The Genomic and Phenotypic Characterization of the Sym2A Introgression Line A33.18 of Pea (Pisum sativum L.) with the Increased Specificity of Root Nodule Symbiosis

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In pea (Pisum sativum L.), alleles of the Sym2 gene determine the specificity of the interaction with nodule bacteria (rhizobia). The Sym2A allele present in landraces from Afghanistan provides higher selectiveness toward rhizobia than the Sym2E allele present in European cultivars. Rhizobial strains possessing the nodX gene can interact with both Sym2A and Sym2E peas, while strains lacking nodX can interact only with Sym2E peas. Here, we studied the previously obtained introgression line A33.18 bearing Sym2A in a homozygous state in the genome of the European pea cultivar ‘Rondo’. A33.18 has proved its high selectiveness in pot experiments. Genome sequencing has shown that A33.18 possesses an 18.2 Mb region inherited from Afghanistan pea with 63 genes, including 5 receptor kinase genes, among which was the Sym2 candidate gene LykX. In a field experiment, under inoculation with the nodX+ strain TOM, over 95% of nodules of A33.18 contained TOM, as opposed to less than 8% of nodules containing TOM in the parental European cultivar ‘Rondo’. Thus, introgression of Sym2A enabled peas to interact specifically with the nodX+ strain, favoring the formation of nodules by the strain from the inoculum and protecting peas from the indigenous soil microbiota.