Vernalization of Thlaspi arvense 1. results in the alteration of gibberellin (CAI metabolism such that the metabolism and turnover of the CA precursor ent-kaur-16-en-19-oic acid (kaurenoic acid) is dramatically increased. lhis cold-induced change in CA 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. l h e 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°C. l h e endogenous levels in shoot tips declined 10-fold by 2 d after the plants were returned to 21'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 ent7a-hydroxykaur-16-en-19-oic acid (7-OH kaurenoic acid). l h e 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. l h e 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. lhese results corroborate our hypothesis that the conversion of kaurenoic acid to 7-OH kaurenoic acid is the primary step in CA metabolism regulated by vernalization in Thlaspi shoot tips.Environmental factors such as daylength and temperature have an important role in the control of reproductive development in plants. Species requiring a period of low temperature (O-10°C) or vemalization before the initiation of flowering form a large and evolutionarily diverse group that includes members of the Gramineae, Cruciferae, Compositae, Chenopodiaceae, and Solanaceae. The change in developmental state induced by vemalization is referred to as ther-
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