Plant stem cells Flashcards
(115 cards)
1
Q
What is the divergence angle between successive primordia in Arabidopsis plants?
A
137.5 degrees.
2
Q
What are the differences between plant development and animal development?
A
- Due to the presence of a cell wall, plant cells must communicate differently.
- Large-scale cell movements/migrations don’t occur in plants therefore different mechanisms must be used.
- The environment has a much larger impact on plant development than animal development.
- The majority of development in plants occurs not in the embryo but in the growing plant, unlike animal development, the plant embryo isn’t a mini version of the adult plant, most adult structures like shoots and roots are produced once the seed has germinated.
3
Q
Why isn’t any major shape change in plant embryos achieved by movements of sheets of cells?
A
The presence of cells walls means there is a lack of cell migration meaning movements of sheets of cells don’t occur.
4
Q
What is the lifecycle of Arabidopsis?
A
6-8 weeks.
5
Q
How many protein-coding genes does the Arabidopsis genome have?
A
27,000
6
Q
What type of plant is Arabidopsis?
A
angiosperm.
7
Q
Describe the first cell division of the plant embryo?
A
The first cell division of the zygote is at right angle to the long axis dividing it into an apical and basal cell.
8
Q
What happens to the apical cell of the plant embryo?
A
Division of the apical cell vertically produces a two-celled proembryo.
9
Q
What happens to the basal cell of the plant embryo?
A
The basal cell gives rise to the suspensor, the top most suspensor cell gets recruited into the emrbyo where it becomes the hypophysis contributing to the RAM and root cap.
10
Q
What kind of divisions occur to the apical cell?
A
Stereotyped divisions in which the cleavage is in a precise pattern and particular plane.
11
Q
What are the three concentric rings that generate the radial axis of a plant.
A
- Outer epidermis.
- Ground tissue.
- Vascular tissue.
12
Q
At what stage does the radial axis of a plant become apparent?
A
Octant stage.
13
Q
What is the relevance of periclinal and anticlinal divisions in establishment of the radial axis?
A
Periclinal divisions give rise to different rings of tissue.
Anticlinical divisions increase the cell number in each tissue ring.
14
Q
What is the dermatogen?
A
The epidermal layer.
15
Q
When is the dermatogen estbalished?
A
16-cell stage.
16
Q
Which mutant demonstrates that cell lineage isn’t crucial.
A
The fass mutant.
17
Q
What can we learn from the fass mutant?
A
In the fass mutant, the regular pattern of cell division is altered and cell divisions occur in random orientation but nonetheless, despite being mishapen, the fass mutant has roots, shoots and flowers in correct places and the radial axis is maintained showing that cell division and pattern formation can be uncoupled.
18
Q
Which hormone is essential for the establishment of the apical-basal axis?
A
Auxin
19
Q
How does auxin establish apical-basal polarity in the embryo?
A
Following the first cell division, auxin is actively transported from the basal to apical cell via auxin efflux protein PIN7 located in the apical surface of the basal cell where auxin accumulates in the apical cell/proembryo. PIN1 facilitates auxin transport between cells of the proembryo. Auxin specifies apical cell fate giving rise to apical structures like the cotyledon and SAM.
20
Q
What is the role of PIN7 in plant embryogenesis?
A
PIN7 is located in the apical surface of the basal cell and actively transports auxin from the basal to apical cell where auxin accumulates.
21
Q
What is the role of PIN1 in plant embryogenesis?
A
PIN1 facilitates auxin transport between cells of the proembryo.
22
Q
At what stage in plant embryogenesis is auxin transport reversed?
A
Globular stage/32-cell stage.
23
Q
How is auxin transport reversed in plant embryogenesis and what is the relevance?
A
The PIN7 transporter is translocated to the basal surface of the cells and in concerted action with other PIN proteins including PIN1 and PIN4, auxin is transported to the basal region of the globular embryo from which teh hypocotyl, root meristem and embryonic root are dervied.
24
Q
Which two homeobox genes are essential for establishing the auxin gradient?
A
WOX2 and WOX8.
WOX2 later becomes restricted to the apical cell.
WOX8 later becomes restricted to the basal cell.
25
What are the WOX genes and what is their role?
WOX2 and WOX8 are critical for establishment of the auxin gradient in plant embryogenesis.
26
What is a major process in plant cell growth and morphogenesis?
