PvUPS1 plays a role in source–sink transport of allantoin in French bean (Phaseolus vulgaris)

Nodulated tropical legumes such as French bean (Phaseolus vulgaris L.) receive their nitrogen via N-fixing rhizobia. The principal products of fixed nitrogen are the ureides allantoin and allantoic acid that are synthesised in root nodules and then translocated to the mature leaves of the shoot via the xylem. By feeding 14Callantoin to mature leaves and roots of French bean plants we showed that this ureide is transported over long distances by xylem and then phloem to developing organs such as pods, root tips and growing leaves. For analysis of allantoin partitioning within the plant, concentrations of allantoin in French bean organs and xylem sap were determined. The amounts of allantoin varied between organs, with the highest levels being detected in the stems. Differences in levels of allantoin were found between nodulated and non-nodulated plants, with generally higher allantoin concentrations in tissues and xylem sap of nodulated plants. RNA and protein expression of the recently identified French bean allantoin permease PvUPS1 (AY461734) was detected in all plant organs indicating a function in allantoin transport throughout the plant. The levels of PvUPS1 expression were consistent with the allantoin concentrations in the different organs. In situ RNA hybridisation studies were carried out and showed that PvUPS1 is expressed in the phloem throughout the plant. Together, our results indicate that in French bean allantoin is transported from source to sink and that PvUPS1 plays a role in phloem loading and in allantoin transport to developing sinks.