lecture 28 Flashcards
1
Q
How did multicellularity evolve?
A
- independent origin of multicellularity in plants and animals
- is there a uniform logic in multicellularity despite different origins?
- independent origin of:
- pattern formation
- cell-cell communication
- evolved under different evolutionary constraints
- plants - sessile and cell wall
- same mechanism of gene regulation due to shared unicellular ancestor
2
Q
Approximately how long ago did the separation of plants and animals occur?
A
1.6 billion years
no direct evidence for the time of this split
ancestor was a unicellular eukaryote
3
Q
What higher plants are used for genetic studies?
A
- most plants belong to the angiosperms (flowering plants) family
- short lifecycle - annual
- self fertile
- diploid (a lot of plants are polyploid, annoying for geneticists)
- small genome - 125 Mbp, 390 Mbp
- sequenced genome
- transformable with Agrobacterium
- e.g. Arabidopsis thaliana (thale cress) [Dicotyledonous plant]
- e.g. oryza sativa (rice) [Monocotyledonous plant]
- more than 250,000 species of flowering plants
- just a couple of reference species
4
Q
What is flowering (dicot) plant embryogenesis?
A
- asymmetric cell division of egg - apical/basal axis
- body plan established by oriented cell divisions:
- cell divisions occur longitudinally or transversally
- radial patterning (epidermis, cortex, vasculature, ‘formation of germ layers’)
- axial patterning (shoot-root) (defines apical and basal pole)
- rigid cell wall - lack of cell movement migration
- hence you get oriented cell divisions
- no gastrulation
- locked in place by cell wall - can’t move
- no adult body structures present - limited organogenesis
- cf vertebrate organogenesis
- no germline established
- fertilised egg - polarised, cytoplasm dense on the apical side - asymmetric division
- two cell - apical cell with dense cytoplasm, basal sense not so dense, different developmental trajectories, apical cell will go on to develop embryo, basal cell will form a structure called the suspensor, tethers developing embryo to the maternal tissue - transfer of nutrients (umbilical cord), early events characterised by ordered cell divisions
- octant
- globular
- triangular
- embryonic leaves arise
- main function is the storage of food utilised by the germinating seedling
- ‘cotyledons’
- also stem like - hypocotyl
- root
- apical meristem - heart
- torpedo
- mature embryo
5
Q
What is post-embryonic plant growth?
A
- organ formation and growth occurs post-embryonically from primary apical meristems
- organogenesis is plastic and highly responsive to environmental fluctuations - adaption to local conditions
6
Q
What are the primary apical meristems of the plant?
A
- continuous organogenesis requires the activity of stem cells that are located in the 1º meristems and are maintained by signals from the organising centre
- shoot apical meristem: stem cells located in the tip of the dome, divide continuously
- root apical meristem: doesn’t give rise to organs, gives rise to the radial pattern cells in the growing root
7
Q
What is plant organogenesis?
A
- organogenesis is modular and continuous - reiterative body plan
- phytomer - leaf, axillary bud (small meristem, inactive in most species), internode
- rhizomer - root, lateral roots, highly divergent
- flowers = modified axillary bud
8
Q
What is the potency of plant somatic cells?
A
- totipotent
- cell determination and differentiation are reversible in plants
- leaf cells can dedifferentiate and then form embryonic-like clusters in liquid culture when exposed to plant hormones
- the fact that a single cell can give rise to a whole plant would suggest that plant embryogenesis is unlikely to rely on maternal morphogens
- this ability to dedifferentiate is absolutely crucial for transgenesis
9
Q
Compare animal and plant development.
A
Animals vs plants:
Body plan:
- unitary vs reiterative
Embryogenesis
- major axes established (same)
- most tissue types established vs basic tissue types established
- extensive vs limited organogenesis
- initially controlled by maternal gene products then zygotic gene products vs controlled by zygotic gene products
post embryonic development
- limited organogenesis in species displaying metamorphosis vs extensive organogenesis
- growth: cell division/expansion (same)
cell movement
- yes vs no - rigid cell wall
cell differentiation
- permanent vs mostly reversible
10
Q
What is evidence for signals involved in plant patterning?
A
- transplantation experiments and genetic analysis in animals show that signals provide positional information during development
- examine patterning in mutants with disrupted patterns of cell division
- fass, tonneau
- embryonic body plan is not disrupted in fs or ton mutants despite serious disorder of cell division
- this suggests they are still receiving positional cues that tell them how to develop/differentiate etc despite jumbled cell division
- positional information is involved in pattern formation during embryogenesis
11
Q
What is the screen for apical-basal patterning mutants?
A
- stereotypical (invariant) pattern of cell division during embryogenesis
- simple correspondence between the embryonic and adult body plan
- screen for mutants with disruptions in apical basal (A-B) patterning
triangular stage
- cells that will form cotyledons are in apical part of developing embryo
- sub-apical layer –> hypocotyl/stem like structure
- pre-basal/basal layers –> roots
12
Q
What were the four classes of mutants identified in the screen?
A
- apical: gk
- central: fk
- basal: mp
- terminal: gn
disrupted apical/basal patterning
13
Q
What are mutants lacking a basal pole?
A
- mutants lack embryonic root - hypophysis is not specified
- genes encode proteins involved in hormonal response
- e.g. monopterous (mp), bodenlos (bdl)
- during their triangular stage of development - lack hypophysis, lens shaped
- if you do not specify the hypophysis you do not form the root
- top cell of the suspensor, only cell that is recruited into embryogenesis
- mp and bdl are components of a hormone signalling pathway
14
Q
What are hormones regulating growth and development in plants?
A
Auxins:
- stimulates growth in response to light/gravity (tropism, e.g. bending toward light)
- cell division (in combination with cytokinin)
- developmental patterning (embryogenesis, organ formation)
- differentiation (vascular tissue)
- apical dominance - restrict axillary bud growth
Cytokinins:
- stimulates cell division in conjunction with auxins
- promote axillary bud growth
- e.g. zeatin
Gibberellins:
- stimulates cell enlargement and division in stem
- induce seed germination
- induces flowering
- e.g. gibberellin A1 (GA1)
and Brassinosteroids, ethylene, jasmonic acid
15
Q
What is Auxin?
A
- hormone controlling development
- synthesised in shoots and roots
- moved directionally between cells by PIN-FORMED (PIN) efflux transporter proteins
- subcellular localisation of PIN proteins determines site of auxin accumulation
- PIN localised in basal regions of cells
- determines direction of auxin flow
- auxin is flowing down the root
