-3 mutant compared to WT (Fig. 9C). 303
Although phb3-3 affects stressed-induced nitric oxide accumulation in roots (Wang et al., 304
2010), we did not observe differences in the levels of NOA1, a chloroplast protein involved 305
in nitric oxide regulation (Mandal et al., 2012), between phb3-3 and WT leaves (Fig. 9C). 306
These data indicate that phb3-3 does not have a general effect on chloroplast proteins. 307
These results strongly suggest that PHB3 regulates SA accumulation through the 308
formation of complexes and stabilization of ICS1 protein under stress conditions. 309
310
311
DISCUSSION 312
Several lines of evidence from this work suggest that increased SA production 313
during UV-C stress involves the induction of active ICS1-containing multiprotein 314
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14
complexes. First, in WT Arabidopsis ICS1 protein levels increase upon UV-C stress. 315
Second, ICS1 forms complexes with PHB3/4 and other PHBs. Third, during stress, ICS1 316
protein (but not RNA) levels and the downstream accumulation of SA show alterations that 317
are correlated with the amount of functional PHB3 in plants. Our findings support the view 318
that PHB3-ICS1 complexes are needed for the full production of SA during stress caused 319
by UV-C. 320
The role of PHB3 in promoting SA production extends beyond the response to UV-321
C stress to the biotic interaction of Arabidopsis with the strongly SA-inducing (avirulent) 322
bacterial strain Pst/AvrRpm1. The phb3-3 mutant is hypersusceptible to this strain as well 323
as the avirulent strain Pst/AvrRpt2. Thus, it is possible that PHB3 is necessary to promote 324
maximal SA production when the need for this defense signal is high, such as during stress 325
and other biotic interactions. 326
It is interesting that even though there is residual SA accumulation in the phb3-3 327
plants after infection, the mutant is as susceptible to two avirulent Pst strains and shows 328
similar hypersensitive cell death suppression as sid2-2 (an SA biosynthesis mutant). One 329
possible explanation for the strong phenotypes of phb3-3 plants is that this mutant fails to 330
sense SA adequately, therefore a normal defense response is not activated. However, the 331
phb3-3 plants respond normally to exogenous SA treatment, indicating that SA signaling is 332
not altered. Instead, the strong phenotype of phb3-3 might be the result of a delay in these 333
plants to fully induce SA. We speculate that there is a threshold level of SA that is needed 334
early after infection to promote the induction of defenses that suppress the growth of P. 335
syringae. Alternatively, SA-independent signal(s) that are affected in the phb3-3 plants 336
under some conditions, such as nitric oxide (Wang et al., 2010) or ethylene (Christians and 337
Larsen, 2007), may also contribute to plant defense. However, other Arabidopsis plants 338
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15
with reduced SA production, generated by altered expression/mutation of genes that impact 339
SA biosynthesis, exhibit enhanced susceptibility to virulent and avirulent pathogens, 340
similar to the phenotype of phb3-3 (Delaney et al., 1994; Nawrath and Métraux, 1999; 341
Dewdney et al., 2000; Lu et al., 2003; Song et al., 2004; Lee et al., 2007). 342
In addition to identifying the localization of PHB3 in mitochondria (Van Aken et al., 343
2007; Piechota et al., 2010), this work established that there is a pool of PHB3 that resides 344
in chloroplasts, the organelle where SA is made and where ICS1 is targeted (Fragnire et al., 345
2011; Strawn et al., 2007). PHB3 has a hydrophobic region at the N-terminus that likely 346
serves to anchor it to membranes. In agreement with this, PHB3 fractionated with 347
chloroplast membranes and was mostly protected from thermolysin proteolysis, while 348
PHB3-GFP fluorescence was detected at the periphery of chloroplasts, similar to an 349
envelope marker. This study thus establishes that PHB3 is targeted to chloroplasts, similar 350
to PHB2 and PHB4, which were previously reported to be in the envelope fraction of 351
chloroplasts in a proteomics study (Kleffmann et al., 2004). 352
PHB3 was previously found to co-purify with PHB1, PHB2, PHB4, and PHB6 (Van 353
Aken et al., 2007). It was also described as residing in two very large (1-2 MDa) 354
mitochondrial inner membrane complexes with this same group of PHBs (Piechota et al., 355
2010). These five PHBs all co-immunoprecipitate with ICS1. The mitochondrial matrix 356
AAA protease FTSH10 immunoprecipitates four of the five PHBs (except PHB4) that we 357
identified in ICS1 complexes as well as FTSH3 (Piechota et al., 2010). This result suggests 358
that at least four PHBs may form complexes together in more than one subcellular 359
compartment along with one or more non-PHB protein (FTSH10 and FTSH3 in 360
mitochondria and ICS1 in chloroplasts). PHBs form ring-like structures in the inner 361
mitochondrial membrane (Tatsuta et al., 2005; Piechota et al., 2010). If PHBs are capable 362
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16
of forming similar structures/complexes in the chloroplast envelope, they may facilitate the 363
import of ICS1 to this organelle or affect its stability after import. Future work will focus 364
