ORGANOGENESI: DIFFERENZIAMENTO DEI PRIMORDI FOGLIARI

ORGANOGENESI: DIFFERENZIAMENTO

DEI PRIMORDI FOGLIARI

Le foglie si formano nello sviluppo post-embrionale

Germination

Embryonic development

zygote

Cotyledons (embryonic leaves)Embryo inside seed

Post-embryonic development

True leavesCotyledons (embryonic leaves)

Sono formate dal meristema apicale del germoglio (SAM)

Germination

zygote

SHOOT APICAL

MERISTEM

Leaf formation

True leaves

Il SAM si forma durante l’embriogenesi

Laux, T, Jurgens, G. (1997) Plant Cell 9: 989-1000

TOP DOWN

Apical

Basal

Shoot apical meristem

Shoot apical meristem

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. Long, J.A., et al., 379: 66-69, copyright 1996.

Dopo la germinazione il SAM forma le foglie

Cotyledons

Leaves

Cotyledons

Leaves

Post-embryonic leaf formation

Shoot apex at germination

http://www.nature.com/nature/index.html

diversi stadi nello sviluppo dei primordi

Organogenesi: poche cellule negli strati L1 e L2 nella zona periferica acquistanol’identità di iniziali (founder cells) della foglia. Cominciano a dividersi più rapidamentedelle cellule circostanti e formano una zona distinta dal resto del doma (primordio).

Sviluppo di differenti regioni nella foglia: regioni dei primordi acquisiscono l’identità delle diverse parti della foglia. Tre assi di sviluppo: adaxiale/abaxiale; prossimale/distale; mediale/laterale

Differenziamento di cellule e tessuti: Con la crescita della foglia cellule e tessutisi differenziano: L1 epidermide; L2 mesofillo; L3 elementi vascolari e cellule della guaina del fascio

DIVISIONI CELLULARI NELLA FORMAZIONE DEL PRIMORDIO (arabidopsis)

divisioni periclinali nello strato più interno della tunica

divisioni periclinali anche negli strati meno interni della tunica e divisioni meno orientate

divisioni anticlinali nelllo strato esterno della tunica per formare il protoderma

Le foglie in formazione hanno una loro polarità intrinseca

Leaf

Leaf

Cot Cot

Peripheral

Central

Assi di asimmetria nella foglia

La polarità è evidente fin dagli stadi iniziali

PeripheralCentral Adaxial Abaxial

The adaxial side is towards the center of the plant

In quale posizione si formano sul germoglio?

(fillotassi)

Alternate Opposite Whorled Spiral

Alternata

Candela, H. et al. (2008) Plant Cell 20: 2073-2087; Itoh, J.-I., et al. (2000) Plant Cell 12:2161-2174

One leaf at a time, 180° apart, as in rice or other grasses.

TEM of rice apex

Cross section of rice apex

Opposta

Two at a time, 180° apart at each node. Sometimes pairs alternate by 90° at successive nodes.

Verticillata

Three or more leaves at each node, as in the horsetail (Equisetum).

Photos courtesy of tom donald

http://www.clearwood.co.uk/

Spiralata

In most plants, such as this succulent, leaves form in a regular spiral pattern.

Photos courtesy of tom donald

http://www.clearwood.co.uk/

spiralata

137°

In plants with spiral phyllotaxy, leaves form at about 137° apart.

Spiral phyllotaxy

Spiral phyllotaxy

A line through sequential leaves makes a spiral.

Spiral phyllotaxy

The NEXT leaf to form is called the Incipient primordium (I1).

I1

Spiral phyllotaxy

The one that will form after that is called I2….etc.

I1I2

Spiral phyllotaxy

Fillotassi spiralata in apice di tabacco

Poethig , R.S. and Sussex ,I.M. (1985) The developmental morphology and growth dynamics of the tobacco leaf. Planta 165: 158-169. Copyright (1985) Planta. Reprinted with kind permission of Springer Science+Business Media.

I1

Cosa determina la posizione del primordio incipiente?

Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.

Surgical experiments demonstrate that leaf placement is determined by pre-existing primordia

http://dev.biologists.org/cgi/content/full/132/1/15

I2

This tomato apex shows the positions of several primordia (P) and incipient primordia (I).



P3

P2

P1

I1

I2

This tomato apex shows the positions of several primordia (P) and incipient primordia (I).

