Plant Development Flashcards
Multicellularity evo in plants?
No common multicellular ancestor w animals
Independent
Development may be different from animals then
In plants - cells joined by cell walls so don’t change neighbours
-no cell migration
-so development potentially simpler?
-done by directional growth instead
Why no central control unit?
land Plants stationary
Also get bigger to compete
Need to harvest nutrients from ground w roots so can’t move
Can’t run away
Perpetually getting eaten partially
So if had central unit and it was eaten whole plant done for
So instead have many stem cell pockets - and the stem cells are more flexible
Higher flexibility plant stem cells?
Animal adults usually have just multipotent progrnitor types that can just make specific tissue
Plant stem cells are much more totipotent
Makes cloning easier too
Plant organ origin?
Apical meristems
Shoots and roots grow from tips
Right in middle of meristem are stem cells
Make organs - eg leaves and flowers
SAM makes?
Basically everything above ground
Germ cells
Leaves
Flowers
Often times parts of roots?
Modularity of plant development
Iterated developmental unit
Primary shoot meristem makes PHYTOMERS - functional units of the plant -can give rise to other shoots??
If bit is taken off and eaten - another dormant meristem is activated and get another healthy plant
Plant germ cell origin differences
In animals - cells are put away v early to become germ cells - strict differentiation
Plant germ cells develop much later
Multicellular diploid embryo evolution
Evolved in land plants
Land plants all evolved from freshwater aquatic algae
Multicellular gen in FW algae is haploid
Fertilisation to make diploid zygote but that immediately undergoes mitosis to make 4 haploid spores
Bryophyte embryo
In land plants - bryophytes
Diploid zygote divides to make multicellular embryo
Benefiti as swimming sperm cannot easily swim on land (too dependent on water)
Then that can produce many haploid spores (many more than 4)
Benefitial as increases odds of successful spore on land
Vascular land plant embryo
Have specialised cells for carrying water and nutrients
So vascular plants can grow bigger
Embryos in vascular plants covered in seed
Seed can store nutrients for development as seed is likely underground when germinating
Helps with dispersal
Plant embryo development within seed
-double fertilisation
2 gametes within female ovule
-haploid egg cell - 1 speed fertulises this to make zygote -> goes on to become embryo
-2 sperm fuse with homoploid central cell - makes endosperm which fills seed and is consumed as embryo grows - is ephemeral and consumed by embryo
Maternal tissue in seed
Integuments
Surround seed
Suspensor
Holds embryo in seed
Nutrients can go up
Undergoes apoptosis during embryo development
Somatic embryogenesis
Possible due to totipotent stem cells being present dispersed around body
Can be activated by hormones or muse pressing certain embryo genes
Can be used for cloning
Occurs naturally in a few species
Kalanchoe (“mother of thousands”) forms embryos around its leaves
Apomixis
eg Dandelion - is triploid
Are sterile due to triploidy - usually cannot make seed
Except they do it with unfertilised flowers - embryo ends up with no paternal info
Often meiosis breaks down giving diploid egg cell - parthenogenesis
Gives clinal propagation by seed - APOMIXIS
The PAR gene confers parthenogenesis
Is expressed in sperm cells - so sworn cell delivers product to mother which kicks off parthenogenesis
Mutant in dandelion which expresses PAR in ovule ends up w no seed in embryo
Embryo Germination
Stem cells at what will be SAM and RAM
Cotyledons - embryonic leaf
Hypocotyl - embryonic stem
So apical basal axis set up in embryo
But many adult structures (seed, fruit, flowers) are not
So development is continuous throughout life(flowers develop in adult plant)
Plant developmental patterning timing
Most occurs post embryonically
Embryogenesis establishes the:
-apical/basal pattern (shoot/root)
-Inside/outside pattern (epidermis/ground tissue/vasculature)
-and stem cells (shoot and root meristems)
Most of plant body plan produced after embryogenesis
Post embryonic patterning
Most of plant body plan produced after embryogenesis
Flowers
Germline
Lateral roots
Branches
Most leaves
Tubers
Continued patterning from groups of stem cells termed meristems
Different to animals where body plan largely formed in embryo
Gives flexibility
Predation problem
Difference to plants - Drosophila embryo patterning
Counter to plants. - patterning of adult stuff set up in embryo
AP and DV axes prefigured in egg
So maternal info in egg determine zygote axes
Localised determinants are localised at piles of egg/zygote (eg bicoid)
Kinase signalling pathways determine terminal elements AP pattern
AP fates determined early on
Localise TF expression (homeodomain proteins) specify regional fate on AP axis
Cell-cell interactions and signals important for segment polarity (wingless etc)
Apical basal pattern appearing in arabidopsis
Egg cell long and thin
The asymmetric cell division giving small apical cell and large basal cell
Apical cell:
Divides longitudinally to give 4 then 8 cells
Then makes layer around to make future epidermis
Apical cell goes on to form embryo
Basal:
Basal cell forms the suspensor and parts of root meristem
Apical basal fate determination
If after zygote 1st division kill apical cell
Basal cell becomes apical in nature
So fate of the cells are due to signaling
If signalling changes then fate of cell changes
Basal cell is capable of producing embryo but is blocked by inhibitory signal from apical cell that inhibits embryo identity/promoted basal fate
Auxin in apical and basal cells
Auxin normally accumulates in apical cell
Important for establishing the vertical/longitudinal division planes and setting off 3D growth
Auxin accumulation promotes vertical rather than horizontal division patterns?
