Regulation of Development in Plants

Question 1

Plant vs Animal Development

Answer 1

• 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

Question 2

Double Flower Mutants

Answer 2

• 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

Question 3

What are some examples of model plants?

Answer 3

Arabidopsis thaliana
Antirrhinum majus (snapdragon)

Question 4

Model Plants

Arabidopsis thaliana

Answer 4

• 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)

Question 5

Model Plants

Antirrhinum majus

Answer 5

• snapdragon
• quite big
• quite large genome with lots of non-coding DNA
• genome contains transposable elements
• flowers are zygomorphic (bilaterally symmetrical)

Question 6

Ariel Structures

Answer 6

• 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

Question 7

Vegetative Growth

Answer 7

• 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

Question 8

What are the two mechanisms of morphogenesis in plants?

Answer 8

• cells can’t move relative to each other in plants
  i) control of orientation of cell division
  ii) orientation and extent of cell elongation

Question 9

Reproductive Development

Answer 9

• 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

Question 10

floricaula gene

Answer 10

• 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

Question 11

leafy gene

Answer 11

• 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

Question 12

Flowers

Answer 12

• 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

Question 13

Homoeotic Mutants Causing Transformations of Floral Organs

Answer 13

• classified using the ABC model

- each mutation causes a change in two adjacent whorls

Question 14

Class A Mutants

Answer 14

• sepals become carpels

- petals become stamens

Question 15

Class B Mutants

Answer 15

• petals become sepals

- stamens become carpels

Question 16

Class C Mutants

Answer 16

• stamens become sepals

- carpels become petals

Question 17

ABC Model

Description

Answer 17

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

Question 18

ABC Model

Transcription Factors

Answer 18

• 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

Question 19

MADS-box Proteins

Answer 19

• 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

Question 20

ABD Model

D and E

Answer 20

• 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

Question 21

The ABC genes of plants…

Answer 21

…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