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Short title: BABY BOOM regulates LAFL gene expression
Corresponding author details: Kim Boutilier ([email protected]); 1
Wageningen University and Research, Bioscience; Droevendaalsesteeg 1, 2
6708 PB Wageningen, Netherlands 3
Article title:
The BABY BOOM transcription factor activates the LEC1-ABI3-FUS3-
LEC2 network to induce somatic embryogenesis
Anneke Horstmana,b, Mengfan Lia,b, Iris Heidmannb,c,1, Mieke
Weemena, Baojian Chena,b, Jose M. Muinod, Gerco C. Angenenta,b and
Kim Boutiliera
4
a Wageningen University and Research, Bioscience, Droevendaalsesteeg 1, 5
6708 PB Wageningen, Netherlands; 6
b Wageningen University and Research, Laboratory of Molecular Biology, 7
Droevendaalsesteeg 1, 6708 PB Wageningen, Netherlands; 8
c Enza Zaden Research and Development B.V, Haling 1-E, 1602 DB 9
Enkhuizen, Netherlands; 10
d Max Planck Institute for Molecular Genetics, Ihnestraße 63-73 11
14195 Berlin, Germany 12
1 Current address: Acepo - Analysis & Advice around Cells & Pollen, 13
Seyndersloot 20, 1602 HA Enkhuizen 14
15
Author contributions: A.H., K.B., G.A. designed research; A.H., M.L., I.H., 16
B.C. and M.W. performed research, J.M. analysed data; and A.H. and K.B. 17
wrote the paper with input from the co-authors. 18
19
Plant Physiology Preview. Published on August 22, 2017, as DOI:10.1104/pp.17.00232
Copyright 2017 by the American Society of Plant Biologists
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Funding information: This work was funded by grants from the Technology 20
Top Institute-Green Genetics and the Netherlands Organisation for Scientific 21
Research (NWO) program, NWO-Groen (project number 870.15.110). 22
23
One sentence summary: 24
25
BBM and other AIL transcription factors induce somatic embryogenesis in a 26
dose- and context-dependent mechanism and through direct transcriptional 27
regulation of major embryo identity genes. 28
29
30
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Abstract 32
Somatic embryogenesis is an example of induced cellular totipotency, where 33
embryos develop from vegetative cells rather than from gamete fusion. 34
Somatic embryogenesis can be induced in vitro by exposing explants to 35
growth regulator and/or stress treatments. The BABY BOOM (BBM) and 36
LEAFY COTYLEDON1 (LEC1) and LEC2 transcription factors are key regulators 37
of plant cell totipotency, as ectopic overexpression of either transcription 38
factor induces somatic embryo formation from arabidopsis (Arabidopsis 39
thaliana) seedlings without exogenous growth regulators or stress 40
treatments. Although LEC and BBM proteins regulate the same 41
developmental process it is not known whether they function in the same 42
molecular pathway. We show that BBM transcriptionally regulates LEC1 and 43
LEC2, as well as the two other ‘LAFL’ genes, FUSCA3 (FUS3) and ABI 44
INSENSITIVE3 (ABI3). LEC2 and ABI3 quantitatively regulate BBM-mediated 45
somatic embryogenesis, while FUS3 and LEC1 are essential for this process. 46
BBM-mediated somatic embryogenesis is dose- and context-dependent, and 47
the context-dependent phenotypes are associated with differential LAFL 48
expression. We also uncover functional redundancy for somatic 49
embryogenesis among other arabidopsis BBM-like proteins, and show that 50
one of these proteins, PLETHORA2, also regulates LAFL gene expression. 51
Our data places BBM upstream of other major regulators of plant embryo 52
identity and totipotency. 53
54
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Introduction 55
56
Plant cells show a high degree of developmental plasticity and can be 57
induced readily to regenerate new tissues or organs (pluripotency) and even 58
embryos (totipotency) from in vitro-cultured explants. Somatic 59
embryogenesis is a type of plant totipotency in which embryos are induced to 60
form on vegetative explants, usually in response to exogenous hormones, 61
especially auxins, or stress treatments (Feher, 2014). Somatic 62
embryogenesis is extensively used as a clonal propagation tool in 63
biotechnology applications (Lelu-Walter et al., 2013; Sharma et al., 2013; 64
Park and Paek, 2014), and while the tissue culture requirements for somatic 65
embryo induction are well known (Gaj, 2004), only a few of the molecular 66
components that drive this process have been described (Elhiti et al., 2013). 67
A number of plant transcription factors have been identified that can 68
convert somatic cells into embryogenic, totipotent cells. One of these 69
transcription factors, Brassica napus BABY BOOM, encodes an 70
AINTEGUMENTA-LIKE (AIL) APETALA2/ethylene-responsive element–binding 71
factor (AP2/ERF) (Boutilier et al., 2002). In arabidopsis, AIL genes form a 72
small, eight-member clade within the AP2/ERF transcription factor family, 73
which, in addition to BBM, comprises AINTEGUMENTA (ANT), AIL1, 74
PLETHORA1 (PLT1), PLT2, AIL6/PLT3, CHOTTO1 (CHO1)/EMBRYOMAKER 75
(EMK)/AIL5/PLT5 and PLT7. AIL genes are expressed in dividing tissues, 76
including root, shoot and floral meristems, where they act in a redundant 77
manner to maintain a meristematic state (Horstman et al., 2014). Single AIL 78
knock-out mutants show no or only minor defects, but double or triple 79
mutants have stronger phenotypes related to reduced cell proliferation or 80
altered cell identity, including smaller floral organs with partial loss of 81
