Salt-tolerant plant growth-promoting Pseudomonas atacamensis KSS-6 in combination with organic manure enhances rice yield, improves nutrient content and soil properties under salinity stress.

In the current study salt tolerant-plant growth-promoting rhizobacteria (ST-PGPR) Pseudomonas atacamensis KSS-6, selected on the basis of prominent plant growth-promoting (PGP) and stress tolerance properties was tested as bioinoculant to improve yield of rice grown in saline soil. The ST-PGPR KSS-6 was capable of maintaining the PGP traits up to 200 mM NaCl, however, higher salt stress conditions affected these activities. The study was designed to determine the effect of developed talc-based bioformulation using KSS-6 along with organic manure (OM) on growth and yield of paddy under saline conditions. Bioformulation broadcasting was also done to examine the effect on soil properties. It was found that the combinatorial treatment showed positive impact on growth and yield of rice under saline conditions. Co-application of KSS-6 with OM showed maximum increment in growth, chlorophyll content, plant fresh weight, and dry weight as compared to untreated control plants. Furthermore, the combinatorial treatment improved the nutrient content (P, K, Zn, Fe, Mg, and Mn) by more than 35% and enhanced the biochemical parameters such as proline, flavonoids, carbohydrates, protein, dietary fiber, and antioxidant content of rice grains by more than 32%. Soil parameters including pH and electrical conductivity (EC), moisture content, total organic carbon, OM, sodium, and chloride ions were also improved upon treatment. There was significant lowering of EC from 7.43 to 4.3 dS/m when combination of OM and bacteria were applied. These findings suggest that the application of KSS-6 in the form of bioinoculant could be a promising strategy to mitigate negative impacts of salt stress and enhance the yield and nutritional properties of rice grown in degraded and saline soil.

Salt-Tolerant Plant Growth Promoting Rhizobacteria for Enhancing Crop Productivity of Saline Soils.

Soil salinity has emerged as a serious issue for global food security. It is estimated that currently about 62 million hectares or 20 percent of the world's irrigated land is affected by salinity. The deposition of an excess amount of soluble salt in cultivable land directly affects crop yields. The uptake of high amount of salt inhibits diverse physiological and metabolic processes of plants even impacting their survival. The conventional methods of reclamation of saline soil which involve scraping, flushing, leaching or adding an amendment (e.g., gypsum, CaCl2, etc.) are of limited success and also adversely affect the agro-ecosystems. In this context, developing sustainable methods which increase the productivity of saline soil without harming the environment are necessary. Since long, breeding of salt-tolerant plants and development of salt-resistant crop varieties have also been tried, but these and aforesaid conventional approaches are not able to solve the problem. Salt tolerance and dependence are the characteristics of some microbes. Salt-tolerant microbes can survive in osmotic and ionic stress. Various genera of salt-tolerant plant growth promoting rhizobacteria (ST-PGPR) have been isolated from extreme alkaline, saline, and sodic soils. Many of them are also known to mitigate various biotic and abiotic stresses in plants. In the last few years, potential PGPR enhancing the productivity of plants facing salt-stress have been researched upon suggesting that ST-PGPR can be exploited for the reclamation of saline agro-ecosystems. In this review, ST-PGPR and their potential in enhancing the productivity of saline agro-ecosystems will be discussed. Apart from this, PGPR mediated mechanisms of salt tolerance in different crop plants and future research trends of using ST-PGPR for reclamation of saline soils will also be highlighted.

Alleviation of Heavy Metal Stress in Plants and Remediation of Soil by Rhizosphere Microorganisms.

Increasing concentration of heavy metals (HM) due to various anthropogenic activities is a serious problem. Plants are very much affected by HM pollution particularly in contaminated soils. Survival of plants becomes tough and its overall health under HM stress is impaired. Remediation of HM in contaminated soil is done by physical and chemical processes which are costly, time-consuming, and non-sustainable. Metal-microbe interaction is an emerging but under-utilized technology that can be exploited to reduce HM stress in plants. Several rhizosphere microorganisms are known to play essential role in the management of HM stresses in plants. They can accumulate, transform, or detoxify HM. In general, the benefit from these microbes can have a vast impact on plant's health. Plant-microbe associations targeting HM stress may provide another dimension to existing phytoremediation and rhizoremediation uses. In this review, applied aspects and mechanisms of action of heavy metal tolerant-plant growth promoting (HMT-PGP) microbes in ensuring plant survival and growth in contaminated soils are discussed. The use of HMT-PGP microbes and their interaction with plants in remediation of contaminated soil can be the approach for the future. This low input and sustainable biotechnology can be of immense use/importance in reclaiming the HM contaminated soils, thus increasing the quality and yield of such soils.

