Early action of pea symbiotic gene NOD3 is confirmed by adventitious root phenotype

A supernodulating and Nts (nitrate-tolerant symbiosis) symbiotic mutation of pea (Pisum sativum L.) line RisfixC was found to retain its expression in the distant genetic background of pea lines Afghanistan L1268, Zhodino E900, and cv. Arvika. This finding allowed for reliable scoring for the trait in mapping crosses. The RisfixC mutation was localized 8.2 cM apart from SYM2 and cosegregated with molecular markers for SYM2–NOD3 region Psc923 and OA-1. Grafting experiments showed that supernodulation is root-determined, consistently with mutants in the NOD3 locus. Therefore, the mutation of RisfixC can be localized in gene NOD3. Like in other published nod3 alleles, the RisfixC mutation determines supernodulation when it is expressed in the root but not in the shoot. Supernodulated adventitious roots that are spontaneously formed in the wild-type scions on mutant rootstocks indicate that the descending systemic signal, which is inhibitory to nodule formation, is absent in this type of chimeric plants. Since the descending signal production in the wild-type shoot reflects the presence of the ascending root signal, the nod3-associated lesion must be located in the beginning of the systemic circuit regulating nodule number.

Research highlights▶ Supernodulation/Nts mutation of pea line RisfixC maps to the NOD3 locus. ▶ Consistent with other nod3 alleles, the phenotype is determined by the root. ▶ The phenotype of adventitious roots formed in the scions correspond to the rootstock. ▶ Nodule number regulation is affected in the beginning of the systemic circuit.