lecture 28

Question 1

How did multicellularity evolve?

Answer 1

• 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

Question 2

Approximately how long ago did the separation of plants and animals occur?

Answer 2

1.6 billion years
no direct evidence for the time of this split
ancestor was a unicellular eukaryote

Question 3

What higher plants are used for genetic studies?

Answer 3

• 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

Question 4

What is flowering (dicot) plant embryogenesis?

Answer 4

• 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

1. fertilised egg - polarised, cytoplasm dense on the apical side - asymmetric division
2. 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
3. octant
4. globular
5. triangular
   - embryonic leaves arise
   - main function is the storage of food utilised by the germinating seedling
   - ‘cotyledons’
   - also stem like - hypocotyl
   - root
   - apical meristem
6. heart
7. torpedo
8. mature embryo

Question 5

What is post-embryonic plant growth?

Answer 5

• organ formation and growth occurs post-embryonically from primary apical meristems
• organogenesis is plastic and highly responsive to environmental fluctuations - adaption to local conditions

Question 6

What are the primary apical meristems of the plant?

Answer 6

• 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

Question 7

What is plant organogenesis?

Answer 7

• 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

Question 8

What is the potency of plant somatic cells?

Answer 8

• 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

Question 9

Compare animal and plant development.

Answer 9

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

Question 10

What is evidence for signals involved in plant patterning?

Answer 10

• 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

Question 11

What is the screen for apical-basal patterning mutants?

Answer 11

• 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

Question 12

What were the four classes of mutants identified in the screen?

Answer 12

• apical: gk
• central: fk
• basal: mp
• terminal: gn

disrupted apical/basal patterning

Question 13

What are mutants lacking a basal pole?

Answer 13

• 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

Question 14

What are hormones regulating growth and development in plants?

Answer 14

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

Question 15

What is Auxin?

Answer 15

• 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

Question 16

How does auxin signal?

Answer 16

• auxin accumulation leads to derepression of auxin response genes
• activation induced by Auxin Response Factors (ARFs) - transcription factors belonging to a multi-gene family
• IAA repressors keep ARFs inactive through dimerisation - multi-gene family
• IAA repressors are degraded by the 26S proteasome following auxin accumulation in the cell
• auxin response gene expression is low when low auxin level in the cell
• high auxin concentration in the cell –> auxin response gene expression is high
• rapid transcriptional responses - within 5 to 10 minutes

Question 17

What is the role of auxin signalling in hypophysis formation?

Answer 17

• MP is an auxin response factor and BDL is an IAA repressor
• auxin accumulation in basal pole triggers Bdl degradation
• Mp activates genes for root formation
• mp mutant: absence of mp protein, therefore unable to respond to auxin accumulation
• bdl mutant: bdl mutant protein resistant to auxin-induced degradation - constitutive repressor
• unable to respond to auxin accumulation

Question 18

Is there an embryonic auxin gradient?

Answer 18

• visualising PIN protein localisation during embryogenesis reveals an important role for auxin in axial patterning
• auxin produced in apical embryo cells
• PIN proteins localised in basal membranes of embryo cells
• auxin accumulates in basal pole of developing embryo
• auxin accumulation correlates with hypophysis specification

Question 19

What is a model for hypophysis formation?

Answer 19

• auxin accumulates in basal region of embryo
• Mp/Bdl are NOT expressed in hypophysis precursor cell, expressed in cells just above the precursor
• mutation of Mp resulted in less PIN expression suggested that it normally promotes PIN expression

1. MP is activated in basal cell of embryo
2. MP promotes PIN expression
3. PIN transport triggers high-level auxin accumulation in hypophysis precursor
4. hypophysis fate established through activation of unidentified ARFs

Question 20

Is auxin a morphogen?

Answer 20

• a morphogen is a positional signal that forms a concentration gradient as it spreads through a field of cells
• • gradient of auxin observed along the apical-basal axis (active transport)
• signalling molecule acts directly on cells in a concentration dependent manner
• • auxin regulates gene expression through ARF activation

Low auxin conc. : IAAs more abundant: ARF less active

High auxin conc. : IAAs less abundant : ARF more active

–> differential gene expression, specification of cell identity

Question 21

What is the morphogenetic trigger concept?

Answer 21

• factor or signal that induces, through unequal distribution of its activity, acquisition of a new developmental fate in a cell or a group of cells that were originally similar to their neighbours
• patterning mechanism establishes unequal distribution of signal
• acts directly on cell and changes developmental fate
• only one threshold concentration
• no direct evidence that there is a range of ARF activity along the apical-basal axis, all we know is that there is a high concentration of auxin in the basal pole and that leads to hypophisis formation
• trigger instead of morphogen?

Question 22

What pathways does auxin control?

Answer 22

• auxin controls multiple developmental pathways
• different combinations of ARFs expressed in different tissues of the plant
• ARFs activate distinct developmental programmes
• 22 other auxin response factors in the plant (apart from Mp)