New sources of Sym2(A) allele in the pea (Pisum sativum L.) carry the unique variant of candidate LysM-RLK gene LykX

At the onset of legume-rhizobial symbiosis, the mutual recognition of partners occurs based on a complicated interaction between signal molecules and receptors. Bacterial signal molecules named Nod factors (“nodulation factors”) are perceived by the plant LysM-containing receptor-like kinases (LysM-RLKs) that recognize details of its structure (i.e., unique substitutions), thus providing the conditions particular to symbiosis. In the garden pea (Pisum sativum L.), the allelic state of Sym2 gene has long been reported to regulate the symbiotic specificity: for infection to be successful, plants with the Sym2(A) allele (for “Sym2 Afghan”, as these genotypes originate mostly from Afghanistan) require an additional acetylation of the Nod factor which is irrelevant for genotypes with the Sym2(E) allele (for “Sym2 European”). Despite being described about 90 years ago, Sym2 has not yet been cloned, though phenotypic analysis suggests it probably encodes a receptor for the Nod factor. Recently, we described a novel pea gene LykX (PsLykX) from the LysM-RLK gene family that demonstrates a perfect correlation between its allelic state and the symbiotic specificity of the Sym2(A)-type. Here we report on a series of Middle-Eastern pea genotypes exhibiting the phenotype of narrow symbiotic specificity discovered in the VIR plant genetic resources gene bank (Saint-Petersburg, Russia). These genotypes are new sources of Sym2(A), as has been confirmed by an allelism test with Sym2(A) pea cv. Afghanistan. Within these genotypes, LykX is present either in the allelic state characteristic for cv. Afghanistan, or in another, minor allelic state found in two genotypes from Tajikistan and Turkmenistan. Plants carrying the second allele demonstrate the same block of rhizobial infection as cv. Afghanistan when inoculated with an incompatible strain. Intriguingly, this “Tajik” allele of LykX differs from the “European” one by a single nucleotide polymorphism leading to an R75P change in the receptor part of the putative protein. Thus, our new data are in agreement with the hypothesis concerning the identity of LykX and the elusive Sym2 gene.