L19 - Developmental plasticity in the shoot and root system Flashcards
What is the primary root of a plant?
Draw a diagram of the different zones of the primary root and describe the function of each zone
- The first root made by the plant
(See diagram on pg 12)
Root Cap:
- Covers apical meristem (protects it from damage as root tip pushes into soil)
- Perceives gravity for gravitropism
- Secretes compounds to help soil penetration and nutrient mobilisation
Meristematic Zone:
- Lies just under root cap
- Contains Root Apical Meristem (RAM), stem cell pool around quiescent centre
- Stem cells divide infrequently, forming progenitor cells
- Progenitor cells divide more rapidly in division zone
Elongation Zone:
- Site of rapid and extensive cell elongation
- Still some division of cells but decreases further from the meristem
Maturation Zone:
- Division and elongation ceased
- Cells acquire differentiated characteristics (despite potentially beginning differentiation earlier)
- Root hairs + lateral organs form
Describe the root apical meristem (RAM) generally and more specifically in Arabidopsis
- RAM contains stem cell niche, maintained whilst feeding cells into growing root
Arabidopsis RAM:
- Concentric rings of initial cells, producing radial layers of cells
- Stem cells surround quiescent centre (QC), small group of organising cells
- Cells in QC don’t divide
Is the identity of cells in the root conferred by spatial position or the identity of the parent stem cell?
Describe an example of this in the specification of Endodermis and Cortex layers
What TFs are involved in this specification and how do they move?
- Identity conferred by spatial position, not lineage
E.g. Specification of Endodermis and Cortex layers:
- Cortex/endodermal initial divides transversely to regerate itself + produce cortex/endodermal initial daughter
- Daughter undergoes longitudinal division
- One layer becomes cortex, the other endodermis
- Endodermis and cortex layers specified by TFs Short Root (SHR) and Scarecrow (SCR)
- shr and scr mutants don’t longitudinally divide, only 1 cell type formed
- SHR mRNA only in stele, protein moves 1 cell layer to endodermis, sequestered by SCR
- SCR doesn’t move, in initials + endodermis
How can the rate of root growth be changed?
- By shifting the timing of the transitions of cells between all the different zones within the roots
- Transition times sensitive to auxin and cytokinin
How does auxin concentration vary within the root and why?
- PIN proteins polarised in specific patterns in cells of root tip
- Creates reflux of auxin down root centre, back up the sides and back to the centre
- Bottleneck at root tip causes auxin gradient
- High levels of auxin in stem cell niche, levels reduce further up root
- Gradient regulates transitions between zones
How is root length controlled by auxin and cytokinin?
- Cytokinin promotes cell differentiation = root growth stopped (smaller root)
- Auxin promotes cell division + elongation
Mechanism:
- TF SHORT HYPOCOTYL2 (SHY2) disrupts PIN and auxin gradient
- Auxin promotes PIN expression by degrading SHY2, maintaining growth
- Cytokinin impedes PIN expression by stimulating SHY2 expression
- Cytokinin causes auxin redistribution + cell differentiation = smaller meristem
How do lateral roots form?
- Develop post embryonically
- Initiate from founder cells deep within existing roots
- Founder cells divide, forming LR primordium
- Primordium grows through overlaying tissue layers to emerge
- Active meristem w/ auxin maxima acquired after emergence
Describe the mechanism behind root gravitropism
- Change in root angle relative to gravity causes statoliths (amyloplasts) in root cap to sediment differently
- Triggers reorientation of PIN proteins
- Causes auxin accumulation on lower side
- In this case this inhibits cell growth + elongation so root grows down
How fo lateral roots respond to gravity and what is their general orientation?
- LRs have a gravitropic set point angle (GSA)
- Always grow at this angle relative to gravity e.g. 30˚
- Return to this angle when tilted so do sense gravity + respond
- Similar auxin mechanism w/ different auxin distribution to achieve angle
How does phosphorous deficiency change the root architecture of plants?
Phosphate usually immobile and at higher levels in topsoil
Low P reduces primary root length
- Meristem not maintained
- Less auxin at RAM
- Loss of quiescent cell markers
- Less cell division (shown in reduction of cycB)
Low P increases no. of lateral roots
- No. of LR increased
- Angle changed to exploit top soil
- Angle change auxin regulated + can be mimicked by auxin treatment
How does Nitrogen availability change the root architecture of plants? Give
Nitrate highly mobile in soil, so N deprivation elicits different response to P deprivation
- N increases cytokinin production which reduces meristem size
- So low N promotes primary root elongation
- Mutants in ahk3-3 CK receptor don’t respond to CK = longer roots and larger meristem
How can locally high nutrients change root patterns? Use nitrogen as the example
No. of lateral roots can change locally to exploit local accumulation of nutrients
E.g. nitrate:
- Lateral root growth suppressed on general high N
- If N starved, local high N promotes LR growth
Nitrate regulated auxin transporter NRT1.1 partially responsible:
- Nitrate absence means NRT1.1 transports nitrate, preventing accumulation and LR growth
- High nitrate means nitrate transported, not auxin, allowing accumulation + LR growth
How does the distribution of water in the soil affect root architecture?
- ABA upregulated when root not in contact with water
- ABA inhibits lateral root formation
- Growth not wasted on “dry” area
What is apical dominance?
Explain the role of auxin in determining apical dominance
Describe an experiment that showed this
Apical dominance is the degree to which the central trunk/stem dominates over side branches in a plant
- Auxin exported from young expanded leaves in primary apex to main stem by polarly localised auxin PIN transporters
- High auxin levels from this reduces sink strength of main stem for auxin
- Reduced sink strength means auxin flux from lateral buds to stem low, inhibiting bud activity
- Demonstrated by Sachs - applying auxin to epicotyls of decapitated pea plant didn’t connect to to vasculature if auxin also added to vasculature (if added alone it did connect)
Describe a different plant hormone that regulates branching
- Strigolactone causes PIN1 depletion, reducing auxin transport canalisation
- Strigolactone synthesis mutants that over accumulate PIN1 have more branches
Give an example of how the environment of a plant can influence branching
- Low N increases auxin export from active shoot apices
- This increases auxin in polar auxin transport stream of main stem
- Inhibits formation of new branches
- Less branches on low N
(Also depends on strigolactone)
