The role of Nodule Cysteine Rich secreted peptides in controlling nodulation

Legumes house nitrogen-fixing endosymbiotic rhizobia in specialised polyploid cells within root nodules. The model legume Medicago truncatula, one of the members of the inverted repeat lacking clade (IRLC) of legumes is known to have a class of multi-gene family encoding for NCRs (Nodule Cysteine Rich peptides). Until now, studies have suggested that this large family acts as antimicrobial defensins keeping the rhizobial population in balance. However, these functional insights come from only a few of more than 500 NCRs that actually have diversified spatio-temporal expression. Due to their occurrence in large numbers from genome amplification, it is possible that these NCR genes have evolved gain of novel functions beyond just bacterial regulation. Microarray transcriptomic data and promoter sequences revealed six novel conserved promoter motifs that are over-represented in subsets of NCRs that are regulated in different ways by combinations of rhizobia and nitrogen. Our hypothesis is that the NCR promoter motifs may be acting as transcription factor (TF) binding sites for the regulation of nodulation. Using the Arabidopsis TF database, we identified CCA1, RVE1, ATHB15, ATHB16, AHL20 and AHL25 as putative TF regulators. Possible existence of a nodule circadian clock in the Medicago truncatula-rhizobium symbiosis has been investigated and altogether our results support an expanded role of NCRs in signalling and nodule development during symbiosis