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PLANT PHYSIOLOGY , Vol 112, Issue 2 549-558, Copyright © 1996 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
The Effects of Exogenous Auxins on Endogenous Indole-3-Acetic Acid Metabolism (The Implications for Carrot Somatic Embryogenesis)
D. M. Ribnicky, N. Ilic, J. D. Cohen and T. J. Cooke
Horticultural Crops Quality Laboratory, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705-2350 (D.M.R., N.I., J.D.C.)
The effect of auxin application on auxin metabolism was investigated in
excised hypocotyl cultures of carrot (Daucus carota). Concentrations of
both free and conjugated indole-3-acetic acid (IAA), [2H4]IAA,
2,4-dichlorophenoxyacetic acid, and naphthaleneacetic acid (NAA) were
measured by mass spectroscopy using stable-isotope-labeled internal
standards. [13C1]NAA was synthesized for this purpose, thus extending the
range of auxins that can be assayed by stable-isotope techniques.
2,4-Dichlorophenoxyacetic acid promoted callus proliferation of the excised
hypocotyls, accumulated as the free form in large quantities, and had minor
effects on endogenous IAA concentrations. NAA promoted callus proliferation
and the resulting callus became organogenic, producing both roots and
shoots. NAA was found mostly in the conjugated form and had minor effects
on endogenous IAA concentrations. [2H4]IAA had no visible effect on the
growth pattern of cultured hypocotyls, possibly because it was rapidly
metabolized to form inactive conjugates or possibly because it mediated a
decrease in endogenous IAA concentrations by an apparent feedback
mechanism. The presence of exogenous auxins did not affect tryptophan
labeling of either the endogenous tryptophan or IAA pools. This suggested
that exogenous auxins did not alter the IAA biosynthetic pathway, but that
synthetic auxins did appear to be necessary to induce callus proliferation,
which was essential for excised hypocotyls to gain the competence to form
somatic embryos.
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