on whether PHBs act as chaperones and whether they form large complexes within 365
chloroplasts. 366
How might PHBs regulate ICS1 levels and/or activity? Using an assay in which 367
radiolabeled ICS1 was imported into isolated pea chloroplasts and then fractionated, Strawn 368
et al. (2007) showed that the majority of ICS1 partitioned with the soluble stroma. However, 369
a fraction of the imported ICS1 protein also partitioned with chloroplast membranes, 370
indicating that ICS1 activity or stability might involve its association with the chloroplast 371
envelope where PHBs most likely reside. ICS1-GFP signals were also visible in the 372
peripheries of chloroplasts, including in stromules (Garcion et al., 2008). In some systems, 373
PHBs play a role in the stabilization of protein complexes. Disruption of mitochondrial 374
PHBs usually destabilizes their interactors and leads to various mitochondrial malfunctions 375
(Van Aken et al., 2007). In yeast, the PHB complex binds to two newly synthesized 376
subunits of respiratory complex IV and protects them against degradation (Steglich et al., 377
1999; Nijtmans et al., 2000). In Arabidopsis, we demonstrate that disruption of PHB3 378
function affects ICS1 protein levels without affecting ICS1 RNA accumulation. Therefore, 379
PHB3 may mainly act by stabilizing the ICS1 protein, although it may also affect ICS1 380
activity. Given the large number of PHBs found in ICS1 complexes, it is possible that 381
several PHBs affect ICS1 and SA production. 382
We focused here on how PHB3 affects ICS1, and downstream production of SA and 383
defense-related signaling outputs such as PR1 and cell death induction. Plants with the 384
phb3-3 mutation show phenotypes that are distinct from those caused by reduced ICS1/SA 385
levels. In particular, the small plant phenotype characteristic of phb3 mutants was not 386
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17
observed in sid2 plants. The product of ICS1, isochorismate, is also a precursor for 387
phylloquinone, which is barely detectable in the Arabidopsis sid2ics2 double mutant; this 388
mutant displays a dwarf phenotype similar to phb3-3, although the cause of the dwarfism is 389
not known (Garcion et al., 2008). Disruption in the penultimate step in phylloquinone 390
synthesis also results in a modest reduction in plant size (Lohmann et al., 2006). Future 391
work will determine whether PHB3 is required for phylloquinone biosynthesis. It seems 392
possible that the different phb3-3 mutant phenotypes (reduced size, stress-induced SA and 393
NO) are due to reduced levels of different specific complexes that contain PHB3 in 394
chloroplasts and mitochondria, respectively. 395
An unanticipated finding resulting from this study is the phenotype of the ICS1-V5-396
expressing plants, which have very high levels of ICS1 relative to WT. These plants have 397
only moderately elevated basal SA levels that are further increased in response to UV-C. 398
Why are the SA levels in ICS1-V5 plants low without stress? One possibility is that the 399
upstream chorismate precursor is limiting under basal conditions. This seems unlikely 400
given that basal chorismate production from the shikimate pathway is usually high 401
(Herrmann and Weaver, 1999). Another possibility is that ICS1 needs activation (for 402
example, by posttranslational modification(s) or conformational change), or another step 403
required for SA production is limiting. 404
In conclusion, this work demonstrated that ICS1 is regulated post-transcriptionally 405
by PHB3. During stress or infection, highly increased SA production in Arabidopsis likely 406
requires an increase in ICS1 protein levels. Upon stress, this increase in ICS1 protein levels 407
is mainly produced by transcriptional activation of the ICS1 gene (Wildermuth et al., 2001; 408
Martínez et al., 2004; Garcion et al., 2008). PHB3 seems to contribute to this process 409
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18
through stabilization of the protein. Whether PHB3 is involved in SA production beyond 410
stabilizing ICS1 will be an interesting topic for future studies. 411
412
413
MATERIALS AND METHODS 414
Plant growth conditions and UV-C treatments 415
Arabidopsis (Arabidopsis thaliana) wild type (WT), phb3-3 (Wang et al., 2010), 416
sid2-2 (Wildermuth et al., 2001), sid2-2/ICS1-V5 (transgenic complementation line SM156 417
in (Strawn et al., 2007)), phb3-3/pUBQ:PHB3-V5 and phb3-3/p35S:PHB3-GFP lines (this 418
report) were in the Columbia (Col-0) background. For plants grown in vitro, growth 419
medium contained 0.5X MS supplemented with 10 g/l sucrose and 2.6 g/l Phytagel (Sigma) 420
and plants were incubated in a growth chamber (16 h light, 100 μmoles m-2 s-1, 22 ± 2ºC). 421
Nicotiana benthamiana plants were grown in soil at 24°C with 16 h light/8 h dark cycle. 422
Fresh spinach was purchased from a store. 423
For fractionation and immunoprecipitation experiments, plants were grown in soil in 424
conditions described in (Pattanayak et al., 2012). Briefly, plants were grown in 50:50 425
Fafard C2/Metro-Mix soil, 16 h light/8 h dark cycle at 20°C. Otherwise, plants were grown 426
in turba soil mixed with vermiculite 1:1 in a growth room under controlled conditions (16 h 427
light/8 h dark cycle, 100 μmoles m-2 s-1, 22 ± 2ºC). 428
