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• FO12 did not exert any evident pathogenic effects on eight herbaceous hosts. • FO12 promoted plant growth in cotton, cucumber, pepper, sunflower, and wheat. • FO12 colonized the root and stem tissues of all herbaceous hosts as an endophyte. • Confocal laser scanning microscopy imaging showed that FO12 root colorization progressed. The nonpathogenic strain Fusarium oxysporum FO12 has been characterized as an effective biological control agent against Verticillium dahliae in olive. Nevertheless, the endophytic role of FO12 in herbaceous hosts is still uncertain. Thus, the aim of this study was to demonstrate the nonpathogenic character of FO12 in eight V. dahliae -susceptible herbaceous hosts and to evaluate its ability to promote plant growth. To this end, plants of each host were grown in pots filled with sterile peat moss and inoculated by irrigation with a conidial suspension of FO12 (106 conidia ml−1) two times—at sowing and at the emergence of the first true leaves. Many plants irrigated with sterile distilled water were included as controls. Sampling was conducted at 7, 14, 21, and 28 days after the second inoculation. The fresh weight and length of the aerial parts, roots, and whole plants were determined at each sampling time. Fungal colonization was assessed via quantitative and qualitative methods, including microbiological isolation from roots and stems and confocal laser scanning microscopy (CLSM) imaging at each sampling time. FO12 exhibited the ability to endophytically colonize the roots and stems of the inoculated plants. However, the effect on growth and the degree of colonization varied depending on the host. FO12 did not exert any evident pathogenic effects or negative effects on plant growth or development. The growth of the eggplant, lettuce, and tomato plants in the FO12 treatment group was similar to that in the control group, whereas the relative area under the growth progress curve was significantly greater than that in the control group for plant length in cotton and cucumber; for root fresh weight in pepper, sunflower, and wheat; and for plant fresh weight in sunflower and wheat. These results highlight the need to continue researching the interaction between FO12 and V. dahliae in a wider range of plant species of agronomic interest. [Display omitted] [ABSTRACT FROM AUTHOR]

Azospirillum sp. is a plant growth-promoting rhizobacteria largely recognized for its potential to increase the yield of different important crops. In this work, we present a thorough genomic and phenotypic analysis of A. argentinense Az39T to provide new insights into the beneficial mechanisms of this microorganism. Phenotypic analyses revealed the following in vitro abilities: growth at 20–38 °C (optimum, 28 °C), pH 6.0–8.0 (optimum, pH 6.8), and in the presence of 1% (w/v) NaCl; production of variable amounts of PHB as intracellular granules; nitrogen fixation under microaerophilic conditions; IAA synthesis in the presence of L-tryptophan. Through biochemical (API 20NE) and carbon utilization profiling (Biolog) assays, we proved that A. argentinense Az39T is able to use 15 substrates and metabolize 19 different carbon substrates. Lipid composition indicated a predominance of medium and long-chain saturated fatty acids. A total of 6 replicons classified as one main chromosome, three chromids, and two plasmids, according to their tRNA and core essential genes contents, were identified. Az39T genome includes genes associated with multiple plant growth-promoting (PGP) traits such as nitrogen fixation and production of auxins, cytokinin, abscisic acid, ethylene, and polyamines. In addition, Az39T genome harbor genetic elements associated with physiological features that facilitate its survival in the soil and competence for rhizospheric colonization; this includes motility, secretion system, and quorum sensing genetic determinants. A metadata analysis of Az39T agronomic performance in the pampas region, Argentina, demonstrated significant grain yield increases in wheat and maize, proving its potential to provide better growth conditions for dryland cereals. In conclusion, our data provide a detailed insight into the metabolic profile of A. argentinense Az39T, the strain most widely used to formulate non-legume inoculants in Argentina, and allow a better understanding of the mechanisms behind its field performance. [ABSTRACT FROM AUTHOR]