The expected position for I3 (*) can be found by tracing the spiral.

I2



P3

P2

P1

I1

I2

I1 (shown in black) was surgically isolated from the rest of the meristem, by cutting along the red line.



P3

P2P1

I1

I2

Two days later, the apex was examined.


P3

P2P1

I1

I2

I3 Instead of emerging at its expected position (star), I3 shifted towards I1.

This experiment shows that I1 influences I3 position.


P3

P2P1

I1

I2

I3

Positions of I2 and I3; older leaves have been cut away.


The older primordia control the placement of the incipient primordia.

P3

P2P1

I1

I2

I3

What kind of signal or information is involved?

L’auxina è coinvolta nella formazione del primordio

Reprinted from Current Opinion in Plant Biology, 8 (1), Byrne, M.E., Networks in leaf development , 59-66, Copyright (2005), with permission from Elsevier .

Wild-type Arabidopsis shoot apex. The meristem is covered by the leaves it has produced.

L’auxina è coinvolta nel determinare la posizione del primordio

The apex of the pin1 mutant is bare – it fails to produce lateral organs.

meristem

Reprinted from Current Opinion in Plant Biology, 8 (1), Byrne, M.E., Networks in leaf development , 59-66, Copyright (2005), with permission from Elsevier ; Reinhardt D et al., (2000) Plant Cell 12: 507- 518

Wild-type Arabidopsis shoot apex

pin1 shoot apex

The pin1 mutant is defective in the transport of auxin.

Auxin transport

pH 7pH 5

IAAH

Auxin (IAA) is a weak acid. At the low pH of cell walls, it is protonated and uncharged (IAAH), allowing it to move through the plasma membrane.

Cell wall

Cytoplasm

Auxin transport

IAA-

pH 7pH 5

IAAH

In the less acidic cytoplasm, it loses the proton, becomes charged (IAA-), and cannot exit the call by diffusion through the plasma membrane.

Cell wall

Cytoplasm

Auxin transport

IAA-

pH 7pH 5

IAAH

PIN1 protein

Auxin efflux through PIN1

The PIN1 protein is an auxin efflux carrier, transporting charged auxin back out of the cytoplasm.

IAA-

Auxin transport

The subcellular localization of PIN proteins can be polar and coordinated between cells, causing directed auxin transport.

In this diagram, the accumulation of PIN1 to the right of each cell causes a net flow of auxin towards the right.

Net flow of auxin

IAA-

pH 7pH 5

IAAH

Un massimo localizzato di auxina è richiesto per l’organogenesi

Reinhardt D et al., (2000) Plant Cell 12: 507- 518

Applying a spot of exogenous auxin (shown as a red blob) stimulates outgrowth of primordium in the pin1 mutant.

38 hours after application 4 days after application

Reinhardt D et al., (2000) Plant Cell 12: 507- 518

IAA-

pH 7pH 5

IAAH

Conclusion - Auxin transport and a local auxin maximum contribute to organ initiation.

This conclusion is supported by imaging PIN1 distribution in living plants.

GFP

PIN

1

GFP

Green fluorescent protein (GFP) emits green light when excited by blue light.

Emitted light

A protein’s position within a cell can be determined by making a fusion protein of it with GFP, and then looking for GFP fluorescence.

Visualizing PIN1 localization

Excitatory light

PIN1pro PIN1 GFPmRNA

Fusion protein

GFP

GFP

PIN

1P

IN1

Reporter gene in the nucleus

Insertion into membrane

PIN

1

Translation


GFP

Using a confocal laser scanning microscope, PIN1:GFP protein distribution can be imaged in the shoot apical meristem. In this image, the green lines show the position of PIN1:GFP at cell membranes.

PIN

1

GFP

Reproduced with permission - Development Gordon, S.P., Heisler, M.G., Reddy, G.V., Ohno, C., Das, P., Meyerowitz, E.M. Development, 2007, 134 (19): 3539-3548.


PIN1pro PIN1 GFPmRNA

La distribuzione di PIN1 è dinamica durante l’organogenesi

PIN1:GFP Positions of primoridia and incipient primordia

The orientation of PIN1 within cells is shown by white arrows, and indicates auxin flow.

Auxin accumulates at I1 position

II11

Reprinted from Current Biology 15: Heisler, M.G., Ohno, C., Das, P., Sieber, P., Reddy, G.V., Long, J.A., and Meyerowitz, E.M. Patterns of auxin transport and gene expression during primordium development revealed by live imaging of the Arabidopsis inflorescence meristem,1899-1911, Copyright (2005), with permission from Elsevier.

