Plant Response to Stimuli other than Light By: Catherine Dombroski Minki Beak Aime Flores Allea Cauilan

Gravity Gravitropism: a plant’s response to gravity Roots: display positive gravitropism Shoots: display negative gravitropism Plants detect gravity by the settling of statoliths: specialized plastids containing dense starch grains Located in the cells of the root cap Thought to trigger redistribution of calcium → lateral transport of auxin Some plants lack statoliths; their mechanism for detecting gravity: Cells pull proteins and stretch them on the “up” side and compress them on the “down” side of the root cells Dense organelles distort the cytoskeleton when they are pulled by gravity

Mechanical Stimuli Thigmomorphogenesis: changes in form that result from mechanical disturbance Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls Thigmotropism: growth in response to touch Occurs in vines and other climbing plants Action potentials: rapid leaf movements in response to mechanical stimulation Transmission of electrical impulse Sensitive Plant

Environmental Stresses Abiotic: nonliving Biotic: living Abiotic/biotic factors: flooding, drought, extreme temperature, herbivory Plant that cannot tolerate stresses will succumb or be outcompeted by other plants Local extinction Defenses for biotic factors (pathogens, herbivores, etc.)

Drought Plants stressed by water deficiency → losing water through transpiration faster than it can be restored through the soil → kills plant When there is a deficit of water, plant cells lose their turgor, which closes the stomata, slowing transpiration The leaf also releases abscisic acid → closing stomata Inhibits the growth of young leaves- decreases leaf surface Wilting plant → leaves roll up to decrease leaf surface During a drought, shallow roots stop growing & deeper roots continue to grow (to reach moist water sources)

Flooding Over-watering a plant can suffocate it because the soil lacks air spaces which are used for cellular respiration in roots Oxygen deprivation stimulates ethylene- Causes some cells in the root cortex to undergo apoptosis Destruction of these cells cause cells to create air tubes that function as “snorkels”, providing oxygen for submerged roots

Salt Stress Excess salt in soil threatens plants because: 1) Lowering water potential of soil solution 2) Sodium and other ions are toxic to plants when their concentrations are relatively high Water potential becomes negative which reduces water uptake Intake of harmful ions can be stopped, but that means less water intake

Heat Stress Heat can kill plants by damaging and denaturing enzymes/damaging metabolism Results in the closing of stomata → response to stress in order to prevent dehydration Plants back-up response to heat is the synthesis of heat-shock proteins → help protect other proteins from heat stress Chaperone proteins (chaperonins), unstressed cells help other proteins fold to their functional shape

Cold Stress When temperature falls there is a change in the fluidity of the cell membranes When membrane cools below critical point the fluidity is lost as the lipids lock in a crystalline structure which alters transport Response is altering lipid composition of membranes Cold= more stressful to plants because of the amount of time it takes for membrane modification At lower temperatures, lipids increase proportions of unsaturated fatty acids which keep the membrane fluid by stopping crystalline formation