SAM

What are the "ways of feedbacks" in auxin action?

1. Concentration based model 2. Flux based model 3. Mechanical feedback-based model

Mechanism of SAM stem cell maintenance?

1. WUSCHEL migrate from organizing centre to central zone to activate CLV3 transcription. 2. CLV3 restricts WUS expression in OC. 3. WUS and CLV interacts with cytokinin regulators like A-type ARR TFs 4. A-type ARR limit SAM size 5. WUS supress negative A-type regulators, which enhances downstream cytokinin response in SAM. 6. OE of ARR7 reduce WUS expression = arrested STM activity. 7. ARR7 inhibit CLV3 expression.

Concentration based model

All cell "senses" the level of auxin in the neighbouring cells and orient PINs toward the cell with highest auxin level.

Role of SAM

Produce above ground parts

Cell wall loosening

- Auxin activates MP/ARF5 which cause cell wall loosening - Weaker walls increase in tensile stress - PIN1 is reorientated which causes auxin concentrations to increase in the initial auxin accumulating cell and helps trigger organogenesis.

Mechanical feedback-based model

New local auxin maxima, initiate slight cell deformations. Differntial cell growth creates mechanical stress that orient pin polarity.

Explain why clv3 (mutant) has enlarged stem cell niche and SAM

When the CLV3 gene is mutated, the negative feedback loop is disrupted. This disruption can lead to an overproduction of stem cells in the central zone of the meristem. Without the normal regulatory influence of CLV3, the stem cells might continue to divide and accumulate, leading to the enlargement of the stem cell niche. The overproliferation of stem cells in the central zone also affects the overall growth and organization of the SAM. An excessive number of stem cells can contribute to an enlarged SAM, as the overactive stem cells result in more rapid and extensive tissue growth.

Types of phyllotaxis

whorled, decussate, distichous, spiral

What are some ways of understanding how SAM and shoot development works?

1. Genetics through mutant screens 2. Functional analysis through reporters 3. Pertubations through chemical and mechanical mechanisms 4. Morphometrics like shape and size and 3D 5. Morphodynamics like 4D

Mersitem

A region of plant tissue consisiting of undifferentiated cells that are actively dividing

The role of auxin in primordium formation

Activator of primordium formation. When auxin is blocked, like in pin1 mutants, there is a pin formed phenotype lacking primordium development and a bare meristem. Rescuing of phenotype by auxin application.

What genes are required for boundary and SAM formation?

CUP-SHAPED COTYLEDON 1 (CUC1) and CUC2 genes are expressed in boundaries between floral organ primordia and in the boundary between the cotyledons. Mutations in CUC1/CUC2 defects in the establishment of boundaries, resulting in organ fusion (aka no organ separation). Mutations of them also affect intiation of SAM. SHOOTMERISTEMLESS (STM) is also invovled in defining the borders of the newly formed primordium. CUC induces STM ecpression at the border, setting up a new meristem at each border.

How might the cell wall influence meristem activity? (pectin)

De-methyl-esterification of pectin HG, is necessary and sufficient for new organ emergence at the shoot apex. Auxin leads to tissue softening through the de-methyl-esterification of HG pectin and this chemical modification of the cell wall is required for auxin-induced organ formation.

Zones of inhibition

Determines placement of new organs. The zones could be caused by production of inhibitor or depletion of acrtivator. Inhibitory fields where the incipient primordium emerge where inhibitopry effect is lowest.

shootmeristemless1 mutant

Fails to initiate SAM. Can be rescued by cytokinin application. Looks like mickey-mouse ears.

Role of MONOPTEROS/ARF5 in primordia formation?

High auxin levels accumulate at primordia sites activate auxin response. MP/ARF5 activate ARF responsible for auxin response and primordia formation at SAM. Has to do with cell wall loosening.

What is meant by boundary?

In dicots, two cotyledonary primordia arise through increased cellular proliferation in two apical regions of the globular embryo, flanking a region of restricted proliferation. The latter region forms the boundary between the developing cotyledons and harbours the cells that contain the SAM. Proliferating leaf primordia are separated from the shoot meristem by slower proliferating boundary regions.

What happens during polymer creeping?

In expanding walls, the cellulose microfibrils slide past each other under stress during polymer creeping, which leads to irreversible growth.

