Inoculation and nitrate alter phytohormone levels in soybean roots: differences between a supernodulating mutant and the wild type

The levels of different cytokinins. indole-3-acetic acid (IAA) and abscisic acid (ABA) in roots of Glycine max [L.] Merr. cv. Bragg and its supermodul ating mutant nts382 were compared for the first time. Forty-eight hours aft er inoculation with Bradyrhizobium, quantitative and qualitative difference s were found in the root's endogenous hormone status between cultivar Bragg and the mutant nts382. The six quantified cytokinins, ranking similarly in each genotype, were present at higher concentrations (30-196% on average f or isopentenyl adenosine and dihydrozeatin riboside, respectively) in mutan t roots. By contrast, the ABA content was 2-fold higher in Bragg, while the basal levels of IAA [0.53 mu mol (g DW)(-1), on average] were similar in b oth genotypes. In 1 mM NO3--fed Bragg roots 48 h post-inoculation, IAA, ABA and the cytokinins isopentenyl adenine, and isopentenyl adenosine quantita tively increased with respect to uninoculated controls. However, only the t wo cytokinins increased in the mutant. High NO3- (8 mM) markedly reduced ro ot auxin concentration, and neither genotypic differences nor the inoculati on-induced increase in auxin concentration in Bragg was observed under thes e conditions. Cytokinins and ABA, on the other hand, Were little affected b y 8 mM NO3-. Root IAA/cytokinin and ABA/cytokinin ratios were always higher in Bragg relative to the mutant, and responded to inoculation (mainly in B ragg) and nitrate (both genotypes). The overall results are consistent with the auxin-burst-control hypothesis for the explanation of autoregulation a nd supernodulation in soybean. However, they are still inconclusive with re spect to the inhibitory effect of NO3-.