Conditional nodule-level sanctions provide long-term stability to the Pea-Rhizobiumsymbiosis

Abstract

SummaryLegumes sanction root nodules containing rhizobial strains with low nitrogen fixation rates (ineffective fixers). Pea (Pisum sativum) nodules contain both undifferentiated bacteria and terminally differentiated nitrogen-fixing bacteroids. It is critical to understand whether sanctions act on both bacteria and bacteroids. As only undifferentiated bacteria are able to regrow from senescent nodules, if sanctioning only acts on bacteroids then ineffective strains might evade sanctioning. In addition, ineffective strains could potentially evade sanctioning by entering the same nodule as an effective strain i.e., piggybacking.P. sativumwas co-inoculated with pairwise combinations of three strains of rhizobia with different effectiveness, to test whether ineffective strains can evade sanctions in this way.We assessed the effect of sanctions on nodule populations of bacteria and bacteroids using flow cytometry and confocal microscopy.We show that sanctioning affects bacteroids and bacteria, altering survival of ineffective strains by reducing their numbers and hastening nodule senescence. In nodules containing two strains differing in their nitrogen-fixing ability, both were sanctioned equally. Thus, peas sanction whole nodules based on their nitrogen output, but do not sanction at the cellular level.Our results demonstrate that conditional sanctioning in peas provides long-term stability to symbiosis by effectively reducing fitness of ineffective strains.