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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
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
Spiralata
In most plants, such as this succulent, leaves form in a regular spiral pattern.
Photos courtesy of tom donald
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
I2
This tomato apex shows the positions of several primordia (P) and incipient primordia (I).
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
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
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
P3
P2
P1
I1
I2
I1 (shown in black) was surgically isolated from the rest of the meristem, by cutting along the red line.
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
P3
P2P1
I1
I2
Two days later, the apex was examined.
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
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.
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
P3
P2P1
I1
I2
I3
Positions of I2 and I3; older leaves have been cut away.
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
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
Visualizing PIN1 localization
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.
Visualizing PIN1 localization
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
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
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
Reinhardt, D., Frenz, M., Mandel, T., and Kuhlemeier, C. (2005) Development 132: 15-26. Reproduced with permission.
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.
Zhang, G-H. et al., (2009) Plant Cell 21:719-735
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
Zhang, G-H. et al., (2009) Plant Cell 21:719-735
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