lective herbicide. However, the trend in recent years is to use natural plant hormones to regulate crop
growth for greater production. This is because they act in low concentration and are fully degraded, there-
fore do not pose environmental and/or ecological threats.
B. Metabolism
The hormone IAA has been studied for more than six decades, yet it remains unclear how it is synthesized
or degraded in plants. Because of the structural similarities, the amino acid tryptophan is commonly con-
sidered to be a precursor to IAA. To date, biosynthetic pathways from L-tryptophan by way of tryptamine
[18], indole-3-pyruvate and indole-3-acetaldehyde [19], indole-3-aldoxine and indole-3-acetonitrile [20],
and indole-3-acetamide [21] have been proposed. Nevertheless, some workers have reported that D-tryp-
tophan may also be an effective precursor for IAA biosynthesis [7,21]. However, in Lemna gibba,D-tryp-
tophan was not converted to IAA and also the rate of conversion from L-tryptophan was far lower than
expected for a direct precursor [22]. However, it remains unclear which pathway does function in plants.
Wright et al. [23], using the tryptophan auxotroph maize mutant orange pericarp, have questioned the
idea that tryptophan is a precursor of auxin and suggest a nontryptophan pathway as a primary route of
IAA biosynthesis. It is therefore possible that plants and crops use more than one route for in vivo IAA
biosynthesis.
It is reasonable to assume that plants have mechanisms to regulate the levels of auxin to maintain bal-
anced growth. This is done by controlling the rate of synthesis as well as by degradation or by forming
conjugates (bound). The enzyme IAA oxidase with its several isoenzymes, which usually have the char-
acteristics of peroxidases, is known to catalyze the reaction. Two pathways of degradation are known in
many plants. The first involves oxidation by O 2 , leading to loss of the carboxyl group as CO 2 and usually
3-methyleneoxyindole as a principal product. In the second pathway the carboxyl group of IAA remains
intact, but carbon at the second position of the heterocyclic ring is oxidized to oxindole-3-acetic acid. In
some species, however, carbons 2 and 3 are oxidized to form dioxindole-3-acetic acid [24]. Lee and Star-
ratt [25] have shown that soybean (Glycine max) callus and hypocotyl tissues were capable of oxidizing
[^14 C]IAA via the carboxylative pathway to indole-3-methanol glucoside as a major product. However, de-
tails of these degradative pathways are still unclear. Synthetic auxins and IAA conjugates are also not de-
stroyed by these enzymes.
In auxin conjugates the carboxyl group is covalently combined with other molecules in the cell to
form derivatives that do not allow easy extraction. Many IAA conjugates are known, including the indole-
3-acetylaspartic acid (IAAsp), indole-3-acetylglutamic acid (IAAGlu), and the esters IAA-myo-inositol
(IAIns) and indole-3-acetylglucose (IAGlu). These conjugates, along with the free IAA, have also been
found in IAA-overproducing transgenic and wild-type tobacco (Nicotiana tabacum) plants [26]. These
conjugates are not active per se but on hydrolysis release free IAA.
C. Transport
The integrity of the complicated structure of plants depends, to a great extent, on regulations that co-
ordinate the various parts of the whole plant. Because production centers and action sites are often lo-
cated at different places in the plant body, auxin transport takes place. Ever since Went first demon-
strated the basipolar movement of auxin and its quantitative description by van der Weij [13],
physiologists have been interested in studying various parameters of its transport as a way to under-
stand the general phenomena of polarity in plant development. With the availability of high-specific-
activity^14 C-labeled auxin coupled with liquid scintillation spectrometry (90% counting efficiency), it
became possible to perform more complex experiments and to obtain more reliable data from a single
segment than was feasible previously [27,28]. In dicots and monocots alike, auxin moves predomi-
nantly in the basipolar direction [28]. However, in the young vegetative Coleus blumeiinternode, the
auxin applied moves with a 3:1 ratio in the basipetal to acropetal direction, but this changed to 1.3:1.0
when the plants flowered [27,29]. Using paper chromatography, evidence was obtained, for the first
time, that the auxin collected apically was the auxin applied at the basal end [27–29]. In excised root
segments the polar transport was in the acropetal direction, but it appears that in the apical segments of
intact roots (with caps), auxin moves basipetally [30]. However, in both organs, polarity was main-
tained regardless of the tissue orientation.
504 NAQVI