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

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. Sanctions are predicted to work at the whole-nodule level. However, whole-nodule sanctions would make the host vulnerable to mixed-nodule infections, which have been demonstrated in the laboratory and observed in natural settings. Here, we present and test a cell-autonomous model of legume sanctions that can resolve this dilemma.

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 abilities 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. In plants that received mixed inoculations, only the plant cells harboring ineffective rhizobia exhibited features consistent with programmed cell death, including collapsed vacuoles, ruptured symbiosomes, and bacteroids that are released into the cytosol. These features were consistently linked with ultrastructural evidence of reduced survival of ineffective rhizobia in planta.

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.