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PLANT PHYSIOLOGY , Vol 102, Issue 2 547-552, Copyright © 1993 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Thermoinductive Regulation of Gibberellin Metabolism in Thlaspi arvense L. (II. Cold Induction of Enzymes in Gibberellin Biosynthesis)
J. P. Hazebroek, J. D. Metzger and E. R. Mansager
United States Department of Agriculture, Agricultural Research Service, Biosciences Research Laboratory, State University Station, Fargo, North Dakota 58105-5674
Vernalization of Thlaspi arvense L. results in the alteration of
gibberellin (GA) metabolism such that the metabolism and turnover of the GA
precursor ent-kaur-16-en-19-oic acid (kaurenoic acid) is dramatically
increased. This cold-induced change in GA metabolism is restricted to the
shoot tip, the site of perception of cold in this species (J.P. Hazebroek,
J.D. Metzger [1990] Plant Physiol 94: 157-165). In the present report
additional biochemical information about the nature of this
low-temperature-regulated process is provided. The endogenous levels of
kaurenoic acid in leaves and shoot tips of plants were estimated by
combined gas chromatography-chemical ionization mass spectrometry at
various times after 4 weeks of vernalization at 6[deg]C. The endogenous
levels in shoot tips declined 10-fold by 2 d after the plants were returned
to 21[deg]C; this decline continued such that there was nearly 50-fold less
kaurenoic acid by 10 d after the end of vernalization. No effect of
vernalization on the endogenous levels of kaurenoic acid in leaves was
observed. An in vitro enzyme assay was developed to monitor changes in the
ability of tissues to convert kaurenoic acid to
ent-7[alpha]-hydroxykaur-16-en-19-oic acid (7-OH kaurenoic acid). The
activity of this enzyme rapidly increased in microsomal extracts from shoot
tips following the end of vernalization. No thermoinduced increase in
activity was observed in leaves. The enzymic oxidation of ent-kaurene to
ent-kaurenol was also induced in shoot tips by vernalization. However, this
reaction does not appear to be rate limiting for GA biosynthesis, because
substantial amounts of kaurenoic acid accumulated in noninduced shoot tips.
These results corroborate our hypothesis that the conversion of kaurenoic
acid to 7-OH kaurenoic acid is the primary step in GA metabolism regulated
by vernalization in Thlaspi shoot tips.
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