CONTROL OF PLANT GROWTH AND DEVELOPMENT BI-2232 RIZKITA R E

CONTROL OF PLANT GROWTH AND DEVELOPMENT BI-2232 RIZKITA R E

The development of a plant the series of progressive changes that take place throughout its life is regulated in complex ways. Factors take part in this regulation: The plant senses and responds to environmental cues. In order to sense some environmental cues, the plant uses receptors such as photoreceptors, molecules that absorb light. Chemical signals, or hormones, mediate the effects of the environmental cues including those sensed by receptors. The plant s genome encodes enzymes that catalyze the biochemical reactions of development.

Signal transduction pathways mediated by : hormone and enviromental receptor action

GENERAL MODE

Stucture and activity

Auxin is transported basipetally

Selected auxin roles 1. Increase stem & coleoptile growth 2. Inhibits root growth 3. Increase cell wall extension via acidification 4. Tropisms Phototropism via lateral auxin distribution Gravitropism, possibly via statoliths 5 Apical dominance 6. Delay leaf abscission 7. Promotes formation of lateral & adventitious roots 8. Promotes fruit development 9. Promotes vascular differentiation

Gibberellins (GAs) 1. There are >100 different GAs, with the chemical identified by a subscript (GA1) 2. There are few bioactive GAs (GA1, GA, GA3) 3. Others are intermediates or inactivated

Selected effects of Ga s Regulate transition from juvenile to adult,primarily in woody perennials Controls transition both ways Influence floral initiation and sex determination Promote fruit set Stimulate seed germination

CYTOKININ Forming bound cytokinins (deactivation,inactive form) Free form is the active form Cytokinin oxidase (inactivation) Activity is controlled by:

Selected cytokinin roles 1. Necessary for shoot growth: Regulation of cell division Promote cell expansion in leaves & cotyledons 2. Suppresses root growth 3. Interaction with auxin dictate root and shoot development in cell culture. 4. Delay leaf abscission 5. Promotes lateral bud development (opposes apical dominance) 6. Promotes nutrient redistribution

The steps in cytokinin signaling: A cytokinin, like zeatin, binds to a receptor protein embedded in the plasma membrane of the cell. The internal portion of the receptor then attaches a phosphate group to a protein in the cytosol. This protein moves into the nucleus where it activates one or more nuclear transcription factors. These bind to the promoters of genes. Transcription of these genes produces mrnas that move out into the cytosol. Translation of these mrnas produces the proteins that enable the cell to carry out its cytokine-induced function

ETHYLENE

Selected ethylene functions 1. Fruit ripening in climacteric fruit 2. Leaf Leaf epinastic 3. Root and root hair development 4. Leaf senesence and abscission 5. Defense responses The presence of ethylene is detected by transmembrane receptors in the endoplasmic reticulum (ER) of the cells. Binding of ethylene to these receptors unleashes a signaling cascade that leads to activation of transcription factors and the turning on of gene transcription

Abscisic acid Found in all vascular plants and some non-vascular plants (mosses) Found in all tissues Synthesized in cells with chloroplasts or amyloplasts

Selected roles for ABA 1. ABA closes stomata during water stress 2. Under low water : Root growth up, Shoot growth down ABA inhibits ethylene 3. initiating and maintaining seed dormancy. Levels peak during embryogenesis Desication tolerance and promote accumulation of seed s material Release from dormancy is controlled by the ratio of ABA to GA

The mechanism: ABA binds to receptors at the surface of the plasma membrane of the guard cells. The receptors activate several interconecting pathways which converge to produce (1) a rise in ph in the cytosol (2)transfer of Ca2+ from the vacuole to the cytosol. The increased Ca2+ in the cytosol blocks the uptake of K+ into the guard cell while the increased ph stimulates the loss of Cl and organic ions (e.g., malate2 ) from the cell. The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor. The stomata close.

Effects include: stem and cell elongation Unrolling and bending of grasses H+ activation Ethylene production Photomorphogenesis Induction of cell division Root initiation Embryogenesis Flower development Fruit ripening Inhibitor of seed and pollen germination; root growth Inducer of plant defenses Inducer of fruit ripening Retarding senescence dueto inhibition of ethylene Induction of flowering Induction

Several Light-Induced Reactions of the Mustard Seedling (Sinapis alba) All these photomorphisms can be traced back to the formation of P FR (according to H. MOHR and P. SCHOPFER, 1978) Inhibition of the elongation of the hypocotyl Inhibition of translocation from the cotyledons Increase of the surface area of the cotyledons Unfolding of the cotyledons lamina Development of hairs at the hypocotyl Opening of the hypocotyl s hook Development of the primary leaves Development of mature leaf primordia Increase in the negative geotropic reaction of the hypocotyl Development of xylem elements Differentiation of the stomata within the epidermis of the cotyledons Development of super-etioplasts in the cotyledons mesophyll Changes in the intensity of the cell respiration Synthesis of anthocyane in the cotyledons and the hypocotyl Increase in the synthesis of carotenoids Increase in the capacity of the chlorophyll synthesis Increase in the RNA synthesis within cotyledons Increase in the protein synthesis within cotyledons Intensification of the storage fat breakdown Intensification of the Storage protein breakdown Increase in the synthesis of ethylene Acceleration of the Shibata-shift within the cotyledons Determination of the cotyledons capacity to photophosphorylate Modulation of the cotyledons enzyme synthesis

Phytochrome activates chlorophyll synthesis in proplastids

Blue light photoreceptors Cryptochrome Phototropin Anthocyanins Zeaxanthin Cryptochromes, found in hypocotyl cells, inhibit hypocotyl elongation. Phototropin is involved in most phototropic (differential growth)responses. Zeaxanthin functions in stomatal opening. All activate signal transduction pathways that induces the auxin migration.

Interacting Factors in Plant Development The development of a plant the series of progressive changes that take place throughout its life is regulated in complex ways. Four factors take part in this regulation: The plant senses and responds to environmental cues. In order to sense some environmental cues, the plant uses receptors such as photoreceptors, molecules that absorb light. Chemical signals, or hormones, mediate the effects of the environmental cues including those sensed by receptors. The plant s genome encodes enzymes that catalyze the biochemical reactions of development. No matter what cues regulate development, ultimately the plant s genome determines the limits within which the plant and its parts will develop.

An Overview of Plant Development Hormones and other substances control the growth of the pollen tube that brings sperm and egg together.

Enzyme in GA3 biosynthesis GERMINATION Seed dormancy must be broken before germination DORMANCY : 1. Exclusion of water or oxygen from the embryo by means of an impermeable seed coat 2. Mechanical restraint of the embryo by means of a tough seed coat 3. Chemical inhibition of embryonic development

Phytochrome and germination

Etiolation in bean The apical hook is maintained through an asymmetrical production of ethylene gas, which inhibits the elongation of cells on the inner surface of the hook. There appears to be a link between phytochrome light perception and brassinosteroids in the etiolation response. Etiolation is an energy conservation strategy to help plants growing in the dark reach the light before they die. They don't green-up until there is light,and they divert energy to growing as tall as possible through internode elongation.