identity (Krizek, 2009; Krizek, 2015), embryo arrest (Galinha et al., 2007) 82
and impaired root and shoot meristem maintenance (Aida et al., 2004; 83
Galinha et al., 2007; Mudunkothge and Krizek, 2012). BBM is expressed in 84
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the embryo and root meristem, where it regulates cell identity and growth 85
together with other AIL proteins (Aida et al., 2004; Galinha et al., 2007). In 86
line with their loss-of-function phenotypes, overexpression of arabidopsis AIL 87
transcription factors induces pluripotency, totipotency and/or cell 88
proliferation, with different functions being assigned to specific AIL proteins 89
(Krizek, 1999; Nole-Wilson et al., 2005; Galinha et al., 2007; Tsuwamoto et 90
al., 2010; Krizek and Eaddy, 2012). However, unlike other AIL genes, the 91
genetic pathways in which BBM functions have not been well characterized 92
(Horstman et al., 2014), and it is not known how this single protein controls 93
both pluripotent and totipotent growth. 94
Overexpression of native or heterologous BBM genes also induces 95
regeneration in other species (Morcillo et al., 2007; Deng et al., 2009; El 96
Ouakfaoui et al., 2010; Heidmann et al., 2011; Lutz et al., 2011; Bandupriya 97
et al., 2014; Yang et al., 2014; Florez et al., 2015; Lowe et al., 2016) and is 98
therefore used as a biotechnology tool to improve plant transformation in 99
model and crop species (Deng et al., 2009; Heidmann et al., 2011; Lutz et 100
al., 2011; Florez et al., 2015; Lowe et al., 2016). 101
Other transcription factors also induce somatic embryogenesis when 102
ectopically expressed in seedlings, including LEAFY COTYLEDON1 (LEC1), 103
which encodes subunit B of a nuclear factor Y protein (NF-YB), and the B3 104
domain protein LEC2 (Lotan et al., 1998; Stone et al., 2001). LEC1 and 105
LEC2, together with LEC1-LIKE (LIL), and the B3 domain proteins ABSCISIC 106
ACID (ABA)-INSENSITIVE3 (ABI3) and FUSCA3 (FUS3) are collectively 107
referred to as the LAFL network (for LEC1/L1L, ABI3, FUS3 and LEC2) (Jia et 108
al., 2014). LAFL proteins function throughout embryogenesis, where they 109
redundantly regulate early developmental processes such as embryo identity 110
and later processes such as embryo maturation (storage product 111
accumulation and desiccation tolerance) and dormancy (Jia et al., 2013). 112
Although FUS3 and ABI3 do not induce somatic embryogenesis when 113
overexpressed, they do activate embryo traits in seedlings (Parcy et al., 114
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1994; Parcy and Giraudat, 1997; Gazzarrini et al., 2004). LEC1 and LEC2 115
directly regulate expression of seed maturation- and auxin response and 116
biosynthesis genes (Lotan et al., 1998; Braybrook et al., 2006), and both of 117
these functions could play a role in inducing a totipotent state (Braybrook 118
and Harada, 2008). Moreover, LEC2 directly activates the MADS-box 119
transcription factor gene AGAMOUS-LIKE15 (AGL15), which enhances 120
somatic embryogenesis from immature zygotic embryos when overexpressed 121
(Harding et al., 2003; Braybrook et al., 2006). LAFL gene expression is 122
controlled by the chromatin remodeller PICKLE (PKL) and by B3 domain-123
containing VIVIPAROUS1/ABI3-LIKE (VAL)/HIGH-LEVEL EXPRESSION OF 124
SUGAR-INDUCIBLE GENE (HSI) transcription factors. Mutations in PKL or VAL 125
genes lead to increased LAFL gene expression in seedlings and maintain 126
embryo identity (Ogas et al., 1999; Dean Rider et al., 2003; Henderson et 127
al., 2004; Suzuki et al., 2007). 128
While both BBM and LEC promote totipotency, it is not known whether 129
they function in the same developmental pathways. To gain insight into 130
signalling pathways regulated by BBM we performed a global analysis of BBM 131
DNA binding sites in somatic embryo tissue (Horstman et al., 2015). Here we 132
show that BBM induces cell totipotency during seed germination through 133
transcriptional activation of the LAFL network, and that BBM induces somatic 134
embryogenesis in a dose- and context-dependent manner. 135
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Results 136
137
BBM binds and transcriptionally activates LAFL genes 138
To understand BBM regulatory networks during embryogenesis, we 139
identified genes that were bound by BBM in somatic embryo cultures using 140
chromatin immunoprecipitation of BBM-YFP (BBM:BBM-YFP) and BBM-GFP 141
(35S:BBM-GFP) fusion proteins followed by next-generation sequencing 142
(ChIP-seq) (Horstman et al., 2015). BiNGO analysis (Maere et al., 2005) of 143
the top 1000 potential target genes in these ChIP experiments was 144
performed to identify overrepresented gene ontology (GO) categories 145
(Dataset S1, a selection of which is shown in Table S1). Genes involved in 146
auxin biosynthesis, transport and signalling, as well root development, 147
meristem initiation and maintenance were overrepresented in the ChIP-seq 148
data sets, as expected from BBM’s function in the root, and in line with 149
studies on other AIL proteins (Horstman et al., 2014; Santuari et al., 2016). 150
Genes involved in adaxial/abaxial polarity specification and shoot 151
development were also overrepresented among BBM–bound genes (Dataset 152
S1, Table S1). Notably, BBM bound to the promoter regions of genes with 153
known functions in embryo identity and maturation, including the LAFL genes 154