UV-C treatments for immunoprecipitation of ICS1-V5 complexes and fractionation 429
experiments were performed with 21-day-old WT and sid2-2/ICS1-V5 plants grown in soil, 430
by exposing them to 254 nm light from an 18.4 W UVP Mineralight UVGL-55 multiband 431
lamp for 45 minutes at a distance of 30 cm in a growth chamber. Leaves were collected 432
from untreated plants and from plants 8, 24 and 32 h after UV-C exposure. For UV-C 433
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irradiation of in vitro-grown plants (Nawrath and Métraux, 1999), 14-day-old plants were 434
exposed for 20 min in a chamber equipped with two UV-C fluorescent tubes of 8 Watts 435
each one (λ= 254 nm) at a distance of 30 cm above the plates. UV-treated plants were 436
subsequently placed in a growth chamber under controlled conditions for the indicated 437
periods of time. 438
For SA treatments, 15-day-old seedlings were floated in a solution of 0.5 mM SA in 439
0.5X MS medium (treatment), or 0.5X MS medium as a control, and incubated for the 440
indicated periods of time under continuous light (80 μmoles m-2 s-1). For immunoblot 441
analysis, whole seedlings were immediately frozen in liquid nitrogen and stored at -80ºC 442
until total protein extraction. 443
444
Bacterial strains, plant inoculation conditions, bacterial proliferation and cell death 445
assays 446
Pseudomonas syringae pv. tomato DC3000 virulent strain (Pst), and its isogenic 447
avirulent strains expressing the AvrRpm1 (Pst/AvrRpm1) or AvrRpt2 (Pst/AvrRpt2) 448
effectors (Mackey et al., 2002) were used. Bacterial strains were grown at 28°C on King’s 449
B medium supplemented with kanamycin at 50 μg/ml, sedimented by centrifugation and 450
then resuspended in 10 mM MgCl2. Bacterial inoculation was performed by the flooding of 451
14-days-old plants (Ishiga et al., 2011) using a suspension of 1 x 106 CFU/ml (to evaluate 452
SA and PR-1 levels) or 1 x 105 CFU/ml (for bacterial proliferation assays). Alternatively, 453
bacteria were inoculated by syringe infiltration on the abaxial side of the leaves of 4-week-454
old soil-grown plants (Greenberg et al., 2000), using a suspension of 1 x 105 CFU/ml (for 455
bacterial proliferation assays) or 1 x 108 CFU/ml (for cell death assays). 456
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For bacterial proliferation assays, leaf discs from 12 independent inoculated plants 457
were taken at 0, 2 and/or 3 days post-inoculation (dpi), ground in 10 mM MgCl2, serially 458
diluted 1:10, and plated onto LB agar plates supplemented with 50 μg/ml kanamycin and 459
50 μg/ml rifampicin. Plates were incubated at 28°C for 2 days and the colony forming units 460
(CFUs) were manually counted. Each bacterial proliferation assay was repeated 3 times. 461
Cell death was visualized in leaves of 4-week-old soil-grown plants 10 h after 462
bacterial or mock (control) infiltration, using trypan blue staining (Pavet et al., 2005). 463
Leaves immersed in freshly prepared lactophenol solution (water, basic phenol, lactic acid, 464
and glycerol in a 1:1:1:1 ratio) containing trypan blue at 250 μg/ml were placed in boiling 465
water for 3 min, cleared in lactophenol without trypan blue at room temperature for 16 h, 466
mounted in 50% glycerol and observed using a compound microscope (Nikon Eclipse 80i 467
microscope) at ×100 magnification. This experiment was repeated 3 times. In each 468
experiment, leaves from 4 independent plants per genotype were analyzed. 469
470
471
Light microscopy of leaf sections 472
Small slices of the sixth leaf of 3-week-old plants were prepared as described in 473
(Leon et al., 2007). Briefly, samples were fixed overnight in 2.5% (v/v) glutaraldehyde in 474
0.1 M sodium cacodylate buffer, pH 7.0, and postfixed in 1% osmium tetroxide for 2 h. 475
Then samples were stained with 1% (w/v) uranyl acetate for 90 min, dehydrated with 476
acetone solutions (50, 70, 95 and 100%) and embedded in Epon resin. Polymerization was 477
performed in a heater at 60ºC for 48 h. Fine cuts (80 nm) obtained with a Sorvall MT5000 478
ultramicrotome were stained with 4% (w/v) uranyl acetate and lead citrate in methanol. 479
Images were taken with a Nikon Eclipse 80i microscope at 40X magnification. 480
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21
481
Genetic constructs and stable plant transformation 482
To obtain phb3-3 mutant plants complemented with PHB3, pUBQ10:PHB3-V5 and 483
p35S:PHB3-GFP constructs were generated using the Gateway technology following the 484
manufacturer’s instructions (Invitrogen, Carlsbad, CA, USA). The PHB3 coding region was 485
obtained by amplification of cDNA, using the primers described in Supplemental Table S2. 486
The PCR fragment was cloned into the pENTR/SD/D-TOPO vector (Invitrogen, Carlsbad, 487
CA, USA) and then recombined into the pB7m34gW vector (Karimi et al., 2005) to express 488
the PHB3 protein fused to a V5 tag controlled by the ubiquitin 10 promoter (pUBQ10). For 489
live imaging experiments of PHB3 fused to the eGFP reporter protein, the amplified PHB3 490
coding sequence cloned into the pENTR/SD/D-TOPO vector was recombined into the 491
pKGWG2 vector (Karimi et al., 2002). 492
Final constructs were verified by sequencing and introduced into the Agrobacterium 493
tumefaciens C58 strain. Stable Arabidopsis phb3-3 transformants were obtained using the 494
