Plants Flashcards
What is the difference between embryogenesis in animal and plants
Animals all tissues and organs are formed in embryogenesis - as organism grows tissues and organts
* Plant- Seed leaves- only base body plan formed- and primitive root- adult tissues and organs form post-embryonically from meristems
What is the shoot apical meristem?
Plant stem cells population come from this- plant tissues and organs made from this - so it is key that the population is maintained
What are the key features of plant growth and development?
• MOst development is post embryonic
• Cells are fixed (cell wall) and do not migrate
• Cell position determines cells identity (not cell lineage)
• Cells divide, then expand, then differentiate
• Self organising meristems contain stem cells
• Stem cells allow indeterminate growth
How care cell organised into complex 3D structures?
• all cells derive from shoot or root meristems
• Iterative nature of development
• Morphogens
• Cell to cell signalling
• Fundamental pattern establishment
What are fractals?
Fundamentals properties of how plants grow- they keep branching
Describe shoot development in plants
Shoot development in plants:
All above ground tissues and organs derive from the shoot apical meristem (SAM)
SAM contains pluripotent stem cells
These stem cells give rise to PHYTOMERS
Phytomer = leaf (Organ), axillary bud, and internode
Shot growth is the iterative (repeated) formation of phytomers
what is the shoot apical meristem?
Begin as a bulge stem cells population in centre never involved in organogenesis
What is Phyllotaxis?
The pattern of organ emergence
What is the Fibonacci spiral?
Spirals in opposite direction.
DIVERGENCE ANGLE OF 137.5 degrees
What specifies cell identity and fate?
Cell position not cell lineage
What regulates lateral root emergence?
Phytohormone auxin
What is the shoot apical meristem (SAM)
Arabidopsis Thaliana: the principle model organism for plant development
Small,easy to grow, with short life cycle (6-8 weeks)
Fully sequenced genome 135mb on 5 chromosomes
Large selection of online research resources
Easy to transform
Mutants available in every gene. Used extensively as a model organism for plant development
The arabidopsis SAM produced leaves during vegetative growth and flowers during productive growth
What is the phytomer concept?
Plant bodies are assembled from developmental subunits called phytomers which are produced by SAM
What are the three layers in SAM?
Three layers in SAM
Epiderminal tissue
Supeipdermal/ ground tissue
Vasculature tissue and stem pith
What is Knotted1?
Like homeobox genes (KNOX genes)- encoded homeodomain TFs - homeodomain is highly conserved between animals, plants and fungi.
KNOX proteins ave an extended tale homeodomain (three amino acid loop extension (Tale)
Bind DNA via homeodomain and activate or repress transcription of target gene via MEINOX domain
What is the role of KNOX genes in shoot meristems?
The KNOX gene shoot meristemless (STM)
* STM is expressed in non-organogenic cells of shoot meristems
* STM promotes. Pluripotency in cells in which it is expressed
* Switched of in leaf founder cells, primordia and mature leaves
* Down-regulation in leaf primordia allows cells to enter differentiation pathways
What is the role of STM transcription factor in SAM development and function?
• STM is required for formation of SAM during embryogenesis
• STM required for maintained (continued self-renewal_ of the SAM during post embryonic adult growth
• STM is sufficient to activate de novo shoot meristems formation
State the levels of floral induction signals and LEAFY needed for primordial to form flowers
both must be high. opposite needed for primordial to form leaves
Where do floral meristems form in the SAM
on the flanks
Are floral meristems determinate or indeterminate and why
indeterminate because they form new organs
Where are the stem cells which form female gametes located
the centre of floral meristems
List what structures form from the 4 whorls
1 and 2 = sterile organs = sepals and petals
3 and 4 = reproductive = stamens and carpels
What is the role of the LEAFY TF
regulates the transition to being a flowering plant, trigger transition to reproductive growth and the production of floral meristems instead of leaf primordial. activates floral identity genes in developing floral meristem
Describe the leafy mutant
lacks floral organ identity
Describe the AP1 mutant phenotype
AP1 expressed in whorls 1 and 2. sepals and petals not formed. new flower forms in place of petals. stamens and carpels form normally
Describe the AP2 phenotype
AP2 exppressed in whorls 1 and 2
Sepals are carpelloid – have a stigmatic papillae normally found on carpels, caused by ectopic C-class actiivty
Petals are either missing, reduced in number or replaced by stamens or stamen-like petal hybrid organs, or in some alleles carpels are formed
Inner stamens and carpels are formed in correct place, but often there are fewer stamens
Describe the AP3 mutant phenotype
`AP3 expressed in whorls 2 and 3
Ap3 mutant lacks petals – replaced by sepal-like organs
Stronger alleles also lack stamens, which are replaced by carpels
Describe the pistillata mutant phenotype
PI expressed in whorls 2 and 3
Pi mutant lacks petals, which are replaced by sepal-like organs
Also lack stamens which are replaced by carpels
Describe the agamous mutant phenotype
