PLANT PHYSIOLOGY , Vol 106, Issue 1 343-351, Copyright © 1994 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Metabolism and Synthesis of Indole-3-Acetic Acid (IAA) in Zea mays (Levels of IAA during Kernel Development and the Use of in Vitro Endosperm Systems for Studying IAA Biosynthesis)
P. J. Jensen and R. S. Bandurski
Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824-1312
Kernels of Zea mays on an intact plant accumulate indole-3-acetic acid
(IAA) at the rate of 190 ng g-1 fresh weight h-1. Of the IAA synthesized,
97% is in the esterified form and less than 3% remains as the free acid.
The site of biosynthesis of the IAA, whether synthesized in the leaf and
transported to the kernel, or in the kernel and remaining in the kernel,
has not been established. In an attempt to determine the locus of
synthesis, we grew isolated kernels on agar media not containing tryptophan
or other possible aromatic precursors of IAA and observed IAA synthesis of
99 ng g-1 fresh weight h-1, approximately 52% of the in situ rate. Thus,
the kernel contains all of the enzymes required for de novo aromatic
biosynthesis of IAA and its ester conjugates. Furthermore, endosperm cells
in suspension culture, grown on hormone-free media and in the absence of
aromatic precursors, are able to synthesize IAA at a rate of 9.2 ng g-1
fresh weight h-1, or 4.8% of the in situ rate. This finding establishes
that all of the enzymes of IAA biosynthesis occur in the endosperm and that
the endosperm is a site of IAA biosynthesis. Isolated endosperm, prepared
from developing kernels, synthesized IAA from labeled anthranilate at a
rate of 8.6 ng g-1 fresh weight h-1, or 4.5% of the in situ rate. Frozen
endosperm preparations maintained the ability to synthesize labeled IAA
from labeled anthranilate. The identity of the synthesized IAA was
established by mass spectral analysis. We suggest that endosperm
preparations of Z. mays are suitable for study of the mechanism(s) of IAA
biosynthesis because they (a) have high rates of synthesis; (b) show
stability to freezing, enabling enzyme storage; (c) provide a system with a
known rate of in situ synthesis; and (d) are available in large amounts for
use as an enzyme source.
