Alleviation of salt stress of symbiotic Galega officinalis L. (goat's rue) by co-inoculation of Rhizobium with root-colonizing Pseudomonas

Background and aims

Salt stress is an increasing problem in agricultural soils in many parts of the world, and salt tolerant cropping systems are in great demand. We investigated the effect of co-inoculation of Galega officinalis with Rhizobium galegae and two plant growth promoting Pseudomonas species on plant growth, nodulation, and N content under salt stress.

Methods

The effect of inoculation with R. galegae sv. officinalis HAMBI 1141 alone and in combination with the root-colonizing Pseudomonas extremorientalis TSAU20 or Pseudomonas trivialis 3Re27 on the growth of G. officinalis exposed to salt stress (50 and 75 mM NaCl) was studied under gnotobiotic and greenhouse conditions.

Results

The growth of goat’s rue was reduced at 50 and 75 mM NaCl both in the gnotobiotic sand system and in low-fertilized potting soil in the greenhouse. Co-inoculation of unstressed and salt-stressed goat’s rue with R. galegae HAMBI 1141 and either P. extremorientalis TSAU20 or P. trivialis 3Re27 significantly improved root and shoot growth and increased nodulation of plant roots in both growth systems compared with plants inoculated with R. galegae alone. The nitrogen content of co-inoculated plant roots was significantly increased at 75 mM NaCl in potting soil. Co-inoculation of G. officinalis with either of the two plant growth promoting (PGPR) Pseudomonas strains also improved root tip-colonization by R. galegae cells.

Conclusions

The co-inoculation of goat’s rue with Rhizobium and PGPR Pseudomonas strains alleviated salt stress of plants grown in NaCl-amended gnotobiotic sand systems and in potting soil in the greenhouse.     

<Emphasis Type="Italic">Pseudomonas chlororaphis</Emphasis>: a salt-tolerant bacterial inoculant for plant growth stimulation under saline soil conditions

Saline soils constitute a serious production problem for vegetable crops as they are known to suppress plant growth. One of the possible measures to improve crop health in such conditions is to use salt-tolerant bacterial inoculants which can control diseases and promote plant growth. In the present work the ability of Pseudomonas chlororaphis isolate TSAU13 to promote cucumber and tomato plant growth and to improve fruit yield by protecting these plants against soil-borne pathogens in salinated soil were investigated. The bacterial strain stimulated shoot growth (up to 32%), dry matter (up to 43%), and the fruit yield of tomato and cucumber (up to 16%) compared to the uninoculated control plants under saline conditions. The strain was able to survive on the root of 2-month-old plants. 29% of the cucumber and 27% of the tomato plants which had grown in soil to which no Fusarium solani spores had been added were diseased, whereas in the presence of the pathogenic fungus 58% of the cucumber and 52% tomato plants had disease symptoms. P. chlororaphis TSAU13 showed statistically significant disease reduction in comparison to the Fusarium-uninfected and infected control plants. Those results showed that P. chlororaphis TSAU13 has a great biotechnological potential in improvement of vegetable production in commercial greenhouses under saline conditions.     

Salt tolerant Methylobacterium mesophilicum showed viable colonization abilities in the plant rhizosphere

The source of infection has always been considered as an important factor in epidemiology and mostly linked to environmental source such as surface water, soil, plants and also animals. The activity of the opportunistic pathogens associated with plant root, their adaptation and survival under hostile environmental condition is poorly understood. In this study the salt tolerance ability of Methylobacterium mesophilicum and its colonization in the root and shoot of plants under severe drought and salt stress conditions were investigated. The colonization of plant by M. mesophilicum was investigated in a gnotobiotic sand system, and their survival in pots with saline soil. Bacterial strain was found to colonize rhizosphere of cucumber, tomato and paprika grown under normal and salt stress condition and reached up to 6.4 × 10⁴ and 2.6 × 10⁴ CFU/g root. The strain was resistant to Gentamicin, Ampicillin, Amoxicillin plus Clavulanic acid, Cefotaxime, neomycin, penicillin and was also tolerant to salinity stress (up to 6% NaCl). These abilities play important roles in enabling persistent colonization of the plant surface by M. mesophilicum strains. In conclusion, this study provides background information on the behaviour of opportunistic pathogen M. mesophilicum on plants and their survival in harsh environmental conditions.     

