Organismal Physiology Lecture No.12: Plant Hormones
Tuesday October 23 , 2012
-A hormone is an organic compound produced in small amounts in one part of a plant and transported
to target cells in another part, where it causes a physiological response. A plant hormone can also elicit
its response in the same cell it was produced. Research on hormones is complex because a single
hormone may affect many target tissues. Moreover, hormones are usually present in low concentrations
and may exist in many chemical forms (inactive, active, etc.). In addition, several hormones may affect
the same response, and hormones may interact with one another rather than working independently.
-Different plant hormones that are known today include: Gibberellins (GAs), Cytokinins (CKs),
Brassinosteroids (BRs), Auxins, Ethylene, Abscisic acid (ABA), Salicylic acid (SA), and Jasmonic acid (JA).
The last two are often categorized as one class of plant hormones since they more or less perform the
-Gibberellins make up a large family (36 known gibberellins) of structurally-related compounds that are
found in a wide variety of fungi and plants. Gibberellic acid (GA ) is a gibberellin that appears to promote
cell elongation and to increase rates of cell division in roots. The importance of gibberellin in plant
development was observed in the Arabidopsis plant where dwarf mutants (deficient in gibberellin) could
attain normal size if treated with gibberellin. This is why gibberellins are used commercially to increase
fruit size in table grapes and to regulate citrus flowering and rind maturation.
-Gibberellins are also key players when it comes to seed dormancy and germination. As a seed begins to
absorb water and commences germination, gibberellins diffuse from the embryo to the outer aleurone
layer. This is the site where gibberellin binds to the receptor of an aleurone cell’s plasma membrane.This
sends a signal to the nucleus that eventually produces the Myb protein. The Myb protein binds to the
promoter of the α-amylase gene and activates its transcription. As amylase is produced and exported to
the starchy interior of the seed, the enzymes digest starch, releasing sugars and other molecules to the
-Evidence for gibberellin activating the Myb transcription factor is observed in a Northern blot analysis
whereby only seeds that attain access to water (or diffuse GA) express the Myb transcript in excess.
Cytokinins & Cell Division:
-Cytokinins are a group of plant hormones that promote cell division. Cytokinins are synthesized in root
tips, young fruits, seeds, growing buds, and other developing organs. Cytokinins regulate growth by
activating the genes that keep the cell cycle going. In the absence of cytokinins, cells arrest at the G1
checkpoint in the cell cycle and cease growth. Cytokinins promote the expression of genes that start S
phase (DNA synthesis). Without cytokinins, cells remain in G1 and do not divide Brassinosteroids & Growth In The Dark:
-Brassinosteroids are plant hormones that are important during etiolation (a process in flowering plants
grown in partial or complete absence of light). Among wild type plant species, there is observed to be a
much greater development in the dark than would be achieved by mutants deficient in brassinosteroids.
-Hardly any plants have been discovered that can grow in auxin-deficient conditions. This is because
auxin is responsible for the development of the apical meristem and lateral shoots from apical and
lateral buds respectively. Auxin is produced in the apical meristems and young leaves (in case meristem
is cleaved off) and always travels down the stem by polar transport all the way down to the root of the
plant. It is only in this small area (the very tip of the root) that auxin will be transported upwards.
-As auxin concentration decreases in the apical bud, it increases in the lateral bud (and vice versa).
During polar transport, auxin enters a cell through the apical end by way of cotransporters that bring
auxin into the cell. Throughout its journey in the cell, auxin is deprotonated (travels in ionized form) and
some auxin molecules are destroyed by enzymes. The protons are pumped into the cell wall as well as
used to make ATP. At the bottom of the cell, carrier proteins help auxin exit the cell. The hormone gets
protonated as it leaves the cytoplasm and enters the next cell in its uncharged form.
-Because auxin is responsible for an excess of protons being pumped into the cell wa