The Department of Microbiology and Plant Pathology's weekly Plant Pathology 250 seminar series is presented this week by Dr. Joel Sachs, from UC Riverside.

Seminar Title: “Rapid evolutionary shifts from mutualist to cheater in plant-associated bacteria”

Abstract: Bacterial symbioses generate major fitness benefits for eukaryotic hosts, reshaping the host phenotype and its interactions with the environment. Yet symbiont populations are predicted to generate mutants that defect from providing costly services to hosts while maintaining the capacity to exploit host resources. We examined the mutualist service of symbiotic nitrogen fixation in a metapopulation of root-nodulating Bradyrhizobium spp. that associate with the native legume Acmispon strigosus. We quantified symbiosis traits of Bradyrhizobium isolates gathered from a 700km transect in California spanning ten sampled A. strigosus populations. We clonally inoculated each Bradyrhizobium isolate onto Acmispon strigosus hosts and quantified nodulation capacity and net effects of infection, including host growth and isotopic nitrogen concentration. Phylogenetic reconstruction inferred multiple losses of symbiotic effectiveness and nodulation capacity, consistent with coevolutionary conflict between symbiont exploitation and host control.

To maximize benefits from symbiosis, legumes must limit physiological inputs into ineffective rhizobia that nodulate hosts without fixing nitrogen. The capacity of legumes to decrease the relative fitness of ineffective rhizobia – known as sanctions – has been demonstrated in several legume species, but its mechanisms remain unclear. We analyzed histological and ultrastructural evidence of sanctions in two legume species, Acmispon strigosus and Lotus japonicus. For the former, we inoculated seedlings with rhizobia that naturally vary in their ability to fix nitrogen. In the latter, we inoculated seedlings with near-isogenic strains that differ only in the ability to fix nitrogen. In both hosts, plants inoculated with ineffective rhizobia exhibited evidence for a cell autonomous and accelerated program of senescence within nodules. Our data suggest an elegant cell autonomous mechanism by which legumes can detect and defend against ineffective rhizobia even when nodules harbor a mix of effective and ineffective rhizobial genotypes.

Seminar Host: Dr. Jason Stajich; jason.stajich@ucr.edu