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

In pea (<i>Pisum sativum</i> L.), alleles of the <i>Sym2</i> gene determine the specificity of the interaction with nodule bacteria (rhizobia). The <i>Sym2^A</i> allele present in landraces from Afghanistan provides higher selectiveness toward rhizobia than the <i>Sym2^E</i> allele present in European cultivars. Rhizobial strains possessing the <i>nodX</i> gene can interact with both <i>Sym2^A</i> and <i>Sym2^E</i> peas, while strains lacking <i>nodX</i> can interact only with <i>Sym2^E</i> peas. Here, we studied the previously obtained introgression line A33.18 bearing <i>Sym2^A</i> 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 <i>Sym2</i> candidate gene <i>LykX</i>. In a field experiment, under inoculation with the <i>nodX⁺</i> 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 <i>Sym2^A</i> enabled peas to interact specifically with the <i>nodX⁺</i> strain, favoring the formation of nodules by the strain from the inoculum and protecting peas from the indigenous soil microbiota.