Molecular Mechanisms Determining the Role of Bacteria from the Genus Azospirillum in Plant Adaptation to Damaging Environmental Factors

Agricultural plants are continuously exposed to environmental stressors, which can lead to a significant reduction in yield and even the death of plants. One of the ways to mitigate stress impacts is the inoculation of plant growth-promoting rhizobacteria (PGPR), including bacteria from the genus Azospirillum, into the rhizosphere of plants. Different representatives of this genus have different sensitivities or resistances to osmotic stress, pesticides, heavy metals, hydrocarbons, and perchlorate and also have the ability to mitigate the consequences of such stresses for plants. Bacteria from the genus Azospirillum contribute to the bioremediation of polluted soils and induce systemic resistance and have a positive effect on plants under stress by synthesizing siderophores and polysaccharides and modulating the levels of phytohormones, osmolytes, and volatile organic compounds in plants, as well as altering the efficiency of photosynthesis and the antioxidant defense system. In this review, we focus on molecular genetic features that provide bacterial resistance to various stress factors as well as on Azospirillum-related pathways for increasing plant resistance to unfavorable anthropogenic and natural factors.

Two New Species of Filamentous Sulfur Bacteria of the Genus Thiothrix, Thiothrix winogradskyi sp. nov. and ‘Candidatus Thiothrix sulfatifontis’ sp. nov

The metagenome of foulings from sulfidic spring “Serovodorodny” (Tatarstan, Russia), where members of the genus Thiothrix was observed, was sequenced. Representatives of the phyla Gammaproteobacteria, Cyanobacteria and Campilobacteriota dominated in the microbial community. The complete genome of Thiothrix sp. KT was assembled from the metagenome. It displayed 93.93–99.72% 16S rRNA gene sequence identity to other Thiothrix species. The average nucleotide identity (ANI) и digital DNA-DNA hybridization (dDDH) showed that the genome designated KT represents a new species within the genus Thiothrix, ‘Candidatus Thiothrix sulfatifontis’ sp. nov. KT. The taxonomic status has been determined of the strain Thiothrix sp. CT3, isolated about 30 years ago and not assigned to any of Thiothrix species due to high 16S rRNA gene sequence identity with related species (i.e., 98.8–99.4%). The complete genome sequence of strain CT3 was determined. The ANI between CT3 and other Thiothrix species was below 82%, and the dDDH values were less than 40%, indicating that strain CT3 belongs to a novel species, Thiothrix winogradskyi sp. nov. A genome analysis showed that both strains are chemo-organoheterotrophs, chemolithotrophs (in the presence of hydrogen sulfide and thiosulfate) and chemoautotrophs. For the first time, representatives of Thiothrix showed anaerobic growth in the presence of thiosulfate.

Reclassification of Sphaerotilus natans subsp. sulfidivorans Gridneva et al. 2011 as Sphaerotilus sulfidivorans sp. nov. and comparative genome analysis of the genus Sphaerotilus

Filamentous iron oxides accumulating bacteria Sphaerotilus natans subsp. natans and S. natans subsp. sulfidivorans were described as subspecies based on 99.7% identity of their 16S rRNA sequences, in spite of important physiological difference. The ANI between their genomes was 94.7%, which indicate their assignment to different species. S. natans subsp. sulfidivorans and S. montanus possess genes for a complete SOX system, while S. natans subsp. natans encode only SoxYZ. There are genes for the Calvin cycle in the genomes of S. hippei DSM 566^T, S. natans subsp. sulfidivorans D-501^T, and S. montanus HS^T. Lithoautotrophy on reduced sulfur compounds is probably possible for S. natans subsp. sulfidivorans and S. montanus, but not for S. natans subsp. natans. Considering significant differences in the genome characteristics and metabolic potential of S. natans subsp. sulfidivorans and S. natans subsp. natans, we propose their classification as different species, S. natans and S. sulfidivorans sp. nov.

The role of the "Thiodendron" consortium in postulating the karyomastigont chimaera of the endosymbiosis theory by Lynn Margulis

The endosymbiosis theory of the origin of eukaryotic cell was first proposed more than a hundred years ago. In the second half of the 20th century, Lynn Margulis suggested a new interpretation of the origin of the nucleus in modern eukaryotes. The background was the study of the consortium "Thiodendron", a symbiotic bacterial community, which includes anaerobic aerotolerant motile spirochaetes and sulfidogenic bacteria (sulfidogens) of vibrioid form with a fermentation type of metabolism. Spirochaetes supply sulfidogens with metabolites (pyruvate and, probably, organic nitrogenous products of cell lysis) and get hydrogen sulfide from sulfidogens that helps to maintain a low redox potential. At low oxygen concentrations, spirochaetes are able to assimilate glucose more efficiently. Margulis hypothesized about the symbiotic origin of the nucleus by adding the bacterium Spirochaeta to the Thermoplasma-like archaea. She considered the "Thiodendron"-like consortium to be an intermediate stage in evolution. According to Margulis, the conversion of carbohydrates and the oxidation of Н₂S to S⁰ by the bacterium provided the archaea with electron acceptors for anaerobic respiration, as shown for modern thermoplasmas and products saturated with carbon. The use of carbon sources increased by attaching the floating bacterium to the archaea. More efficient microaerobic oxidation of glucose pre-adapted the spirochaetes for association with Thermoplasma. However, modern "Thiodendron"-like consortia are not in stable symbiosis and a sulfidogenic component of the consortium is capable for fermentation, rather than anaerobic respiration, which makes the theory by Margulis disputable.

Thioflexithrix psekupsensis gen. nov., sp. nov., a filamentous gliding sulfur bacterium from the family Beggiatoaceae

A sulfur-oxidizing, filamentous, gliding micro-organism, strain D3^T, was isolated from a sulfidic spring in Goryachy Klyuch, Krasnodar, Russia. The cell walls were Gram-negative. The new isolate was a microaerophilic facultative anaerobe and an obligate chemolithoautotroph. The pH range for growth was pH 6.8-7.6, with an optimum at pH 7.2. The temperature range for growth was 10-46 °C, with an optimum at 32 °C. The G+C content of DNA was 42.1 mol%. Phylogenetic analysis of the 16S rRNA gene showed that strain D3^T belongs to the family Beggiatoaceae, order Thiotrichales and was distantly related to the genera of the family Beggiatoaceae(86-88 % sequence similarity). The major respiratory quinone was ubiquinone-6. Major fatty acids were C18:1 ω7 (37.6 %), C16 : 0 (34.7 %) and C16: 1 ω7 (27.7 %). On the basis of its physiological properties and the results of phylogenetic analysis, strain D3^T is considered to represent a novel species of a new genus, for which the name Thioflexithrix psekupsensis gen. nov., sp. nov. is proposed. The type strain is D3^T (=KCTC 62399=UNIQEM U981).