La polarità delle proteine PIN 1 determina un massimo di auxina in I1

II11

II11

This observation is consistent with the emergence of a primordium at the site of auxin application

dopo l’inizio della formazione del primordio la distribuzione di PIN1 cambia e orienta il flusso di auxina nel tessuto vascolare

Poethig , R.S. and Sussex ,I.M. (1985) The developmental morphology and growth dynamics of the tobacco leaf. Planta 165: 158-169. Figure 3 Copyright (1985) Planta. Reprinted with kind permission of Springer Science+Business Media. Adapted by permission from Macmillan Publishers, Ltd: Nature Reinhardt D., Pesce, E.-R., Stieger, P., Mandel, T., Baltensperger, K., Bennett, M., Trass, J., Friml, J., Kuhlemeier, C . Regulation of phyllotaxis by polar auxin transport. Nature 426, 255-260; copyright (2003).

I1P1

I1

P1

I1

P1

TIME

Una successiva inversione nella polarità di PIN1 cambia la posizione del picco di auxina e specifica la posizione del

nuovo primordio

P3

P2

P1

I1

P3

P2

P1

I1I2

time

Summary

• Organ initiation at the shoot apical meristem is determined by auxin distribution and PIN1

• An auxin maximum is necessary and sufficient to specify the site of primordium formation

• Primordia affect auxin distribution and so placement of incipient primordia

• Auxin has been proposed to act as a morphogen – a generator of form

Come viene acquisita l’identità di foglia?

The meristem is a population of small, undifferentiated, dividing cells.

A leaf primordium is a population of small, undifferentiated, dividing cells.

The differ in their expression of critical regulatory genes; the meristem expresses meristem-specific genes, and the leaf primordium expresses primordium-specific genes.

Poethig , R.S. and Sussex ,I.M. (1985) The developmental morphology and growth dynamics of the tobacco leaf. Planta 165: 158-169. Figure 3 Copyright (1985) Planta. Reprinted with kind permission of Springer Science+Business Media.

Ruolo dei fattori di trascrizione

KNOX-1

Geni di identità meristematica

Class I KNOX genes (KNOX-1)•(KNOX means Knotted-like homeobox)•Expressed in meristem•Not expressed in incipient primordia•Help maintain indeterminate growth

Class I KNOX genes

KNOX genes are Knotted-like homeobox genes that encode homeodomain transcription factors.

Hao Yu, H., Yang, S.H., and Goh, C. J. (2000) DOH1, a class 1 knox gene, Is required for maintenance of the basic plant architecture and floral transition in orchid. Plant Cell 12: 2143-2160.

Espressione di KNOTTED

KNOTTED (a KNOX-1 gene) mRNA accumulates in the meristem but not the leaf primordia (arrows) of Zea mays.

Jackson, D., Veit, B., and Hake, S. (1994) Expression of maize KNOTTED1 related homeobox genes in the shoot apical meristem predicts patterns of morphogenesis in the vegetative shoot. Development 120: 405–413. Reproduced with permission.

STM un gene di classe KNOX1 è necessario per la formazione del meristema

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. Long, J.A., et al., 379: 66-69, copyright 1996.

Wild-type plant showing leaf formation at the shoot apex

The shootmeristemless mutant (stm) fails to form a shoot apical meristem during embryogenesis; notice the absence of leaf formation.

Geni Primordio-specifici

ARP genes •“ARP” is derived from three genes, ASYMMETRIC LEAF1, ROUGH SHEATH2, and PHANTASTICA •ARP genes encode MYB transcription factors •Expressed in cells of leaf primordia• Promote determinate growth and differentiation

ARP

ARP

ARP

ARP

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. Byrne, M.E., et al., 408: 967-971. Copyright 2000..

ASYMMETRIC LEAF1 (AS1) mRNA is expressed in cotyledons but not in the meristem .

ARP

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. Byrne, M.E., et al., 408: 967-971. Copyright 2000..

In the stm mutant, AS1 is expressed in the meristem (arrow).

