Control of Plant development Flashcards
Plant growth regulators
PGRs play an important role.
They may be generated by development in one part of the plant.
Other parts of the same plant may be programmed to produce an appropriate developmental response.
The same PGR is produced in response to certain stimulus. But the plant response to the PGR is species, tissue and time specific.
PGR Synthesis
PGR Synthesis may be synthesized in response to an environmental signal. Other parts of the same plant may be programmed to produce an appropriate developmental response.
Control of primary Growth (Cell Expansion)
The most important co-ordinating influence on primary vegetative growth and differentiation, shoot branching and cambial activity is auxin synthesized
The auxin is swept root tip wards by a process termed polar transport (1cm per hour) as the xylem differentiates and the content of the vessels lyse through the living parenchyma cells of the stele.
Cell expansion determining internode length is auxin dependent, so the developing leaf influences the size of internodes below it that will support and supply it.
Tropic curvature
Enables the expanding internodes to respond to light and gravity, but the mechanism remains obscure. The environmental signals are detected by specialised plastids (chloroplasts, chromoplasts, amyloplasts & aleoplasts).
Statoliths
Large amyloplasts which do not disappear when the plant is starved and, unlike other starch grains, are ‘loose’ inside the cell, rolling down to the lowest point when the plant is tilted.
Phototropism
Growth in response to the direction of light is mediated by a yellow flavoprotein photoreceptor maximally sensitive to blue light.
Sensitivity to phototropic stimuli in coleoptiles correlates with special bright yellow plastids unique to cells of the bundle-sheath. Leaves have statoliths in the bundle sheaths that only stems consist of in the endodermis (but in roots they are in the root cap)
The most likely explanation for this is that the specialised plastids influence the transport of auxin from the polar transport stream in the stele to the expanding tissues of the cortex, blocking the radial outwards movement of auxin on the illuminated or upper side.
Gibberellins
Differentiating leaves export gibberellins which promote cell expansion (& directly regulate the activity of the subapical meristem, promoting cell divisions that increase internode length.
Cytokinins
Growing root tips export cytokinins, which promote cell expansion in leaves, but not in stems. When more mineral nutrients are available in the soil, more cytokinin is exported from roots, and leaves are larger in response.
Ethylene
Cell expansion is also sensitive to ethylene, part of a response to physical stress. The production of ethylene by plant tissues increases when you bend, shake or stress them. Elevated ethylene alters the orientation of cortical utubules in expanding cells, which alters the orientation of cellulose microfibrills synthesized.
Subsequently, so as to increase radial expansion at the expense of longitudinal expansion-> stouter axis, able to push up paving stones.
Vascular tissue
Primary xylem requires auxin from developing leaves in order to differentiate.
Primary phloem requires cytokinin from growing roots.
In addition, cells in the appropriate position do not differentiate into fibres unless supplied with both auxin and gibberellin. These PGRs influence the program of differentiation.
Increased gibberellin GA -> larger fibres increased auxin IAA-> thicker walls.
Meristemoids
Firstly, the nucleus moves to one end of the cell, which may show a gradient of organelles across it, oriented to some outside influence e.g. a meristem.
Then cells are formed: a small cell, from the dense end which goes onto something exciting, & a large cell from the other end of the gradient which usually does not.
The small cell usually re-commences cell division & acting as tiny meristem (meristemoid) goes on to generate one of the various types of glands, hairs, & other little structures which are dotted throughout & especially over the surface of plants.
Control
The purpose of meristemoids seems to be to allow independent control of the development of these ‘tiny’ organs, e.g. the timing of maturation and spacing of guard cells.
Control of secondary growth: Branching
Lateral roots arise deep inside the root. From a single layer- the PERICYCLE - just inside the endodermis so that the integrity of the stele isnt breached.
Shoot branches arise from the similarly arrested cells, but in pockets on the outside surface of the axis where they monitor light.
The mechanisms of control of shoot branching vary from plant to plant.
Collective Developmental Transitions
Plant organs and especially meristems, sometimes undergo a major shift in biochemistry and/or pattern of growth.
Rather like differentiation for a single cell, but this time for a whole group of cells there is a coordinated, synchronous change from the expression of one set of genes to the expression of a new set.
Such changes are referred to as ‘transformations’ which are large scale qualitative transitions which form the life cycle of organs and individuals.
Senscence
This is a terminal transformation for any tissue/individual, involving the breakdown of cell structures & macromolecules - but note that the initiation of this phase is by gene de-repression & synthesis of a range of hydrolytic enzymes = energy-consuming, so waterlogged, dried or poisoned leaves stay green
The onset of tissue senescence is controlled by auxin levels in the tissue which, post differentiation slowly declines until eventually is at very low auxin. Tissues begin to respond to the tiny amounts of ethylene they produce all the time by producing excess ethylene which activates the senescence process.