Salinity is among the major environmental factors affecting the performance of both crop plants and soil bacterial inoculants beneficial to plant growth. Based on previous screening studies, a novel halotolerant biofilm-forming PGPR strain, namely Pantoea agglomerans -FAP10, was studied for its potential to protect wheat (Triticum aestivum ; var. 343) against salinity stress. The ability of this strain to produce biofilms on glass surfaces, 96 well microtiter plates, and seedling roots was characterized qualitatively and quantitatively using light and scanning electron microscopy (SEM). The FAP10 strain was tested for performance by inoculating wheat in a pot-soil system under varied salinity stresses (75, 125, 250, and 500 mM NaCl). The FAP10 strain exhibited discrete and multifarious plant growth-promoting traits as well as efficient rhizosphere and root colonization which could sustain wheat growth under salinity stress. The FAP10-induced modifications conferred enhanced plant salinity tolerance by regulating photosynthetic attributes (gs, Ci, E, iWUE, P N , PSII, and Rubisco enzyme activity), antioxidant system (SOD, CAT, GR, APX, GSH, MDA, and proline), and sulfur metabolism (sulfur and cysteine content, ATP-S, and SAT activity), and also sustained soil physicochemical characteristics and hydrolytic enzymes including urease, protease, DHA, ALP, ACP and β glucosidase. The findings of the current study can support future efforts to improve plant salinity tolerance by engineering rhizobacterial biofilms and exploiting the potential of native strains for agricultural applications in stressed environments. • Pantoea agglomerans FAP10 a novel biofilm forming and salt tolerant PGPR strain protects inoculated wheat plants against salinity stress • The impact of treatment of FAP10 on wheat plant showed restoration of physiological attributes to normal level from the salinity stress level. • Role of biofilm in effective rhizoplane and rhizosphere colonization by FAP10 strain further supported protection mechanisms against salinity stress and improved soil hydrolytic enzymes • Findings of this study strongly support future efforts in the management of plant salinity stress with the development of effective bioinoculant for agricultural use. [ABSTRACT FROM AUTHOR]

Salinity ranks among the most severe environmental stressors that limit crop productivity. Use of microbial inoculants with desired traits is believed to be effective strategy to combat plant abiotic stress. Therefore, the objective was to isolate salt tolerant Bacillus spp. with multifunctional plant growth promoting traits including biofilm development and to evaluate its performance under salt stress conditions. We isolated and characterized a novel salt tolerant isolate of Bacillus sp. FAB10 with multifunctional traits by screening of 56 selected Bacillus isolates from rhizospheric soils. The isolate FAB10 was identified as B. pumilus based on 16S rRNA gene sequence analysis. The FAB10 isolate produced strong biofilm, enhanced amount of exopolysaccharides, IAA, ACC-deaminase activity and solubilized phosphate in vitro. The isolate FAB10 forms biofilm and expressed its associated traits at the different NaCl concentrations in vitro. Biofilm development on Wheat root surface was also demonstrated at 250 mM salt (NaCl) concentration. Successful root colonization by the FAB10 isolate was demonstrated via scanning electron microscopy and viable counts. Wheat plant var. 343 inoculated with Bacillus pumilus isolate FAB10 grown in the presence of different concentrations (0 to 250 mM) of NaCl under pot house conditions. At elevated concentration of NaCl adverse effect on wheat growth parameters and other biochemical attributes pertaining to photosynthesis, transpiration, and content of proline in plant tissue was recorded in uninoculated plants. However, inoculated plants showed a significant (p < 0.005) improvement in most of the above parameters. Similarly a significant (p < 0.005) reduction in antioxidant enzyme activities (catalase, superoxide dismutase, and glutathione reductase) and malonaldehyde content in wheat was observed in FAB10 inoculated plants than uninoculated plants in the presence of salt. The findings indicated that multifunctional traits of the FAB10 contribute to NaCl stress alleviation in wheat plants through multiple mode of action and it could be exploited under field condition. • Screening of rhizospheric Bacillus isolates resulted in selection of novel salt tolerant and biofilm forming strain of Bacillus pumilus FAB10 exhibiting multifunctional PGP traits. • The strain FAB10 colonized rhizosphere and formed biofilm on the root surface in the presence of NaCl upto 250 mM. • Modulation of plant physiological attributes and antioxidant enzymes level were observed by FAB10 inoculation under saline condition. • B. pumilus FAB10 could be effectively used for salt stress alleviation in wheat plant under pot house condition. [ABSTRACT FROM AUTHOR]

High soil nitrate concentrations can lead to the secondary salinization of soils. Bacillus megaterium NCT-2 is a wild-type strain isolated from secondary salinized soil and is very effective in reducing nitrate. Laboratory and greenhouse experiments were carried out to investigate its nitrate reduction capabilities, colonization pattern, and plant growth promotion responses to nitrate content in the soil. B. megaterium NCT-2 was marked with a green fluorescent protein ( gfp ) gene and was left to successfully colonize maize roots and the rhizosphere. Inoculation with gfp -tagged NCT-2 significantly promoted nitrate removal from the soil and improved plant growth. Confocal microscopy results revealed that NCT-2 is an endophyte that can colonize the meristematic and elongation zones of the root tip, and the middle segment of the root. Soil nitrate concentration had no significant effect on NCT-2 distribution. The gfp -tagged NCT-2 populations in the roots and rhizosphere soil first increased, but then decreased, and at the end of the experiment, colonization levels in the rhizosphere soil stabilized at ∼5 × 10 4  CFU g −1 soil. However, the levels in the roots increased again to 1–3 × 10 4  CFU g −1 root in the different treatments. The NCT-2 population in the roots was significantly affected by nitrate content. A nitrate-nitrogen concentration of 72 mg kg −1 was the optimum concentration for NCT-2 colonization of maize roots. This study will improve the agricultural application of NCT-2 as a biofertilizer for nitrate removal and plant growth promotion. [ABSTRACT FROM AUTHOR]