floral dip method (Zhang et al., 2006). Transgenic seeds were selected in 0.5X MS solid 495
medium supplemented with 50 μg/ml kanamycin for the GFP reporter lines or with 15 496
μg/ml glufosinate-ammonium for the PHB3-V5-expressing lines. Homozygous transgenic 497
lines were selected and used for further analyses. 498
499
500
Protein extracts, immunoblot and immunoprecipitation. 501
Total protein extracts were obtained from WT and transgenic lines by homogenizing 502
0.5 g tissue (each sample was composed of 12-15 seedlings grown on plates, or several 503
mature leaves of plants grown in soil) per 1 ml extraction buffer (50 mM sodium phosphate 504
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22
buffer pH 8.0, 150 mM NaCl, 0.2% Igepal, 5 mM EDTA, 1X complete Protease Inhibitor 505
Cocktail solution (Roche Diagnostics Corporation, Indianapolis, IN, USA)). After 506
centrifugation at 2,350 g for 10 min, supernatants were transferred to clean tubes and stored 507
at –80°C. Protein concentration was determined by the Bradford method, using the BioRad 508
Protein Assay (Bio-Rad Laboratories, Inc., CA, USA). 509
For immunoblot analysis, total or chloroplast (see below) protein extracts (30 µg of 510
protein per lane) were separated in 12% SDS–polyacrylamide gels (PAGE) and transferred 511
to ImmobilonTM PDVF membranes (Millipore Co., Bedford, MA, USA) by semi-dry 512
electroblotting. The membranes were blocked for 1 h in phosphate-buffered saline solution 513
(PBS-T: 137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 2 mM KH2PO4 and 0,1% Tween-514
20) with 5% non-fat dried milk at room temperature. To detect the ICS1-V5 and PHB3-V5 515
fusion proteins, the mouse monoclonal V5 antibody (Invitrogen, Carlsbad, CA, USA, 516
#R96025) was used at a 1:5,000 dilution. To detect PR1, the rabbit polyclonal PR1 517
antibody (Agrisera, Vännäs, Sweden, #AS10 687) was used at a 1:2,500 dilution. PHBs 518
were detected with the PHB3/4 rabbit polyclonal antibody described in (Snedden and 519
Fromm, 1997; Van Aken et al., 2007) at a 1:10,000 dilution. 520
ICS1 was detected with the rabbit polyclonal ICS1 antibody UCB68 at a 1:10,000 521
dilution. This antibody was made by immunizing rabbits with purified recombinant mature 522
ICS1 protein (described in Strawn et al., 2007). The titer and specificity of the antibody 523
were tested by immunoblot analysis. The antibody can detect as low as 31 pg of 524
recombinant ICS1 protein, with ready detection of ICS1 protein in UV-C-induced leaf 525
extracts using a 1:10,000 dilution. 526
Proteins used as markers for subcellular localization were detected using rabbit 527
polyclonal antibodies against: ATP synthase β (AtpB, chloroplast, 1:10,000, a gift from Dr. 528
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23
Dominique Drapier, CNRS/UPMC, Paris, France (Drapier et al., 2007)), peroxiredoxin II F 529
(Prx II F, mitochondria, 1:3,000; a gift from Dr. Karl-Josef Dietz, Bielefeld University, 530
Germany (Finkemeier et al., 2005)), cytosolic fructose-1,6.bisphosphatase (cFBPase, 531
cytoplasm, 1:25,000, Agrisera, Vännäs, Sweden, #AS04043), H+ATPase (plasma 532
membrane, 1:2,000, Agrisera, Vännäs, Sweden, #AS07260), Tic110 (chloroplast inner 533
envelope, 1:3000, a gift from Masato Nakai, Osaka University, Japan (Kikuchi et al., 534
2006)), NOA1 (1:10000, a gift from Pradeep Kachroo, University of Kentucky, USA 535
(Mandal et al., 2012)), chloroplast lipooxygenase 2 (LOX-C, 1:30,000, Agrisera, Vännäs, 536
Sweden, #AS07258), light harvesting complex II (LHCII, thylakoid, 1:5000, a gift from 537
Roberto Bassi, Università di Verona, Italy (Di Paolo et al., 1990)), SFR2 N and C 538
(chloroplast outer envelope, 1:1000, a gift from Christoph Benning, Michigan State 539
University, USA (Roston et al., 2014)) and a guinea pig antibody against chloroplast Hsp70 540
(1:12000, a gift from Thomas Leustek, Rutgers University, USA (Wang et al., 1993)). 541
After washing in PBS-T (three times), the membranes were incubated with the anti-542
mouse horseradish peroxidase-conjugated secondary antibody (KPL, Gaithersburg, MD, 543
USA. #074-1806, 1:10,000 and Thermo Fisher, Waltham, MA, USA, #31440, 1:1,000), 544
anti-guinea pig-peroxidase antibody (Sigma, St. Louis, MO, USA, #A5545, 1:100,000) or 545
the anti-rabbit horseradish antibody (Invitrogen, Carlsbad, CA, USA. #65-6120, 1:10,000 546
and Thermo Fisher, Waltham, MA, USA, #32460, 1:1,000). After washing in PBS-T (15 547
min three times), proteins immunodetected were visualized using chemiluminescence kits 548
(Thermo Fisher Scientific, Waltham, MA, USA, #34087 and #34095) according to the 549
manufacturer’s instructions. 550
All immunoblots were repeated 2 or 3 times with new samples, as indicated in the 551
corresponding figure legends. 552
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24
For immunoprecipitation of ICS1-V5-containing complexes, total protein extracts of 553
untreated and UV-C treated WT and sid2-2/ICS1-V5 plants (see above) were prepared as 554
described (Jelenska et al., 2010). Immunoprecipitation was performed using anti-V5 555
agarose (Sigma, St. Louis, MO, USA, #A7345). After immunoprecipitation, proteins were 556
separated by SDS/PAGE and analyzed either by Coomassie blue staining and LC-MS/MS 557