Explain why clv3 (mutant) has enlarged stem cell niche and SAM

In wild-type plants, CLV3 restricts the number of stem cells in the meristem by inhibiting their proliferation. It promotes differentiation and prevents the stem cells from self-renewing excessively. This negative feedback loop helps maintain the balance between stem cell maintenance and differentiation. When there is a mutation in the CLV3 gene, the negative feedback regulation is disrupted. As a result, the signaling pathway that controls stem cell proliferation and differentiation is compromised. This leads to the overproliferation and expansion of stem cells in the niche and SAM. In the absence of functional CLV3, the stem cells in the niche continue to divide without proper regulation. This uncontrolled cell division leads to the accumulation of excess stem cells, causing the niche to enlarge. The enlarged stem cell niche, in turn, affects the size and activity of the shoot apical meristem. With more stem cells present, the SAM expands, resulting in an enlarged shoot apex. This enlargement is a consequence of increased cell division and growth within the meristem.

Mechanism of SAM stem cell initation?

Initiation The shoot meristem becomes histologically visible during mid-stage embryogenesis. WOX2 + cytokinin HDZIP 111s regulate aux-ck

Layers of the SAM

Layer 1 = shoot epidermis Layer 2 and 3 = inner tissues

Name the different mechanisms in SAM development which are influenced by mechanical signals

Mechanical signals, such as physical forces and stresses, can play a significant role in shaping the development of the SAM. Cell Division and Proliferation: Mechanical forces can influence cell division rates and orientations within the SAM. Cell Polarity: Mechanical signals can impact the establishment and maintenance of cell polarity within the SAM. Hormonal Signaling: Mechanical forces can affect hormone distribution and signaling within the plant. Hormones like auxins and cytokinins play essential roles in SAM development, and their distribution can be influenced by mechanical cues.

PIN / AUXIN feedback

PIN genes are activated by auxin, such that more auxin accumulation leads to more auxin efflux and vice versa. PIN1 protein polarized towards cells with higher auxin levels, pumping more auxin into those cells and depletes surrounding areas of auxin.

What genes are important in apical patterning?

PIN1 and MP. They regulate the expression of CUC1 and CUC2. PIN1 is required for cotyledon separation during A. thaliana embryogenesis (pic).

Principal protein for primordium development

PIN1. Pin1 mutants lack primordia. PIN1 is expressed in epidermal layer - L1, and polarized downards toward new primordia. In young primordia, PIN1 is polarized downards into epidermal layers. In epidermis below SAM, PIN1 is polarized upwards toward SAM - a process dependent on PID (PINOID) proteins.

Flux based model

PINs orient the direction of the pre-existing auxin flux

Role of cortical microtubules (CMT)

Present in cell cortex, decisive in deposition of new microfibrils on the cell wall, mediates the movement of cellulose synthase complexes. The complexes move along the track lined by CMTs and alight the cellulose microfibrils along the directions of microtubules. The oritentation of CMTs itself is strongly linked with mechanical sterss on the walls.

Phyllotaxis

The arrangement of organs on an axis or stem. Helps predict where new organs will be formed.

Stem cell niche

The cellular micro-environment that provides the signals and physical support to maintain stem cells.

What is the role of central zone, peripheral zone and rib zone

The central zone comprises the stem cells and the organizing center and contains cells that remain undifferentated. The peripheral zone surrounds the central zone, and its cells are competent to form organs, even though not all of its cells share that eventual fate. The rib zone lies below the central zone and gives rise to the central tissues of the stem.

Stem cells

Uncommitted slowly dividing initial cells that produce all the cells in the meristems and thereby in the entire plant body (stem and root). Upon cell division, one daughter cell remains a stem cell, while the other becomes committed to a developmental pathway.

Stem cell autofeedback loop

WUS and CLV3 regulate each other. 1. More stem cells -- promote transcription of CLV3 2. CL3 binds to CLV1 -- suppresses WUS (required for maintance of stem cell number) 3. When WUS is repressed, stem cell number decreases. Level of CLV3 is reduced, allowing expression of WUS -- increase in stem cell number.

Zones of the SAM

central zone (CZ), peripheral zone (PZ), and the rib zone (RZ)

What are ways of understanding how SAM and shoot development works?

· Genetics (through mutants) · Functional analysis (through reporters) · Perturbations (through chemical and mechanical mechanisms) · Morphometrics (shape and size and 3D) · Morphodynamics (4D and predictions)