Cell enlargement.
27
What are the two key events in establishment of the apical-basal axis?
Asymmetrical cell division of the zygote into apical and basal cell and auxin signalling.
28
As opposed to cell lineage, what is important in determining cell fate in plants?
Cell position.
29
How many cells comprise the central zone in Arabidopsis?
12-20.
30
What is meant by the term phyllotaxis?
```
As shoots grow, leaveas are generated within the meristem at regualr intervals and with particular spacing, this is phyllotaxis.
This time delay in the initiation of leaf production from the SAM results in a repetitive modular patter in teh leaf each module consisting of an internode, a node and its associated leaf and axillary bud.
```
31
How does is the pattern of auxin circulation estbalished and how does this generate a regular pattern of leaf formation around the SAM?
Auxin produced below the meristem, the direction of auxin flow is determined by PIN1 which is located on the side of the cell nearest the neighbour with the highest auxin concentration facilitating direction flow, auxin is always transported towards a region of higher concentration. This depletes the surrounding cells of the primordium of auxin and so there is generated sequential peaks in auxin at regular positioning which prefigures future leaves.
32
What is the equivalent of the organising centre in the SAM in the RAM?
The quiescent centre.
33
How are root hair cells specified?
By a combination of positional information and lateral inhibition. On the surface of the developing root, the files of cells that will make root hairs alternate with the files of cells that will produce non-hair structures.
If a root hair cell overlies the junction between two cortical cells, it forms a root hair, but if it is just contacting one cortical cell, it doesn't, this is the positional information.
34
How many whorls are there in an Arabidopsis plant?
4
35
What are the KNOX genes?
KNOX genes like KNOTTED-1 encode homeodomain transcription factors, these have an extended TALE homeodomain and bind DNA via the homeodomain to activate/repress target genes.
Class-1 KNOX genes are important for SAM function.
Higher plants have Class-1 and Class-2 KNOX genes.
KNOX genes form dimers with closely related BELL homeodomain proteins and form homodimers with themselves or heterodimers with other KNOX proteins.
36
What is the most important KNOX gene in the SAM?
STM.
37
What does the STM mutant look like?
The SAM fails to develop in STM LOF mutants.
The meristematic cells aren't maintained in an undifferentiated state, they are incorporated into developing organs e.g. cotyledons during embryogenesis.
No true leaves form in STM mutants due to the loss of the SAM.
STM LOF mutants generated by RNAi exhibits ectopic organ formation and depletion of meristem cells.
The whole meristem is consumed in one go and turned into a terminal leaf because meristematic fate cannot be maintained.
38
What is the role of KNOX genes like STM?
They promote and maintain pluripotency in the SAM.
STM is required for formation of the SAM during embryogenesis and is required for the maintenance (continued self-renewal) of the SAM in post-embryonic adult growth.
The STM is suffcieint to activate de novo SAM formation.
39
What does an STM over-expressor look like?
Leaf growth and expansion is inhibited and pavement cell expansion and differentiation is inhibited.
Ectopic shoot meristems form on the upper leaf surface from undifferentiated cells.
Epidermal cells fail to adopt their characteristic jigsaw differentiated shape.
Over-expressing STM prolongs mitotic division.
40
How does STM promote pluripotency?
STM inhibits cell expansion and differentiation.
STM coordinates undifferentiated cells and organises them into new meristems.
The stem cell population of the meristem is within the STM expressio domain and within these undifferentiated cells, there are other factors acting to maitnian the identity of a small subset of cells as stem cells.
41
Where abouts in the SAM are the stem cells found?
The central zone.
42
Which area of the SAM is responsible for maintaining stem cell identity?
The organising centre.
43
Which regulatory feedback loop controls stem cell numbers in the SAM?
Describe it.
The CLV-WUSCHEL regulatory feedback loop.
WUSCHEL is expressed in the SAM by the organising centre, WUS promotes stem cell identity in the cells above the organising centre.
WUS also induces expression of peptide ligand CLAVATA3 (CLV3) which binds to the CLAVATA1 receptor kinase (CLV1) leading to repression of WUS.
WUS is specifying cells above to be stem cells and promotes expression of CLV3 in the stem cells which diffuses down and meets peptide ligand receptor CLV1 and these events conspire to switch off WUS.
44
What is WUSCHEL?
