Plant Growth Regulators Hormones

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Plant Growth Regulators (Hormones) include five
categories of long-studied hormones auxins gibb
erellins cytokinins ethylene abscisic acid
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AUXINS - includes more than one specific
chemical - most well-studied indole acetic
acid (IAA)
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- 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
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IAA is synthesized from tryptophan IAA often
stored in plant tissues as auxin conjugates
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auxin is transported basipetally in shoots by
specialized polar transport process pola
r transport process - is independent of gravity
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- includes auxin efflux from one cell and
auxin influx into another (i.e., is neither
symplastic nor apoplastic) - requires metabolic
energy
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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- -
cant diffuse directly out of cell 4) IAA-
can exit cell via efflux carriers (PIN
proteins) located in membrane only at base of
each cell
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polar auxin transport establishes polarity of
overall plant
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auxin effects include at low conc. (lt 0.1
mM) stimulation of stem elongation stimulation
of root elongation stimulation of fruit
growth at high conc. (gt 1 mM) inhibition of
stem and root elongation stimulation of lateral
root formation stimulation of adventitious root
formation
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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
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auxin is involved in developmental processes
including phototropism gravitropism apical
dominance differentiation of vascular tissue
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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
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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
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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)
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ABSCISIC ACID (ABA) synthesized throughout the
plant often involved in stomatal closing in
response to stress also promotes seed maturation