or by immunoblotting. This experiment was repeated 3 times with different protein extract 558
samples. 559
560
Database searching of peptides identified by LC-MS/MS. 561
LC-MS/MS protein identification was performed at the Stanford University Mass 562
Spectrometry Facility. Protein bands in ICS1-V5 immunoprecipitations and the 563
corresponding regions of WT samples were cut from the gel; UV-C treated and untreated 564
samples were combined for further analysis. Proteins in gel bands were digested with 565
trypsin, extracted, and peptides were identified as described (Jelenska et al., 2010). MS/MS 566
samples were analyzed using Sequest (Thermo Fisher Scientific, San Jose, CA, USA; 567
version v.27, rev. 11) set up to search the NCBI_Arabidopsis_thaliana database (222041 568
entries) assuming the digestion enzyme stricttrypsin. Scaffold (version Scaffold_3.1.4.1, 569
Proteome Software Inc., Portland, OR, USA) was used to validate MS/MS-based 570
identifications of peptides (at greater than 95.0% probability) and proteins (at greater than 571
95.0% probability and contained at least one identified peptide). 572
573
Chloroplast isolation, fractionation and thermolysin treatment 574
Three-week-old WT and sid2-2/ICS1-V5 plants grown in soil were treated with UV-575
C and chloroplasts were isolated in a percoll gradient as described (Kubis and Lilley, 2008). 576
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25
Purity of the chloroplast fractions was evaluated by immunoblot analysis as described 577
above. N. benthamiana chloroplasts were isolated and treated with thermolysin (Sigma, St. 578
Louis, MO, USA) according to (Lamppa, 1995). Arabidopsis chloroplasts for thermolysin 579
treatment experiment were isolated as described (Aronsson and Jarvis, 2002). For 580
membrane association, spinach, N. benthamiana and Arabidopsis chloroplasts were isolated 581
as described in (Kieselbach et al., 1998) and partitioned to membrane and soluble fractions 582
according to (Lamppa, 1995). 583
584
Confocal microscopy 585
Leaves and cotyledons of ~10-day-old seedlings of phb3-3/PHB3-GFP (line #6.5) 586
Arabidopsis were used for imaging. Nicotiana benthamiana plants grown in soil were 587
transiently transformed with Agrobacteria harboring the constructs p35S:PHB3-GFP (see 588
above), pdex:OEP7-RFP (Cecchini et al., 2015), pd35S:COX4-mCherry (mt-rb, (Nelson et 589
al., 2007)) and p35S:COX4-dsRed (from Dr. Guo-Liang Wang, Ohio State University, USA 590
(Li et al., 2017)) as described in (Cecchini et al., 2015; Jelenska et al., 2017). Agrobacteria 591
with different constructs were infiltrated together and N. benthamiana leaves were imaged 592
2 days later. Leaves transformed with pdex:OEP7-RFP construct were sprayed with 3 µM 593
dexamethasone 1 day after agroinfiltration. Three-week-old N. benthamiana plants were 594
used for images with mitochondrial markers and 5–6-week-old plants for other images. 595
Confocal images were obtained using a Zeiss LSM710 laser-scanning confocal 596
microscope (Zeiss, Germany) as described in (Kang et al., 2014; Cecchini et al., 2015; 597
Jelenska et al., 2017). Fluorescence was visualized as follows: GFP excitation 488 598
nm/emission 496-557 nm; RFP ex 561/em 570-619, mCherry/dsRed ex 561/em 580-629, 599
chlorophyll autofluorescence ex 633/em 655-747. Fluorescence in different channels was 600
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26
acquired for the same field using a sequential acquisition mode for PHB3 + OEP7 and 601
simultaneously for PHB3 + COX4 to allow imaging of moving mitochondria. Z series 602
images (14-18 µm for N. benthamiana and 21 µm for Arabidopsis) and time series videos 603
(60 image series) were processed with ImageJ (https://imagej.nih.gov/ij). 604
605
Quantitation of SA 606
Free and glycosylated SA (SAG) extracted from leaf tissues were quantified by 607
HPLC using an Atlantis T3 column, as described previously (Verberne et al., 2002). 608
Results show data from 3 independent experiments. Each sample consisted of 0.5 g of 609
tissue collected from a pool of 12-15 seedlings grown on plates or mature leaves from three 610
plants grown in soil. 611
612
RT-qPCR analysis 613
Whole UV-C-treated plants (4 seedlings per genotype) grown in vitro were frozen 614
in liquid nitrogen and stored at -80ºC until RNA isolation. Total RNA was obtained from 615
frozen samples using the TRIzol® Reagent (Invitrogen) according to the manufacturer’s 616
instructions. cDNA was synthesized from each sample (2 μg of total RNA) with an ImProm 617
II Kit (Promega). qPCR was performed using the Brilliant III Ultra-Fast SYBR® Green 618
QPCR Master mix (Agilent Technologies) reagents on a AriaMx real-time PCR system 619
equipment. The expression levels of the ICS1 gene were calculated relative to the YLS8 620
endogenous control. Primers used for each gene are listed in Supplemental Table S2. Data 621
represent results from 3 biological replicates (independent experiments). In each 622
experiment, 3 technical replicates from the same cDNA sample were used. 623
624
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27
Accession numbers 625
Sequence data from this article are found in the GenBank/EMBL data libraries under the 626
following accession numbers: 627
-Arabidopsis thaliana: AT1G74710 (ICS1/SID2), AT5G40770 (PHB3), AT1G03860 628