AG is expressed in whorls 3 and 4
Promotes development of reproductive organs
Sepals In whorls 1 and petals in whorl 2 develop normally
Stamens in whorl 3 replaced by petals (caused by ectopic A class activity)
Carpels in whorl 4 replaced by another ag flower, and whorl 4 of that flower replaced by another ag flower, and so on…
What is teh relationship between WUS and AG
WUS induces AG
AG represses WUS
negative feedback loop activates carpel development and terminates stem cell identity
What are SEPALLATA MADS box genes
SEPALLATA genes encode MADS domain TFs
Expressed throughout developing floral meristem
Four members in the gene family – sep1,2,3,4
Function only revealed in triple and quadruple mutant. mutants have floral organs converted to sepals
What does LEAFY activation of ABC genes require
LEAFY activation of B class gene expression requires the F box protein UNUSUAL FLORAL ORGANS (UFO)
LEAFY activation of C class gene expression requires stem cell factor WUSCHEL
Floral organ development is directed by tetramic complexes of MADS domain proteins comprising different combinations of ABCE TFs
What does the MADS domain complex binding to CArG boxes in genomic DNA cause
Causes looping of DNA between adjacent CArG boxes
MADS complexes associate with chromatin remodelling enzymes and TF co-factors to alter local chromatin accessibility (pioneer factors)
This leads to activation or repression of target gene expression
How do KNOX genes ensure STM is expressed in the SAM in reproductive growth
STM is initially down-regulated as FM primordia are initiated
STM then reactivated across the apical region of the developing FM
Gradually downregulated as each type of floral organ is formed
Remains active in centre of FM (whorl 4)
How is STM needed for the development of carpels in the gynoecium
Downregulation of STM expression using RNA-interference leads to a loss of whorl4 carpel development
No other organs are formed as whorl 4 is missing so stamens reside at the centre of the flower
What does overexpression of STM in flowers cause
Sowllen gynoecium
Ectopic carpels (other floral organs form normally, but eventually degenerate and form new carpelloid organs)
STM promotes homeotic conversion of ovules (in the gynoecium which form seeds) to carpels
What are two meristems present in embryonic development of plants? What do they do?
Shoot apical meristem- produces all above ground tissues and organs. Root apical meristem- produces all roots (primary root and lateral root)
What happens if a cell divides and ends up in the wrong layer?
It will take up the identity of the layer it is in
Describe the fass mutant?
A functional plant but is disorganised cell divisions. Plant body plant is present- so highly ordered cell divisions in the embryo are not required for embryonic pattern formation
What occurs during embryogenesis of maize?
In the monocots species maize does not involved ordered cell divisions - a fully function embryo is made
How is SAM development activated during embryogenesis
Activating SAM development during embryogenesis: CUC transcription factors promote STM expression
* CUP-SHAPED cotyledons (CUC) genes (CUC1,CUC2 AND CUC3) encode NAC- domain TFs
* Functionally redundant (requires loss of at least two CUC genes to lose function)
* CUC activates STM expression in a stroller across the apical embryo domain STM expression is not activated in CUC1 nd cuc2 double mutants
* STM expression is then restricted to the cells that form the SAM, while CUC is restricted to the boundary region between the cotyledons and the SAM is required for proper separation of meristem and organs
How do KNOX genes such as STM promote pluripotency
How do KNOX genes such as STM promote pluripotency
One mechanisms is through promotion of CYTOKININ (CK) buosytnthesis
Cytokinin is phytohormone derived from adenin
Cytokinin has an opposing function auxin
Promote cell division and promote shoot formation
One of key enzymes classes involved in CK synthesis is the Isopentyl Transferase (IPT) family of enzymes
These enzymes are encoded by the IPT genes
In particular, STM activates transcription of IPT3 and IPT7, leading to enhancedCK synthesis in the SAM
Describe the cytokinin signalling pathway
- Cytokinin binds to AHK receptors in the plasma membrane
- AHK receptor transfers a phosphate group to AHP proteins
- AHPs translocates to the nucleus
- AHP transfers its phosphate to ARR-B proteins activate CK-responsive gene expression
- Arr-a PROTEINS REPRESSOR arb EXPRESSION LEADING TO REPRESSION OF CTOKINKIN ESPONSUVE GENE EXPRESSION
Describe the specification of the stem cell niche during embryonic SAM development?
EXPRESSION OF WUS (red) is activated during early embryogenesis
Gradually confined to cells that will form the SAM organising centre (OC)
CLAVATA3 (CLV3) expression is activated by WUS in cells overlying the OC
These become the stem cells, organs are never formed directly from these stem cells. Ste cells numbers are regulated by the WUS-CLV feedback loop
WUS promote stem cell identity in particular by promoting cytokinin responses
What organ primordium specific genes do primordia express?
AINTEGUMETA, ASYMMETRIC LEAVES1
CLASS2 TCPs
What does WUS promote
Promotes stem cell identity in the cells immediately above the organising centre
Includes expression of the peptide ligand CLAVATA3 which binds to CLAVATA1 receptor kinase leading to repression of WUS