Wild type

stm mutant

L’espressione dei geni KNOX nel meristema reprime quella dei geni ARP

ARP

KNOX-1

L’espressione dei geni ARP reprime quella dei geni KNOX

ARP

KNOX-1

La sovraespressione di KNOX-1aumenta la complessità della foglia e la sua indeterminazione

Chuck G et al., (1996) Plant Cell 8: 1277-1289. Reprinted by permission from Macmillan Publishers, Ltd: NATURE GENETICS 31: 121 – 122. Hake, S., and Ori, N. Plant morphogenesis and KNOX genes. Copyright (2002).

Arabidopsis Tobacco Maize

WT OX WT OX WT OX

Mutazioni loss of funcion arp hanno fenotipo simile alla sovraespressione di KNOX-1

Reprinted by permission from Macmillan Publishers, Ltd: NATURE 408: 967-971. Byrne, M.E., Barley, R., Curtis, M., Arroyo, J.M., Dunham, M., Hudson, A., and Martienssen, R.A. Asymmetric leaves1 mediates leaf patterning and stem cell function in Arabidopsis. Copyright (2000). Reproduced with permission Development Schneeberger, R., Tsiantis, M., Freeling, M., Langdale, J. Development (1998) 125: 2857-2865.

rs2 WT rs2 WT rs2

Arabidopsis Maize

Nei primordi i geni ARP agiscono come repressori trascrizionali dei geni di classe KNOX

Guo, M., et al. (2008) Plant Cell 20:48-58

ARP heterodimer KNOX gene

Geni di confine (boundary) sono necessari per la separazione degli organi

Boundary genes •Ensure a sharp boundary between leaf and meristem•Expressed at organ boundaries•Loss-of-function leads to “jagged” or fused organs

GENI CUC

JLO expressionJAGGED LATERAL ORGANS (JLO) is a boundary gene.

Loss-of-JLO function causes fused or jagged organs.

JLO coordinates KNOX-1 and PIN activities.

Loss-of-function phenotype

JAGGED LATERAL ORGAN (JLO) (LOBD gene family)

Summary

• A leaf acquires identity by turning OFF meristem genes and turning ON leaf genes

• KNOX-1, ARP and boundary genes encode transcriptional regulators that control expression of other genes

• Precise control of cell fates involves tight control of transcription by developmentally regulated activators and repressors

COME VIENE ACQUISITA LA POLARITA’?

Polarità anatomica e funzionale

Juarez, M. T., Twigg, R.W., and Timmermans, M.C.P. (2004) Development 131:4533-4544. Reproduced with permission.

O2CO2

Most leaves have polarity – they are functionally and anatomically different on their upper and lower surfaces

Abaxial surface - transpirational water loss, respiratory gas exchange

Adaxial surface – light harvesting

Leaves have an inherent polarity because one side is more central and one more peripheral.

Leaf

Leaf

Peripheral

Central

Leaf

Peripheral

Central

Adaxial

Abaxial

The central side is adaxial, and peripheral is abaxial.

How does a leaf “know” which side is central?

The Sussex signal

In the 1950s, Ian Sussex showed that a signal from the meristem is required for proper leaf polarity.

Reprinted, with permission, from the Annual Review of Plant Physiology and Plant Molecular Biology, Volume 49 (c) 1998 by Annual Reviews www.annualreviews.org

Incipient primordia were surgically isolated from the rest of the meristem by a small incision

P3

P2

P1

I1

I2

I3


The isolated primordium lost polarity (it became entirely abaxialized) and became radially-symmetrical.

P3

P2

P1

I1

I2

I3

P3

P2

P1

I1

I2

I3


QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

A more recent experiment showed that laser ablation of only the epidermal cell layer is sufficient for the primordium to lose its adaxial polarity.

•A signal from the meristem moves through the epidermis into the incipient primordium.

•The signal conveys the adaxial positional information.

•The nature of the signal is not known.

Waites, R., and Hudson, A. (1995) Development 121: 2143 – 2154. Reproduced with permission.

The phantastica mutant of Antirrhinum (snapdragon) gives important clues to the basis of leaf polarity.

Controllo genetico della polarità

Wild-type phan

The phantastica mutant has radially symmetrical leaves.

Waites, R., and Hudson, A. (1995) Development 121: 2143 – 2154. Reproduced with permission.

phan mutant leafWild-type leaf

Mutant phan leaves are abaxialized, indicating that PHAN is necessary for adaxial cell fate.

phan mutant leafWild-type leaf

phan mutant leaf

P3

P2

P1

I1

I2

I3

Surgical isolation

The phan mutant leaves resemble the surgically isolated leaf primordia –

Tutte le foglie radialmente simmetriche sono abaxializzate?

