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 production, transport, and accumulation of ureides are regulated in common bean is not understood, yet 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 vary 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 reciprocal grafts of these four genotypes. The greatest accumulation of nitrogen was in leaves, while least accumulation was observed in petioles. Ureide content and total N were greater in R32, Puebla and Eagle (nodulating) than in R99 (non-nodulating). R32 (super-nodulating) scions greatly increased nodulation when grafted onto normal nodulating rootstocks.  Conversely, grafting normal scions onto R32 roots suppressed nodulation. Grafting normal scions onto R99 rootstocks has no impact on nodulation, total ureide content, or total nitrogen accumulation. In general,  tissue ureide and total N content were positively associated, but increasing nodule number did not result in greater accumulation of tissue N or ureides.  Certain scion-root stock combinations, however, had a positive impact on N accumulation per nodule, compared to non-grafted or self-grafted controls.  These results suggest shoot regulation of nodulation and nodule effectiveness for ureide production can be manipulated to improve BNF and biomass accumulation in common bean.