A WOX-family homeodomain transcription factor.
45
What is encoded by the genes CLV1 and CLV2?
Membrane-associated leucine-rich repeat (LRR) receptor kinases.
46
What is encoded by CLV3?
CLV3 encodes a small CLE-type peptide ligand that binds CLV1/2.
47
Describe the regulation of lateral organ initiation?
Lateral organs such as leaves are initiated from the periphery of the SAM.
Organ founder cells are specified by the accumulation of auxin and expression of organ-specific transcription factors.
The organ primordium then develops through cell division, expansion and differentiation.
Repression of KNOX gene expression is one of the first steps in the acquisition of organ identity.
48
What is one of the first steps in the acquisition of organ identity?
Repression of KNOX genes allows subsequent differentiation of cells.
49
Describe what is promoting phyllotaxis?
There is a feed-forward process resulting in the slight accumulation of auxin in one cell causing auxin to be pumped into that cell from surrounding cells due to preferential transport by PIN1 which locates itself on the side nearest the cell with the highest auxin conc.
This depletes the region around this cell of auxin - remember auxin is involved in regulation of lateral organ initiation and therefore, this ensures two leaves don't form too close to each other.
A second new primordium will form in the nearest region where there is sufficient auxin and this regulates the spiral phyllotactic pattern and the divergence angle.
50
List 2 genes involved in organ formation?
1. Asymmetric leaves 1 (AS1).
| 2. Aintegumenta (ANT).
51
What is meant by polar auxin transport?
Auxin is a weak acid so becomes deprotonated in the neutral pH cytoplasm making it polar. It can now no-longer cross the lipid bilayer which is apolar and so auxin is said to be acid-trapped.
It can enter the cell by passive diffusion or active transport by AUX1 importers but must be exported from the cell by PIN or PGPs.
The PIN efflux carriers line up where auxin will be pumped out and efflux auxin int the next cell = polar auxin transport because it is only in one direction.
52
What is AS1 and what does it do?
AS1 is a member of the R2R3 MYB domain protein family of transcriptional repressors.
AS1 promotes differentiation and represses KNOX gene expression and forms a repressive complex with other factors expressed in the leaf.
53
Describe the AS1 mutant?
The AS1 mutant has undifferentiated leaves, similar to a mild KNOX over-expressor.
54
What is ANT and what does it do?
ANT is a member of the AP2 gene family and encodes a transcription factor that promotes cell division in leaf primordia and controls constituent organ cell number.
55
What is the role of KNOX genes?
Meristematic competence - inhibition of organ specification, repression of cell expansion and differentiation.
56
What is the role of the WUS/CLV loop?
Controls stem cell identity and number.
57
What is the role of auxin?
Acquisition of organ-specific cell fate.
58
What is the role of ANT?
Regulation of constituent organ cell number.
59
What is the role of AS1 and TCP?
Repression of KNOX genes and promotion of cell differentiation.
60
What is the role of the CUC genes?
Separation of the eristem from the organ cells (boundary formation) and establishment of the SAM and auxiliary meristems.
61
How do KNOX genes promote pluripotency?
They are transcription factors meaning they activate and repress target gees.
KNOX genes promote the production of cytokinin biosynthesis, cytokinin is a phytohormone derived from adenine with an opposing function to auxin.
Cytokinin promotes cell division and shoot formation.
It also stimulates chloroplast division and is involved in greening.
STM activates expression of biosynthetic genes IPT3 and IP7 leading to enhanced cytokinin synthesis in the SAM.
62
What key family of enzymes are involved in cytokinin biosythesis?
Isopentyl transferase family.
63
What genes encode the isopentyl transferase enzymes
IPT genes.
64
Which 2 IPT genes does STM activate?
IPT3 and IPT7.
65
Describe the basic steps in the cytokinin signalling pathway?
Cytokinin repsonse is controlled by the ARR-B family of transcription factors.
Cytokinins bind to arabidopsis histidine kinase (AHK) receptors int eh plasma membrae which transfers a phosphate from its C-terminus to Arabidopsis histidine phosphotransfer proteins (AHP).
The AHPs trasnloate to the nucleus where the phosphate group is transferred to two types of transcription factors, ARR-A and ARR-B.
ARR-A antagonise the ARR-Bs and switch the cytokinin response off whereas ARR-Bs activate the genes associated with cytokinin response.
