AUTHOR=Safronova Vera , Belimov Andrey , Sazanova Anna , Chirak Elizaveta , Kuznetsova Irina , Andronov Evgeny , Pinaev Alexander , Tsyganova Anna , Seliverstova Elena , Kitaeva Anna , Tsyganov Viktor , Tikhonovich Igor 

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

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00514

DOI=10.3389/fmicb.2019.00514

ISSN=1664-302X

ABSTRACT=<p>Two bacterial strains Ach-343 and Opo-235 were isolated, respectively from nodules of Miocene-Pliocene relict legumes <italic>Astragalus chorinensis</italic> Bunge and <italic>Oxytropis popoviana</italic> Peschkova originated from Buryatia (Baikal Lake region, Russia). For identification of these strains the sequencing of 16S rRNA (<italic>rrs</italic>) gene was used. Strain Opo-235 belonged to the species <italic>Mesorhizobium japonicum</italic>, while the strain Ach-343 was identified as <italic>M. kowhaii</italic> (100 and 99.9% <italic>rrs</italic> similarity with the type strains MAFF 303099<sup>T</sup> and ICMP 19512<sup>T</sup>, respectively). Symbiotic genes of these strains as well as some genes that promote plant growth (<italic>acdS</italic>, gibberellin- and auxin-synthesis related genes) were searched throughout the whole genome sequences. The sets of plant growth-promoting genes found were almost identical in both strains, whereas the sets of symbiotic genes were different and complemented each other with several <italic>nod</italic>, <italic>nif</italic>, and <italic>fix</italic> genes. Effects of mono- and co-inoculation of <italic>Astragalus sericeocanus</italic>, <italic>Oxytropis caespitosa</italic>, <italic>Glycyrrhiza uralensis</italic>, <italic>Medicago sativa</italic>, and <italic>Trifolium pratense</italic> plants with the strains <italic>M. kowhaii</italic> Ach-343 and <italic>M. japonicum</italic> Opo-235 expressing fluorescent proteins mCherry (red) and EGFP (green) were studied in the gnotobiotic plant nodulation assay. It was shown that both strains had a wide range of host specificity, including species of different legume genera from two tribes (Galegeae and Trifolieae). The effects of co-microsymbionts on plants depended on the plant species and varied from decrease, no effect, to increase in the number of nodules, nitrogen-fixing activity and plant biomass. One of the reasons for this phenomenon may be the discovered complementarity in co-microsymbionts of symbiotic genes responsible for the specific modification of Nod-factors and nitrogenase activity. Localization and co-localization of the strains in nodules was confirmed by the confocal microscopy. Analysis of histological and ultrastructural organization of <italic>A. chorinensis</italic> and <italic>O. popoviana</italic> root nodules was performed. It can be concluded that the strains <italic>M. kowhaii</italic> Ach-343 and <italic>M. japonicum</italic> Opo-235 demonstrate lack of high symbiotic specificity that is characteristic for primitive legume-rhizobia systems. Further study of the root nodule bacteria having complementary sets of symbiotic genes will contribute to clarify the evolutionary paths of legume-rhizobia relationships and the mechanisms of effective integration between partners.</p>