Taxonomically Different Co-Microsymbionts of a Relict Legume, Oxytropis popoviana, Have Complementary Sets of Symbiotic Genes and Together Increase the Efficiency of Plant Nodulation

Abstract

Ten rhizobial strains were isolated from root nodules of a relict legume Oxytropis popoviana Peschkova. For identification of the isolates, sequencing of rrs, the internal transcribed spacer region, and housekeeping genes recA, glnII, and rpoB was used. Nine fast-growing isolates were Mesorhizobium-related; eight strains were identified as M. japonicum and one isolate belonged to M. kowhaii. The only slow-growing isolate was identified as a Bradyrhizobium sp. Two strains, M. japonicum Opo-242 and Bradyrhizobium sp. strain Opo-243, were isolated from the same nodule. Symbiotic genes of these isolates were searched throughout the whole-genome sequences. The common nodABC genes and other symbiotic genes required for plant nodulation and nitrogen fixation were present in the isolate Opo-242. Strain Opo-243 did not contain the principal nod, nif, and fix genes; however, five genes (nodP, nodQ, nifL, nolK, and noeL) affecting the specificity of plant-rhizobia interactions but absent in isolate Opo-242 were detected. Strain Opo-243 could not induce nodules but significantly accelerated the root nodule formation after coinoculation with isolate Opo-242. Thus, we demonstrated that taxonomically different strains of the archaic symbiotic system can be co-microsymbionts infecting the same nodule and promoting the nodulation process due to complementary sets of symbiotic genes.

Search for Ancestral Features in Genomes of Rhizobium leguminosarum bv. viciae Strains Isolated from the Relict Legume Vavilovia formosa

“…The first report about isolation rhizobia from Vavilovia nodules [16] demonstrated a spectrum of microorganisms from Bosea, Tardiphaga, and Phyllobacterium genera. It was shown later that some rhizobia isolated from other relict legumes may harbor the incomplete sets of sym genes and can effectively infect their host only in the presence of other microsymbionts [65]. A similar type of complementation has been described firstly for different types of non-infective Sinorhizobium meliloti mutants [66].…”

Two Broad Host Range Rhizobial Strains Isolated From Relict Legumes Have Various Complementary Effects on Symbiotic Parameters of Co-inoculated Plants

“…It can also be assumed that products of different genes responsible for plant nodulation may interact with each other if they are secreted by bacteria into the external environment or localized on the outer membrane of cells. Acceleration of the root nodule formation has been previously observed in the symbiosis between the relict legume O. popoviana and its two co-microsymbionts, one of which could not induce nodules but contained nodPQ, nolK , and noeL genes that could affect the specificity of plant-rhizobia interactions via the sulfation and fucosylation of NFs (Safronova et al, 2018a).…”

“…Nine other strains belonging to M. japonicum and M. kowhaii were also isolated in our previous work from the nodules of O. popoviana plants (Safronova et al, 2018a). Among them the isolate Opo-235 was chosen as the object of this research and presented in Figure 1, having 100% rrs sequence similarity with the type strain M. japonicum MAFF 303099 T isolated from Lotus japonicus (Martínez-Hidalgo et al, 2016).…”

“…All strains were deposited in the Russian Collection of Agricultural Microorganisms (RCAM, WDCM 966) and stored at -80°C in the automated Tube Store (Liconic Instruments, Lichtenstein) as described previously (Safronova and Tikhonovich, 2012). Isolation of rhizobial strains from nodules of O. popoviana plants originated from the same region as A. chorinensis was carried out in our previous work using similar methods (Safronova et al, 2018a).…”

“…On the other hand, two rhizobial strains Mesorhizobium japonicum Opo-242 and Bradyrhizobium sp. Opo-243 isolated from the same nodule of a relict legume O. popoviana significantly accelerated the root nodule formation on the host plant after a combined inoculation (Safronova et al, 2018a). The whole genome sequence analysis of this pair of strains showed the presence in Opo-243 additional genes nodPQ, nolK , and noeL involved in the modification of Nod factors (NFs) and affecting the specificity of plant-rhizobia interactions.…”