LEC1, LEC2, ABI3 and FUS3 (but not L1L), the MADS box transcription factor 155
gene AGL15, which enhances somatic embryogenesis and functions in a 156
positive feedback network with LEC2 (Harding et al., 2003; Braybrook et al., 157
2006; Zheng et al., 2009), and NF-YA9, which also induces somatic 158
embryogenesis when overexpressed (Mu et al., 2013) (Dataset S1, Table S1, 159
Figure 1A). Here we focus our efforts on characterization of the interaction 160
between BBM and members of the LAFL/AGL15 network. We confirmed BBM 161
binding to the promoters of LEC1, LEC2, ABI3 and AGL15 by independent 162
ChIP-qPCR experiments in somatic embryos (Figure S1A). We did not 163
observe BBM binding to the FUS3 promoter, which is consistent with its lower 164
and atypically-shaped BBM ChIP-seq peak (Figure S1). 165
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ANT/AIL DNA binding motifs were previously determined in vitro by 166
SELEX and EMSA (Nole-Wilson and Krizek, 2000; Yano et al., 2009; O'Malley 167
et al., 2016; Santuari et al., 2016). MEME analysis (Bailey and Elkan, 1994) 168
of in vivo BBM-bound regions identified and overrepresented sequence motif 169
that resembles the ANT/AIL DNA binding motif (Figure S1B)(Santuari et al., 170
2016). The BBM-bound region in LEC1, LEC2 and FUS3 contains this BBM 171
binding motif, while the ABI3 bound region contains a slightly degenerated 172
version thereof (Figure S1C). The BBM binding motif was not found in the 173
region bound by BBM in the AGL15 gene, which suggests that BBM binds 174
AGL15 using a different motif or via an intermediate protein. 175
We determined whether BBM regulates LAFL/AGL15 gene expression 176
during somatic embryo induction from imbibed seeds using a steroid 177
(dexamethasone, DEX)-regulated 35S::BBM-GR line in combination with 178
qRT-PCR and reporter gene analysis. qRT-PCR analysis in the presence of 179
DEX and the translational inhibitor cycloheximide (CHX) showed that LEC1, 180
LEC2, FUS3 and ABI3 expression, but not AGL15 expression, was 181
upregulated after BBM-GR activation in imbibed seeds (Figure 1B). 182
LAFL/AGL15 genes were not differentially regulated upon BBM-GR activation 183
in the same material that was used for the ChIP (2,4-D-induced embryo 184
cultures; data not shown). An explanation for this lack of transcriptional 185
response could be the use of different promoters to drive BBM in the two 186
experiments. BBM might only bind and activate LAFL/AGL15 genes during 187
the induction/early stages of somatic embryogenesis (pBBM:BBM-YFP used 188
for ChIP is expressed during early embryogenesis; (Horstman et al., 2015)), 189
but not at later stages of somatic embryogenesis (p35S:BBM-GR used for 190
transcription analysis (qRT-PCR) is expressed during late embryogenesis; 191
(Johnson et al., 2005)). 192
Next, a LEC1:LEC1-GFP reporter (Li et al., 2014) was used to chart the 193
dynamics of LEC1 expression during BBM-induced somatic embryogenesis 194
(Figure 1C). 35S:BBM-GR seedlings initially form somatic embryos on the 195
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cotyledon tip and later from the SAM and cotyledon margins. LEC1-GFP was 196
observed one day after BBM-GR activation, in small patches of cells on the 197
abaxial side of the cotyledon (Figure 1C, +1) and one day later at the 198
cotyledon tip and in patches of cells on the adaxial cotyledon blade (Figure 199
1C, +2). LEC1-GFP became stronger in the cotyledon tip and margin at the 200
time when the tip began to swell (Figure 1C, +3), and could be found later in 201
the somatic embryos that formed at the cotyledon tip and at the margin 202
(Figure 1C, +6). 203
Our results demonstrate that BBM overexpression can ectopically 204
activate LAFL gene expression during seed germination. 205
206
LAFL proteins modulate BBM-induced embryogenesis 207
We investigated the genetic relationship between BBM and its 208
LAFL/AGL15 gene targets. Since outcrossing BBM overexpression lines 209
silences the BBM phenotype (Figure S2), we introduced the 35S:BBM-GR 210
construct into the lec1-2, lec2-1, fus3-3, agl15-3 and abi3 (three alleles) 211
mutant backgrounds by transformation (Figure 2). The lec1-2 and fus3-3 212
seeds are desiccation intolerant (Meinke et al., 1994), therefore 213
heterozygous mutants (lec1-2/+ and fus3-3/+) were used for 214
transformation. 215
In wild-type arabidopsis, 6-7% of the primary (T1) 35S:BBM-GR 216
transformants were embryogenic when grown on DEX (Figure 2). 217
Transformation of the lec1-2/+, lec2-1, fus3-3/+ and agl15-3 mutants with 218
the 35S:BBM-GR construct resulted in a significantly reduced percentage of 219
embryogenic seedlings compared to transformed wild-type seedlings (Figure 220
2). We determined the genotype of the few embryogenic seedlings that were 221
generated after transformation of 35S:BBM-GR to the lec1-2/+ and fus3-3/+ 222
backgrounds. Only one of the embryogenic lec1-2/+ progeny contained the 223
lec1-2 mutant allele, while none of the embryogenic fus3-3/+ progeny 224
contained the fus3-3 mutant allele (Table S2). To determine whether BBM-225
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GR activation can induce somatic embryogenesis in a homozygous lec1-2 226
background, we rescued immature zygotic embryos from the single 227
embryogenic lec1-2/+ 35S:BBM-GR line to bypass the lec1-2 desiccation 228
