Abstract
Within legume root nodules, rhizobia differentiate into bacteroids, which reduce N2 into NH3 for secretion to the plant. Bacteroids may be swollen and terminally differentiated or non-swollen and can regenerate outside nodules. It is unclear why these different endosymbiotic lifestyles exist and whether they differ in symbiotic efficiency. Here, we compared N2 fixing bacteroids of the near isogenic strains Rhizobium leguminosarum bv. phaseoli 4292 (Rlp4292) and R. leguminosarum bv. viciae A34 (RlvA34), nodulating Phaseolus vulgaris (common bean) and Pisum sativum (pea), respectively. The larger bean plants fixed more N2, but peas fixed 1.6-3-fold more per unit nodule mass. Values per unit volume were similar between bean and pea because bean nodules are 2.7-fold denser (i.e. mass per unit volume). Bean nodules have higher numbers of smaller (∼1/5 the volume) bacteroids than peas. Bean bacteroids are denser (i.e. 2.5-fold protein per unit volume) although less closely packed than pea bacteroids (i.e. more space between bean bacteroids). Critically, pea bacteroids fix N2 at higher rates versus bean per unit bacteroid protein, as protein expression is skewed toward N2 fixation and TCA-cycle enzymes. Pea bacteroids infect 1.6 times the percentage of nodule volume of beans (i.e. 14.2% versus 9.1%). Overall, the increased packing density of pea bacteroids, as well as the bias of their proteome to nitrogenase, associated N2 fixation processes, and dicarboxylate metabolism, contributes to their greater symbiotic efficiency, which is likely driven by plant antimicrobial peptides.