Polar PIN7 efflux protein localisation in basal cell causes this accumulation
Isolating basal cell from both apical and maternal tissue means it does not respectfully like if just apical is ablated
So some +ve signal must be coming from maternal tissues
Basics - how is apical and basal axis and shoot root suspensor fates specified?
Kinase mediate signalling pathways
Localised determinants
Localised TFs
Polar transport of auxin
Zygote polarisation properties
Is long and thin
Nucleus toward one end
Vacuole towards other
Zygote is transiently symmetrical after fertilisation but elongates and becomes polar
Promoter of basal fate in embryo
YODA Activity promoted basal fate
Loss of yoda causes suspensor cells to divide longitudinally like in embryos
YDA gene encodes a MAP3K
Component of signalling pathways
Phosphorylase’s downstream MAP kinases such as MPK3 and MPK6
Constitutive activation of YDA gives opposite effect of loss of function
Hyperactive YODA phenotype
Have defective more suspensor like embryos
Lose YDA - base become embryo like
Hyperactive YDA - apical becomes suspensor like
YDA kinase signalling promotes basal fates and inhibits apical fates in early embryo
Stomata formation
Stomata with 2 guard cells forming mouth like structure
Involves asymmetric divisions in epidermis - smaller daughter remains in stomata lineage - divides again to form 2 daughters becoming guard cells
Bigger daughter becomes elidermis pavement cell
YDA and stomata
YDA inhibits stomata fate
YDA- = more stomata
Hyperactive YDA = all pavement cells
Breaking of asymmetry in stomat lineage BASL
BASL determines asymmetric division in stomatal lineage
Mutant gives lots of nearby stomata instead of spread out
because BASL needed for asymmetric division
No good BASL = symmetric stomatal lineage division -> both go on to guard cell lineage causing close by stomata
BASL localisation
Is in polar localisation on cell periphery
Predicts the asymmetric division plane
Localised to the nucleus and the edge of the cell
Cell will then divide so that the cell was forms distal to the cortical BASL crescent
The larger daughter cell inherits cortical BASL and the smaller daughter enters the stomatal lineage
YDA and BASL
YDA required for BASL polar localisation
YDA mediates BASL phosphorylation and is required for its polar localisation
A similar pathway may control asymmetrical division in zygote
YDA and zygote asymmetrical division
(BASL not present here)
Shirt suspensor (ssp) mutant resembles YDA mutants
Ssp encodes a PM localised pelle-like receptor kinase
Misexoresson of ssp in leaves inhibits stomata development - so confirms intersction wit YDA?
Ssp genetics
Self heterozygote for mutation and WT plants
50% if resusltung embryos give mutant phenotype not expected 25% that would be mutant homo
Cross male WT to homozygous mutant females
All progeny is WT
Cross ssp mutant males with WT females
All embryos are mutants
Suggests that ssp is active in male side (sperm)
Male activity of ssp
Ssp active in male side (sperm)
Pollen expresses ssp RNA
Deposits that RNA in the ovule where it is translated to protein
Explains the odd genetics as maternal is never active
Paternally supplied SSP protein
Localised to one side?