intolerance phenotype and cultured them on DEX-containing medium. 229
Embryos were separated phenotypically into lec1-2 homozygous mutant and 230
combined lec1-2 heterozygous/wild-type classes. Somatic embryos formed in 231
lec1-2 heterozygous/wild-type seedlings, but not in the homozygous lec1-2 232
seedlings (Table S2). 233
These results suggest that LEC1, LEC2 and FUS3 are positive 234
regulators of BBM-mediated somatic embryogenesis, and that LEC1 and 235
FUS3 are essential for this process. Surprisingly, we found that AGL15 is also 236
a positive regulator of BBM-induced somatic embryogenesis, even though it 237
is not transcriptionally regulated by BBM at the start of somatic embryo 238
induction; AGL15 might be transcriptionally regulated by BBM at a later time 239
point. 240
In contrast to the results obtained with the fus3, lec and agl15 mutants, 241
transformation of the 35S:BBM-GR construct to three different abi3 mutants 242
enhanced the number of embryogenic seedlings (Figure 2). abi3 is the only 243
LAFL mutant that is insensitive to ABA and overexpression of ABI3 does not 244
induce somatic embryogenesis (Parcy et al., 1994; Parcy and Giraudat, 245
1997). To separate the effects of ABA-insensitivity and other embryo defects 246
of abi3 mutants on the BBM phenotype, we tested another ABA-insensitive 247
mutant, abi5-7, which does not show mutant embryo phenotypes other than 248
ABA-insensitivity (Nambara et al., 2002). As with the abi3 mutants, the abi5-249
7 mutant also enhanced the frequency of the BBM phenotype (Figure 2), 250
suggesting that BBM-mediated totipotency is suppressed by ABA signalling, 251
rather than by other functions of the ABI3/ABI5 proteins. 252
A number of regulatory proteins repress LAFL gene expression during 253
the transition from seed to post-embryonic growth. Seedlings with loss-of-254
function mutations in these proteins ectopically express LAFL genes and 255
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retain embryo identity (Jia et al., 2014). We determined whether loss-of-256
function mutants for two of these proteins, the CHD3 chromatin remodeller 257
PKL and the B3 domain protein VAL1/HSI2 influence the penetrance of BBM-258
induced embryogenesis (Figure 2). The pkl-1 and val1-2 (hsi2-5) mutants 259
improved the efficiency of BBM-mediated somatic embryogenesis, as 260
measured by a higher percentage of embryogenic primary transformants. 261
Together, the data show that members of the LAFL network, as well their 262
upstream negative regulators are important direct and indirect components 263
of the BBM signalling pathway. 264
265
AIL/PLT proteins promote totipotency and regulate LAFL gene 266
expression 267
BBM is expressed in the embryo and the root meristem where it 268
regulates cell identity and growth along with other AIL proteins (Aida et al., 269
2004; Galinha et al., 2007). PLT1 and PLT2 induce spontaneous root 270
organogenesis (Aida et al., 2004) and have roles in hormone-mediated 271
regeneration (Kareem et al., 2015), while BBM and CHO1/EMK/AIL5/PLT5 272
(Tsuwamoto et al., 2010) are the only genes reported to induce somatic 273
embryogenesis. We generated 35S:AIL overexpression lines for the six AIL 274
genes that have not been reported to induce somatic embryogenesis when 275
overexpressed, namely ANT, AIL1, PLT1, PLT2, PLT3/AIL6 and PLT7, and 276
found that overexpression of all these genes except the phylogenetically-277
distinct ANT and AIL1 (Kim et al., 2006) induced somatic embryogenesis in 278
the primary transformants (Figure S3, A, B). A PLT2-GR fusion protein 279
directly activated LEC1, LEC2 and FUS3 gene expression, but not ABI3 and 280
AGL15 expression (Figure S3C). Together these data suggest extensive 281
overlap in AIL protein function. 282
283
BBM- and PLT2-mediated somatic embryogenesis is dose-dependent 284
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Ectopic AIL expression induces spontaneous adventitious growth 285
including somatic embryos, ectopic shoots and roots, and callus formation 286
(Boutilier et al., 2002). These phenotypes are correlated with the amount of 287
nuclear-localized BBM protein in DEX-treated 35S:BBM-(GFP)-GR lines 288
(Figure 3 and Figure 4A). A relatively low BBM dose induced the formation of 289
small seedlings with epinastic cotyledons and leaves (Figure 3B and Figure 290
4B) that showed reduced cell differentiation (Figure 4C and Figure 4D). 291
Intermediate DEX concentrations also induced ectopic trichome-bearing 292
protrusions or ectopic leaves on their cotyledon petioles (Figure 3, C-E and 293
Figure 4B), and ectopic roots appeared after longer exposure to DEX (Figure 294
3, F, G). A low frequency of seedlings with somatic embryos on their 295
cotyledons (Figure 3H) was also observed at intermediate DEX 296
concentrations and this phenotype became highly penetrant at the highest 297
effective DEX concentration (Figure 4B). PLT2 also directs the same dose-298
dependent overexpression phenotypes (Figure S4) as BBM overexpression, 299
although ectopic root formation was evident earlier in the PLT2-GR lines than 300
in the BBM-GR lines. These data suggest that AIL protein dose drives the 301
developmental fate of regenerating tissues. 302
303
BBM promotes context-specific embryogenesis 304
Previously we identified direct BBM target genes in four-day-old 305
35S:BBM-GR seedlings using microarray analysis (Passarinho et al., 2008). 306
