Sajid Mahmood Nadeem

The role of mycorrhizae and plant growth promoting rhizobacteria (PGPR) in improving crop productivity under stressful environments

Both biotic and abiotic stresses are major constrains to agricultural production. Under stress conditions, plant growth is affected by a number of factors such as hormonal and nutritional imbalance, ion toxicity, physiological disorders, susceptibility to diseases, etc. Plant growth under stress conditions may be enhanced by the application of microbial inoculation including plant growth promoting rhizobacteria (PGPR) and mycorrhizal fungi. These microbes can promote plant growth by regulating nutritional and hormonal balance, producing plant growth regulators, solubilizing nutrients and inducing resistance against plant pathogens. In addition to their interactions with plants, these microbes also show synergistic as well as antagonistic interactions with other microbes in the soil environment. These interactions may be vital for sustainable agriculture because they mainly depend on biological processes rather than on agrochemicals to maintain plant growth and development as well as proper soil health under stress conditions. A number of research articles can be deciphered from the literature, which shows the role of rhizobacteria and mycorrhizae alone and/or in combination in enhancing plant growth under stress conditions. However, in contrast, a few review papers are available which discuss the synergistic interactions between rhizobacteria and mycorrhizae for enhancing plant growth under normal (non-stress) or stressful environments. Biological interactions between PGPR and mycorrhizal fungi are believed to cause a cumulative effect on all rhizosphere components, and these interactions are also affected by environmental factors such as soil type, nutrition, moisture and temperature. The present review comprehensively discusses recent developments on the effectiveness of PGPR and mycorrhizal fungi for enhancing plant growth under stressful environments. The key mechanisms involved in plant stress tolerance and the effectiveness of microbial inoculation for enhancing plant growth under stress conditions have been discussed at length in this review. Growth promotion by single and dual inoculation of PGPR and mycorrhizal fungi under stress conditions have also been discussed and reviewed comprehensively.

Potassium-Solubilizing Bacteria and Their Application in Agriculture

Potassium (K) is one of the major macronutrients which play an important role in plant growth and development. Total soil potassium reserves are generally large; however, major portion of it exists in insoluble K minerals and very little potassium becomes available to plants. There are certain microorganisms which use a number of biological processes to make potassium available from unavailable forms. These potassium-solubilizing bacteria (KSB) can be used as a promising approach to increase K availability in soils, thus playing an important role for crop establishment under K-limited soils. Owing to naturally available source of potassium in soil and high price of synthetic potassium fertilizers, the importance of KSB is increasing day by day. The use of chemical fertilizers can be decreased by using KSB in agriculture that can lead to sustainable agriculture. A number of workers have demonstrated the role of KSB in crop improvement. The present review highlights the importance of KSB for enhancing crop production. The mechanisms used by KSB for K solubilization have been discussed. The work of various scientists regarding plant growth promotion through KSB has been reviewed in detail, and present constraints and future scope of this technology have also been discussed.

Microbial ACC-Deaminase: Prospects and Applications for Inducing Salt Tolerance in Plants

Salinity is one of the most important stresses that hamper agricultural productivity in nearly every part of the world. Enhanced biosynthesis of ethylene in plants under salinity stress is well established. Higher ethylene concentration inhibits root growth and ultimately affects the overall plant growth. Overcoming this ethylene-induced root inhibition is a prerequisite for successful crop production. Recent studies have shown that ethylene level in plants is regulated by a key enzyme 1-aminocyclopropane-1-carboxylicacid (ACC)-deaminase. This enzyme is present in plant growth-promoting bacteria (PGPR) and lowers the ethylene level by metabolizing its precursor ACC into α-ketobutyrate and ammonia (NH3). Inoculation of plants under salinity stress with PGPR having ACC-deaminase activity mitigates the inhibitory effects of salinity on root growth by lowering the ethylene concentration in the plant. This in turn results in prolific root growth, which is beneficial for the uptake of nutrients and maintenance of growth under stressful environment. The present review critically discusses the effects of salinity stress on plant growth with special reference to ethylene production and the effects of rhizobacteria containing ACC-deaminase on crop improvement under salinity stress. It also discusses how much progress has been made in producing transgenic lines of different crops over-expressing the gene encoding ACC-deaminase and how far such transformed lines can tolerate salinity stress.

Yeast extract promotes decolorization of azo dyes by stimulating azoreductase activity in Shewanella sp. strain IFN4.

Biological treatment of azo dyes commonly requires a combined anaerobic-aerobic process in which initial decolorization is achieved by reductive cleavage of azo bonds on the parent molecule. The present study was conducted to examine the relative importance of co-substrates for driving reductive decolorization of azo dyes by Shewanella sp. strain IFN4 using whole cells and enzyme assays. Results showed that the dye decolorization by strain IFN4 was faster in medium containing 1gL(-1) yeast extract (YE) as compared to nine other co-substrates. Moreover, only YE stimulated azoreductase activity (increased from 1.32 to 4.19U/mg protein). Increasing the level of YE up to 8gL(-)(1) resulted into 81% decolorization of the dye in 1h along with an increase in azoreductase activity up to 6.16U/mg protein. Among the components of YE, only riboflavin stimulated the decolorization process as well as enzyme activity. Moreover, strain IFN4 demonstrated flavin reductase activity, and a significant correlation (r(2)=0.98) between flavin reduction and dye reduction by this strain emphasized the involvement of flavin compounds in the decolorization process. The results of this study show that YE serves both as a source of reducing equivalents and an electron shuttle for catalyzing dye reduction.