promotes YDA activity
WUS gene (WUSCHEL)
Encodes homeodomain TF
Required for stem cell niche in shoot meristem
Wox homeodomain genes
Patterned on apical basal axis
Due to auxin localisation
Important for apical basal fate specification
WOX2 WOX8
WOX2
WOX8
WOX8 WOX9
WOX9
Homeosis
Assumption by one member of a meristic series of the form or characters proper to other members of the series
Meristic series. -series of repeated homologous units
Homeotic mutations - one member of a repeating series is replaces by another member
Homeotic genes control
Differences between these repeated units
Eve difference between segments in drosophila
Whorl organs
Most flowers have organs in them
Organised in whorls
Whorl number in flowers
1- sepals - form bud
2- petals
3- stamens - male
4- carpel - female
Homeotic mutations in floral organ identity
Class a - affect identity of organs in whorls 1 and 2
Class b- 2 and 3
Class c - 3 and 4
ALWAYS 2 adjacent whorls
Class a mutant example
Arabidopsis
1- sepal - transformed to carpel
2- petals - transformed to stamen
Apetal1
Sepals more leaf like
Petals absent
Class B mutants example
Sepals sepals carpels carpels
Arabidopsis- apetala3 ap3, pistillata pi
Antirhinnum- deficiens def and globosa glo
2 and 3 affected
Class C mutants
Agamous
No sex organs
Whorls 1 and 2 fine
Stamen replaced by extra whorl of petals
Carpel also converted - and unlike WT where stem cells stop and only get 4 whorls - in agamous keep getting more and more whorls of sepals and petals - double flower
Eg plena in antirrhinum
abc model of floral organ identity
3 Homeotic functions
a b and c
-a function in whorl 1 and 2
-b in 2 and 3
-c in 3 and 4
Combinations on Homeotic functions in a whorl specifies the identity of the organs that form there
a =sepal
a+b=petal
b+c=stamen
c=carpel and STOP
Mutual inhubition in a and c
Class c mutations give sepals and petals everywhere even though A function is needed there and is not normally everywhere
Same with C expression in whorls 1 and 2 in A mutants
Lead to idea that A and C are mutually inhibitory
So mutating one expands range of other into the two other whorls
A and C functions are antagonistic
Triple mutant for a b and c
ap2 pi ag triple mutant
Organs are leaf like
Flowers are modified shoots
Floral organs are modified leaves
Like shoots flowers also the compressed internodes, organs in concentric rings
Flowers are determinate
Shoots usually indeterminate
Genes in the abc model
TFs
MADS box mediating DNA binding
C function genes
Agamous RNA expressed where whorls 3 and 4 will be
B function genes
Apetella3 RNA active where whorls 2 and 3 will be not others
A function genes
On where whorl 1 and 2 will be
Are the floral Homeotic genes sufficient for floral identity?
Ectopic expression of them in leaves does not transform them
So not sufficient
SEP genes also needed
SEP genes
SEP1 2 and 3 isolated
Sequence similarity to AGAMOUS
Expressed in whorls 1-4
Single mutants have no phenotype (redundancy)
Triple mutant all floral organs are like sepals
SEP1-3 genes are needed for activity of b and c class Homeotic genes
SEP1-4 quadruple mutant - all organs are leaf mike and have branched trichomes
SEP genes also needed for class a activity too
SEP protein function
Interact with the Homeotic gene products
Need SEP to be expressed in leaves along with abc genes to transform to flower
Have MADS domains like the abc genes
Form tetramer with abc genes at c termini
with aa, ab, bc, cc
MADS domains at either end of tetramer interact with DNA
Model of MADS tetramer binding DNA
Binding of tetramer of MADS TF complexes results in DNA looping
Change in 3D chromatin structure may be necessary for triggering expression of specific genes and therefore different floral organogenesis depending on tetramer structure (abc contained within it?)