(PHB2), AT3G27280 (PHB4), AT2G20530 (PHB6), AT4G28510 (PHB1), AT2G14610 629
(PR-1), AT5G08290 (YLS8), AT3G52420 (OEP7), AT3G06050 (PRX IIF), AT1G43670 630
(cFBPase), AT4G30190 (H+-ATPase), AT1G06950 (TIC110), AT4G24280 (cpHSP70-1), 631
AT5G49910 (cpHSP70-2), AT3G06510 (SFR2), ATCG00480 (AtpB), AT3G47450 632
(NOA1), AT3G45140 (LOX2). 633
-Saccharomyces cerevisiae: YGL187C (ScCOX4). 634
-Nicotiana benthamiana: DQ121387 (NbPHB1) and DQ121388 (NbPHB2). 635
-Spinacia oleracea: AF035456, AF039083, M99565 (So-cpHSP70s), 2715576 (SoAtpB). 636
637
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28
SUPPLEMENTAL MATERIAL 638
The following supplemental materials are available: 639
Supplemental Figure S1. Spectrum count of peptides detected by LC-MS/MS in WT and 640
sid2-2/ICS1-V5 samples. 641
Supplemental Figure S2. PHB3/4 protein levels in UV-C treated WT, phb3-3 and phb3-642
3/PHB3-V5 plants. 643
Supplemental Figure S3. PHB3 colocalizes with chloroplasts and mitochondria markers. 644
Supplemental Video S1. Video of N. benthamiana leaves expressing PHB3-GFP. 645
Supplemental Video S2. Video of N. benthamiana leaves expressing PHB3-GFP and 646
mitochondrial marker COX4-mCherry. 647
Supplemental Table S1. List of peptides matching PHB protein family members and ICS1 648
and the number of spectra of peptides found from the top enriched proteins identified in 649
ICS1-V5 complexes by LC-MS/MS analysis 650
Supplemental Table S2. List of primers 651
652
ACKNOWLEDGMENTS 653
We thank Hillel Fromm (Department of Plant Sciences, Tel Aviv University, Israel) for 654
providing the PHB3/4 antibody, Dominique Drapier (CNRS/UPMC, Paris, France) for the 655
AtpB antibody, Karl-Josef Dietz (Bielefeld University, Germany) for the Prx II F antibody, 656
Pradeep Kachroo (University of Kentucky, USA) for the NOA1 antibody, Gopal 657
Pattanayak (University of Chicago, USA) for antibodies against Tic110 (from Masato 658
Nakai, Osaka University, Japan) and LHCII (from Roberto Bassi, Università di Verona, 659
Italy), Thomas Leustek, (Rutgers University, USA) for the cpHSP70 antibody, Christoph 660
Benning (Michigan State University, USA) for the SFR2 antibody, Andreas Nebenführ 661
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29
(University of Tennessee, Knoxville, USA) for the COX4-mCherry marker, and Guo-Liang 662
Wang (Ohio State University, USA) for the COX4-dsRed marker. We also thank Jacquelyn 663
Freeman for help with IP experiments and Michael Allara for help with western blots. 664
665
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30
Table 1. Prohibitins found in ICS1-V5 complexes purified from sid2-2/ICS1-V5 plants. 666
Peptides were identified by LC-MS/MS analysis. (*) Number of different peptides that 667
match the indicated protein and may also match one or more other isoforms. (†) Number of 668
peptides specific to the protein isoform indicated. The column % Coverage shows the 669
percentage of the protein sequence represented by the peptides found in the LC-MS/MS 670
analysis. 671
672
673
Proteins AGI Unique peptides*
Specific peptides† % Coverage
ICS1 AT1G74710 16 13 42 PHB3 AT5G40770 19 11 69 PHB2 AT1G03860 16 9 64 PHB4 AT3G27280 12 4 53 PHB6 AT2G20530 9 4 37 PHB1 AT4G28510 8 4 39
674 675
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FIGURE LEGENDS 676 677
Figure 1. PHB3/4 form complexes with ICS1-V5 under normal and stress conditions. 678
A, Plants grown in soil were treated with UV-C light for 45 min and assayed 24 h later (+). 679
Untreated plants were used as controls (-). Total proteins were extracted and ICS1-680
containing complexes were immunoprecipitated with V5 antibody. The presence of ICS1-681
V5 and PHB3/4 proteins in the total protein extracts (input, left panel) and in the 682
immunoprecipitated ICS1-containing complexes (IP αV5, right panel) was confirmed by 683
immunoblotting using V5 and PHB3/4 antibodies. Coomassie-stained membranes (bottom) 684
show similar loading. Panel A shows a representative blot from three independent 685
experiments (two from total extracts and one from chloroplast extracts). B and C, 686
Quantitation of ICS1-V5 (B) and PHB3/4 (C) protein levels from three IP experiments as 687
in (A), using densitometry with the Image J program. RU, relative units with respect to an 688
average signal of untreated samples for each antibody. Error bars are SEM. Statistical 689
analyses using t-tests showed that no significant differences were detected (p=0.78 for B 690
and; p=0.97 for C). 691
692
Figure 2. ICS1 protein levels are induced by UV-C treatment and constitutively 693
elevated in sid2-2/ICS1-V5 plants. Plants were grown on plates (A, B, C, F) or soil (D), 694
treated with UV-C for 20 min (A, B, C, F) or 45 min (D), and assayed 0, 8 and 24 h post-695
treatment (hpt). Untreated plants were used as controls (–). A, ICS1 transcript levels were 696
measured by RT-qPCR in WT (black bars) and sid2-2/ICS1-V5 (gray bars) plants. 697
Expression is relative (R.) to YLS8 gene (n=3 biological replicates). B, Immunoblot 698
analysis showing that ICS1 is induced by UV-C treatment in WT and is not detectable in 699
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32
sid2-2 plants. C, ICS1 protein levels detected by immunoblot in WT and sid2-2/ICS1-V5 700
plants using ICS1 or V5 antibodies. D, PR1 and ICS1-V5 protein levels in WT and sid2-701
