Chapter 16. Plant Hormones

Chapter 16. Hormones in Plant Development

first hormone class to be identified and characterized
Indole acetic acid (IAA) is principal natural auxin (there are others and several synthetic auxins of agricultural importance)
IAA is synthesized in shoot/root apices, translocated in parenchyma
Some effects

Shoot elongation through cell expansion - classic experiment floats explants on IAA solutions of different concentrations to determine dose/response relationship. For example concentrations of IAA that stimulate shoot elongation, inhibit root elongation.
Apical dominance

Shoot decapitation releases dormant axillary buds (suggests chemical communication from shoot apex)
IAA can substitute for apex, suppress axillary buds
Mechanism? Hypothesis - optimum concentration of IAA for buds is low, IAA from apex suppresses growth. This is example of mediation of hormone effect by concentration.
Interaction with other hormones
Cytokinins applied to intact apical meristems will release axillary buds from dormancy
ABA of bud may suppress release from dormancy
Experiment:
decapitate, bud ABA decreases, bud released
decapitate, apply IAA to cut end, ABA level maintained, bud suppressed
hypothesis: bud ABA concentration is controlled by IAA from apex.

Experiment - remove lamina, compare leaf life span to intact leaves (delaminated leaf abscises sooner)
Application of IAA to cut end of petiole can delay abscission
Hypothesis: export of IAA from lamina to petiole delays formation of abscission zone.

Flower and fruit development: a multitude of effects, depending on the species
Exogenous auxin can suppress flowering (evidently through stimulation of ethylene production)
In species with unisexual flowers, exogenous auxin stimulates production of female flowers.
Exogenous auxin can stimulate parthenocarpy (resulting in seedless fruits)

This was how gibberellins were first discovered - fungal metabolite caused rice seedlings to elongate excessively and topple
Dwarf peas (e.g.. Mendel's) carry a mutation in a gene encoding a protein required for GA synthesis. Exogenous GA can substitute for lack of endogenous hormone, cause plants to take on “normal” phenotype.

Bolting in rosette plants - GA can substitute for environmental cue (e.g. chilling, photoperiod), induce bolting (in turn required for flowering)
Seed germination

GA stimulates mobilization of seed reserves by activating gene encoding amylase.
GA can substitute for environmental cue in dormant seeds of some species.

GA is not the flowering hormone, but may play a part in induction of flowering
In species with unisexual flowers stimulates production of male flowers. Hypothesis: sex ratio is a function of auxin/gibberellin balance

Plant tissue culture - differentiation of callus into shoot or root can be controlled by manipulating auxin/cytokinin concentration ratio.
High ration encourage root growth
Low ratio encourages root growth
Senescence - application of cytokinins will delay senescence, perhaps by stimulating metabolism.

Apparently not associated with leaf abscission or bud dormancy as previously thought.
Seed germination

Suppresses precocious seed germination (ABA-deficient Arabidopsis mutants produce viviparous seeds)
Mechanism = ABA interferes with GA synthesis (required for seed reserve mobilization)

Well watered plants have low ABA concentration in leaves
Apply exogenous ABA, stomata close
Subject plant to water stress, ABA concentration increases
Drought signal can initiate in leaves or roots (see sketch of spit-root experiment, Figure 22.4)

The “stress hormone”, but there are numerous other species-specific effects:
Ripening fruit (autocatalytic)
Maintaining epicotyl hook in dark-grown seedlings
Breaking seed dormancy in some species
Stimulate abscission (effect can be countered by auxin)
Stimulate flowering in Bromeliaceae (primarily a family of tropical epiphytes, also the pineapple.

Is all this confusing? You bet it is! We need to elucidate mechanisms to make sense of the bewildering pattern of hormone effects.