Floral quartet model
Quartet - depends on the abc and Sep genes present
Abc genes and Sep genes come together into tetramer to confer that identity in that whorl
Male reproductive organs
Located in stamens inside anther is where meiosis occurs and gametes form
Male gametophytes form in anthers
Microsporocyte
-meiosis
4 haploid microspores
-divide (pollen mitosis I)
Vegetative cell (makes pollen capsule) and generative cell
Generative cell divides (pollen mitosis II)
Generative cell divides to give 2 sperm cells
Plant sperm motility
Cannot swim
Has no flagella
Pollen and pollen tube structure
Vegetative nucleus and generative cells that will divide to make 2 sperms
At one point pollen capsule will swell up and grow a tube quickly
Grows fast to reach and fertilise female
Grows with tip growth
Female reproductive organs
In middle of flower
-Carpel goes on to make fruit
Sopecific to flowering plants (as opposed to naked ovules)
-at top of carpel is stigma where pollen lands and germinates dependent on communication
-pollen tube goes down style and arrive at ovary where gametes are formed
Plant Ovary
Contains the ovules
-funiculus - pollen tube grows up this
-micropyle - where pollen tube enters ovary
-then sperm delivered
-so no need for water or sleek swimming
Female gametophyte formation
Maegasporocyte
-meiosis
4 haploid megaspores
3 abort
Survivor forms gametophyte, divides 3 times
-mitosis - 2 nuclei
-mitosis - 4 nuclei
-mitosis - 8 nuclei in 7 cells of embryo sac
Central cell
Has 2 polar nuclei
Fertilised by sperm cell and forms endosperm
Synergids location in gametophyte
2 of them right by the micropyle
Egg cell location in gametophyte
Just behind the synergids
Antipodal cells location in gametophyte
3 of them
Opposite end from synergids/micropyle
Double fertilisation event
2 sperm nuclei from pollen
One fertilises egg cell
Gives 2N zygote
The other fertilises the polar nuclei of the central cell
Gives 3N endosperm
Ephemeral tissue - dies and does not contribute to next gen
Pollen germination
Pollen lands:
Female can determine self and non self
Closely related pollen is refused
Less related/unrelated individuals’ pollen is refused
If communication is successful Pollen germinates - tube grows through style
Tubes go through the funiculus and then through micropyle
How do these cells meet?
Various signalling pathways
Self incompatibility
Pollen attraction by ovule
Pollen ovule recognition to trigger sperm release
Pollen tube guidance basic
Pollen tube goes down transmitting tract liens with probably signals that direct pollen
Get into overt and go past ovule they grow to side
Out of transmitting tract
Into ovule
Through funiculus - funicular guidance
Through micropyle - micropyles guidance
Female gametophyte presence is necessary for funicular and micropyles guidance
Something produced by haploid female tissue
Male tissue senses this
Transmitting tract role in pollen sensing of ovule
Things in T tubes that switch on genes in pollen though receptors
So if no female tissue - no finding ovules
Need to go through T tract to sense ovules
Pollen tube attraction by female gametophyte - which cells?
Both synergids ablated - no pollen tube growth towards it
Synergids produce pollen tube attractant
Die after fertilisation so no more pollen attracted (not needed)
LURE
Pollen tube growth mechanism
Plant cell pressure is important for fast growth of pollen tubes
The tip growing cells loosen wall at top to allow it to grow
Too stiff - cannot grow
Too soft - explodes
Attractant in synergids
LURE proteins
Cysteine rich peptides - similar structure to many plant ligands
Signal peptides
This LURE:
Specifically expressed in ovary
Specifically in synergids
present Right at tip of synergids
Involuted cell walls here to increase signaling surface
Shown that this peptide is sufficient for pollen tube attraction
Express arabidopsis LURE on Torennia synergids
Attracts arabidopsis pollen
But no bursting
So something else signals bursting
Pollen specific receptor for LURE 1 in arabidopsis
PRK6
KO reduces male side fertility only
So pollen specific
Receptor kinase active at tip of pollen tube
Affects growth
Expressing this on other species pollen makes it attracted to arabidopsis LURE
Signalling event once it reaches ovule
LURE attracts pollen tube to ovule
THEN
Is recognised
Signals to synergids
If successful
One of the synergids does
Pollen tube bursts
Sperm released into ovule
This is why the other species didn’t burst in the experiments earlier - unsuccessful signalling at this point
Sirene/Feronia mutant
Ovules are not fertile
No pollen tube bursting
Overgrowth of pollen and synergids continue to attract other pollen tubes
Female active
Receptor kinase
Unique receptor domain - carbohydrate molecule receptor - may have role in cell wall perception
Is also in leaf and plays a role in growth and regulating immunity
Feronia function
Female receptor kinase
Senses signal from male
Has malectin binding domains
Promotes ROS production at tip of ovule in Filiform apparatus
Mutants have no ROS burst
ROS production is important for pollen tube burst
If ROS scavenger used to remove them then no pollen tube burst
ANXUR1 and 2
Expressed in pollen tube
Closely related to Feronia but instead male specific
Expressed at pollen tube tip
Double mutant pollen burst prematurely
Affects male fertility only
So Involved in maintaining pollen tube integrity during growth through female tissues
Receptors are active to prevent this burst (as their absence causes premature burst)
RALF function
Rapid alkalinisation factor
Small secreted cysteine rich secreted peptide family
Encode ligands for FER type receptors
Feronia mutants don’t respond to RALF so suggests Feronia is receptor
RALF binds FER
FER autophosohorylates
Inhibits activity of membrane H+ ATPase, promotes Ca2+ ingress, activates ROS signalling (& cell wall properties, actin cytoskeleton…)
RALF4 and 19
Expressed in pollen tubes
Single mutants are infertile male
Double mutants cause premature burst like anxur mutants
Prevent pollen burst via ANXURE1/2 signalling
Bind anxur 1 and 2
Autocrine signalling stiffening tube and preventing bursting
RALF6/7/16/36/37 function
Mediate pollen reception
Required in male
In absence pollen not recognised by synergid cell, no burst & overgrowth
Paracrine signalling from pollen to ovule?