66
What family of transcription factors is controlled by cytokinin signalling?
ARR-B.
67
What are ARR-B proteins?
MYB domain transcription factors, they activate many genes associated with the cytokinin response and they activate their own repressors ARR-A.
This is a negative feedback loop emplyed by the system to dampen down cytokinin response and ensure system stability.
68
What transcription factors repress the ARR-B?
ARR-A.
69
List some functions controlled by cytokinin?
```
Seed development.
Leaf senescence.
Stress tolerance.
Nutrient balance.
Root, shoot and inflorescence growth and branching.
Chloroplast biogenesis.
Vascular differentiation.
Stem cell control in root and shoot.
```
70
How does STM promote meristem development?
STM regulates the genes involved in the cytokinin synthesis pathways and response.
This promotes cytokinin biosythesis e.g. activates IPT3 and IPT7 and response in the SAM.
STM promotes cytokinin biosyntehsis and expression of the cytokinin receptor e.g. AHK4 and responses.
STM also inhibits catabolism of cytokinins e.g. cytokinin oxidase.
71
How does cytokinins promote cell division?
Principally through the activation of D3-type cyclin (CycD3) expression.
D-type cyclins promote cell division and enhance cytokinin sensitivity possibly through repression of ARR-A sot here is a poistive reinforcement loop ensuring cells do not differentiate.
CycD3 forms a complex with cyclin dependent kinase which phsophroylates the retinoblastoma protein releasing E2F transcription factors which acitvate expression of core cell cycle enzymes e.g. genes involved in S phase.
When the cell isn't activity dividing, E2F transcription factors are bound by retinoblastoma protein.
Cytokinin stimualtes cdc25 which removes cdk phosphorylation thus removing the cell cycle halt.
The presence of CycD3 increases cytokinin sensitivity driving a positive feedback loop giving a strong response keeping the cells of the meristem dividing.
72
How is WUS linked to cytokinins?
As part of its role in promoting stem cell identity, WUS promotes cytokinin responses by directly repressing ARR-A gene expression.
ARR-A gene expression is induced by cytokinin (as a regulatory mechanism to dampen down cytokinins signalling), but ARR-As repress cytokinin repsonses by repressing ARR-B expression.
WUS binds the promoter of ARR-A genes and represses their trasncirption hence promoting cytokinin resposne.
73
What does WUS do?
WUS (expressed in the organising centre) promotes stem cell identity via the WUS/CLV3 feedback loop/mechanism.
WUS also represses ARR-As which normally switch of cytokinin responses, therefore, WUS is promoting cytokinin response to maintain division of the meristematic cells.
74
What is the cellular role of auxin?
Promotes cell division at high concentration and promotes cell expansion at low concentration.
75
What is the cellular role of cytokinin?
Promotes cell division.
76
What is the morphogenic role of cytokinin?
Cytokinin promotes shoot devleopment and the formation of chloroplasts (greening).
77
What is the morphgenic role of auxin?
Promotes root development.
78
What is the localisation of cytokinin?
Cytokinin is associated with pluripotent stem cell populations and the early stages of organ formation where cell division dominates.
The centre of the meristem expresses cytokinin and the cytokinin response is greatest in the middle.
79
What is the localisation of auxin?
Auxin is associated iwth tissues and organs that will undergo terminal differentiation e.g. leaves.
80
What is the difference in the localisation of auxin and cytokinin?
Auxin is expressed in the cell that are about to undergo organogenesis and cytokinin is expressed in the cells that won't undergo organogenesis.
81
How does STM repress auxin biosynthesis and response in the SAM?
STM represses synthesis of giberellic acid which promotes polar cell expansion and differentiation by directly repressing expression of GA-20 oxidase biosynthetic gene, in this response GA acts with auxin.
STM represses auxin biosynthesis by transcirptionally repressing genes involving in auxin biosythesis such as TAA1 and YUCCA.
82
Which two genes classes interact to control primordium specification?
Lateral organ primordia develop on the flanks of the SAM.
Organ primordia formation is initiated by the phytohormone auxin and auxin synthesis and response is repressed by KNOX genes.
KNOX gene expression must be turned off to allow primordium formation.
In turn organ-specific transcription factors are activated in the primordium including the TCP family of transcription factors.
