Regulation of Development in Plants Flashcards
Plant vs Animal Development
- just as in animal genes we can identify genes that regulate plant development because they give rise to homoeotic phenotypes
- e.g. understanding of regulation of flower development has been aided by this kind of analysis
Double Flower Mutants
- Theophrastus described double flowers
- Gerard describes ‘hose in hose’ primula mutant, c function mutant
- Linnaeus observed mutants of toadflax whose flowers were radially instead of bilaterally symmetrical
What are some examples of model plants?
Arabidopsis thaliana Antirrhinum majus (snapdragon)
Model Plants
Arabidopsis thaliana
- small, ~15cm in height
- self fertile and can be crossed
- rosette leaves and cauline leaves
- prolific - each silique contains ~200 seeds
- flowers are actinomorphic (radially symmetrical)
Model Plants
Antirrhinum majus
- snapdragon
- quite big
- quite large genome with lots of non-coding DNA
- genome contains transposable elements
- flowers are zygomorphic (bilaterally symmetrical)
Ariel Structures
- derived from the shoot apical meristem
- top two layers of cells L1 and L2 divide perpendicular to the surface of the plant to form the epidermis and subepidermis
- the cells underneath this divide in all directions
Vegetative Growth
- vegetative growth is be cell divisions on the flanks of the meristem generating successive leaf primordia
- in Arabidopsis this generates the vegetative rosette
- the vegetative shoot apical meristem is intermediate, it continuously produces new leaf primordia
What are the two mechanisms of morphogenesis in plants?
- cells can’t move relative to each other in plants
i) control of orientation of cell division
ii) orientation and extent of cell elongation
Reproductive Development
- initiated when an environmental cause a developmental transition at the shoot apical meristem (SAM)
- SAM is now an inflorescence meristem, it remains intermediate producing floral meristems in the place of leaf primordia
- each floral meristem is determinate, it will only produce one flower by successively generating the floral organ primordia
- first sepals, then petals and stamens, then the centre of the meristem terminally differentiates to form the carpel
floricaula gene
- FLO
- found in snapdragon
- corresponding gene in Arabidopsis is lfx
- following floral transition, the inflorescence meristem generates more indeterminate meristems instead of determinate floral meristems
- so flowers never grow
leafy gene
- LFX
- found in Arabidopsis
- corresponding gene in snapdragon is flo
- has a similar mutant phenotype
- the lfy/flo gene encodes a unique plant specific transcription factor
- the DNA sequences that are targets for this transcription factor are found in promoters of genes regulating organ identity
Flowers
- made up of concentric whorls (rings) of organs when looking down on them from above
- from outside in
1) sepals
2) petals
3) stamen
4) carpels
Homoeotic Mutants Causing Transformations of Floral Organs
- classified using the ABC model
- each mutation causes a change in two adjacent whorls
Class A Mutants
- sepals become carpels
- petals become stamens
Class B Mutants
- petals become sepals
- stamens become carpels
Class C Mutants
- stamens become sepals
- carpels become petals
ABC Model
Description
1) there are three classes of homoeotic gene, A B C
2) each class is expressed in two adjacent whorls of floral meristem
3) class A is expressed in whorls 1 and 2
4) class B is expressed in whorls 2 and 3
5) class C is expressed in whorls 3 and 4
6) cells only expressing class A develop as sepals
7) cells expressing classes A and B develop as petals
8) cells expressing classes B and C develop as stamens
9) cells only expressing class C develop as carpels
10) the class A and class C genes repress each other and the C-function encodes a stop developing determinacy signal
ABC Model
Transcription Factors
- A, B and C are functions encoded by transcription factors expressed in floral meristem
- these transcription factor genes mutate to give homoeotic phenotypes like animal homeobox genes
- but homeobox genes and the A, B and C genes are not molecularly homologous
- there are plant transcription factors that contain the homeobox domain, but they control other aspects of plant development
MADS-box Proteins
- most floral regulators are members of a class of proteins called ‘‘MADS-box proteins’
- they contain domains first discovered in yeast gene Mcm1, the mammalian Serum response factor, arapbidopsis Agamous gene and snapdragon Deficiens
ABD Model
D and E
- we now know that there are additional functions also required for correct floral morphogenesis, a D-function and an E-function
- these are also encoded by MADS-box proteins
The ABC genes of plants…
…encode DNA binding transcription factors that coordinate floral organ formation
…are expressed in the organs they specify
…interpret positional information at the shoot meristem to specify formation of the right organ in the right place