RALF34 function
Promoted pollen tube burst
Expressed in ovule
Out competed RALF4/19 (male autocrine ones)
Signalling from ovule to pollen
Summary of RALF model
Autocrine signalling by pollen tube
4/19 to stiffen and prevent burst
Pollen signals with RALF 6/7/16/36/37 to synergid (paracrine)
If successful Synergid releases RALF34 - outcompetes RALF4/19 for ANXUR1/2 - triggers pollen bursting and sperm release
Phyllotaxy types
Spacing of leaves/other leaf like organs around the stem
Spiral
Alternate (aka distichous)
Decussate
Spiral phyllotaxy
One leaf at each node
Processive leaves arranged 137.5 degrees from last
From oldest leaf and go up - forms spiral around stem
Can be R or L handed
R= clockwise
Common one
Alternate phyllotaxy
Single leaf at each node
Leaves at processive nodes are spaced 180 degrees from each other
So alternate sides
Decussate phyllotaxy
2 leaves at each node 180 degrees from each other (opposite sides of stem)
Next node’s leaves are at 90 degrees from the last
Phyllotaxy and growth pattern
Spiral and Decussate make sense for upward growth
Leaves at each node offset so they are in gaps in shading by leaves above
Distichous/alternate usually found in stems that grow horizontally
Phyllotaxy switch during development
Antirhinnum starts as Decussate
But once sexual development begins and flowering genes activate - it switches to 137.5 degree spiral
Flowers are in whorled arrangement
Look inside flower and see that primordial that give rise to the flower structures are spaced in gaps of last ring
See switch in vegetative growth
Start with Decussate
But then changes and starts releasing 3 leaves at each node instead of 2
Can see that leaves at one node are offset in gaps of leaves at last node still
Means that the pattern is meta stable - can switch and then maintain itself once switched
Spiral phyllotaxy and golden ratio
Angle between one leaf and next is 137.5 degrees
The golden angle
No of spirals in the phyllotaxy represent consecutive numbers in Fibonacci sequence
Can trace left and right handed sets of spirals of organs
The number of these spirals always represents consecutive nos in sequence
Eg 3L 5R
21L 34R
Leaf production at shoot tip
Originate from SAM
Dome of cells in SAM that contains the stem cells
Division of stem cells pushes daughters to side and that’s where they differentiate
Can eithe produce internode
Or go out to produce lateral organs
Leaf primordial grows out and assumes leaf shape
Shoot goes through cycles of producing leaf primordia in cycles leaving them behind in the phyllotaxy structure
Leaves behind some stem cells in the axils of leaf primordia
Axillary meristems
Left behind in leaves from SAM lateral organ production
These allow main stem to branch
Goes through cycle of initiating leaves and repeatedly growing out
Activation of axillary meristems is repressed by auxin from the apex (apical dominance)
Meaning that branching out is repressed toward apex
Transitions to flowering development
Activation of inflorescence meristem (the SAM In sexual development)
Stem cells can stay and make internode or go out and make lateral organs
Domes of stem cells in middle producing lateral organs on flanks
Lateral organs have diff identities
Floral meristems go out of side and make the lateral organ flowers (different from inflorescence meristem)
Sets:
Sepal primordia
Petal
Stamen
Carpel
Carpel production uses up the floral meristem’s stem cells which differentiate into carpel
SAM in embryogenesis
Formed in embryogenesis
Stem cells set aside during embryogenesis
In the torpedo stage? Loooks like it in diagram
visible Stages of plant embryogenesis
Octant
Globular
Heart
Torpedo
Then the inverted U (I think)
Central zone of SAM
Part of SAM (other is peripheral zones)
Low rate of cell division - once a week
Weakly histologically staining cells (not metabolically active)
Stem cells
Peripheral zone(s) of SAM
High rate cell division
Densely staining cells (metabolically active
Region of leaf formation
Leaf primordia come from here?