No: I mutanti Loss of function kanadi hanno foglie radiali adaxiali

Eshed Y et al., Izhaki, A., Baum, S.F., Floyd, S.K., and Bowman, J.L. (2004) Development 131: 2997-3006. Reproduced with permission.

•A triple mutant kanadi 1,2 and 3 has radial, adaxialized leaves

•KANADI genes promote abaxial cell fate

La perdita della identità adaxiale o abaxiale determina la radializzazione

Wild type

phan mutant

kan mutant

Eshed Y et al., Izhaki, A., Baum, S.F., Floyd, S.K., and Bowman, J.L. (2004) Development 131: 2997-3006. Reproduced with permission.

Fenotipo del mutante Gain-of-function phb-1d

Gain-of-function phb-1d mutants have radial, adaxialized leaves.

McConnell, J.R. and Barton, M.K. (1998) Development 125: 2935-2942. Reproduced with permission.

Cross sectionSEM

In gain-of-function phb-1d mutants, PHB is expressed everywhere, resulting in adaxialized, radially symmetric leaves.

Longitudinal section Cross section

In wild-type plants, PHB expression is restricted to the adaxial side of the leaves

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. McConnell, J.R., Emery, J., Eshed, Y., Bao, N., Bowman, J., and Barton, M.K. Nature 411: 709-713, copyright 2001.

La mutazione The phb-1d mutation inluenza la distribuzione del mRNA di PHB

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. McConnell, J.R., Emery, J., Eshed, Y., Bao, N., Bowman, J., and Barton, M.K. Nature 411: 709-713, copyright 2001.

PHB promuove l’identità adaxiale

Wild-type leaf: phb-1d leaf

PHB expression = Adaxial

PHB expression = Adaxial

No PHB expression = Abaxial

come PHB, PHV and REV promuovono l’identità adaxiale

Like PHB, REVOLUTA (REV) and PHAVOLUTA (PHV) are expressed in the adaxial domain.

Reprinted from Current Biology 13, Emery, J.F., et al., Radial patterning of Arabidopsis shoots by Class III HD-ZIP and KANADI genes, 1768–177, Copyright (2003), with permission from Elsevier .

come PHB, PHV and REV promuovono l’identità adaxiale

Loss of function triple phb / phv / rev mutant has radial, abaxialized leaves

Reprinted from Current Biology 13, Emery, J.F., et al., Radial patterning of Arabidopsis shoots by Class III HD-ZIP and KANADI genes, 1768–177, Copyright (2003), with permission from Elsevier .

La polarità richiede la corretta espressione di PHB

Too much PHBToo little PHB

Borghi, L., et al.,(2007) Plant Cell 19:1795-1808.

AAAAAAA

AGO

AAAAAAA

AAAAAAA

miRNAs are short (~21-22 nt) RNAs that, in association with ARGONAUTE (AGO) target specific mRNAs for degradation (or interfere with translation).

La espressione di PHB è regolata da miRNA

AAAAAAA

In phb-1d plants, base changes in the PHB mRNA prevent miR166 from binding to it, allowing it to accumulate throughout the leaf primordium.

x

PHB-1D mRNA

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. Kidner, C.A. and Martienssen, R.A. Nature 428: 81-84, copyright 2004.; McConnell, J.R., Emery, J., Eshed, Y., Bao, N., Bowman, J., and Barton, M.K. Nature 411: 709-713, copyright 2001.

AAAAAAA

AGO

In wild-type plants, miR166 binds to the PHB mRNA and degrades it on the abaxial side of the leaf primordium.

miR166 PHB mRNA

Controllo della espressione di PHB da parte di miRNA

Additional support for role of miRNA in leaf polarity comes from the fact that the ago1 mutant has radial leaves; AGO is needed for miR166 function.

ago mutant phb-1D mutant

In ago mutants, as in phb-1D mutants, PHB mRNA accumulates throughout the leaf primordia.

Reprinted by permission from Macmillan Publishers, Ltd: NATURE. Kidner, C.A. and Martienssen, R.A. Nature 428: 81-84, copyright 2004; McConnell, J.R., Emery, J., Eshed, Y., Bao, N., Bowman, J., and Barton, M.K. Nature 411: 709-713, copyright 2001.

