3.2 Plant Sensory Systems and Responses PDF

Preview

Summary

3.2 Plant Sensory Systems and Responses...

Description

Plant Sensory Systems and Responses I. Plant responses to light A. Pigment molecules detect light and mediate the plantʼ s response 1. Photomorphogenesis: growth and development in response to light B. The phytochrome system and the red/far-red response 1. Phytochrome molecule has two interconvertible forms: a) Pr – absorbs red light at 660 nm (1) Biologically inactive (2) Converted to Pfr when red photons available b) Pfr – absorbs far-red light at 730 nm (1) Active form (2) Converted back to Pr when far-red photons available 2. Activated phytochrome molecule turns-on genes that lead to plant response a) Pr is found in the cytoplasm b) When it is converted to Pfr it enters the nucleus c) Pfr binds with other proteins that form a transcription complex, leading to the expression of light-regulated genes 3. Seed germination a) Inhibited by far-red light and stimulated by red light in many plants b) Only germinate when exposed to direct sunlight 4. Detection of plant spacing a) Plants measure the amount of far-red light bounced back from neighboring plants C. Photoperiodism: ability to use light to track time D. Photoperiod (day-length) may trigger flowering 1. Short-day plants: bloom in fall or spring 2. Long-day plants: bloom in summer 3. Day-neutral plants: do not respond to day length E. Blue light responses 1. Phototropism directional growth responses to light 2. Reversible phototropism is caused by changes in turgor pressure; plant movement is caused by water entering or leaving cells 3. Some turgor movements are triggered by light F. Circadian Clocks ~ 24-hour rhythms 1. Day pollinators - open day/close night 2. Night pollinators - open night/ close day II. Plant responses to gravity: gravitropism A. Roots have positive gravitropism 1. The cap is the site of gravity perception 2. Signaling triggers differential cell elongation and division in the elongation zone 3. Gravity-sensing cells are located in the root cap 4. Cells that actually undergo asymmetrical growth are in the elongation

III.

zone closest to root cap 5. Auxin may be involved a) Still occurs if auxin transport is suppressed B. Shoots exhibit negative gravitropism, gravity is sensed along the length of the stem in endodermal cells surrounding the vascular tissue Responses to mechanical stimuli directional growth of a plant or plant part in response to contact A. Threshold refers to the level of stimulation that must be present for reaction to occur B. Morphogenesis results from gene expression – genes can be turned on or off by plant generated stimuli or from environmental stimuli C. Cells secrete chemicals that stimulate or inhibit activities of nearby cells. Concentration of the signal molecule decreases with Increased distance away from signal cell D. Dosage-dependent responses 1. Same chemical messenger may have different effects at different concentrations 2. A given concentration may have different effects on different target cells E. Auxin 1. Inhibits abscission 2. Promotes salt intake 3. Promotes callus tissue (wound healing) 4. Promotes dormancy 5. Inhibits growth of lateral buds 6. Stimulates primary growth of stems and root 7. Stimulates cell division in vascular cambium 8. Promotes formation of secondary xylem 9. Stimulates formation of root meristems, (apical, lateral & adventitious) 10. Promotes cell elongation 11. Promotes fruit development F. Human uses synthetic auxins 1. Herbicides a) 2,4,D b) Agent orange 2. Stimulate root growth of cuttings 3. Delay fruit and leaf drop of apples and berries 4. Promote flowering and fruiting of pineapples 5. Seedless fruits G. Cytokinins 1. Produced in the root apical meristems and developing fruits, young leaves and seeds. In combination with auxin, stimulates cell division and differentiation a) Natural functions (1) Among others; Stimulates cell division

b) Human applications (1) With gibberellins - promote fruit - apples (2) With auxin, tissues culture 2. Gibberellins a) Produced in apical buds, roots, young leaves and embryos - stem elongation b) Natural functions; (1) Stimulate elongation of stem internodes c) Human application (1) Promote growth for more open grapevines (2) Increase malt production (3) Faster germination 3. Ethylene a) Produced by fruits, flowers, seeds leaves and roots - triggers aging processes (1) Natural functions (a) Promote fruit ripening...