Cardiolipin synthesis in Pseudomonas fluorescens UM270 plays a relevant role in stimulating plant growth under salt stress.

Membrane cardiolipin (CL) phospholipids play a fundamental role in the adaptation of bacteria to various environmental conditions, including saline stress. Here, we constructed deletion mutants of two CL synthetase genes, clsA (UM270 ∆clsA) and clsB (UM270 ∆clsB), in the rhizobacterium Pseudomonas fluorescens UM270, and evaluated their role in plant growth promotion under salt stress. UM270 ∆clsA and UM270 ∆clsB mutants showed a significant reduction in CL synthesis compared to the P. fluorescens UM270 wild-type (UM270 wt) strain (58% ∆clsA and 53% ∆clsB), and their growth rate was not affected, except when grown at 100 and 200 mM NaCl. Additionally, the root colonization capacity of both mutant strains was impaired compared with that of the wild type. Concomitant with the deletion of clsA and clsB genes, some physiological changes were observed in the UM270 ∆clsA and UM270 ∆clsB mutants, such as a reduction in indole acetic acid and biofilm production. By contrast, an increase in siderophore biosynthesis was observed. Further, inoculation of the UM270 wt strain in tomato plants (Solanum lycopersicum) grown under salt stress conditions (100 and 200 mM NaCl) resulted in an increase in root and shoot length, chlorophyll content, and dry weight. On the contrary, when each of the mutants were inoculated in tomato plants, a reduction in root length was observed when grown at 200 mM NaCl, but the shoot length, chlorophyll content, and total plant dry weight parameters were significantly reduced under normal or saline conditions (100 and 200 mM NaCl), compared to UM270 wt-inoculated plants. In conclusion, these results suggest that CL synthesis in P. fluorescens UM270 plays an important role in the promotion of tomato plant growth under normal conditions, but to a greater extent, under salt-stress conditions.

Arsenic and mercury tolerant rhizobacteria that can improve phytoremediation of heavy metal contaminated soils.

Background: Mining deposits often contain high levels of toxic elements such as mercury (Hg) and arsenic (As) representing strong environmental hazards. The purpose of this study was the isolation for plant growth promoting bacteria (PGPBs) that can improve phytoremediation of such mine waste deposits. Methods: We isolated native soil bacteria from the rhizosphere of plants of mine waste deposits and agricultural land that was previously mine tailings from Tlalpujahua Michoacán, Mexico, and were identified by their fatty acid profile according to the MIDI Sherlock system. Plant growth promoting traits of all bacterial isolates were examined including production of 3-indoleacetic acid (IAA), siderophores, biofilm formation, and phosphate solubilization. Finally, the response of selected bacteria to mercury and arsenic was examined an in-vitro assay. Results: A total 99 bacterial strains were isolated and 48 identified, representing 34 species belonging to 23 genera. Sixty six percent of the isolates produced IAA of which Pseudomonas fluorescens TL97 produced the most. Herbaspirillum huttiense TL36 performed best in terms of phosphate solubilization and production of siderophores. In terms of biofilm formation, Bacillus atrophaeus TL76 was the best. Discussion: Most of the bacteria isolates showed high level of tolerance to the arsenic (as HAsNa2O4 and AsNaO2), whereas most isolates were susceptible to HgCl2. Three of the selected bacteria with PGP traits Herbispirillum huttiense TL36, Klebsiella oxytoca TL49 and Rhizobium radiobacter TL52 were also tolerant to high concentrations of mercury chloride, this might could be used for restoring or phytoremediating the adverse environmental conditions present in mine waste deposits.

Bacillus toyonensis COPE52 Modifies Lipid and Fatty Acid Composition, Exhibits Antifungal Activity, and Stimulates Growth of Tomato Plants Under Saline Conditions.

Salinity is one of the most important factors that limit the productivity of agricultural soils. Certain plant growth-promoting bacteria (PGPB) have the ability to stimulate the growth of crop plants even under salt stress. In the present study, we analysed the potential of PGPB Bacillus toyonensis COPE52 to improve the growth of tomato plants and its capacity to modify its membrane lipid and fatty acid composition under salt stress. Thus, strain COPE52 increased the relative amount of branched chain fatty acids (15:0i and 16:1∆9) and accumulation of an unknown membrane lipid, while phosphatidylethanolamine (PE) levels decreased during growth with 100 and 200 mM NaCl. Importantly, direct and indirect plant growth-promoting (PGP) mechanisms of B. toyonensis COPE52, such as indole-3-acetic acid (IAA), protease activity, biofilm formation, and antifungal activity against Botrytis cinerea, remained unchanged in the presence of NaCl in vitro, compared to controls without salt. In a greenhouse experiment, tomato plants (Lycopersicon esculentum 'Saladette') showed increased shoot and root length, higher dry biomass, and chlorophyll content when inoculated with B. toyonensis COPE52 at 0 and 100 mM NaCl. In summary, these results indicate that Bacillus toyonensis COPE52 can modify cell membrane lipid components as a potential protecting mechanism to maintain PGP traits under saline-soil conditions.

Draft genome analysis of the endophyte, Bacillus toyonensis COPE52, a blueberry (Vaccinium spp. var. Biloxi) growth-promoting bacterium.

In this work, we report an analysis of the draft genome of the blueberry (Vaccinium spp. var. Biloxi) growth-promoting endophyte Bacillus toyonensis, strain COPE52. The genome of COPE52 consists of a single 5,806,513 bp replicon, with a 35.1% G + C content. Strain COPE52 was strongly affiliated to B. toyonensis species, based on species delimitation cut-off values established for average nucleotide identity (> 95-96%), genome-to genome distance calculator (> 70%) and phylogenomic analysis. The RAST genomic annotation of the COPE52 strain revealed a total of 5979 total genes, including 5631 protein-coding genes, 11 rRNA genes, 5 ncRNAs, 81 tRNA genes, and 251 pseudogenes. To further validate the in silico analysis results, experiments were carried out to detect the production of indoleacetic acid, protease activity, and the emission of volatiles like acetoin, 2,3-butanediol and dimethyl disulphide as potential plant growth-promoting mechanisms. COPE52 also showed antifungal action against the grey mould phytopathogen, Botrytis cinerea, during in vitro bioassays. In addition, inoculation with strain COPE52 promoted growth biomass and chlorophyll content in blueberry plants (Vaccinium spp. var. Biloxi) under greenhouse conditions. To our knowledge, this is the first study showing genomic and experimental evidence of B. toyonensis as plant growth-promoting bacteria (PGPB).

Data on the effect of Pseudomonas stutzeri E25 and Stenotrophomonas maltophilia CR71 culture supernatants on the mycelial growth of Botrytis cinerea.

Plant growth-promoting bacterial endophytes (PGPBEs) produce volatile and diffusible compounds that inhibit phytopathogens (Santoyo et al., 2016) [1]. A recent work by Rojas-Solis and colleagues [2] demonstrated the antifungal effect of volatile organic compounds exerted by the Pseudomonas stutzeri E25 and Stenotrophomonas maltophilia CR71 endophytes, highlighting the production of sulfur-containing compounds such as the antimicrobial volatile dimethyl disulfide (DMDS). The data presented in this article include the effect of two culture supernatants from the same strains, E25 and CR71, on the mycelial growth of the gray mold phytopathogen Botrytis cinerea. These data may help to further evaluate the specific compounds produced by endophyte isolates E25 and CR71 with antifungal activity. This article is submitted as a companion paper to Rojas-Solís et al. (2018) [2].