Managing the specificity of legume-rhizobial symbiosis to increase the efficiency of nitrogen fixation

A team of researchers from the All-Russia Research Institute of Agricultural Microbiology (ARRIAM) led by Dr. Vladimir A. Zhukov, in collaboration with scientists from the Sirius University of Science and Technology and the Federal Research Center “N.I. Vavilov All-Russian Institute of Plant Genetic Resources” (VIR) studied the problem of the specificity of interaction between common pea (Pisum sativum L.) and nodule bacteria and demonstrated the possibility of managing this specificity to increase pea yields under conditions of using biopreparations containing nodule bacteria.

Modern pea varieties are capable of interacting with a wide range of strains of nodule bacteria that inhabit the soils of the Russian Federation, but these strains are characterized by low efficiency of nitrogen fixation. Selected strains used in new generation biopreparations, on the contrary, effectively fix nitrogen, but lose the competition with native strains when introduced into the soil. The solution to this problem may be the approach proposed by Academician of the Russian Academy of Sciences, Professor Igor A. Tikhonovich: artificial narrowing of the specificity of interaction of peas with nodule bacteria so that only certain strains can penetrate the nodules. Wild varieties of peas from Afghanistan have exactly this property – they are able to interact with a very narrow range of strains of nodule bacteria. This feature is determined by the allelic state of the Sym2 gene, encoding the receptor of signaling molecules secreted by bacteria. Previously, the introgression line A33.18 was obtained, carrying the "Afghan" allele of the Sym2 gene from wild pea in the genome of the European cultivar Rondo. In the present study, the genome of this line was sequenced and it was found that the A33.18 line inherited an 18.2 Mb fragment containing 63 genes, including Sym2, from the “Afghan pea”. In a field experiment, plants of the original cv. Rondo and the introgression line A33.18 were inoculated with the TOM strain of nodule bacteria capable of penetrating the nodules of the “Afghan pea”. Using a specially developed test system based on PCR and restriction digest, it was found that the presence of the “Afghan” Sym2 allele in a homozygous state in A33.18 ensures penetration of the TOM strain used for inoculation into 95% of the nodules, while in plants of the original cv. Rondo, the TOM strain was found in only 7% of the nodules, and the remaining nodules were formed by native bacterial strains. Thus, the possibility of manipulating the specificity of symbiosis has been experimentally confirmed, allowing to protect plants from native microflora and ensure the formation of nodules with highly effective strains introduced as part of biopreparations.

Further work will be aimed at developing a technology for genomic editing of the Sym2 gene sequence, as well as at developing approaches to marker-mediated selection to impart the property of high specificity of symbiosis to modern pea varieties.

The results of the study are published in the journal Plants: https://www.mdpi.com/2223-7747/14/3/427.