Plant Hormones & response to environment

Hormones = signaling molecules carry info from cell to cell 1) Auxins – lengthen shoots & apical dominance - decrease abscission (leaf dropping) - fruit formation 2) Ethylene – ripen fruit (is a gass) 3) ABA = Abscisic Acid – induce dormancy - drought tolerance by - closing stomata

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Seed Dormancy A) mature seed produces ABA 1. induces production of proteins to resist dehydration 2. induce seed dormancy B) To germinate ABA levels must come down 1. heavy rain washes ABA out of seed 2. light and cold exposure may also be needed

Auxin Mechanism Auxin concentrations control gene transcription genes controlled regulate cell differentiation Auxin only produced in the apical meristem Auxin is transported down the shoot by Polar transport (much faster than diffusion) (Apical meristem = tip of root/shoot that is growing : doing mitosis)

Polar because auxin transport proteins are only located at the base of the cells. (at one pole)

Acid Growth Hypothesis High concentration of Auxin cause elongation of apical cells (growth) Called Acid Growth because high Auxin conc. causes protein pumps to actively transport H+ out of the cell (making the cell wall low pH)

Effects of high H+ conc. in cell wall Membrane potential created (voltage across memb.) Memb. Pot. Induces ion uptake by cell Cell becomes hypertonic Osmosis increases turgor pressure Low pH Activates expansin : Enzymes that break crosslinks (H-bonds) btwn cellulose fibers Allows cell wall to expand

Result = cell elongates (grows) Under uniform light cells grow equally Directional light causes dark side cells to elongate more http://bcs.whfreeman.com/thelifewire/conten t/chp38/3802003.html http://bcs.whfreeman.com/thelifewire/conten t/chp38/3802002.html

Auxin concentrations also respond to gravity http://www.kscience.co.uk/animations/auxin. htm