Essential roles of nodule cysteine-rich peptides in maintaining the viability of terminally differentiated bacteroids in legume–rhizobia symbiosis

Abstract

Investigations into the nitrogen-fixing symbiosis between legumes and rhizobia can yield innovative strategies for sustainable agriculture. Legume species of the Inverted Repeat-Lacking Clade (IRLC) and the Dalbergioids, can utilize nodule cysteine-rich (NCR) peptides, a diverse family of peptides characterized by four or six highly conserved cysteine residues, to communicate with their microbial symbionts. These peptides, many of which exhibit antimicrobial properties, induce profound differentiation of bacteroids (semi-autonomous forms of bacteria) within nodule cells. This terminal differentiation endows the bacteroids with the ability to fix nitrogen, at the expense of their reproductive capacity. Notably, a significant number of NCR peptides is expressed in the nodule fixation zone, where the bacteroids have already reached terminal differentiation. Recent discoveries, through forward genetics approaches, have revealed that the functions of NCR peptides extend beyond antimicrobial effects and the promotion of differentiation. They also play a critical role in sustaining the viability of terminally differentiated bacteroids within nodule cells. These findings underscore the multifaceted functions of NCR peptides and highlight the importance of these peptides in mediating communications between host cells and the terminally differentiated bacteroids.