Alleviation of salt stress by plant growth regulators and IAA producing bacteria in wheat

The action of phytohormone producing bacteria and plant growth regulators on germination and seedling growth of wheat under saline conditions were studied. Seed dormancy enforced by salinity (100 mM NaCl) was substantially alleviated and the germination was promoted by gibberellin, auxin, zeatin, and ethephon from 54 to 97%. The IAA producing bacterial strains Pseudomonas aureantiaca TSAU22, Pseudomonas extremorientalis TSAU6 and Pseudomonas extremorientalis TSAU20 significantly increased seedling root growth up to 25% in non-salinated conditions and up to 52% at 100 mM NaCl, compared to control plants. It is concluded that growth regulators considerably alleviated salinity-induced dormancy of wheat seeds. The facts mentioned above make it possible to recommend root colonizing bacteria that produce phytohormone to alleviate salt stress of wheat grown under conditions of soil salinity.     

Genome Analysis of Pseudomonas fluorescens PCL1751: A Rhizobacterium that Controls Root Diseases and Alleviates Salt Stress for Its Plant Host

Pseudomonas fluorescens PCL1751 is a rod-shaped Gram-negative bacterium isolated from the rhizosphere of a greenhouse-grown tomato plant in Uzbekistan. It controls several plant root diseases caused by Fusarium fungi through the mechanism of competition for nutrients and niches (CNN). This mechanism does not rely on the production of antibiotics, so it avoids the concerns of resistance development and is environmentally safe. Additionally, this bacterium promotes plant growth by alleviating salt stress for its plant host. To investigate the genetic mechanisms that may explain these observations, we determined the complete genome sequence of this bacterium, examined its gene content, and performed comparative genomics analysis with other Pseudomonas strains. The genome of P. fluorescens PCL1751 consisted of one circular chromosome that is 6,143,950 base-pairs (bp) in size; no plasmid was found. The annotation included 19 rRNA, 70 tRNA, and 5,534 protein-coding genes. The gene content analysis identified a large number of genes involved in chemotaxis and motility, colonization of the rhizosphere, siderophore biosynthesis, and osmoprotectant production. In contrast, the pathways involved in the biosynthesis of phytohormones or antibiotics were not found. Comparison with other Pseudomonas genomes revealed extensive variations in their genome size and gene content. The presence and absence of secretion system genes were highly variable. As expected, the synteny conservation among strains decreased as a function of phylogenetic divergence. The integration of prophages appeared to be an important driver for genome rearrangements. The whole-genome gene content analysis of this plant growth-promoting rhizobacterium (PGPR) provided some genetic explanations to its phenotypic characteristics. The extensive and versatile substrate utilization pathways, together with the presence of many genes involved in competitive root colonization, provided further support for the finding that this strain achieves biological control of pathogens through effective competition for nutrients and niches.     

High incidence of plant growth-stimulating bacteria associated with the rhizosphere of wheat grown on salinated soil in Uzbekistan

Soil salinization is increasing steadily in many parts of the world and causes major problems for plant productivity. Under these stress conditions, root-associated beneficial bacteria can help improve plant growth and nutrition. In this study, salt-tolerant bacteria from the rhizosphere of Uzbek wheat with potentially beneficial traits were isolated and characterized. Eight strains which initially positively affect the growth of wheat plants in vitro were investigated in detail. All eight strains are salt tolerant and have some of the following plant growth-beneficial properties: production of auxin, HCN, lipase or protease and wheat growth promotion. Using sequencing of part of the 16S rDNA, the eight new isolates were identified as Acinetobacter (two strains), Pseudomonas aeruginosa , Staphylococcus saprophyticus , Bacillus cereus , Enterobacter hormaechei , Pantoae agglomerans and Alcaligenes faecalis . All these strains are potential human pathogens. Possible reasons for why these bacteria present in the rhizosphere and establish there are discussed.     

Arbuscular mycorrhizal fungi regulate the oxidative system,hormones and ionic equilibrium to trigger salt stress tolerance in Cucumis sativus L.

