Control of Plant development Flashcards

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1
Q

Plant growth regulators

A

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.

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2
Q

PGR Synthesis

A

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.

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3
Q

Control of primary Growth (Cell Expansion)

A

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.

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4
Q

Tropic curvature

A

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).

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5
Q

Statoliths

A

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.

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6
Q

Phototropism

A

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.

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7
Q

Gibberellins

A

Differentiating leaves export gibberellins which promote cell expansion (& directly regulate the activity of the subapical meristem, promoting cell divisions that increase internode length.

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8
Q

Cytokinins

A

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.

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9
Q

Ethylene

A

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.

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10
Q

Vascular tissue

A

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.

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11
Q

Meristemoids

A

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.

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12
Q

Control

A

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.

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13
Q

Control of secondary growth: Branching

A

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.

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14
Q

Collective Developmental Transitions

A

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.

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15
Q

Senscence

A

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.

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16
Q

Onset of tissue senescence

A

Adaptively entrained to: nutrient stress via CKs, water stress via ABA & daylength via GIA, the effective levels of which increase with daylength.

17
Q

Abscission

A

The ethylene also activates abscission, the final stage of tissue senescence. Scattered through the plant are groups of cells which do not respond to auxin in cell expansion.
Strategically placed, e.g. in petiole bases, they respond to ethylene when mature by cell expansion & partial autolysis of cell walls.

18
Q

Induction of flowering (Development control)

A

In some species, the entire plant may be ‘switched over’ into senescence.

For many plants the switch is part of the internal development of the individual.
Sometimes the transition is observed to be related to the accumulation of a fixed number of leaves.

19
Q

Photoperiodism

A

For plants such as biennials, the entertainment is to a prolonged period of chilling: vernalization

For photoperiodism, the detector system is phytochrome as a primary light sensor, linked to the endogenous circadian rhythm, which connects phytochrome -> apex at a fixed time after sunset every night.

20
Q

Detector system

A

In the developing leaves which produce the bulk of the GA & auxin & as they mature they produce the bulk of the ABA & relay all the CK which arrives from the root, onto the sh apical meristem.

The ‘detector’ system triggers an altered pattern of GR production/ relay by the leaf where the sh meristems are adapted to interpret by transforming into floral meristems.
Sometimes the connection is simple e.g. effective (GA) rises in long days and in some plants which flower in long days exogenous GA will induce flowering.

21
Q

Complex interactions

A

Complex & prolonged because the transition to flowering is critical for survival & therefore subject to careful, multiple cross-checking

22
Q

Species-specific

A

species specific, because the transition to flowering forms part of the machinery for adaptation to the environment by individual species in a very wide range of environments.

23
Q

Germination

A

Many seeds will germinate as soon as the chemical inhibitors of the fruit sap have been washed off & conditions are favourable: water & warmth.
Other types stubbornly refuse- they have an innate dormancy. There are 3 basic types, characterised by the treatment required to break dormancy.

24
Q

After ripening

A

no germination unless dry at room temp for several weeks, e.g. grasses & cereals, clover & the evening primrose (Oenothera)

25
Q

Chilling

A

Seed needs to have imbibed before it responds & several weeks at 0 to 5 oC are required= stratification. Same ‘detector’ system as vernalisation.