2/ICS1-V5 plants. In sid2-2/ICS1-V5 plants, basal PR1 is elevated and UV-C further 702
induces the PR1 level. B-D, Panels show representative results from three (B, C) or two (D) 703
independent experiments. Coomassie stained membranes (bottom) show similar loading. E, 704
F, Free SA (gray bars) and glycosylated SA (SAG, white bars) levels in the indicated plants. 705
E, untreated plants grown in soil, showing higher SA levels in the sid2-2/ICS1-V5 plants 706
than WT (n=5). Similar results were found in an independent trial. F, Free SA and SAG 707
levels are induced by UV-C in plants with constitutive ICS1 levels (n=3). Statistical 708
analyses in A, E and F were performed using ANOVA/Fisher’s LSD test. Each letter group 709
differs from other letter groups at p<0.05. Error bars show SEM. 710
711
Figure 3. PHB3-GFP localizes to chloroplasts and mitochondria. A, Morphological 712
phenotype of the phb3-3 mutant is complemented by the PHB3-GFP transgene. Images 713
show 3-week-old WT, phb3-3 and phb3-3/PHB3-GFP #6.5 plants grown in soil. B-E, 714
Untransformed (B) and transiently transformed (C, E) N. benthamiana leaves, and stably 715
transformed Arabidopsis cotyledons (D), were imaged by confocal microscopy in green 716
(PHB3-GFP), red (OEP7-RFP and COX4-mCherry), chl (chlorophyll auto-fluorescence, 717
blue), and DIC (bright field) channels. Maximum intensity projections of Z series images 718
are shown. Scale bar = 20 µm. C, Co-expression of PHB3-GFP and OEP7-RFP 719
(chloroplast outer envelope marker). D, Expression of PHB3-GFP in Arabidopsis (phb3-720
3/PHB3-GFP #6.5 plants). E, Co-expression of PHB3-GFP and COX4-mCherry 721
(mitochondrial marker). Insert shows higher magnification. Co-localization with COX4-722
dsRed was also performed with similar results. White arrowheads show examples of PHB3-723
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33
GFP localization in chloroplasts (C-E). Experiments were done two (D) or three times (B, 724
C and E) with similar results. 725
726
Figure 4. PHB3 is a chloroplast membrane protein. A, Immunodetection of PHB3/4 in 727
total and chloroplast protein extracts obtained from WT and sid2/ICS1-V5 plants, grown in 728
soil, treated (+) or not (–) with UV-C as described in Figure 1. ICS1-V5 was detected with 729
V5 antibody and PHBs were detected with a PHB3/4 antibody (n=2). The purity of the 730
chloroplast fraction was evaluated by immunodetection of markers for specific subcellular 731
localizations (AtpB for chloroplast; Prx II F for mitochondria; cFBPase for cytoplasm; 732
H+ATPase for plasma membrane). B, PHB3 localizes to chloroplast membranes. Spinach 733
(So), Arabidopsis (At) and N. benthamiana (Nb) chloroplasts were partitioned into 734
membrane and soluble fractions. PHB was detected with a PHB3/4 antibody and fractions 735
were confirmed with antibodies against pea Tic110 (membrane) and spinach cpHsp70 736
(soluble). Coomassie blue (A) and Ponceau S (B) staining of proteins show similar loading. 737
C-D, PHB is mostly protected from thermolysin digestion in N. benthamiana (C) and 738
Arabidopsis (D) chloroplasts. Chloroplasts isolated from N. benthamiana transiently 739
expressing AtPHB3-GFP (C, upper 4 panels), control N. benthamiana (C, lower 2 panels) 740
or from Arabidopsis leaves (D) were incubated without (-) or with (+) thermolysin (Therm). 741
Sensitivity to digestion was evaluated for PHB (PHB3/4 antibody), AtPHB3-GFP (GFP 742
antibody and PHB3/4 antibody, C) and for the inner envelope Tic110, the outer envelope 743
SFR2 (D), the thylakoid-associated AtpB, and LHCII and the stromal cpHsp70 (C). Black 744
arrowheads indicate AtPHB3-GFP and white arrowhead shows NbPHB (C). Similar results 745
were obtained for 2-4 chloroplast preparations. 746
747
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34
Figure 5. phb3-3 mutant plants have lower SA levels and PR1 accumulation after UV-748
C treatment. A-C, Leaf tissue from plants grown on plates treated with UV-C light for 20 749
min and harvested at 2, 8, and 24 hpt. Untreated plants were used as controls (–). A, Levels 750
of free SA (gray bars) and glycosylated SA (SAG, white bars). FW indicates fresh weight; 751
n/d indicates not detected. Each bar represents the mean of three independent experiments; 752
error bars indicate SEM. Statistical analysis was performed using ANOVA/Fisher’s test. 753
Different letters denote statistically significant differences at p<0.05. B-C, PR1 protein was 754
detected by immunoblot analysis using a PR1 antibody. A non-specific band in the 755
immunoblot shows similar loading in each lane. Panels show representative results from 756
three independent experiments. D, Two-week-old WT, phb3-3, and sid2-2 plants grown on 757
plates were treated with liquid MS growth medium alone (Control) or MS medium 758
supplemented with 0.5 mM SA for 2, 8, and 24 hours. (–) indicates untreated plants. PR1 759
protein was detected by immunoblot analysis as in (B-C). An independent trial of this 760
experiment showed similar results. 761
762
Figure 6. phb3-3 mutant plants have reduced SA accumulation and PR1 protein levels 763
after infection with Pseudomonas syringae pv tomato DC3000/AvrRpm1. Two-week-764
old plants grown on plates were flooded with Pst/AvrRpm1 bacteria at a concentration of 765