Stem cell vs leaf identity in SAM
Stem cell:
KNOX homeobox genes
Giberellin down (due to KNOX)
Cytokinin up (due to KNOX)
Represses ARP genes (due to KNOX)
Leaf:
ARP genes
ARP do:
-myb TFs
-represses KNOX genes
KNOX KO?
SAM differentiates
ARP gene expression extends to central zone of SAM
Stem cells differentiate
So cotyledons present
But no developing shoot cause no stem cells
ARP KO
Gives meristem characteristics to leaves
Inhibitory field model of phyllotaxis
3 rules
CZ cannot form primordia
Existing primordia produce inhibitor
Inhibitors effect decreases with distance
Region of least inhibition will correspond with the greatest space available from nearest leaves
So new prinordium isn’t inhibited and so it can develop there
Now produces an inhibitory signal here
Next primordium will form in the new greatest space
Different phyllotaxy depending on jnhibitory field range
Inhibitory field surgical experiments
Ablate youngest primordium (nearest top)
Position of next primordium establishes nearer to where the ablated
Widens angle to 157 degrees in spiral
Inhibitory field and switching metastability
Have decissate
Inhibitory field opposite each other from opposite promordia at node
Next 2 at next node 90 degrees from last because of greatest space available
If a field forms that is larger than it should be
Then next node only has space to form 1 primordium in spiral form
Inhibitory fields keep this new pattern metastable
Inhibitory fields and Fibonacci who cares
Shoot as sheet of paper
Promordia as coins
Once stacked so they occupy greatest available state. - Fibonacci numbers appear in the rows of coins
Geometry
Idk
PIN1 auxin efflux transporter localisation role
Localisation of PIN1 often asymmetric
Occurs at one end of cell
Suggests auxin moving from one cell to another
Neighbouring cells have coordinated localisation of PIN1 protein
Means that auxin moves with the tissue from one end to another
Can visualise auxin movement with PIN1 localisation
Auxin flow in protoderm/SAM
Auxin flows in protoderm towards SAM (up the stem to apex)
then to leaf promordia
Comes up to apex in gaps between promordia and is diverted INTO existing promordia
Auxin model of phyllotaxy
Opposite to inhibitory field kind of
Highest auxin concentration area is where promordia will develop
Next promordia will develop in greatert space
Because existing promordia are sucking away auxin from the tissue
Meaning greatest space has the least sucking and so most auxin transport
Auxin localisation in auxin model
model Requires relocation of PIN1
From down the gradient (in protoderm?)
To up the gradient towards the promordia
IDK SPECIFICS OF THAT
question of what determines phyllotaxy is now boiled down to. What determines Auxin relocation change
What allows relocation of PIN1 towards promordia
the Mechanical model for the auxin model
Phyllotaxy is altered when medhanical stresses are changed
Relieving stress locally causes primordium formation
Artificially relaxing cell wall causes formation of ectopic leaf primordium
Auxin loosens cell walls relaxing the cell
Mechanical model for auxin model
All cells equivalent
One cell by chance ends up w more auxin than neighbours
Loosens it’s cell walls -so turgor pressure blows it up
Pushes more on surrounding cells than they push back
As a result
Reorients stresses I’m surrounding cells
Band of stress around loosened cell
This relorients PIN1 protein - moves towards the middle higher mechanical stress cell
Explains the change from down the gradient to up the gradient
This causes auxin accumulation in this area
RAM Basic
Roots develop from RAM
At tip of every root
Contains self renewing stem cell pop
Enable continuous root development through life
Similar to SAM
Also has stem cell pop
But it is easier to teach development in root as model
As forms files of cells
Lots of expansion in shoot so messier
Roots as models
Grow happily in agar (clear)
Can get water and nutrients from it fine
Zones of root tip
Meristematic zone near tip
Elongation zone - has small boxes (cells) which increase in length as they get older/go up the zone
Then beyond that is the differentiation zone where first get differentiated cell types - this bit has root hair cells
Overall cell type organisation in root tip
Promeristem
Root cap around root exterior and below promeristem
Vascular tissue (stele) in centre
Ground tissue around that
Epidermis around ground tissue