Arbuscular mycorrhizal fungi (AMF) association increases plant stress tolerance. This study aimed to determine the mitigation effect of AMF on the growth and metabolic changes of cucumbers under adverse impact of salt stress. Salinity reduced the water content and synthesis of pigments. However, AMF inoculation ameliorated negative effects by enhancing the biomass, synthesis of pigments, activity of antioxidant enzymes, including superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase, and the content of ascorbic acid, which might be the result of lower level lipid peroxidation and electrolyte leakage. An accumulation of phenols and proline in AMF-inoculated plants also mediated the elimination of superoxide radicals. In addition, jasmonic acid, salicylic acid and several important mineral elements (K, Ca, Mg, Zn, Fe, Mn and Cu) were enhanced with significant reductions in the uptake of deleterious ions like Na+. These results suggested that AMF can protect cucumber growth from salt stress.     

Arbuscular mycorrhizal symbiosis and abiotic stress in plants: A review

Abiotic stresses (such as salinity, drought, cold, heat, mineral deficiency and metals/metalloids) have become major threats to the global agricultural production. These stresses in isolation and/or combination control plant growth, development and productivity by causing physiological disorders, ion toxicity, and hormonal and nutritional imbalances. Some soil microorganisms like arbuscular mycorhizal fungi (AMF) inhabit the rhizosphere and develop a symbiotic relationship with the roots of most plant species. AMF can significantly improve resistance of host plants to varied biotic and abiotic stresses. Taking into account recent literature, this paper: (a) overviews major abiotic stresses and introduces the arbuscular mycorrhizae symbiosis (b) appraises the role and underlying major mechanisms of AMF in plant tolerance to major abiotic stresses including salinity, drought, temperature regimes (cold and heat), nutrient-deficiency, and metal/metalloids; (c) discusses major molecular mechanisms potentially involved in AMF-mediated plant-abiotic stress tolerance; and finally (d) highlights major aspects for future work in the current direction.     

Alleviation of cadmium stress in Solanum lycopersicum L. by arbuscular mycorrhizal fungi via induction of acquired systemic tolerance

Experiments were conducted to evaluate cadmium (Cd) stress-induced changes in growth, antioxidants and lipid composition of Solanum lycopersicum with and without arbuscular mycorrhizal fungi (AMF). Cadmium stress (50 μM) caused significant changes in the growth and physio-biochemical attributes studied. AMF mitigated the deleterious impact of Cd on the parameters studied. Cadmium stress increased malonaldehyde and hydrogen peroxide production but AMF reduced these parameters by mitigating oxidative stress. The activity of antioxidant enzymes enhanced under Cd treatment and AMF inoculation further enhanced their activity, thus strengthening the plant’s defense system. Proline and phenol content increased in Cd-treated as well as AMF-inoculated plants providing efficient protection against Cd stress. Cadmium treatment resulted in great alterations in the main lipid classes leading to a marked change in their composition. Cadmium stress caused a significant reduction in polyunsaturated fatty acids resulting in enhanced membrane leakage. The present study supports the use of AMF as a biological means to ameliorate Cd stress-induced changes in tomato.     

Pseudomonas induces salinity tolerance in cotton (Gossypium hirsutum) and resistance to Fusarium root rot through the modulation of indole-3-acetic acid

Abiotic stresses cause changes in the balance of phytohormones in plants and result in inhibited root growth and an increase in the susceptibility of plants to root rot disease. The aim of this work was to ascertain whether microbial indole-3-acetic acid (IAA) plays a role in the regulation of root growth and microbially mediated control of root rot of cotton caused by Fusarium solani. Seed germination and seedling growth were improved by both NaCl and Mg₂SO₄ (100 mM) solutions when treated with root-associated bacterial strains Pseudomonas putida R4 and Pseudomonas chlororaphis R5, which are able to produce IAA. These bacterial strains were also able to reduce the infection rate of cotton root rot (from 70 to 39%) caused by F. solani under gnotobiotic conditions. The application of a low concentration of IAA (0.01 and 0.001 μg/ml) stimulated plant growth and reduced disease incidence caused by F. solani (from 70 to 41–56%, respectively). Shoot and root growth and dry matter increased significantly and disease incidence was reduced by bacterial inoculants in natural saline soil. These results suggest that bacterial IAA plays a major role in salt stress tolerance and may be involved in induced resistance against root rot disease of cotton.