TheMedicago truncatulanodule-specific cysteine-rich peptides, NCR343 and NCR-new35 are required for the maintenance of rhizobia in nitrogen-fixing nodules

Abstract

SummaryIn the nodules of Inverted Repeat-Lacking Clade legumes, includingM. truncatula, nitrogen-fixing rhizobia undergo terminal differentiation resulting in elongated and endoreduplicated bacteroids specialised for nitrogen fixation. This irreversible transition of rhizobia is mediated by host produced nodule-specific cysteine-rich (NCR) peptides, of which about 700 are encoded in theM. truncatulagenome. Some of these NCR peptides, NCR169, NCR211 and NCR247, are essential for nitrogen-fixing symbiosis.The analysis of bacteroid and symbiotic host cell differentiation revealed that the symbiotic phenotype ofM. truncatulamutants,Mtsym19,Mtsym20and NF-FN9363, were defective likewise in the formerly studiedncrmutants,Mtdnf4-1andMtdnf7-2. The incomplete differentiation of bacteroids triggered premature senescence of rhizobia in the nitrogen fixation zones of mutant nodules.Mtsym19andMtsym20mutants are defective in the same peptide NCR-new35 and the lack ofNCR343is responsible for the ineffective symbiosis of NF-FN9363.The activity ofNCR-new35is significantly lower and limited to the transition zone of the nodule compared with other crucialNCRs. The fluorescent protein-tagged version of NCR343 and NCR-new35 localize to the symbiotic compartment. Our discovery added two additional members to the group ofNCRgenes essential for nitrogen–fixing symbiosis inM. truncatula.