TCP transcription factors promote differentiation, repress cell division and repress mitosis-associated genes and the TCP family of transcription factors are associated with auxin accumulation in the primordia.
83
How many genes are in the TCP family of transcription factors?
23.
84
Which gene crucially represses TCP?
STM - crucially it represses TCP3 and TCP4.
85
Which genes conspire to keep KNOX genes switched off in the organ primordia?
TCP and AS1.
86
Pluripotency and differentiation are said to be what?
Antagonistic processes
87
What is the result of ectopic TCP4 expression in the SAM?
Ectopic TCP4 expression in the SAM causes meristeatic termination similar to STM mutants.
Overexpression of TCP4 and STM look very alike.
With both mutants we see the formation of a terminal leaf.
This provides good evidnece that these two transcription factors antagonise each other.
88
Besides cytokinin biosythesis, what are some other roles of STM/KNOX genes in pluripotency/development in the SAM?
STM not only controls cytokinin biosythesis in the SAM but also in controlling cell division in the developing roots and leaves.
STM interacts with other plant hormone pathways, e.g. repression giberellic acid syntehsis - GA promotes cell expansion and differentiation.
Repression of auxin biosyntehsis - auxin promotes cell division and expansion and high auxin levels drive organ formation.
STM promotes de novo SAM organisation from a population of undifferentiated cells.
89
How does STM promote meristem development?
STM regulates genes involved in patterning and differentiation.
Promotes pluripotency and control of phyllotaxis by activation of the PLETHORA genes e.g. PLT7 and AIL7 in the SAM.
Repression cellular differentiation by repression expression of class-2 TCP transcription factors in the SAM.
Controls meristem-orga boundary zone formation by regulation of CUP-shaped cotyledon genes.
90
How does LEAFY expression differ in leaf and floral primordia?
Low levels of LEAFY transcription factor and low levels of floral induction signals expressed in primordia causes them to form leaves.
High levels of LEAFY transcription factor and high levels of floral induction signals expressed in primordia causes them to form flowers.
91
What are the four main organs in Arabidopsis flowers?
1. Sepals - green outer organs.
2. Petals - white organs to attract pollinators.
3. Stamens - male reproductive organs comprising the anther and filaments.
4. Carpels - female reproductive organs comprising the ovules, the carpels are fused into a gynoecium.
92
What are the outer2 whorls?
Sepals and petals, these are sterile organs.
93
What are the inner 2 whorls?
Stamens and carpels, these are the reproductive organs.
94
What is the role of LEAFY?
LEAFY is a transcription factor and is a major regulator of the transition to flowering.
High levels of LEAFY in the primordia trigger the transition to reproductive growth.
LEAFY activates floral organ identity genes in the developing floral meristem.
LEAFY tells the cells that they are in a reproductive state and when expressed highly throughout the primordia, it triggers the primodia to develop into flowers not a leaf.
LEAFY activates floral patterning (ABC) genes in the floral meristem
95
What do we see in LEAFY mutants?
LEAFY mutants lack floral organ identity although they do have some carpelloid features, they are defective in floral organ identity.
96
Where does LEAFY initially accumulate?
The floral meristem.
97
Describe the expression pattern of ABC class genes?
```
The A class genes are expressed in the outer 2 whorls.
The C class genes are expressed in tthe inner 2whorls.
A class gene alone = sepals.
A class + B class = petals.
C class alone = carpels.
C class + B class = stamens.
```
98
What does A class gene expression promote?
The formation of outer sterile organs.
99
What does C class gene expression promote?
The formation of inner reproductive organs.
100
A class and C class expression are what.....?
Mutually antagonistic.
101
Describe the function of apetala 1 and the mutant?
AP1 is an A-class mutant.
Apetala 1 encodes a MADS domain protein.
AP1 is initially expressed throughout the floral meristem and then confined to whorls 1 and 2 like other A class genes.
In the AP1 mutant, sepals are leaf-like and petals are not formed, a new flower forms in place of petals.
The stamens and carpels form normally.
AP1 is necessary to confer to the SAM that a floral organ is forming so in its absence it is possible to get a whole new flower forming.
102
Describe the function of apetala 2 and the mutant?
AP2 is an A-class mutant.
Apetala 2 encodes an AP2 domain transcription factor.
AP2 is expressed in whorls 1 and 2.
