Thermoinductive Regulation of Gibberellin Metabolism in Thlaspi arvense L. (II. Cold Induction of Enzymes in Gibberellin Biosynthesis)

Abstract

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-

Chilling of Dormant Buds HyperinducesFLOWERING LOCUS Tand Recruits GA-Inducible 1,3-β-Glucanases to Reopen Signal Conduits and Release Dormancy inPopulus

“…Possibly, GA production is a downstream effect of chilling, as proposed for dormant seeds (Hazebroek et al, 1993;Yamauchi et al, 2004). Our finding that chilling of dormant buds increased the expression of GA biosynthesis genes ( Figures 6A and 6E) is in support of this.…”

“…Gibberellic acids (GAs) are particularly important, as they may function in the timing of dormancy establishment and chilling induced release (Hazebroek et al, 1993;Zanewich and Rood, 1995;Schrader et al, 2004). Interestingly, in both vernalization and dormancy release, GA application may substitute for chilling (Saure, 1985), although in the case of dormancy results have been equivocal.…”

Arabidopsis ent-Kaurene Oxidase Catalyzes Three Steps of Gibberellin Biosynthesis

“…After growing overnight, 0.5 mL of each culture was removed; the yeast was pelleted and resuspended in 0.5 mL of a reaction mixture containing 100 mm Tris-HCl pH 7.5, 0.5 mm NADPH, and 0.5 mm FAD, according to the method of Hazebroek et al (1993). The substrates added were 25 g of ent-kaurene, 5 g of entkaurenol, or 20 g of [17,17-2 H 2 ]-ent-kaurenal.…”

Regulation of Arabidopsis flower development.

“…A wealth of studies in numerous plant species have implicated the hormone gibberellin as an important regulator of flowering and flower development in higher plants (reviewed by Zeevaart, 1983;Kinet et al, 1985;Chailakhyan and Khrianin, 1987;Bernier et al, 1993, this issue). Moreover, gibberellin levels can be affected by changes in both photoperiod and temperature due to changes in the activities of specific enzymes in the gibberellin biosynthetic pathways (Metzger and Zeevaart, 1980;1982;Gilmour et al, 1986;Hazebroek and Metzger, 1990;Hazebroek et al, 1993). One clue that gibberellin might be involved in the physiological control of floral induction and inflorescence development in Arabidopsis comes from a recent study by Wilson et al (1992), who showed that flowering can be completely suppressed in the gibberellin biosynthesis mutant gal under short-day conditions.…”