B10 Flashcards
What are plant responses?
Evolved mechanisms that allow plants to respond to environmental signals in a way that allows them to maximise growth and survival
What are the phases in plant growth and development?
Seed dormancy
Germination
Seedling growth, differentiation, maturation
Flowering
Seed and fruit production
Clonal growth
Seasonal growth
What are types of plant growth regulators?
Auxins, gibberelins, cytokinins, abscisic acid, ethylene, brassinosteroids.
What are some important properties of plant growth regulators?
Growth promotion and/or inhibition. Act in combination or antagonistically. Transported in different ways, plant growth regulators rather than hormones
What happens in the signal transduction pathway?
Hormones bind to proteins associated with membranes of the cells they will affect. E.g. extra cellular signalling molecule activates a membrane receptor, that in turn alters intracellular molecules. This creates a response.
What are the main effects of auxin and how is it transported?
Cell enlargement and tissue differentiation. Polar transport from apex to base in parenchyma surrounding vascular bundles, approx 1cm per hour, specific auxin transport proteins.
What is the mechanism of auxin transport?
Basipetal transport, transport remains polar if shoot/root turned upside down, faster than diffusion, requires energy and carrier proteins, use of arabidopsis mutants very important in elucidation.
What are some auxin effects?
Apical dominance, tropisms, tissue differentiation.
What happens in apical dominance?
Auxin inhibits auxiliary bud growth, auxiliary bud dormant until apical top is removed. Lateral branches are formed
What are tropisms?
Plant growth responses which may be positive or negative
What are the tropisms in roots and stems?
Roots + gravitropic, stems - gravitropic. The stems of many plants are usually positively phototropic.
What happens with positive phototropism?
Blue light is detected by protein-flavin complex, and auxin accumulates on shaded side.
What is a diffusable growth hormone?
Auxin, can pass through gelatin but not mica.
How does phototropism work?
Light has to be perceived at the tip. Growth response has to be elicited - in the elongation zone.
What is phototropin?
Blue light receptor, it is a flavoprotein
What is the curvature response?
Cholodny-went theory - redistribution of auxin with greater conc on shaded side. Auxins cause acidification of cell wall - loosening of wall matrix (expansins) - turgor pressure results in cell elongation
What do auxins promote and how?
Tissue differentiation, auxin transported by polar transport mechanism to area of wound. Cells in pith differentiate into new vascular tissue and connect with vascular tissue above and below wound.
What is the signal transduction pathway in auxins?
In the absence of auxin, the auxin response gene is repressed by AUX/IAA protein. Auxin receptor is a ubiquitin E3 ligase. Auxin-ubiquitin E3 ligase complex binds to AUX/IAA protein which becomes ubiquitanated. Ubiquinated protein is tagged for proteolysis via the 26S proteasome. Auxin response gene is turned on.
What are the main effects of gibberelins and how are they transported?
Cell division and enlargement, seed germination, environmental effects, maturation, flowering initiation, fruit formation. Transportation is nonpolar bidirectional in phloem.
What effect do gibberelins stimulate on dwarf plants
Internode elongation
What can cause dwarf mutants?
Mutations in GA biosynthesis pathway genes. Mutations in genes that control response to GA.
What is the role of gibberelins in germination?
Many seeds require light or cold periods to break dormancy. GA promoted alpha amylase activity in barley seeds.
What happens in the gibberellin signal transduction pathway?
Inhibited by a repressor, binding to receptor, transcription activated. Repressor has two domains, one binds GA-receptor complex, the other binds to response gene. Mutation in DELLA domain results in no response to GA. Mutation in GRAS domain results in growth response in the abstened of added GA.
What are the main effects of cytokinins and how are they transported?
Cell division, germination, retards senescence, root growth and differentiation, overcomes bud dormancy. Transportation is long distance in xylem
What happens when cytokines interact with other plant regulators?
Cytokinins and auxin interaction in tissue culture, high cytokinin ratio = shoots, high auxin ratio =roots
What are other cytokinin effects?
Promote cell expansion in dicot cotyledons, promote chloroplast development, delay leaf senescence, promote movements of nutrients.
What happens with cytokinins and signal transduction pathway?
Cytokinins receptors are transmembrane protein kinases, binding of cytokinin to the receptor protein initiates a phosphorylation cascade. Leading to transcription of genes.
What happens with cytokinins and signal transduction pathway? Steps
Cytokinin binds to a receptor, activates histidine kinase activity - P transferred to a protein. Phosphorylated protein moves to the nucleus, transfers P. Gene transcription begins in the nucleus
What are the main effects, stress responses and transportation of abscisic acid?
Main effects are that it is a growth inhibitor, seed and bud dormancy, embryo development, senescence. Stress response is stomatal closure. Transportation is root to shoot in xylem, shoot to root in phloem.
What happens in dormancy in abscisic acid?
ABA levels increase during early seed development, promotes production of seed storage proteins, prevents premature germination. Breaking dormancy corresponds with decrease in ABA in seeds. Bud dormancy also under ABA control. ABA effects overcome by GA.
What is the drought stress response in ABA?
Stressed plants contain much higher amounts of ABA than non-stressed plants. Roots respond to drought stress by increased synthesis of ABA. ABA transported from root to shoot in xylem. ABA reaches leaves and induces stomatal closure. ABA promotes efflux of K+ out of guard cells
What are the main effects and transportation of ethylene?
Fruit ripening, leaf and flower senescence, abscission, root hair development, stress responses, inhibition of cell expansion - triple response. Transportation involves diffusion.
What is ethylene triple response?
Exposure of seedlings to ethylene in the dark results in, short hypocotyl, thickened hypocotyl, horizontal growth. In absences of ethylene, seeds exhibit etiolation.
What is the relationship between ethylene triple response and arabidopsis mutants?
Seedlings grown in the dark with atmosphere containing ethylene. Arabidopsis gene ETR1 codes for ethylene receptor protein.
What is the relationship between ethylene and leaf abscission?
Over life span of leaf auxin level decrease whilst ethylene level increases. Synthesis of enzymes that hydrolyze the cell wall polysaccharides, resulting in cell separation and leaf abscission.
What happens with protein kinases and phosphatases in signal transduction pathways?
Nearly all biological signalling systems involve protein phosphorylation and de-phosphorylation in cascade systems. Signal transduction pathways in plants are still incompletely understood. Experiments with arabidopsis mutants has advanced our understanding of these processes enormously.
What happens with vernalisation and stratification?
Plants that have to endure unfavourable winter usually become dormant in autumn and initiate new growth in spring. In temperate zones, many seeds require cold exposure to break their dormancy. Similarly many buds require cold period before they initiate new growth. These processes are under hormonal control. Abscisic acid (ABA) is implicated in dormancy.
What happens in seed germination?
Seed contains a desiccated, dormant embryo. Must absorb water and obtain oxygen. Impervious coat may impose dormancy. Physiological dormancy imposed by hormones. In temperate regions, a cold period may be required. In deserts, seeds remain dormant during the dry season and may require inhibitors to be leached out by rain. Some seeds require light to stimulate germination. A cold period is often required to break bud dormancy or promote flowering.
