Plant Cell Structure and Meristems

Root apical meristem organization

The root apical meristem (RAM) is the zone at the root tip that produces the cells of the root body through continued division. Its organization centres on a small group of slowly dividing cells called the quiescent centre (QC), which lies near the root cap and is surrounded by the initial cells that keep the meristem active. The quiescent centre is a characteristic feature of the root apical meristem and is not found in leaf, shoot apical, or floral meristems.

Key terms:

• quiescent centre — a group of cells that divide very rarely and help maintain the surrounding stem cells. e.g. Removing the QC disturbs the organization of the root tip.
• initial cells — the stem cells that surround the QC and generate tissue layers of the root. e.g. Initials divide asymmetrically to form new cells.
• shoot apical meristem — organized into a central zone and a peripheral zone, not a QC. e.g. Cell fate in the shoot is governed by zone position.

Root development is often analysed by epistasis, which compares single and double mutants to determine whether one gene acts upstream of another and whether its effect is to promote or suppress. If a gain-of-function allele of gene X and a loss-of-function allele of gene Y both block root formation, and the double mutant is also rootless, then Y is required for roots and X reduces Y activity, so X acts upstream and suppresses Y. The double mutant looks like the loss-of-Y single mutant because the suppressed gene is already absent. e.g. A gene whose loss gives the same phenotype as the double mutant is usually the downstream target of the suppressor.

Statoliths and gravitropism

Gravitropism in roots depends on statoliths, which are starch-filled plastids that sediment within the columella cells of the root cap. These organelles are amyloplasts, and their displacement under gravity triggers the redistribution of auxin that bends the root toward the gravity vector.

Plastid types that are often confused:

• amyloplast — a starch-storing plastid that acts as the statolith. e.g. Amyloplasts in root cap cells settle to the lower side of the cell when the root is turned.
• chromoplast — a plastid that accumulates carotenoid pigments, as in ripening fruit. e.g. Chromoplasts give colour to tomato fruit but have no role in gravity sensing.
• chloroplast — the organelle of photosynthesis, not a gravity sensor. e.g. Chloroplasts are the main site of light reactions in leaf mesophyll.
• etioplast — a chloroplast precursor that forms in darkness. e.g. Etioplasts convert to chloroplasts when a seedling is exposed to light.

Plant cytoskeleton components

The plant cytoskeleton is built from two main polymer systems: microtubules, which are made of tubulin, and microfilaments, which are made of actin. Both are present in angiosperm cells and control processes such as cell division, cell expansion, and intracellular transport.

Components that are absent from angiosperm cells:

• intermediate filaments — a class of cytoskeletal fibres found in animal cells that is not part of the plant cytoskeleton. e.g. A plant cell lacks the intermediate filament network that supports an animal cell's nucleus.
• centrioles — paired structures of animal cells that organize spindle microtubules. Centrioles are absent in higher plants, where microtubules are instead organized by other microtubule-organizing centres. e.g. A flowering plant spindle forms without centriolar poles.

When a question asks which components are NOT part of the angiosperm cytoskeleton, the answer is the set that lacks plant homologues, such as intermediate filaments and centrioles.

Cellulose microfibril orientation and cell expansion

The primary cell wall is a load-bearing structure in which cellulose microfibrils are largely inextensible. During expansion, the wall stretches most readily in the direction perpendicular to the microfibril orientation, because the fibrils resist stretching along their own length. The shape a growing cell takes is therefore determined by the arrangement of its microfibrils.