1x106 CFU/mL. Samples were taken at 0, 12, 24, 48, and 72 hours post-infection (hpi). A, 766
The levels of free SA (gray bars) and glycosylated SA (SAG, white bars) are shown; each 767
bar represents the average of three independent experiments. Error bars indicate SEM. 768
Statistical analysis was performed using ANOVA/Fisher’s test. Different letters denote 769
statistically significant differences at p<0.05 for free SA and p<0.001 for SAG. B, The 770
levels of PR1 protein were detected by immunoblot analysis using a PR1 antibody. The 771
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35
membrane was stained with Coomassie blue to show similar protein loading. Similar results 772
were seen in two additional experiments. 773
774
Figure 7. phb3-3 mutant plants are more susceptible to infection by Pseudomonas 775
syringae pv. tomato strains DC3000/AvrRpm1 and DC3000/AvrRpt2. A, Two-week-old 776
plate-grown plants were inoculated by flooding the plants with Pst/AvrRpm1 at a 777
concentration of 1x105 CFU/ml. Bacteria were enumerated at 0, 2, and 3 days post-778
inoculation (dpi). B, Three-week-old plants grown in soil were inoculated with Pst/AvrRpt2 779
by infiltration using a concentration of 1x105 CFU/mL. Bacteria were counted at 0 and 3 780
dpi. For A and B, bars represent average values of twelve replicates; error bars indicate 781
SEM. Statistical analysis was performed using ANOVA/Fisher’s test. Different letters 782
denote statistically significant differences at p values as follows: A, 2 dpi (p<0.0001), 3 dpi 783
(p≤0.0005); B, 3 dpi (p<0.0001). C-D, Cell death was evaluated using trypan blue staining 784
in leaf tissue of four-week-old plants grown in soil, 10 h after infiltration with Pst/AvrRpm1 785
or Pst/AvrRpt2 (1x108 CFU/mL). Cell death was scored as occurring if at least 6 blue 786
stained cells were visible in the area of tissue shown and if similar staining occurred 787
throughout the inoculated leaf. Numbers indicate the total number of leaves showing cell 788
death out of the total number scored in three independent experiments. Arrowheads indicate 789
examples of the collapsed, trypan blue-stained cells. Scale bar = 200 μm. These 790
experiments were repeated 3 times with similar results. 791
792
Figure 8. Expression of PHB3 complements phb3-3 mutant phenotypes. A, Phenotype 793
of 3-week-old plants grown in soil. B, Immunodetection of the PHB3-V5 protein using V5 794
antibody, in extracts from phb3-3 and complemented phb3-3/PHB3-V5 #15 and #17 two-795
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36
week-old plants grown in soil. C, Leaf cross sections of WT, phb3-3, and complemented 796
phb3-3/PHB3-V5 #15 plants. Scale bar = 100 µm. D, Levels of free SA (gray bars) and 797
glycosylated SA (SAG, white bars) in plants grown in plates, treated with UV-C light for 798
20 min and harvested at 24 hpt (+). Untreated plants were used as controls (–). SA levels 799
are expressed as micrograms per gram of fresh weight (FW). Each bar represents the mean 800
value of three independent experiments; error bars indicate SEM. Statistical analysis was 801
performed using ANOVA/Fisher’s test. Different letters denote statistically significant 802
differences at p<0.02 for free SA and p<0.04 for SAG. E, PR1 protein was detected by 803
immunoblot using PR1 antibody in WT, phb3-3, and phb3-3/PHB3-V5 complemented lines, 804
grown on plates, treated with UV-C light for 20 min, then harvested at 8 and 24 hpt. The 805
panel shows a representative result from three independent experiments. F, Bacterial 806
proliferation was quantified in 3-week-old plants grown in soil at 0 and 3 dpi with 1x105 807
CFU/ml Pst/AvrRpt2. Each bar represents the average of 12 replicates; error bars indicate 808
SEM. Statistical analysis was performed using ANOVA/Fisher’s test. Different letters 809
denote statistically significant differences at p<0.05. This experiment was repeated 3 times 810
with similar results. In B and E, membranes were stained with Coomassie blue to show 811
similar protein loading. 812
813
Figure 9. PHB3 regulates ICS1 protein levels. A, ICS1 transcript levels measured by RT-814
qPCR in two-week-old plants grown in plates and treated with UV-C light for 20 min. 815
Whole seedlings were harvested at 2, 8, and 24 hpt. (–) indicates untreated plants. ICS1 816
transcript levels are relative (R.) to YLS8, which was used as endogenous control. Bars 817
show the mean of 3 biological replicates. Error bars show SEM. Statistical analysis was 818
performed using ANOVA/Fisher’s test. Different letters denote statistically significant 819
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37
differences at p<0.05. B, ICS1 was detected by immunoblot analysis with ICS1 antibody in 820
extracts of plants treated as in (A). C, PHB3 does not affect other chloroplast protein levels. 821
The chloroplast proteins Tic110, NOA1, AtpB and LOX2 were detected by immunoblot 822
analysis with specific antibodies. Representative data from three (B) or two (C) 823
independent experiments are shown. Coomassie blue (B) and Ponceau S (C) staining of 824
proteins in the membranes show similar loading. LOX2 immunoblot shows the same 825
membrane as ICS1 blot in B (lower panel). 826
827
828
829
830
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