AAAAAAA

AGO

I miRNA controllano la polarità della foglia

These experiments demonstrate that in Arabidopsis miRNAs contribute to leaf polarity, by controlling the distribution of critical mRNAs.

AAAAAAA

AGO

I miRNA contribuiscono alla determinazione della polarità della foglia

Cosa fanno tutti questi geni?

KANADI 1,2,3 encode GARP transcription factors; YABBY genes have similar function and also encode putative transcription factors

PHANTASTICA encodes a MYB transcription factor; PHB/ PHV/ REV genes encode HD-ZIP III transcription factors

ADAXIALIZING GENES

ABAXIALIZING GENES

The genes regulated by these transcription factors are not yet known.

geni YABBY (FIL, YAB2, YAB3)

promuovono il differenziamento del lato abaxiale delle foglie (SONO FATTORI DI TRASCRIZIONE ZINC –FINGER)

Mutanti yab3 (omozigoti) producono foglie lobate che esprimono KNOX1 e formanomeristemi ectopici e mostrano conversione abaxiale/adaxiale

Il fenotipo yab suggerisce che ci sia incompatibilità tra la funzione KNOX e le funzioni che specificano l’identità ABAXIALE nella foglia

Modello per l’acquisizione della polarità

PHAN or PHB/PHV/REV

miR166 KAN, YAB

Adaxial fate

Abaxial fate

Meristem-derived signal

Il patterning ad/abaxiale può influenzare tratti agronomicamente importanti

Wild-type rice leaf. Sclerenchymatous tissue forms on the abaxial surface and supports the leaf in an open form.

Zhang, G-H. et al., (2009) Plant Cell 21:719-735

SLL1 è una proteina GARP che influenza la polarità e l’arrotolamento della foglia

sll1 mutantWild-type

In sll1 mutants, the supportive sclerenchymatous tissues on the abaxial surface do not form, causing the leaf to roll inwards.


QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Rice plants with rolled leaves (like sll1) can have more erect leaves, reduced water loss by transpiration and higher yields.

Wild-type sll1 mutant


SUMMARY

• Leaves are initiated from cells in the shoot apical meristem• Auxin gradients are important in leaf primordium initiation and

positioning• Leaf identity is determined by a change in expression of

transcription-factor encoding genes• Leaf polarity requires an unknown signal from the meristem

and the domain-specific expression of adaxial- and abaxial- specific transcription factors

GENI DI IDENTITA’ ADAXIALE/ABAXIALE

mutazioni loss of function phan promuovono la conversione adaxiale/abaxiale

(organi a simmetria radiale)

I trascritti sono localizzati nel lato adaxiale

Geni di identità adaxiale

I mutanti phan hanno anche difetti nel mantenimento del meristema

Interdipendenza destino adaxiale / meristematico

PHANTASTICA (anthirrinum)/ AS1 (arabidopsis)

mutazione phan

Homeodomain leucin zipper proteins

HD-ZIPIIIcontenenti un dominio START che lega lipidi

PHABULOSA, PHAVOLUTA, REVOLUTA

Fattori di trascrizione specifici per il latoadaxiale

PHB PHV REV Sono inizialmente espressi in maniera continua dal centro del SAM finoai primordi fogliari; successivamente la loro espressione si restringe al lato adaxiale della foglia

Mutazioni loss-of-function nei geni PHB o PHV o REV determinano conversione adaxiale/ abaxiale e incapacità di formazione o mantenimento del SAM

Le funzioni PHB, PHV, REV sembrano positivamente correlate alla funzionalità dei geni KNOX

Interdipendenza destino adaxiale / meristematico

La localizzazione nella regione adaxiale dei trascritti HD-ZIP III è regolata da:

Geni KANADI (KAN) espressi nella regione abaxiale

(mutazioni recessive KAN determinano adaxializzazione e espressione ectopica di PHAV, PHAB, REV)

microRNA

(mutazioni dominanti PHB e PHV inibiscono il riconoscimento e la degradazione dei trascritti genici ad opera di miRNA espressi specificamente nella regione abaxiale)

Regolazione da micro RNA dei geni HD-ZIP III

Mutazioni in queste regioni danno luogo a fenotipi dominanti con foglie adaxializzate,radiali e meristemi più grandi

miR165

miR166

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ORGANOGENESI: DIFFERENZIAMENTO DEI PRIMORDI FOGLIARI