LAFL/AGL15 genes were not identified as BBM target genes in this study, and 307
in general there was little overlap between these seedling microarray-derived 308
targets and the top 1000 ChIP-seq-derived targets identified in somatic 309
embryos (Table S3). This discrepancy might be explained by the different 310
tissues that were used in each study. We therefore examined the relationship 311
between the developmental competence for BBM-mediated regeneration and 312
LAFL transcription. 313
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We activated BBM-GR at different time points before and after 314
germination. When 35S:BBM-GR seeds were placed directly in DEX-315
containing medium before or during germination (d0-d2), 100% of the 316
seedlings formed somatic embryos directly on their cotyledons after circa one 317
week (Figure 5A and Figure S5A). By contrast, post-germination DEX 318
treatment (d3-d4) induced callus formation on the adaxial side of the 319
cotyledons of circa 40% of the seedlings from which somatic embryos 320
eventually developed (Figure 5B and Figure S5A). We obtained similar results 321
when we activated PLT2-GR before and after germination (Figure S5, B, C). 322
Thus, AIL proteins induce somatic embryogenesis in two ways depending on 323
the developmental stage of the explant: directly from cotyledons in a narrow 324
window before germination, and indirectly via a callus phase after 325
germination. In agreement with our previous microarray results, neither 326
LEC1, LEC2, FUS3, nor ABI3 were expressed when BBM-GR (or PLT2-GR) 327
was activated in post-germination seedlings, although AGL15 expression was 328
slightly upregulated under these conditions (Figure S5D). LEC1-GFP was only 329
detected in this indirect pathway 10 days after DEX-induction (Figure 5B), 330
where it was localized to globular-like embryos that emerged from the callus. 331
The dessication intolerance combined with the lack of or weak 332
embryogenesis phenotypes of the lec1/fus3 35S:BBM-GR lines (above) 333
complicated further genetic analysis of the role of LEC1 and FUS3 in this 334
indirect pathway. 335
Our results highlight the existence of a narrow developmental window 336
of competence for direct embryogenesis, and suggest that tissues outside 337
this window require more extensive reprogramming, e.g. callus formation, 338
before embryogenesis can be induced. LAFL genes are transcriptionally 339
silenced after germination (Zhang et al., 2012; Zhou et al., 2013), therefore 340
these loci might only become transcriptionally accessible after 341
redifferentiation of cotyledon cells to callus. 342
343
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Discussion 344
345
BBM-mediated embryogenesis requires LAFL genes 346
An increasing number of proteins are being identified that regulate cell 347
totipotency in vivo and in vitro, including members of the LAFL network and 348
AIL proteins (Feher, 2014; Horstman et al., 2014). LAFL transcription is 349
regulated at the chromatin level and by extensive transcriptional feedback 350
loops between LAFL proteins. The interactions between LAFL proteins and 351
other regulators of cell totipotency are less well known. Our data now places 352
BBM/AIL proteins directly upstream of the LAFL/AGL15 genes. There is some 353
evidence that LAFL proteins might act upstream of AILs: A Phaseolus vulgaris 354
ABI3-like factor (Pv-ALF) binds and activates arabidopsis 355
CHO1/EMK/AIL5/PLT5 (Sundaram et al., 2013), and arabidopsis FUS was 356
shown to bind BBM, PLT2, AIL6/PLT3 and PLT7, although transcriptional 357
regulation was not investigated (Wang and Perry, 2013). In contrast, the 358
lack of AIL deregulation after inducible overexpression of LEC1, ABI3, FUS3 359
or LEC2 (Braybrook and Harada, 2008; Yamamoto et al., 2010; Junker et al., 360
2012; Monke et al., 2012) suggests that there is no direct feedback of LAFLs 361
on AILs. BBM expression is reduced in lafl mutant seeds (Figure S6), but 362
this genetic interaction could be indirect. Although BBM proteins appear to 363
directly activate LAFL genes, the data is inconclusive as to whether there are 364
additional direct transcriptional feedback loops in the AIL-LAFL cell 365
totipotency network. 366
LAFL/AGL15 proteins are required for BBM function, as BBM 367
overexpression in lec1, lec2, fus3 and agl15 mutants either reduced or 368
eliminated the capacity of seedlings to form somatic embryos. The reduced 369
competence for somatic embryogenesis in these mutants could be explained 370
in two ways: (1) the developmental defects in the mutants change the 371
physiological state of the mature embryo/seedling in such a way that it is no 372
longer responsive for BBM; or (2) BBM-induced embryogenesis relies on 373
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15
transcriptional activation of the LEC1, LEC2, FUS3 and AGL15 genes. Several 374
lines of evidence support the latter scenario. First, we observed a reduced 375
responsiveness to BBM in segregating lec1 and fus3 populations, which 376
contained wild-type and heterozygous plants. However, the few embryogenic 377
transformants in these populations were mainly wild-types, suggesting that 378
the lec1 and fus3 mutations affect BBM function in the heterozygote state. 379
Heterozygous lec1 and fus3 mutants do not show obvious growth defects, 380
suggesting that reduced LEC1 or FUS3 levels (dose) in the heterozygous 381