Can follow individual files of cells from these regions to the promeristem - stem cells somewhere around here
Conserved among species
Arabidopsis promeristem
Zoom in on promeristem
Cell files converging outside
Small number of central cells in promeristem - the quiescent centre
Quiescent centre
Central promeristem region where root’s cell files converge
Could be:
Maintain the undifferentiated state of surrounding initials (organiser)
No evidence of cell division here - is quiescent
Highly conserved in all land plants
Stem cells in root promeristem
Surround the quiescent centre
Are stem cells for each of the major tissue lineages mentioned earlier - called initials
Contain initials for:
Root cap
Ground tissue
Vascular tissue
Testing QC function
Focus on root cap
Root cap is covering tissue
Focus on columellar root cap specifically because it begins differentiation v close to QC just one initial layer away
Know it’s differentiated too due to appearance of starch granules
One of the QC cells was laser ablated
Affected organisations and root patterning:
The columella initial became differentiated. -gained starch granules
Near non destroyed QC cells - initials remain undifferentiated
So even tho QC doesn’t divide it
Maintains undifferentiated initials
How are the cell lineages in root tip specified
Either by lineage
Or by induction
(Is INDUCTION)
Fully ablate QC
look at vascular marker and root cap marker
Shows where QC should be (between the tissues)
But when QC is ablated entirely
Cells above where it should be (should give rise to vascular) instead give rise to root cap tissue
So cell types are not necessarily pre determined
Can double check as this may be due to damage
Use GUS reporter and transposon that activates in only a few cells randomly
Marker activated in one initial
Should stay in one lineage of lineage dependent
BUT instead ends up in root cap and vascular tissue
So an initial divided into cells that gave rise to both
So is not lineage dependent
Levels of patterning in root
Proximo distal (distance to tip)
Circumferential
Radial (along radius of root)
Radial patterning in roots
Root can build girth but indergoing radial divisions
Genes involved in radial patterning
Scarecrow- Scr1 (mutant has short roots)
Short root 1 - Shr1 - similar mutant phenotype
Scarecrow mutant pattern
WT - The endodermis layer towards centre
Wit a cortex surrounding it
Scarecrow mutant - no cortex or epidermis but a single mutant layer
Shr 1 mutant
No endodermis
Just cortex
Scarecrow properties
Expressed in initials
Then after differentiation ONLY in endodermis
Important in endodermis fate decision
Give ability to undergo special anticlinal division
Scr+ initial does this A division
Giving endoderm cell on inside and cortex cell on outside
This division doesn’t occur in scarecrow mutant giving the single mutant layer
Short root properties
Expressed in QC then initial then only:
Expressed in vascular tissue
Moves out of vascular tissue and interacts with Scr in future endoderm
Sequesters it to the nucleus of these cells so Shr cannot reach the cortex layer
Nuclear Scr/Shr upregulates Scr transcription - sequestering all the Scr preventing any from reaching cortex and giving endoderm its cell fate
Absence of Shr in cortex allows cell fate in cortex as Scr/Shr can’t go in nucleus
Shr mutant causes only cortex identity cause no Scr complex can make it knot nucleus to give endoderm fate
Evolution of root system
Non vascular plants have hair like rhizoids
Rhizoid only exist in non vascular plants (bryophytes)
Roots only in vascular
But having roots is a polyphyletic group
So common ancestor of all vascular plants didn’t have roots
Evolved 2 separate times in vascular plants
RHD6, RSL1 function
RDH6 and RSL1 mutants lack root hairs
Both closely related HLH TFs which control root hair development
Single mutants reduce root hair
Redundancy between them cause need double mutant to eliminate hairs completely
RHD6 expressed in developing root hairs
Oberexpression of these genes gives root hairs on hypocotyl - not normally there
So these genes are important in root hair development
Bryophytes and RSL
Mosses and liverworts both have RSL class 1 genes in genome
Expressed in cells that form rhizoids
Constitutive expression turns shoots into rhizoids
Moss/liverwort RSL genes complement arabidopsis - can rescue RHD6-3 mutant
Conserved regulator among land plants
