Leaf Development And Evolution 3: Morphogenesis Flashcards
Size
Uniform growth
Shape
Differential growth
Cell expansion and division are co-ordinated to produce
Growth
Growth can be anisotropic
- orientation; different structure
- must be specified
- simple principles lead to different shapes
How do genes control leaf shape?
Specifying growth rates and orientations
Progressive changes in cell division orientation in maize leaf development
- early: isotropic
- P5: division in base and upper middle are oriented
- at the tip: just expansions
- P7: expansion thru majority of leaf
Division in maize
Contributes to formation of leaf sheath length
Maize leaf general progression
1) wide structure: primordial encircle meristem
2) transition to longer structure
3) v. long and narrow
- all achieved by changes in orientation and division cessation from tip -> base
Differential gene expression regulates
Cell division vs expansion domains
Late development determinants
Duration, rate and size
Cell division arrest has
Feedback regulation at the arrest front
Feedback regulation at the arrest front
- TCP proteins
- miRNAs reinforce boundary
- Arrest front moved basipetally
- Cell division -> expansion
TCP proteins
- promote cell expansion
- activate in expanding part of leaf
- activate miRNA396
- GRF inhibited
Basipetal
Tip -> base
GRF TFs
Promote cell proliferation
Complex shapes
- serrations, lobing
- arise from persistent growth in isolated blade regions
- superimposition
Lobing
- restricted growth in sinus?
- accelerated growth in lobe?
Both?
Serrations
Lobes + sinuses
Lobing in A. species’
- associated w STM activity in leaf
- high: distinct lobes emerge from central petiole
- complexity spectrum
Ectopic lobing is suppressed by
GA
For the indeterminacy that leads to proliferation of lobing to be enabled
- GA must be suppressed
KNOX and GA
- KNOX binds to GA20ox1 promotor
stm1 dynamics
- stm1
- weak stm1: small meristem; some indeterminate growth
- weak stm1 + spy : growth abolished
- STM has to turn GA20ox1 off
spy
Constitutive GA
Determinate
KNOX off, GA on
pin1 / NPA
- no serrations
PIN1 orientation
DR5pro + GFP
CUC2
- pro + GUS
- critically located to sinus
- cuc2: 1x serration
- initiates outgrowth orientation
Simple leaf
- axillary bud part of phytomer
- 4 = 4 leaves
Compound leaf
- single axillary meristem to multiple leaves
- develops from a single primordium
- how?
Kn1 OX in tomato
- super compounds!
Model species w compound leaves
- Solanum lycopersicum; tomato
- Cardamine hirsuta; A. relative
- Pisum sativum
Kn1 in Cardamine
- KO: simple
- KI: super
- necessary and sufficient
Cytokinin in tomato leaves
- FILpro: only active in leaf primordium
- AtIPT7: cytokinin biosynthesis; ^ complexity
- AtCkX3: degrades cytokinin; simple!
Compound leaf indeterminacy requires
Cytokinin
Cytokinin activity is downstream of KNOX
- FILpro»AtCkx3: super
- 35Spro»Kn1: complex
- both; super
???
Cytokinin is necessary for
Formation of extra leaflets
PAT in Cardamine leaflets
- necessary for compound leaf formation
- pin1 + NPA = none
PIN1-generated auxin maxima precede leaflet formation
- DR5pro»YFP
Angiosperm evolution of compound leaves
- multiple independent events
LMI1
- HD-ZIPI TF
- suppresses KNOX @ leaf margins
RCO
- LMI1 duplicate
- lost in secondarily simple brassica (A. Thaliana)
- retained in complex brassica (C. hirsuta, A. lyrata)
- expression shown by promotor GUS reporter transgenic
- in margins + stipules of C. hirsuta
- inhibits cell proliferation in sinuses
rco
- simples leaves
- is it inhibiting KNOX? No; non-statistical expression difference; no interaction
- is it inhibiting PIN1? No.
- proliferation in sinuses
What separates LMI1 and RCO?
- divergence in enhancer elements in promoter expression patterns
Convergent evolution of compound leaves
- CUC2-auxin-PIN1-KNOX shoot module co-opted twice
- Brassicales, Solanales
Fabales
3 different leaves:
- stipule
- leaflets
- tendrils
- no KNOX
unifoliata
Simple leaves
UNI
- encodes pea LFY ortholog (WGD)
- diverged in function by changing TF BD
Convergent roles for KNOX and
LFY orthologs in compound leaf development
Evolution of LFY function
- Physcomotrella: induces first division in zygote; needed for proliferation
- Seratroptris ricardii: maintains activity of single apical cells required for leaf formation; needed for divisions
- angiosperms: promotes in->det
tendrilless
Sweet pea!
TL
- LMI1 ortholog
- suppressed growth @ leaf margin in A.
- expressed throughout tendril primordia in situ hybridisation for P. sativum
- specifies tendril, not leaflet
