Hormone intro & HW assignment



Abscisic acid






With over 80 analogs identified, gibberellins are the largest hormone grouping . They are used commercially to break dormancy in seeds which will not germinate readily,to promote fruit setting in a number of plants, including the production of parthenocarpic ( seedless) varieties and to produce dwarf plants, and in the beer industry...

Discovery: Kurosawa, a Japanese botanist, discovered gibberellin while investigating the rice foolish seedling disease in which spindly seedlings are formed due to GA like compounds produced by the fungus ( Gibberella fujikuroi) infecting the plant.( see text for more information) .

Gibberellin occurs in angiosperms, gymnosperms, mosses, algae, and fungi.

Effects of Gibberellin:

  • Extensive Growth --Produced by roots and young leaves, it increases growth by both division and elongation; promotes elongation of dwarf mutants. It stimulates shoot elongation even in mature regions of trees & shrubs
  • Seed Germination -- breaks dormancy of certain seeds. This is used in beer industry....yeast need a large supply of malt sugars from barley to ferment....

As the seed imbibes water the embryo produces GA

GA attaches to a protein receptor which sends a signal to a transduction pathway to switch on particular genes --> m-RNA.

Through transcription and translation this leads to synthesis of amylase which then is secreted into the endosperm (stored food reserve)

Amylase breaks down starch to glucose units which diffuses to the embryo and are used for plant growth.

  • Juvenility -- juvenile stages of some plants have different shaped leaves than the adult stage. Gibberellins help determine whether a particular part of a plant is juvenile or adult. An example to further explain this is that the buds of adult branches grow into adult branches, but if treated with gibberellin it grows into juvenile branches.
  • Flowering -- Biennial + Gibberellin ---> Annual. Biennials when treated with gibberellin give flowers and fruits in their first year instead of the usual two years.
  • Parthenocarpic -- stimulates pollen germination and growth. results in larger fruits and even bigger flowers.
  • Fruit formation -- increases size of fruits.

Difference between Auxin & Gibberellin:

  • Gibberellin controls elongation in the mature regions of trees and shrubs whereas auxin regulates elongation in grass seedlings and herbs.
  • Gibberellin stimulates cell division and elongation; auxin stimulates only cellular elongation.
  • Plants can tolerate high levels of gibberellin but not of auxin.
  • Gibberellin have little effect on roots whereas auxin has more of an effect on roots.

The interaction of hormones in the evolution of cell types...

Role of Auxin, Gibberellin and Ethylene in the Size, Pattern and Orientation of Regenerative Tracheids in Pinus pinea L.

Aloni, Roni, Kalev, Natalie

The hormonal control of the redifferentiation of tracheids from parenchyma cells was studied in the hypocotyl of young Pinus pinea L. seedlings.

  • Auxin alone induced short regenerative tracheids usually in discontinuous patterns across the hypocotyl.
  • Combinations of both auxin and gibberellin promoted the differentiation of relatively long tracheids in continuous strands parallel to the stem axis.
  • While combinations of ethrel (which releases ethylene) with either auxin, or both auxin and gibberellin, induced more regenerative tracheids many of them with various horizontal, or radial orientations. Neither gibberellin, nor ethrel could induce regenerative tracheids in the absence of an auxin stimulation. These results show that tracheids are induced and controlled by combinations of various morphogenetic stimuli.

From the evolution perspective the vessels and fibers in the xylem of angiosperms have originated from the tracheids of conifers. Therefore, it is suggested that during evolution, the hormonal mechanism that controls tracheid formation has been modified in the higher plants. From a mechanism based on combinations of auxin, gibberellin and ethylene for the differentiation of tracheids, the mechanisms in angiosperms become more specific for each xylem element.

  • Auxin alone is enough to induce vessels, while
  • gibberellin in the presence of auxin induces fibers. Ethylene which promotes tracheid differentiation might repress vessel and fiber differentiation.

The ethylene, which interrupts the longitudinal orientation of the developing regenerative tracheids, is probably involved in the differentiation of radially oriented tracheids in the vascular rays of conifers.


Gibberellin-induced elongation and osmoregulation in internodes of floating rice

T. Azuma , e-mail azuma@icluna.kobe-u.ac.jp), S. Ueno, N. Uchida and T. Yasuda

Internodal elongation in floating rice (Oryza sativa L. cv. Habiganj Aman II) is known to be enhanced by treatment with ethylene or gibberellic acid (GA3) at high relative humidity (RH). However, ethylene-induced internodal elongation is inhibited at low RH, while GA3-induced internodal elongation is hardly affected by humidity. We examined the possible involvement of osmoregulation in the stimulation by GA3 of the elongation of internodes at low RH. Submergence and treatment with ethylene or GA3 at 100% RH increased the osmotic potential in internodes of excised stem segments, while GA3 at 20% RH maintained the osmotic potential at a low level. In internodes of stem segments that had been treated with GA3 at 20% RH, the activity of invertase and the level of soluble sugars were almost 2- and 1.5-fold higher, respectively, than those in internodes that had been treated with GA3 at 100% RH. These results indicate that one of the possible mechanisms by which GA3 promotes elongation of internodes at low RH involves the osmoregulation that is achieved by promotion of the synthesis of invertase.