Cell autonomous sanctions in legumes target ineffective rhizobia in nodules with mixed infections

Legumes versus rhizobia: a model for ongoing conflict in symbiosis

Contents Summary 1199 I. Introduction 1199 II. Selecting beneficial symbionts: one problem, many solutions 1200 III. Control and conflict over legume nodulation 1201 IV. Control and conflict over nodule growth and senescence 1204 V. Conclusion 1204 Acknowledgements 1205 References 1205 SUMMARY: The legume-rhizobia association is a powerful model of the limits of host control over microbes. Legumes regulate the formation of root nodules that house nitrogen-fixing rhizobia and adjust investment into nodule development and growth. However, the range of fitness outcomes in these traits reveals intense conflicts of interest between the partners. New work that we review and synthesize here shows that legumes have evolved varied mechanisms of control over symbionts, but that host control is often subverted by rhizobia. An outcome of this conflict is that both legumes and rhizobia have evolved numerous traits that can improve their own short-term fitness in this interaction, but little evidence exists for any net improvement in the joint trait of nitrogen fixation.

Early nodule senescence is activated in symbiotic mutants of pea (Pisum sativum L.) forming ineffective nodules blocked at different nodule developmental stages

Plant symbiotic mutants are useful tool to uncover the molecular-genetic mechanisms of nodule senescence. The pea (Pisum sativum L.) mutants SGEFix--1 (sym40), SGEFix--3 (sym26), and SGEFix--7 (sym27) display an early nodule senescence phenotype, whereas the mutant SGEFix--2 (sym33) does not show premature degradation of symbiotic structures, but its nodules show an enhanced immune response. The nodules of these mutants were compared with each other and with those of the wild-type SGE line using seven marker genes that are known to be activated during nodule senescence. In wild-type SGE nodules, transcript levels of all of the senescence-associated genes were highest at 6 weeks after inoculation (WAI). The senescence-associated genes showed higher transcript abundance in mutant nodules than in wild-type nodules at 2 WAI and attained maximum levels in the mutant nodules at 4 WAI. Immunolocalization analyses showed that the ethylene precursor 1-aminocyclopropane-1-carboxylate accumulated earlier in the mutant nodules than in wild-type nodules. Together, these results showed that nodule senescence was activated in ineffective nodules blocked at different developmental stages in pea lines that harbor mutations in four symbiotic genes.

Symbiotic nitrogen fixation by rhizobia-the roots of a success story

By evolving the dual capacity of intracellular survival and symbiotic nitrogen fixation in legumes, rhizobia have achieved an ecological and evolutionary success that has reshaped our biosphere. Despite complex challenges, including a dual lifestyle of intracellular infection separated by a free-living phase in soil, rhizobial symbiosis has spread horizontally to hundreds of bacterial species and geographically throughout the globe. This symbiosis has also persisted and been reshaped through millions of years of history. Here, we summarize recent advances in our understanding of the molecular mechanisms, ecological settings, and evolutionary pathways that are collectively responsible for this symbiotic success story. We offer predictions of how this symbiosis can evolve under new influences and for the benefit of a burgeoning human population.

Spatio-temporal control of mutualism in legumes helps spread symbiotic nitrogen fixation

Mutualism is of fundamental importance in ecosystems. Which factors help to keep the relationship mutually beneficial and evolutionarily successful is a central question. We addressed this issue for one of the most significant mutualistic interactions on Earth, which associates plants of the leguminosae family and hundreds of nitrogen (N2)-fixing bacterial species. Here we analyze the spatio-temporal dynamics of fixers and non-fixers along the symbiotic process in the Cupriavidus taiwanensis-Mimosa pudica system. N2-fixing symbionts progressively outcompete isogenic non-fixers within root nodules, where N2-fixation occurs, even when they share the same nodule. Numerical simulations, supported by experimental validation, predict that rare fixers will invade a population dominated by non-fixing bacteria during serial nodulation cycles with a probability that is function of initial inoculum, plant population size and nodulation cycle length. Our findings provide insights into the selective forces and ecological factors that may have driven the spread of the N2-fixation mutualistic trait.

Lotus japonicus alters in planta fitness of Mesorhizobium loti dependent on symbiotic nitrogen fixation

Rhizobial bacteria are known for their capacity to fix nitrogen for legume hosts. However ineffective rhizobial genotypes exist and can trigger the formation of nodules but fix little if any nitrogen for hosts. Legumes must employ mechanisms to minimize exploitation by the ineffective rhizobial genotypes to limit fitness costs and stabilize the symbiosis. Here we address two key questions about these host mechanisms. What stages of the interaction are controlled by the host, and can hosts detect subtle differences in nitrogen fixation? We provide the first explicit evidence for adaptive host control in the interaction between Lotus japonicus and Mesorhizobium loti. In both single inoculation and co-inoculation experiments, less effective rhizobial strains exhibited reduced in planta fitness relative to the wildtype M. loti. We uncovered evidence of host control during nodule formation and during post-infection proliferation of symbionts within nodules. We found a linear relationship between rhizobial fitness and symbiotic effectiveness. Our results suggest that L. japonicus can adaptively modulate the fitness of symbionts as a continuous response to symbiotic nitrogen fixation.