mutants, rather than a change in the physiological state of the tissue, 382
reduces the response to BBM overexpression. Secondly, the abi3 mutant 383
shows overlapping maturation defects with the other LAFL mutants (To et al., 384
2006; Roscoe et al., 2015), yet the abi3 mutations had the opposite effect 385
on BBM-induced embryogenesis. Therefore, we hypothesize that the inability 386
of BBM to induce LEC and FUS gene expression reduces the capacity for 387
embryogenic growth in these mutants. This hypothesis is further 388
strengthened by our observations that mutations in LAFL repressors enhance 389
BBM-mediated embryogenesis, possibly by facilitating elevated LAFL gene 390
expression. 391
392
Embryo induction is a dose-dependent phenotype 393
It was previously suggested that the PLT2 protein regulates root 394
meristem size and maintenance through a protein concentration gradient, 395
with high, intermediate and low AIL concentrations instructing stem cell fate, 396
cell division and differentiation, respectively (Galinha et al., 2007). 397
AIL6/PLT3 overexpression also induces dose-dependent phenotypes in floral 398
organs (Krizek and Eaddy, 2012). We showed that a high BBM/PLT2 dose 399
induces embryogenesis, a lower dose induces organogenesis and the lowest 400
dose inhibits differentiation. Our overexpression data therefore support a 401
general dose-dependent AIL output in plant tissues. This dose-dependency 402
complicates functional complementation studies on AILs, as even 403
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16
complementation with endogenous promoters (Galinha et al., 2007; Santuari 404
et al., 2016) can lead to differences in expression levels among 405
transformants and a range of developmental outcomes. 406
How dedifferentiation, organogenesis and somatic embryogenesis are 407
induced at successively higher AIL doses is not clear, but likely reflects the 408
endogenous roles of AIL proteins during embryo, organ and meristem 409
development. Mechanistically, AIL dose-dependent phenotypes could result 410
from dose-dependent expression levels of the same set of target genes 411
and/or from dose-specific activation of specific target genes. A transcription 412
factor gradient can regulate different sets of target genes through differences 413
in binding site number and affinity (Rogers and Schier, 2011). Target genes 414
with many or high-affinity binding sites are activated by low levels of the 415
transcription factor, whereas genes with few or low-affinity binding sites are 416
only activated at high transcription factor levels. Specificity might also be 417
determined at the level of protein-protein interactions (Horstman et al., 418
2015). Defining the overlapping and unique target genes for each AIL 419
transcription factor at different doses and the protein complexes in which 420
they function will shed light on how AIL dose directs specific developmental 421
fates. 422
423
424
425
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17
MATERIALS AND METHODS 426
427
Plant material and growth conditions 428
The lec2-1 (CS3868), lec1-2 (CS3867), fus3-3 (CS8014), agl15-3 429
(CS16479), and pkl-1 (CS3840) mutants were obtained from the Nottingham 430
Arabidopsis Stock Centre. The val1-2 (hsi2-5)(Sharma et al., 2013), abi3-8, 431
abi3-9, abi3-10 and abi5-7 (Nambara et al., 2002) mutants and the 432
LEC1:LEC1-GFP marker (Li et al., 2014) were described previously. All 433
mutants are in Col-0 background, except lec1-2 and lec2-1, which are in Ws 434
background. 435
Seeds were sterilized with liquid bleach as described previously 436
(Boutilier et al., 2002; Passarinho et al., 2008) and germinated on half-437
strength MS medium containing 1% sucrose and vitamins (½MS-10). 438
Dexamethasone (DEX) and cycloheximide (CHX) (both Sigma) were added to 439
the medium as described in the text. Solid and liquid (rotary shaker, 60 440
rpm/min) cultures were kept at 21 ˚C and 25 ˚C, respectively (16 hour 441
light/8 hour dark regime). lec1-2 35S:BBM-GR mutant embryos were 442
rescued by excising them from sterilized siliques and allowing them to 443
germinate on solid ½MS-10 with DEX and kanamycin (for selection of the 444
BBM transgene). 445
446
Vector construction and transformation 447
The 35S:BBM-GR construct was described previously (Passarinho et al., 448
2008). The ANT, PLT3/AIL6, PLT7 and PLT1 protein coding regions were 449
amplified from Arabidopsis Col-0 genomic DNA and the PLT2 protein coding 450
region from cDNA, using the primers listed in Table S4. 35S:AIL ectopic 451
overexpression constructs were made using the Gateway (GW) binary vector 452
pGD625 (Chalfun et al., 2005). The 35:BBM-GFP-GR construct was made 453
using the GW-compatible destination vector pARC146 (Danisman et al., 454
2012). BBM-GFP used in the 35:BBM-GFP-GR construct was amplified from a 455
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18
BBM:BBM-GFP plasmid (Horstman et al., 2015). Constructs were introduced 456
into wild-type or mutant lines by floral dip transformation (Clough and Bent, 457
1998). 458
459
Confocal microscopy 460
Confocal imaging was performed as previously described (Soriano et al., 461
2014). Propidium iodide (PI, 10 µg/mL) counterstaining (35S:BBM-GFP-GR 462
roots) and autofluorescence (LEC1:LEC1-GFP cotyledons) were used to 463
