Leaf Development And Evolution I: What Is A Leaf

Question 1

Leaf function

Answer 1

1) support
2) photosynthesis
3) defence
4) bearing spores
5) nutrition
6) floatation

Question 2

Monocot grass leaf

Answer 2

• blade, ligule, sheath

Question 3

Eudicot simple leaf

Answer 3

Blade, petiole

Question 4

Seed leaves

Answer 4

Cotyledons

Question 5

Reduced leaves

Answer 5

• cactus spines
• butcher’s broom cladode
• whisk fern

Question 6

What is an angiosperm leaf?

Answer 6

• determinate lateral organ associated with a bud

Question 7

SAMs

Answer 7

• make leaves
• transition from indeterminate to determinate growth via leaf primordia

Question 8

SAM zones

Answer 8

1) central
2) peripheral
3) rib

Question 9

SAM Layers

Answer 9

1) L1 (outer)
2) L2 (sub-surface)
3) L3 (inner)

Question 10

SAM function

Answer 10

• perpetuate stem cell pop
• produce organ dedicates

Question 11

Perturbed SAM mutants

Answer 11

1. wuschel
2. clavata1, 2, 3

Question 12

Wuschel

Answer 12

Homeodomain protein Responsible for proliferating cells

Question 13

wuschel

Answer 13

• defective (reduced) meristem forms defective organs
• can form axillary meristems
• decreased CLV; less cells in central zone

Question 14

clavata1, 2, 3

Answer 14

• enlarged meristem
• more cells in central zone

Question 15

CLAVATA1, 2, 3

Answer 15

Component of receptor-ligand pathway that makes less cells in central zone

Question 16

WUS/CLV interactions

Answer 16

1. WUS promotes CLV expression
2. CLV inhibits WUS zone
   - feedback inhibition

Question 17

CLV OX

Answer 17

~ wus (due to strong inhibition)

Question 18

Expression doesn’t always mean function

Answer 18

Just because the transcript is there, doesn’t mean the protein is

Question 19

Stem cell maintenance

Answer 19

• kept beneath stem cell zone
• easy to self-regulate upon environmental cue
• WUS = essential for stem cell identity

Question 20

Periclinal chimaeras

Answer 20

• one layer has a different identity
• generated by grafting
• helps you work out what contributions each layer makes to final structure
• reveal contributions of SAM layers to leaves

Question 21

Sectorial chimaeras

Answer 21

• for clonal (sector) analysis
• deduce what part of the meristem does what
• reveals no. of leaf founder cells
• shows plastochron

Question 22

Sector induction to determine cell lineage relationships

Answer 22

• in tobacco
• genetic stock: mid-green double heterozygote
• double chance of sector
• one break = green
• one break = yellow

Question 23

Lineage analysis

Answer 23

• pattern of division at each plastochrons
• division continues at leaf base

Question 24

Leaf initiation in tobacco

Answer 24

• 120-180 founder cells (15x2-3x4 in a dome)
• L1, 2 and 3 maintained
• requires co-ordination

Question 25

L1

Answer 25

Upper and lower epidermis

Question 26

L2

Answer 26

Palisade upper mesophyll

Question 27

L3

Answer 27

Inner mesophyll Vascular tissue

Question 28

KNOX genes

Answer 28

- knotted1-like homeobox - Homeodomain proteins bind DNA - green algae and land plants

Question 29

Ectopic cell division in KNOX GOF

Answer 29

- 35S constitutive for expression - necessary and sufficient

Question 30

Simple angiosperms

Answer 30

Maize, antirrhinum, arabidopsis, pea, tobacco

Question 31

Simple leaf formation in angiosperms

Answer 31

- KNOX turns off in primordium - what turns it off? - need recessive LOF

Question 32

rs2

Answer 32

- -ve Kn1 regulator (transverse visualisation) - rough sheath 2 - similar to kn1-0 - ectopic Kn1 accumulation in leaf primordium

