Legume-imposed selection for more-efficient symbiotic rhizobia

Agriculture could reduce the economic and environmental costs of nitrogen (N) fertilizers by relying more on atmospheric N taken up (fixed) by rhizobia bacteria in legume root nodules. Just scaling up current N fixation might not help, however, because many nodules are occupied by rhizobia with high carbon (C) costs relative to N fixed. Given this low efficiency, greater plant investment in nodules can decrease yields (1). Even if we inoculate with more-efficient rhizobia, evolution can favor mutants that divert more resources from N fixation to their own reproduction (2). Could we instead breed legumes that preferentially support the reproduction and release into soil of only the most-efficient rhizobia in their nodules? If so, how much would this benefit future crops grown in the same soil? In PNAS, Westhoek et al. (3) present modeling and convincing results that expand our understanding of host-imposed selection among rhizobia differing in efficiency. Their methods could help plant breeders “select for legumes which are better at discriminating among strains” (3). Although their results are also relevant to broader questions about the evolution of cooperation, this commentary will focus on issues related to that suggested application. Previous modeling showed that, without host-imposed selection, rhizobia strains that invest anything in N fixation would have lower fitness than competing strains in the same plants that allocate resources only to their own reproduction. Fixing strains would only have greater fitness if hosts impose fitness-reducing “sanctions” on less-beneficial strains (2). All legumes tested appear to reduce allocation to nonfixing nodules (4⇓–6), although decreased allocation may not always limit rhizobia reproduction (7). Most … [↵][1]1Email: denis036{at}umn.edu. [1]: #xref-corresp-1-1