Verticillium wilt caused by a highly virulent, defoliating (D) pathotype of Verticillium dahliae is threatening olive production in Spain and other Mediterranean countries. This disease must be managed by an integrated strategy, in which biocontrol agents can play an important role. We have investigated the potential of Trichoderma asperellum strains for antagonism against V . dahliae and suppression of Verticillium wilt of olive caused by the D pathotype. First, we tested the antagonistic potential of T . asperellum strains Bt2, Bt3 and T25 against six V . dahliae isolates, four of the D and two of the nondefoliating (ND) pathotypes, in different in vitro assays. All T . asperellum strains overgrew the colonies of all V . dahliae isolates to a similar extent. However, extracellular compounds from strains Bt3 and T25 showed higher anti- V . dahliae activities than those of Bt2 in membrane assays. Also, growth of Bt2 was reduced by ND V . dahliae whereas that of Bt3 and T25 was not affected by V . dahliae -secreted compounds. In planta assays using strains Bt3 and T25, and ’Picual’ olive plants, showed that the two T . asperellum strains significantly reduced the severity of symptoms and the standardized area under the disease progress curve caused by highly virulent D V . dahliae , but not the final disease incidence. Strain T25 significantly increased growth of ‘Picual’ plants and displayed higher ability for colonizing the olive rhizosphere and establishing endophytic infection in olive roots than Bt3. [ABSTRACT FROM AUTHOR]

Summary: Unlike most bacteria, the nitrogen-fixing rice-associated Pseudomonas stutzeri A15 disposes of three different nitrate reductases that enable conversion of nitrate to nitrite through three physiologically distinct processes, called nitrate assimilation, nitrate respiration and nitrate dissimilation. To study the role of nitrate respiration in rhizosphere fitness, a Pseudomonas stutzeri narG mutant was constructed and characterized by assessing its growth characteristics and whole-cell nitrate reductase activity in different oxygen tensions. Unexpectedly, the Pseudomonas stutzeri A15 narG mutant appeared to be a better root colonizer, outcompeting the wild type strain in a wheat and rice hydroponic system. [Copyright &y& Elsevier]

Summary: Five bacterial strains with phosphate-solubilizing ability and other plant growth promoting traits increased the plant biomass (20–40%) by paper towel method. Glasshouse and field experiments were conducted using two efficient strains Serratia marcescens EB 67 and Pseudomonas sp. CDB 35. Increase in plant biomass (dry weight) was 99% with EB 67 and 94% with CDB 35 under glasshouse conditions. Increase in plant biomass at 48 and 96 days after sowing was 66% and 50% with EB 67 and 51% and 18% with CDB 35 under field conditions. Seed treatment with EB 67 and CDB 35 increased the grain yield of field-grown maize by 85% and 64% compared to the uninoculated control. Population of EB 67 and CDB 35 were traced back from the rhizosphere of maize on buffered rock phosphate (RP) medium and both the strains survived up to 96 days after sowing. [Copyright &y& Elsevier]

Abstract: Isolates of Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are effective biocontrol agents against soilborne pathogens. A previous study showed that the superior (“premier”) root colonizer P. fluorescens Q8r1-96 (genotype D) utilized trehalose, benzoate and valerate as sole carbon sources but average colonizers Q2-87 (genotype B) and 1M1-96 (genotype L) did not. We tested the utilization of these three carbon sources by a collection of 55 2,4-DAPG-producing P. fluorescens strains from 17 genotypes and found no correlation between a strain''s ability to utilize these carbon sources and superior rhizosphere competence on wheat and pea. Of the strains tested, 73%, 48% and 69% were able to utilize trehalose, benzoate and valerate as sole carbon sources, respectively. With some exceptions, we found a correlation between the utilization of these compounds and previous groupings of these strains by BOX-PCR; genotype D strains utilized all three compounds. Twenty-three strains grew efficiently on root and seed exudates from wheat and pea, with doubling times between 0.9 and 1.6h generation−1 and lag phases between 5 and 8h, comparable to growth on glucose as a sole carbon source. Only 10 strains, including those with “premier” (Q8r1-96) and “average” (Q2-87) rhizosphere competence, showed slower growth in wheat root exudates, with lag phases between 16 and 22h. Results were the same when soil was added to the culture medium. Growth of four strains in media containing glucose or wheat or pea seed exudates as a sole carbon source was not influenced by whether the bacterial cells used as inoculum were harvested from wheat seeds or broth culture. We conclude that the superior ability of some strains to colonize the roots of certain crops cannot be explained by the utilization of the carbon sources tested in our study. [Copyright &y& Elsevier]

