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Plant Growth Regulators (Hormones) include five categories of long-studied hormones auxins

Plant Growth Regulators (Hormones) include five categories of long-studied hormones auxins gibberellins cytokinins ethylene abscisic acid. AUXINS - includes more than one specific chemical - most well-studied = indole acetic acid (IAA).

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Plant Growth Regulators (Hormones) include five categories of long-studied hormones auxins

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  1. Plant Growth Regulators (Hormones) include five categories of long-studied hormones auxins gibberellins cytokinins ethylene abscisic acid

  2. AUXINS - includes more than one specific chemical - most well-studied = indole acetic acid (IAA)

  3. - synthesized mostly in shoot tips (young leaves, shoot meristems) - transported by polar pathway towards roots & root tips - creates an auxin gradient throughout length of plant

  4. IAA is synthesized from tryptophan IAA often stored in plant tissues as “auxin conjugates”

  5. auxin is transported basipetally in shoots by specialized polar transport process polar transport process: - is independent of gravity

  6. - includes auxin efflux from one cell and auxin influx into another (i.e., is neither symplastic nor apoplastic) - requires metabolic energy

  7. proposed mechanism: 1) auxin enters cells passively as IAAH 2) IAA- can enter cells via H+-IAA- symporter (possibly AUX1) (secondary active transport) 3) in near neutral pH, most IAAH dissociates to IAA- - can’t diffuse directly out of cell 4) IAA- can exit cell via efflux carriers (PIN proteins); located in membrane only at base of each cell

  8. polar auxin transport establishes polarity of overall plant

  9. auxin effects include: at low conc. (< 0.1 mM) stimulation of stem elongation stimulation of root elongation stimulation of fruit growth at high conc. (> 1 mM) inhibition of stem and root elongation stimulation of lateral root formation stimulation of adventitious root formation

  10. acid growth mechanism auxin may 1) activate H+-ATPases in plasma membrane 2) stimulate synthesis of new H+-ATPases 3) stimulate movement of H+-ATPases to the plasma membrane

  11. auxin is involved in developmental processes including: phototropism gravitropism apical dominance differentiation of vascular tissue

  12. GIBBERELLINS includes many chemically similar molecules (GA1, GA2, GA3, etc.) synthesized at shoot tips and root tips - transported elsewhere through xylem some known effects stimulation of shoot elongation (through increased cell division and cell expansion) stimulation of food mobilization during germination of grass seeds

  13. CYTOKININS includes zeatin, kinetin, others synthesized in root tips, transported elsewhere through xylem some known effects: stimulation of cell division stimulation of leaf expansion delay of leaf senescence

  14. ETHYLENE is a volatile hormone synthesized throughout the plant, often in response to stress OR the presence of auxin some known effects: stimulates stem and root elongation (in most plants; opposite in others) stimulates fruit ripening causes leaf epinasty in flooded plants inhibits flowering (or, in bromeliads, stimulates flowering)

  15. ABSCISIC ACID (ABA) synthesized throughout the plant often involved in stomatal closing in response to stress also promotes seed maturation

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