Rhizosphere Bacteria for Crop Production and Improvement of Stress Tolerance: Mechanisms of Action, Applications, and Future Prospects

Rhizosphere bacteria associated with plant roots can enhance crop productivity through a number of direct and indirect mechanisms. These beneficial bacteria attracted the scientists around the globe due to their significant contribution to mitigate adverse effects of environmental stresses on plants. These plant growth-promoting rhizobacteria (PGPR) have the potential to improve crop production under stress conditions solely and/or in combination with other microbes. The use of PGPR as co-inoculants with symbiotic bacteria is a potential biotechnological approach to promote nodulation for improving crop biomass and soil health. Multi-strain bacterial consortia are also proved useful for enhancing plant growth and development particularly in conditions where single inoculation was not so effective. The objectives of present review are to highlight the basic mechanisms used by such bacteria in general and the applied aspects of these bacteria for improving plant stress tolerance and ultimately crop productivity particularly. Certain examples highlighting their significant role for enhancing plant growth under biotic and abiotic stress conditions have been reviewed. The role of PGPR for improving nodulation when used with nitrogen-fixing bacteria has been discussed. The potential of genetically engineered rhizobacteria that possess the required trait necessary under certain environmental conditions has also been evaluated. The areas that need further research and future perspectives of this technology have been discussed in detail.

Plant–Microbe Interactions for Sustainable Agriculture: Fundamentals and Recent Advances

Coordinated interactions between plants and microbes have supreme importance for improving plant growth as well as maintaining proper soil conditions. Rhizosphere interactions that are based on complex exchange are more complicated than those occurring above soil surface or non-rhizosphere soil. Among diverse microbial population, plant growth promoting rhizobacteria (PGPR) gain special attention owing to their multifarious functional characters like effective root colonization, hormone production, solubilization of nutrients, and production of certain enzymes that are beneficial for sustainable agriculture. An understanding about their ecology, growth-promoting traits, mechanisms of action, and their application for plant growth stimulation has key importance for maximum utilization of this naturally occurring population. The present review highlights the importance of PGPR for enhancing crop production. The mechanisms of plant growth promotions as well as effectiveness of PGPR under different environments have been discussed. The effectiveness of multistrain inocula over single strain has been explained with examples. Also, the limitations related to the use of bacterial inoculants under natural field conditions and some important basics related to their formulation and commercialization have been discussed.

Role of Phytohormones in Stress Tolerance of Plants

Environmental stresses, both biotic and abiotic, cause negative impact on plant growth and development, and plants need to adopt certain strategies for maintaining proper growth under stress conditions. These strategies include certain physiological, biochemical, and molecular mechanisms to cope with these stresses. These mechanisms include the production of hormones (phytohormones) and osmolytes. Phytohormones are organic molecules that affect various plant physiological processes like growth, development, and cell differentiation. Phytohormones regulate key physiological events under normal and stress conditions. They play a vital role for enhancing the ability of plants to adapt to the harsh environmental conditions by mediating a wide range of adaptive responses. These responses enable the plants to acclimatize to adverse soil conditions. Various types of phytohormones play an important function in plants individually or in coordination with each other. The nature and level of these hormones in plants are major factors that influence plant processes and functions. The present chapter describes the potential role of phytohormones for promoting plant growth and development under stress conditions. The major classes of plant hormones and their source of production have been described. Metabolism of phytohormones and their physiological responses with special reference to their concentration-dependent or negative impact on plant growth have been discussed in detail. The impact of these hormones on plant growth under stress conditions has been reviewed and discussed with selected examples. Also, the role of microbes in phytohormone production has been elaborated with examples. Future perspectives of the area have also been discussed.

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.

Arbuscular Mycorrhizas: An Overview

Almost every plant in natural ecosystem forms association with fungi either intracellularly as in arbuscular mycorrhizal fungi (AMF), or extracellularly as in ectomycorrhizal fungi. Arbuscular mycorrhizas (AMs) represent the most widespread symbiosis with land plants. The associated fungi colonize the plant roots and reside in the internal tissues of their host plant. This mutualistic association not only plays key role for enhancing plant growth by facilitating the uptake of water and essential nutrients but also protects the plant from adverse soil conditions. The application of mycorrhizal fungi is a promising alternative strategy for sustainable crop production under normal as well as biotic and abiotic stress conditions. The mycorrhizal plants have an improved ability for nutrient uptake and have ability to tolerate stress environments. There are increasing interest for the application of AM fungus for improving plant growth and enhancing crop production. The AM fungus also has positive impact on crop growth by improving soil quality by increasing water infiltration and retention and therefore reducing soil erosion. This review chapter epitomizes the current knowledge on the significance of AM fungus for improving crop production and maintaining agriculture sustainability.

Comparative Efficacy of Bio-organic and Mineral Phosphate on the Growth, Yield and Economics of Wheat (Triticum aestivum L.) Grown by Different Methods

ABSTRACT This study was conducted to assess the efficacy of bio-organic phosphate (BOP) and diammonium phosphate (DAP) fertilizers for improving the growth, yield, plant phosphorus (P) content and profitability of wheat cultivation under systems of wheat intensification (direct seeding and nursery transplantation) and conventional sowing (broadcasting). BOP was formulated by enriching the compost with rock phosphate and inoculated with plant growth-promoting rhizobacterial (PGPR) strain, Bacillus thuringiensis strain K5. Results revealed the supremacy of BOP over DAP, and the direct seeding method over the other sowing methods in almost all parameters studied. The application of BOP significantly (p ≤ 0.05) increased the plant height, root length, number of tillers per plant, wheat biomass, number of grains per spike, 1000-grain weight, grain yield, straw and grain P contents in direct seeded wheat in comparison to DAP fertilization. Furthermore, application of BOP and direct seeding of wheat generated more profit per hectare compared with the other treatments.