Evaluation of rhizospheric Pseudomonas and Bacillus as biocontrol tool for Xanthomonas campestris pv campestris.

Xanthomonas campestris pv campestris (Xcc), causing black rot, is one of the most yield-limiting and destructive pathogens of cruciferous crops. The intention of this study was to evaluate the potential of rhizobacteria in black rot management. Fifty-four isolates from rhizosphere soil of Brassica campestris were screened against Xcc. Two isolates namely, KA19 and SE, with inhibition radius >11 mm were selected. The combined use of them produced an average inhibition zone of 18.1 ± 1.4 mm radius (P < 0.05). 16S rRNA gene sequencing and phylogenetic analysis identified KA19 and SE as the nearest homologs (>99.4%) of Pseudomonas aeruginosa and Bacillus thuringiensis, respectively. In greenhouse study, both isolates were effective (P < 0.05) in reducing black rot lesions compared to untreated control involving either a foliar spray or the combined seed soak and soil drench. However, the combined strains (KA19 + SE) were significantly more effective (P < 0.05) when the mode of application was combined seed and soil drench. The lipid content of seeds increased significantly with the application of these strains, especially with SE alone and in combination. After 9 weeks, the Xcc population was significantly lower in soil treated with combined strains (P < 0.05). KA19 produced extracellular siderophores, influenced by various carbon sources and identified as 4-hydroxy-2-nonyl-quinoline by NMR. In Bacillus SE, two antibacterial factors corresponding to autolysins (ß-N-acetylglucosaminidase) and AHL-lactonases were established. This study would strengthen our understanding for application of different rhizobacteria with various active principles like Pseudomonas and Bacillus as ingredients of a biocontrol mixture.

Physiologically stressed cells of fluorescent pseudomonas EKi as better option for bioformulation development for management of charcoal rot caused by Macrophomina phaseolina in field conditions.

Bioformulation that supports the inoculant under storage condition and on application to field is of prime importance for agroindustry. Pseudomonas strain EKi having biocontrol activity against Macrophomina phaseolina was used in the study. EKi cells were pretreated by carbon starvation, osmotic stress (NaCl), and freeze drying conditions, and talc-based bioformulation was developed. Combined pretreatment with carbon starvation and osmotic stress was given to Pseudomonas cells. Bioformulation of untreated, freeze dried (FD), carbon starved, osmotic stressed, and combined pre-treated cells showed 50.36, 44.76, 45.95, 34.82, and 27.27% reduction in CFU counts after 6 months of storage. The osmotic stressed cells showed one over-expressed protein (11.5 kDa) in common with carbon starved cells responsible for its better shelf life. The plant growth promotory activity of bioformulations was determined taking Cicer arietinum as a test crop in M. phaseolina infested field. Carbon starved + osmotic stressed cells showed maximum enhancement of dry weight (272.56%) followed by osmotic stressed (230.74%), untreated (155.70%), FD (88.93%), and carbon starved (59.34%) cells over uninoculated control. Carbon starved + osmotic stressed, osmotic stressed, untreated, FD, and carbon starved cells showed 156.60, 100, 75, 40, and 16.67% reduction of charcoal rot disease over uninoculated control. The results clearly showed that combined pretreatment by carbon starvation and osmotic stress provides the bacteria potential of rapid adaptation to different environment conditions.

Suppression of charcoal rot of chickpea by fluorescent Pseudomonas under saline stress condition.

The ability of fluorescent Pseudomonas strain EKi, in production of biocontrol and plant growth promotory (PGP) metabolites under saline stress was evaluated. Strain EKi could tolerate NaCl up to 1,550 mM and showed biocontrol of Macrophomina phaseolina (76.19%) in the presence of up to 400 mM NaCl. Strain EKi was able to produce IAA, siderophore and pyocyanin with gradual reduction of up to 76.31, 45.46, and 48.99%, respectively, as NaCl concentration increased from 0 to 500 mM. Reduced growth rate resulted in delayed induction of IAA, siderophore and pyocyanin by the PGPR. Thin layer chromatography of chloroform extract from non-stressed and salt stressed EKi, and inhibition of M. phaseolina by purified pyocyanin clearly indicated its role in biocontrol. In vitro and in vivo results showed the growth promotion and charcoal rot disease suppression of chickpea by strain EKi under both non-stressed and saline stress. There was 76.75 and 65.25% reduction of disease incidence in non-saline and saline conditions, respectively, in vitro conditions. In presence of M. phaseolina strain EKi brought about 67.65 and 58.45% reduction of disease incidence in non-saline and saline soil, respectively.