Abstract

Bradyrhizobium japonicum bacteroids isolated from root nodules of soybean (Glycine max.) plants converted the gibberellin (GA) precursor [C-14(1)]GA(12) into several products identified by combined gas chromatography-mass spectrometry as [C-14(1)]GA(24), [C-14(1)]GA(9), [C-14(1)]GA(15), GA(9) 17-nor-16-one and unidentified products. The oxidation of GA(12), catalyzed by the GA 20-oxidase, was present in symbiotic bacteroids from plants around flowering, but not in bacteroids from plants at either an early vegetative stage or at late growth stages. Expression of cps and ks genes, involved in ent-kaurene biosynthesis, was also demonstrated in bacteroids from soybean plants around flowering. Earlier precursors of the GA pathway, ent-[C-14(1)]kaurenoic acid or [C-14(4)]GA(12)-aldehyde, were efficiently utilized by B. japonicum bacteroids to give labelled GA(9) plus intermediates partially oxidized at C-20, as well as GA(9) 17-nor-16-one and an unidentified product. No 3 beta or 13-hydroxylated [C-14]GAs were detected in any of the incubations. Moreover the C-19-GAs [C-14(1)]GA(4) or [C-14(1)]GA(20) were recovered unconverted upon incubation with the bacteroids which supports the absence of GA 3 beta-hydroxylase activity in B. japonicum. The bacterial 20-oxidase utilized the 13-hydroxylated substrates [C-14(1)]GA(53),[C-14(1)]GA(44) or [C-14(1)]GA(19), although with less efficiency than [C-14(1)]GA(12) to give [C-14(1)]GA(20) as final product, while the 3 beta-hydroxylated substrate [C-14(1)]GA(14) was converted to [C-14(1)]GA(4) to a very small extent. Endogenous GA9 and GA(24) were identified by GC-MS in methanolic nodule extracts. These results suggest that B. japonicum bacteroids would synthesize GA(9) under the symbiotic conditions present in soybean root nodules. (C) 2013 Elsevier Ltd. All rights reserved.