delineate the tissue. Both fluorophores were excited with a 532 nm laser and 464
detected at 600-800 nm. 465
To quantify the subcellular BBM-GFP-GR localisation in root, confocal 466
images were made of the meristematic region of roots of 35S:BBM-GFP-GR 467
seedlings grown for seven days in medium containing different DEX 468
concentrations. ImageJ was used to calculate the ratio of nuclear to 469
cytoplasmic fluorescence by comparing the average fluorescence intensity in 470
the nucleus with the average fluorescence intensity of an area of equal size 471
in the cytoplasm. 472
473
Leaf imaging and quantification of stomatal development 474
The first leaf pairs of nine day-old seedlings were placed overnight in 475
70% ethanol at 4 ˚C, then transferred to 85% ethanol for 6 hours, and 476
subsequently to 3% bleach overnight or until imaging. Leaves were mounted 477
in HCG solution (80 g chloral hydrate, 10 ml glycerol, 30 ml water) prior to 478
imaging with a Nikon Optiphot microscope. The stomatal, meristemoid and 479
stomatal lineage indices (SI, MI and SLI) were calculated as previously 480
described (Peterson et al., 2013). 481
To calculate stomatal indices, eight images from the abaxial sides of 482
cleared first leaves of nine day-old 35:BBM-GR plants grown with or without 483
DEX were analysed (n= 125 and 350 cells per image). The SI, MI and SLI 484
were calculated as previously described (Peterson et al., 2013). SI = 485
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19
(number of stomata/(total number of stomata + non-stomatal epidermal 486
cells)) x 100. For the SI, only mature stomata with a pore were counted. MI 487
= (number of meristemoids/(total number of stomata + non-stomatal 488
epidermal cells)) x 100. SLI = (number of stomata and stomata 489
precursors/(total number of stomata + non-stomatal epidermal cells)) x 100. 490
491
ChIP-seq 492
The previously published ChIP-seq data and data analysis (Horstman et 493
al., 2015) can be downloaded from GEO (GSE52400). The ChIP-seq 494
experimental set-up is described in (Horstman et al., 2015); briefly, the 495
experiments were performed using somatic embryos from either 2,4-496
dichlorophenoxyacetic acid (2,4-D)-induced BBM:BBM-YFP cultures (with 497
BBM:NLS-GFP as a control) or a 35S:BBM-GFP overexpression line (with 498
35S:BBM as a control). Two independent ChIP-qPCR experiments on 2,4-D-499
induced BBM:BBM-YFP cultures were performed to validate the ChIP-seq 500
results, using the same protocol as previously described (Horstman et al., 501
2015). A fold change was calculated by comparison to an unbound genomic 502
control region (ARR6) and statistically significant differences between BBM-503
bound regions and a second unbound genomic control region (HSF1) were 504
determined using Student’s t-test (p<0.05). The DNA primers are shown in 505
Table S4. 506
507
Expression analysis 508
cDNA from Col-0 and 35S:BBM-GR seeds or seedlings (3 biological 509
replicates of each) were treated as described in the text and used for 510
quantitative real-time RT-PCR (qPCR). qPCR was performed using the 511
BioMark HD System (Fluidigm). The data were normalized against the SAND 512
gene (Czechowski et al., 2005) and relative gene expression was calculated 513
by comparison with similarly-treated wild-type Col-0 or untreated 35S::BBM-514
GR (Livak and Schmittgen, 2001). The DNA primers are shown in Table S4. 515
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20
516
SUPPLEMENTAL MATERIAL 517
518
Table S1. BBM target genes. 519
Table S2. Effect of the lec1 and fus3 mutant backgrounds on BBM-mediated 520
somatic embryogenesis. 521
Table S3. Overlap between BBM targets obtained using microarray and 522
ChIP-seq analysis. 523
Table S4. Oligonucleotide primers used in this study. 524
Figure S1. BBM binds to an ANT-like DNA binding motif. 525
Figure S2. Outcrossing BBM overexpression lines silences the BBM 526
phenotype 527
Figure S3. Ectopic overexpression of AIL proteins induces somatic 528
embryogenesis and activates LAFL expression. 529
Figure S4. PLT2 ectopic overexpression induces dose-dependent 530
phenotypes. 531
Figure S5. AIL proteins induce context-dependent somatic embryogenesis. 532
Figure S6. BBM expression is reduced in lafl mutant seeds. 533
Dataset S1. BBM target genes. 534
535
536
ACKNOWLEDGMENTS 537
We thank Nirmala Sharma for the val1-2 (hsi2-5) mutant and Bas Dekkers 538
for the abi3-8, abi3-9, abi3-10 and abi5-7 mutants. The authors declare no 539
conflict of interest. 540
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21
FIGURE LEGENDS 541
542
Figure 1. BBM binds and transcriptionally regulates LAFL genes. 543
(A) ChIP-seq BBM binding profiles for embryo-expressed genes. 35S:BBM-544
GFP (upper profile) and BBM:BBM-YFP (lower profile). X-axis, nucleotide 545
position (TAIR 10 annotation; black bars indicate exons, lines indicate 546
introns); y-axis, ChIP-seq score (fold-enrichment of the BBM-GFP/YFP ChIP 547
to the control ChIP), as calculated by the CSAR package in Bioconductor; 548
>>, direction of gene transcription; *, peaks with scores that are considered 549
statistically significant (FDR<0.05). (B-C) Transcriptional regulation of LAFL 550
genes. Relative expression was determined by qPCR in 35S:BBM-GR and Col-551
0 seeds one day after seed plating. Samples were treated for three hours 552
with DEX and CHX (both at 10 µM) (B). Error bars indicate standard errors 553
