Appraising the potential of EPS-producing rhizobacteria with ACC-deaminase activity to improve growth and physiology of maize under drought stress.

Rhizobacteria containing 1-aminocyclopropane-1-carboxylic acid-deaminase (ACC-deaminase) and exopolysaccharides (EPS) activity are important to induce stress tolerance in plants. The present study was conducted to screen and characterize plant growth-promoting rhizobacteria (PGPR) with ACC-deaminase and EPS-producing activity for improving maize growth under drought stress. Eighty-five rhizobacterial strains were isolated from the rain-fed areas, among those 69 isolates were able to utilize ACC and 31 strains were found positive for EPS production. These strains containing ACC-deaminase and/or EPS-producing activity were subjected to drought tolerance assay by inducing water stress in media using polyethylene glycol 6000. Based on results of the drought tolerance bioassay, 12 most prominent strains were selected to evaluate their growth-promoting abilities in maize under water-stressed conditions by conducting jar trial. The impact of strains on maize growth parameters was variable. Strains with co-existence of ACC-deaminase and EPS-producing activity showed comparatively better results than those with either ACC-deaminase or EPS-producing activity only. These strains were also significantly better in improving the plant physiological parameters including photosynthesis rate, stomatal conductance, vapor pressure, water-use efficiency and transpiration rate. The strain D3 with co-existence of ACC-deaminase and EPS-producing activity was significantly better in colonizing maize roots, improving plant growth and physiological parameters. The strain was named as Bacillus velezensis strain D3 (accession number MT367633) as confirmed through results of 16S rRNA partial gene sequencing. It is concluded that the strains with co-existence of ACC-deaminase and EPS-producing activity could be better suited for improving crop growth and physiology under drought stress.

Preliminary study on phosphate solubilizing Bacillus subtilis strain Q3 and Paenibacillus sp. strain Q6 for improving cotton growth under alkaline conditions.

BACKGROUND: Low phosphorus availability limits crop production in alkaline calcareous soils in semi-arid regions including Pakistan. Phosphate solubilizing bacteria may improve crop growth on alkaline calcareous soils due to their ability to enhance P availability. METHODS: Twenty rhizobacterial isolates (Q1-Q20) were isolated from rhizosphere of cotton and characterized for their growth promoting attributes in vitro. The selected phosphate solubilizing isolates were further screened for their ability to improve cotton growth under axenic conditions (jar trial). The phosphorus solubilization capacities of selected strains were quantified and these strains were identified through 16S rDNA sequencing. RESULTS: Isolates Q2, Q3, Q6, Q7, Q8, Q13 and Q14 were able to solubilize phosphate from insoluble sources. Most of these isolates also possessed other traits including catalase activity and ammonia production. The growth promotion assay showed that Q3 was significantly better than most of the other isolates followed by Q6. Maximum root colonization (4.34 × 106 cfu g-1) was observed in case of isolate Q6 followed by Q3. The phosphorus solubilization capacities of these strains were quantified, showing a maximum phosphorus solubilization by Q3 (optical density 2.605 ± 0.06) followed by the Q6 strain. The strain Q3 was identified as Bacillus subtilis (accession # KX788864) and Q6 as Paenibacillus sp. (accession # KX788865) through 16S rDNA sequencing. DISCUSSION: The bacterial isolates varied in their abilities for different growth promoting traits. The selected PGPR Bacillus subtilis strain Q3 and Paenibacillus sp. strain Q6 have multifarious growth promoting traits including ability to grow at higher EC and pH levels, and phosphorus solubilizing ability. These strains can efficiently colonize cotton roots under salt affected soils and help plants in phosphorus nutrition. It is concluded that both strains are potential candidates for promoting cotton growth under alkaline conditions, however further investigation is required to determine their potential for field application.

Synergistic use of biochar, compost and plant growth-promoting rhizobacteria for enhancing cucumber growth under water deficit conditions.

BACKGROUND: Limited information is available about the effectiveness of biochar with plant growth-promoting rhizobacteria (PGPR) and compost. A greenhouse study was conducted to evaluate the effect of biochar in combination with compost and PGPR (Pseudomonas fluorescens) for alleviating water deficit stress. Both inoculated and un-inoculated cucumber seeds were sown in soil treated with biochar, compost and biochar + compost. Three water levels - field capacity (D0), 75% field capacity (D1) and 50% field capacity (D2) - were maintained. RESULTS: The results showed that water deficit stress significantly suppressed the growth of cucumber; however, synergistic use of biochar, compost and PGPR mitigated the negative impact of stress. At D2, the synergistic use of biochar, compost and PGPR caused significant increases in shoot length, shoot biomass, root length and root biomass, which were respectively 88, 77, 89 and 74% more than in the un-inoculated control. Significant improvements in chlorophyll and relative water contents as well as reduction in leaf electrolyte leakage demonstrated the effectiveness of this approach. Moreover, the highest population of P. fluorescens was observed where biochar and compost were applied together. CONCLUSION: These results suggest that application of biochar with PGPR and/or compost could be an effective strategy for enhancing plant growth under stress. © 2017 Society of Chemical Industry.

Plant growth-promoting rhizobacteria associated with ancient clones of creosote bush (Larrea tridentata).

Plant growth-promoting rhizobacteria (PGPR) are common components of the rhizosphere, but their role in adaptation of plants to extreme environments is not yet understood. Here, we examined rhizobacteria associated with ancient clones of Larrea tridentata in the Mohave desert, including the 11,700-year-old King Clone, which is oldest known specimen of this species. Analysis of unculturable and culturable bacterial community by PCR-DGGE revealed taxa that have previously been described on agricultural plants. These taxa included species of Proteobacteria, Bacteroidetes, and Firmicutes that commonly carry traits associated with plant growth promotion, including genes encoding aminocyclopropane carboxylate deaminase and ß-propeller phytase. The PGPR activities of three representative isolates from L. tridentata were further confirmed using cucumber plants to screen for plant growth promotion. This study provides an intriguing first view of the mutualistic bacteria that are associated with some of the world's oldest living plants and suggests that PGPR likely contribute to the adaptation of L. tridentata and other plant species to harsh environmental conditions in desert habitats.