Shoot and Root Control of Ureide Accumulation and Partitioning in Phaseolus vulgaris L.

Ureides are manufactured predominantly in legume root nodules and are the primary form of organic N transported to the shoots in common bean (Phaseolus vulgaris L.).  How the production, transport, and accumulation of ureides are regulated in common bean is not understood, but is a fundamental aspect of improving nitrogen fixation in this crop. We used a grafting technique to determine shoot and/or root control of ureide accumulation and partitioning among four genotypes of common beans that have been noted for variation in nitrogen fixation related phenotypic traits (R99, Eagle, Puebla, and R32,). Effect of shoot and/or root on ureide accumulation and partitioning was verified by analyzing differences in root, stem, leaf, and pod biomass, ureide concentration, nitrogen concentration, and nodule numbers among intra-species grafts of these four genotypes. The greatest accumulation of nitrogen was in leaves. Ureide content and total N were greater in R32, Puebla and Eagle (nodulating) than in R99 (non-nodulating). Super-nodulation was observed when R32 (a super-nodulator) scion was grafted onto normal nodulating rootstocks.  Conversely, grafting normal scions onto R32 roots suppressed nodulation. Grafting normal scions onto R99 rootstocks did not affect nodulation, total ureide content, or total nitrogen. Extent of nodulation only indirectly affected ureide and N accumulation, as increasing nodule number did not result in greater accumulation of nitrogen. These results suggest shoot regulation of nodulation, ureide metabolism, and nodule effectiveness would be ideal physiological targets for further investigations aimed at improving BNF and yield in common bean.