Symbiotic peptides modulate rhizobial physiology without terminal differentiation.

Abstract

Symbiotic nitrogen fixation reduces reliance on synthetic fertilizers and is central to agriculture and ecosystem functioning. In legumes, this process occurs in root nodules where rhizobia differentiate into bacteroids that either remain viable or undergo terminal differentiation, a strategy that can enhance nitrogen fixation but limits nodule life span. This irreversible program suits annual legumes and has evolved convergently across multiple lineages, including the inverted repeat-lacking clade (IRLC), where it is enforced by nodule-specific cysteine-rich (NCR) peptides. By contrast, perennial legumes with indeterminate nodules must sustain symbiosis over extended periods, which is incompatible with terminally differentiated bacteroids. Here, we identify a family of nodule-specific proline-glycine-rich peptides (NPGs) in <i>Robinia</i> that are induced upon rhizobial infection. NPGs are highly expressed in nodules, encode intrinsically disordered peptides, and accumulate in infected cells within the fixation zone. Exposure of <i>Mesorhizobium robiniae</i> to recombinant NPGs induces transcriptional changes associated with a fixation-related physiological state while preserving bacterial viability. These findings identify NPGs as candidate host effectors at the plant-microbe interface and point to an alternative mode of symbiont modulation in perennial legumes.