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Plant Responses to Internal and External Signals From experiments with light and the coleoptile-conclusion: tip of coleoptile senses light à some signal was sent from tip to elongating region of coleoptile-cells on darker side elongate faster then cells on brighter side-Auxin = chemical messenger that stimulates cell elongation Hormones = chemical messengers that coordinate different parts of a multicellular organismImportant plant hormones1. Auxin – stimulate cell elongation à phototropism and gravitotropism (high concentrations = herbicide)2. Cytokinins – cell division (cytokinesis) and differentiation3. Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development4. Abscisic Acid – slows growth, closes stomata during water stress, promote dormancy5. Ethylene – promote fruit ripening, (positive feedback); involved in apoptosis (shed leaves, death of annuals) Plant Movement:1. Tropisms: growth responses à SLOW-phototropism – light (auxin)-gravitotropism – gravity (auxin)-thigmotropism - touch2. Turgor movement: allow plant to make relatively rapid and reversible responses – venus fly trap, mimosa leaves, ‘sleep’ movement Ex: Positive gravitotropism in roots – the statolith hypothesis – statoliths settle to bottom of cellsEx: Thigmotropism – rapid turgor movements by mimosa plant à action potentials Plant Responses to Light-plants can detect direction, intensity, and wavelength of light-phytochromes: light receptors, absorb mostly red light --regulate seed germination, shade avoidance Biological Clocks-Circadian Rhythm: biological clocks-persist w/o environmental cues. Frequency = 24 hours.-Phytochrome system + Biological clock = plant can determine time of year based on light/darkness Photoperiodism: physiological response to the relative length of night and day (i.e. flowering)-night length is a critical factor-even interrupting the dark period with a brief exposure to light affects flowering-short-day plants: flower when nights are long (mums, poinsettia)-long-day plants: flower when nights are short (spinach, iris, veggies)-day-neutral plants: unaffected by photoperiod (tomatoes, rice) Plant Responses to Stress: 1. Drought (water deficit)-close stomata-release abscisic acid to keep stoma closed-inhibit growth-roll leaves à reduce SA and transpiration-deeper roots 2. Flooding (oxygen deprivation)–release ethylene à root cell death à air tubes formed to provide oxygen to submerged roots 3. Excess salt-cell membrane – impede salt uptake-produce solutes to decrease water potential and retain water 4. Heat-evaporative cooling via transpiration-heat shock proteins – prevent denaturation 5. Cold-alter lipid composition of membrane (increase unsat. fatty acids, increase fluidity)-antifreeze proteins 6. Herbivores-physical (thorns)-chemicals (garlic, mint)-recruit predatory animals (parasitoid wasps) 7. Pathogens-1st line of defense – epidermis-2nd line of defense – pathogen recognition, host-specific
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