In the AP2 mutant, sepals are capelloid e.g. they have stigmatic papillae (this is due to ectopic C-class expression).
Petals are either missing, reduced in number, replaced by stamens or stamen-petal hybrids or occassionally converted to carpels.
The inner stamen and carpels form normall although often the number of stamen is reduced.
103
Describe the function of apetala 3 and the mutant?
AP3 is a B-class mutant.
AP3 encodes a MADS domain transcription factor.
AP3 is expressed in whorls 2 and 3.
AP3 mutants lack petals which are replaced by sepal-like organs, some also lack stamens.
AP3 is more importnat for petal growth than stamen growth.
You get carpel formation (only requires C class mutant exclusively) but not stamen formation (requires B and C) and we see lack of petals (requires A + B) but the sepals are fine.
104
Describe the function of pistillata and the mutant?
The pistillata mutant is a B-class mutant.
The pistillata gene encodes a MADS domain transcription facotr.
Pistillata is usually expressed in whorls 2 and 3.
The pistillata mutant lacks petals which are replaced by sepal-like organs and lacks stamens replaced by carpels.
105
Describe the function of agamous and the mutant?
The agamous mutant is a C-class mutant.
Agamous encodes a MADS domain transcription factor.
Agamous = lacks gametes.
AG is expressed in whorls 3 and 4 promoting the development of reproductive organs.
The sepals in whorl 1 and petals in whorl 2 develop normally but the stamens in whorl 3 are replaced by petals (due to ectopic A (also have B)) and carpels in whorl 4 are replaced by another flower.
106
What is the agamous mutant defective in?
Agamous mutants are defective in specification of reproductive organs and fail to terminnate central stem cell acitvity resulting in formation of flowers within flowers.
WUS has the same role in the floral meristem as the SAM so it maintains stem cells, an agamous normally turns WUS off.
Agamous is activated by WUS and LEAFY.
Thus in absence of agamous, WUS isn't switched off = too much WUS.
107
What is important to remember about the ABC class genes?
None of them are sufficient to direct the formation of a whole new organ. We also need SEPALLATA genes.
Ectopic expression of ABC genes has no effect on leaves.
108
What is needed besides ABC genes to direct formation of a whole new organ?
SEPALLATA genes.
109
What are the sepallata genes and what is their role?
The sepallata genes encode MADS domain transcription factors and are expressed throughout the developing floral meristem.
There are 4 sepallata genes.
Need to lose at least 3 sepallata genes to get a phenotype.
In sepallata mutants, all floral organs are converted to sepals hence sepallata genes are needed for B and C genes to exert their effects.
Sepallata genes have E-class function.
110
What is needed for conversion of leaves to petals?
Conversion of leaves to petals requires sepallata function and A and B class genes.
111
Are ABC genes necessary and sufficient for floral organ formation?
They are necessary, but they aren't sufficient.
We know this because ectopic expression of ABC genes in leaves has no effect.
But co-expression of ABC genes with SEPALLATA genes is sufficient to convert leaves to petals.
Overexpression of A, B, C and SEPALLATA together is sufficient to generate and direct the formation of an entire new organ.
112
What are D class genes?
D class genes are involved in ovule formation and in carpel seed formation.
113
What do MADS domain complexes do?
MADS domain complexes bind cis-acting elements called CArG boxes which have the consensus sequence 5'-CC(A/T)6GG-3' and causes looping between adjacent CArG boxes.
MADS domain complexes associate with chromatin remodelling enzyme and transcriptional co-factors to alter local chromatin accessibility.
114
What is the CArG box?
A cis-acting element with the consensus 5'-CC(A/T)6GG-3' bound by MADS domain transcription factors.
115
What is the role of KNOX genes in the floral meristem?
STM is expressed in the SAM during reproductive growth and vegetative growth.
STM is initially dowregulated in the floral meristem and primordia are initiated, STM is then re-activated across the apical region of the developing FM to keep cells in a pluripotent state to form sepals, petals etc.
STM is gradually downregulated as the floral organs are formed but expression is maintain in the centre of the FM (whorl 4).
STM is required for maintaining the space where carpels form, STM LOF/downregulation using RNAi = loss of W4 and loss of carpel development.
STM overexpression in flowers results in swollen gynoecium and the formation of ectopic carpels.
STM overexpression also promotes the homeotic conversion of ovules to carpels.