Abstract: Biological control of soil-borne pathogens is frequently based on the application of antagonistic microorganisms selected solely for their ability to produce in vitro antifungal factors. The aim of this work was to select bacteria that efficiently colonize the roots of avocado plants and display antagonism towards Rosellinia necatrix, the causal agent of avocado white root rot. A high frequency of antagonistic strains (ten isolates, 24.4%) was obtained using a novel procedure based on the selection of competitive avocado root tip colonizers. Amplification and sequencing of the 16S rRNA gene, in combination with biochemical characterization, showed that eight and two of the selected isolates belonged to the genera Pseudomonas and Stenotrophomonas, respectively. Characterization of antifungal compounds produced by the antagonistic strains showed variable production of exoenzymes and HCN. Only one of these strains, Pseudomonas sp. AVO94, produced a compound that could be related to antifungal antibiotics. All of the ten selected strains showed twitching motility, a cell movement involved in competitive colonization of root tips. Production of N-acyl-homoserine lactones and indole-3-acetic acid was also reported for some of these isolates. Resistance to several bacterial antibiotics was tested, and three strains showing resistance to only one of them were selected for biocontrol assays. The three selected strains persisted in the rhizosphere of avocado plants at levels considered crucial for efficient biocontrol, 105–106 colony forming units/g of root; two of them, Pseudomonas putida AVO102 and Pseudomonas pseudoalcaligenes AVO110, demonstrated significant protection of avocado plants against white root rot. [Copyright &y& Elsevier]

Since sugarcane is a ratoon crop, genome analysis of plant growth-promoting bacteria that exist in its soil rhizosphere, can provide opportunity to better understand their characteristics and use of such bacteria in turn, may especially improve perennial crop productivity. In the present study, genome of two bacterial strains, one each of B. megaterium (BM89) and B. subtilis (BS87), isolated and reported earlier (Chandra et al., 2018), were sequenced and characterized. Though both strains have demonstrated plant growth promoting properties and enhanced in-vitro plant growth responses, functional annotation and analysis of genes indicated superiority of BS87 as it possessed more plant growth promotion attributable genes over BM89. Apart from some common genes, trehalose metabolism, glycine betaine production, peroxidases, super oxide dismutase, cold shock proteins and phenazine production associated genes were selectively identified in BS87 genome indicating better plant growth performances and survival potential under harsh environmental conditions. Genes for chitinase, d -cysteine desulfhydrase and γ-aminobutyric acid (GABA), as found in BM89, propose its selective utilization in defense and bio-control measures. Concomitant with better settlings' growth, scanning electron micrographs indicated these isolated and characterized bacteria exhibiting healthy colonization within root of sugarcane crop. Kegg pathways' assignment also revealed added pathways namely carbohydrate and amino acid metabolism attached to B. subtilis strain BS87, a preferable candidate for bio-fertilizer and its utilization to promote growth of both plant and ratoon crops of sugarcane usually experiencing harsh environmental conditions. [ABSTRACT FROM AUTHOR]

Pseudomonas chlororaphis subsp. aurantiaca strain JD37 is a plant growth-promoting rhizobacterium (PGPR), which has important biotechnological features such as plant growth promotion, rhizosphere colonization and biocontrol activities. In present study, the genome sequence of JD37 was obtained and comparative genomic analysis were performed to explore unique features of the JD37 genome and its relationship with other Pseudomonas PGPR: P. chlororaphis PA23, P. protegens Pf-5 and P. aeruginosa M18. JD37 possessed a single circular chromosome of 6,702,062 bp in length with an average GC content of 62.75 %. No plasmid was detected in JD37. A total of 5003 functional proteins of JD37 were predicted according to the clusters of orthologous groups (COGs) database. The JD37 genome consisted of various genes involved in plant growth promotion, biocontrol activities and defense responses. Genes involved in the rhizosphere colonization and motility were also found in the genome of JD37, suggesting the common plant growth-promoting traits in PGPR. The identified resistance genes (e.g. those related to metal resistance, antibiotics, and osmotic and temperature-shock) and secondary metabolite biosynthesis revealed the pathways for metabolites it produced. Data presented in present study further provided valuable information on its molecular genetics and adaptive capacity in the rhizosphere niche. [ABSTRACT FROM AUTHOR]