Under water deficit, ureidic legumes accumulate ureides in plant tissues, and this accumulation has been correlated with the inhibition of nitrogen fixation. In this work we used a molecular approach to characterize ureide accumulation under drought stress in Phaseolus vulgaris. Accumulation of ureides, mainly allantoate, was found in roots, shoots and leaves, but only a limited transient increase was observed in nodules from drought-stressed plants. We show that ureide accumulation is regulated at the transcriptional level mainly through induction of allantoinase (ALN), whereas allantoate amidohydrolase (AAH), involved in allantoate degradation, was slightly reduced, indicating that inhibition of this enzyme, key in ureide breakdown in aerial tissues, is not the main cause of allantoate accumulation. Expression of the ureide metabolism genes analysed in this study was induced by abscisic acid (ABA), suggesting the involvement of this plant hormone in ureide accumulation. Moreover, we observed that increases of ureide levels in P. vulgaris drought-stressed tissues were similar in non-nodulated, nitrate-fed plants, and in plants cultured under nitrogen-fixation conditions. Our results indicate that ureide accumulation in response to water deficit is independent from de novo synthesis of ureides in nodules, and therefore uncoupled from nitrogen fixation.
© 2010 Blackwell Publishing Ltd.