Question 33

RS2 and Kn1

Answer 33

Expressed in mutually exclusive domains / zones

Question 34

ASl

Answer 34

- inhibits KNOX in leaves - RS2 ortholog

Question 35

asl

Answer 35

KNOX expression in leaves

Question 36

PHANTASTICA

Answer 36

- ARP gene - turns KNOX off in primordium

Question 37

ARP genes

Answer 37

Encode Myb TFs

Question 38

Timing of leaf evolution

Answer 38

- c425Mya; earliest land plants - c360-409Mya; leaves formed in Devonian - c130Mya; flowers formed in Cretaceous

Question 39

Cooksonia

Answer 39

- leafless fossil - branched axis supporting terminal sporangia - c400Mya

Question 40

Leafless fossils

Answer 40

- Rhynia - Asteroxylon - Psilophyton - study absence/arrangement of vasculature

Question 41

Rhynia

Answer 41

- 410Mya - lycophyte stem lineage

Question 42

Asteroxylon

Answer 42

Lycophyte (pre-microphyls)

Question 43

Psilophyton

Answer 43

- monilophyte stem lineage - pre-euphyllophyte megaphylls

Question 44

At the very least, there are

Answer 44

2x Leaf origins, 50My apart

Question 45

Microphylls

Answer 45

- simple vein - simple shaped leaf

Question 46

Megaphylls

Answer 46

Tries to unify many types of leaf

Question 47

Lycophytes

Answer 47

Microphylls

Question 48

Telome theory of leaf evolution intro

Answer 48

- explains fossil trajectory - leaves evolved from flattened branches

Question 49

Telome theory of leaf evolution

Answer 49

1) branching 2) overtopping (Actinoxylon); dominant branch 3) planation (Archaeopteris); flattening

Question 50

How does the telome theory of leaf evolution explain M/M divide?

Answer 50

Megaphyll - fusion Microphyll - reduction

Question 51

Enation theory of leaf evolution

Answer 51

1) leaves evolved through lateral outgrowth from unbranched axes 2) followed by vascularisation

Question 52

Sterilisation theory of leaf evolution

Answer 52

- leaves evolved through sterilisation of lateral sporangia - Zosterophyll

Question 53

Selaginella kraussiana

Answer 53

- lycophyte (v. hard to transform£ - dichotomous prostrate branching pattern; v. rigorous - meristems bifurcate - major and minor branches in specific leaf pairs w/ specified rhizophores

Question 54

S. kraussiana counting mechanism?

Answer 54

- SkKNOX1 - SkKNOX2 - no 1:1 ortholog - conserved expression patterns - in indeterminate equivalent - where leaf is budding

Question 55

SkKNOX

Answer 55

- present in meristem - different to angiosperms! - conservation of pattern

Question 56

In order to prove function

Answer 56

you have to KO

Question 57

35S:SkARP1

Answer 57

- complements asl - transgene expression matches degree of rescue

Question 58

Independent recruitment of KNOX pathway

Answer 58

- at least twice - >30My apart from - did it arise de novo?

Question 59

KNOX expression in bifurcating shoot

Answer 59

When meristem bifurcates, KNOX does too

Question 60

KNOX hypothesis

Answer 60

- SkARP expression makes KNOX genes switch off in that cleft at the time the meristem needs to branch - facilitates bifurcation - can’t prove until KO

Question 61

Independent recruitment of pathway to enable leaf evolution

Answer 61

1) KNOX promotes cell division in apex of multicellular sporophyte (Bryophyte stem lineage) 2) acquired for branching (Lycophyte stem lineage) 3) KNOX leaves (Lycophytes)

Question 62

How many times has it happened in euphyllophytes?

Answer 62

Open q!

Question 63

Structure:

Answer 63

1. Intro (functions, shapes) 2. SAM 3. WUS/ClV 4. Leaf founder cells 5. (In)determinism 6. Evolution 7. Selaginella