Multicellular plants are complex organisms,
and their orderly development requires an extraordinary measure of coordination
between cells. In order to coordinate their activities, cells must be able to
communicate with each other. The principal means of intercellular communication
within plants are the hormones. Hormones are signal molecules that individually
or cooperatively direct the development of individual cells or carry
information between cells and thus coordinate growth and development. Plant
hormones have been the subject of intensive investigation since auxin was first
discovered almost a century ago. The discussion of each hormone in this and
subsequent chapters will begin with a review of biosynthesis and metabolism. An
understanding of hormone bio- chemistry makes it easier to understand what
kinds of molecules they are and how they may function. In addition, a lot of
what is known about what these molecules do and how they do it is based on
studies of mutants that interfere with their biosynthesis or metabolism. The
metabolic turnover of hormone molecules is also a significant factor in the
regulation of cellular activities.
STRUCTURE OF AUXINS
An auxin is a plant
hormone derived from the amino acid tryptophan. An auxin may be
one of many molecules, but all auxin molecules are involved in some sort of
cellular regulation. Auxin molecules are one of five major types of plant
hormone. The other major groups are the gibberellins, cytokinins, ethylene, and
abscisic acid. Auxin was the first of these groups to be identified, and was
chemically isolated in the 1930’s.
Although a large number of
compounds have been discovered with auxin activity, indole-3-acetic acid (IAA)
is the most widely distributed natural auxin. Several other auxins in higher
plants were discovered later, but IAA is by far the most abundant and physiologically
important. Because the structure of IAA is relatively simple, academic and
industrial laboratories were quickly able to synthesize a wide array of
molecules with auxin activity. Some of these compounds are now used widely as
herbicides in horticulture and agriculture. Although they are chemically
diverse, a common feature of all active auxins is a molecular distance of about
0.5 nm between a fractional positive charge on the aromatic ring and a
negatively charged carboxyl group.
Auxins mainly present in two forms:
Natural or Endogenous auxin and Synthetic Auxins. Many naturally occurring
compounds that exert auxin like effects have been revealed. IAA, an extensively
studied endogenous auxin, is active in all bioassays described above and is
often potent at nanomolar concentrations.
A chlorinated form of IAA with high
auxin activity, 4-Cl-IAA, is found in several plants.
In addition to the indolic auxins,
phenylacetic acid (PAA) has been identified in plants and is another active
auxin.
Certain IAA precursors, such as
indole-3-acetonitrile and indole-3-pyruvic acid, are also active in bioassays,
presumably because of conversion in the tissue to IAA .
Similarly, indole-3-butyric acid
(IBA), identical to IAA except for two additional methylene groups in the side
chain, is effective in bioassays.
Like IAA, exogenous IBA inhibits arabidopsis
root elongation and induces lateral and adventitious root formation.
IBA, originally classified as a synthetic
auxin, is in fact an endogenous plant compound. IBA is more effective than IAA
at lateral root induction, perhaps because, unlike IAA, IBA efficiently induces
lateral roots at concentrations that only minimally inhibit root elongation. IBA
is employed commercially for this purpose. Biochemical analyses in a variety of
plants and genetic studies in arabidopsis indicate that IBA acts primarily
through conversion to IAA in a process resembling peroxisomal fatty acid b-oxidation
though roles for IBA independent of conversion to IAA have been proposed.
Two main types of synthetic plant
growth regulators with auxin-like activity have been described:
1-naphthalacetic acid (NAA) and 2,4-D-related compounds. Both compounds exert
auxin-like influences, including root elongation inhibition and lateral root promotion.
The NAA isomer 2-NAA has little
activity in bioassays and provides a weak acid control for auxin experiments
employing the active 1-NAA.
2,4-Dichlorophenoxybutyric acid
(2,4-DB) is a 2,4-D derivative with two additional methylene groups in the side
chain (analogous to the structural relationship between IBA and IAA) that
elicits similar responses to those observed after 2,4-D treatment. In general,
2,4-dichlorophenoxyacetic acid (2,4-D) and IAA derivatives with even-numbered
carbon side chains have more activity than derivatives with odd numbered carbon
side chains. This result suggests that a process such as b-oxidation could
remove two-carbon units from the side chains, arriving at the active acetate
form if the substrate started with an even carbon number 2,4,5-
Tricholorphenoxybutyric acid (2,4,5-TB) also exerts auxin like activity; the
infamous defoliant herbicide Agent Orange was a mixture of 2,4-D and 2,4,5-TB.
Agent Orange was particularly toxic because of dioxin produced as a by-product
of 2,4,5-TB synthesis.
Today, 2,4-D alone is a widely used
herbicide. In addition to NAA and 2,4-D, several alkylated and halogenated
forms of IAA elicit auxin-like growth responses in various bioassays.
Which of the following hormone deficiency causes dwarfism in plant? (ICAR PG Crop Science) 2022
1. Auxin
2. Gibberellin
3. Brassinosteroid
4. Ethylene
Which of the following hormone act as morphogen during embryogenesis (ICAR PG Plant Biotechnology 2020)
Auxin
Cytokinin
Abscisic Acid
Ethylene
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