of the three biological replicates. Statistically significant differences (*) 554
between DEX+CHX-treated 35S:BBM-GR and DEX+CHX-treated Col-0 were 555
determined using Student’s t-test (p<0.01). (C) LEC1:LEC1-GFP regulation 556
by BBM. Samples were treated with 10 µM DEX one day after plating and 557
imaged on subsequent days as indicated in the panel. The images show the 558
adaxial sides of cotyledons, unless indicated (ab, abaxial side). The green 559
signal in Col-0 and LEC1:LEC1-GFP cotyledon tips is autofluorescence. Scale 560
bars, 250 µm. 561
562
Figure 2. BBM-induced embryogenesis is modulated by LAFL genes. 563
Percentage of primary embryogenic transformants obtained after 564
transformation of the 35S:BBM-GR construct to wild-type Ws or Col-0 565
backgrounds or the indicated mutant lines. Statistically significant differences 566
(*) in the number of embryogenic transgenic lines between the mutant and 567
the corresponding wild-type line were determined using a Pearson’s chi-568
squared test (p<0.05). The total number of transformants per line is 569
indicated above each bar. Somatic embryo formation was not observed in 570
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22
any of the individual mutant backgrounds alone, or mutant + 35S:BBM-GR 571
backgrounds in the absence of BBM-GR activation. 572
573
Figure 3. BBM overexpression phenotypes are dose-dependent. 574
Phenotypes of 35S:BBM-GR seedlings grown for two (A-E, H) or three (F, 575
G) weeks on the DEX concentration (µM) indicated in the panel. (A) A 576
phenotypically normal seedling. (B) A small seedling with epinastic leaves 577
and cotyledons. (C) A small, epinastic seedling with a trichome-bearing 578
ectopic leaf (arrow) on the cotyledon petiole. (D) A seedling with ectopic 579
leaves on the petioles of both cotyledons (arrows). (E) A magnified view of 580
the ectopic leaf in (D). (F) A 35S:BBM-GR seedling with an ectopic root (*). 581
(G) A magnified view of the ectopic root in (F). (H) A seedling with somatic 582
embryos on the cotyledon margins (arrowheads). Scale bars, 2.5 mm. 583
584
Figure 4. Quantification of BBM dose-dependent phenotypes. 585
(A) BBM-GFP-GR nuclear localization increases with increasing 586
dexamethasone concentration. The effect of dexamethasone (DEX) on BBM 587
localization in roots of 35S:BBM-GFP-GR seedlings grown for seven days in 588
medium containing the indicated DEX concentration. Non-DEX treated (Col-589
0) roots are shown as a GFP-negative control. Subcellular BBM-GFP-GR 590
localisation was quantified for each DEX concentration by calculating the 591
average nuclear/cytoplasmic GFP ratio (63-133 cells of 5-8 roots per DEX 592
concentration), which is indicated on the bottom of the images (± standard 593
error). The average ratios are significantly different from each other (p<0.01 594
in Student’s t-test). Green, GFP. Magenta, propidium iodide. (B) Frequency 595
of phenotypes observed in 35S:BBM-GR seedlings from two independent 596
transgenic lines (solid and hatched bars) grown for two weeks on medium 597
containing different DEX concentrations (n= 100 to 350 seedlings). Leaf, 598
ectopic leaves; SE, somatic embryos. Seedlings that showed both ectopic 599
shoots and somatic embryos were scored as ‘SE’. SE refers to both 600
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23
embryogenic tissue (smooth, swollen, bright green in colour and lacks 601
trichomes) or cotyledons, as well as histodifferentiated embryos. (C) A 602
relatively low BBM dose induces smaller and less-lobed leaf pavement cells 603
compared to the control. The abaxial sides of cleared first leaves of nine-day-604
old 35:BBM-GR seedlings grown on medium without DEX (left) or with 0.1 605
μM DEX (right). Scale bars, 25 μm. (D) Stomatal differentiation in DEX-606
treated 35:BBM-GR seedlings is reduced compared to untreated seedlings. In 607
DEX-treated 35S::BBM-GR seedlings, fewer cells are commited to stomatal 608
development, as reflected by a lower stomatal lineage index (SLI). Also, 609
fewer mature stomata were found in leaves of DEX-treated seedlings 610
(stomatal index, SI), while the number of stomatal meristemoids was 611
increased (meristemoid index, MI). Error bars indicate standard errors. *, 612
statistically significant difference compared to the non-DEX treated control 613
(p<0.05 in Student’s t-test). 614
615
616
Figure 5. BBM-induced embryogenesis is context-dependent. 617
(A) 35S:BBM-GR plants were cultured with 10 µM DEX starting at the dry 618
seed stage (d0) or after germination (d4). The culture time after DEX 619
application is indicated on the bottom right of each picture. Arrowheads, 620
callus; arrows, somatic embryos/embryogenic tissue; ‘le’, callused leaf 621
tissue. The lower-most image is a magnification of the boxed region in the 622
‘d4 +14’ image. (B) LEC1:LEC1-GFP regulation by BBM. Seedlings were 623
treated with 10 µM DEX after germination (d4) and imaged on subsequent 624
days as indicated in the panel. The images show the adaxial sides of 625
cotyledons. Arrowhead, callus on the distal end of the cotyledon blade; 626
arrows, GFP-positive embryo clusters. The outline of the cotyledon margins is 627
shown with dashed lines. Autofluorescence (magenta) was used to delineate 628
the tissue. Scale bars, 250 M. 629
630
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24
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