Almas Zaidi

Role of phosphate-solubilizing microorganisms in sustainable agriculture – A review

Compared with the other major nutrients, phosphorus is by far the least mobile and available to plants in most soil conditions. Although phosphorus is abundant in soils in both organic and inorganic forms, it is frequently a major or even the prime limiting factor for plant growth. The bioavailability of soil inorganic phosphorus in the rhizosphere varies considerably with plant species, nutritional status of soil and ambient soil conditions. To circumvent phosphorus deficiency, phosphate-solubilizing microorganisms (PSM) could play an important role in supplying phosphate to plants in a more environmentally-friendly and sustainable manner. The solubilization of phosphatic compounds by naturally abundant PSM is very common under in vitro conditions; the performance of PSM in situ has been contradictory. The variability in the performance has thus greatly hampered the large-scale application of PSM in sustainable agriculture. Numerous reasons have been suggested for this, but none of them have been conclusively investigated. Despite the variations in their performance, PSM are widely applied in agronomic practices in order to increase the productivity of crops while maintaining the health of soils. This review presents the results of studies on the utilization of PSM for direct application in agriculture under a wide range of agro-ecological conditions with a view to fostering sustainable agricultural intensification in developing countries of the tropics and subtropics.

Plant growth promotion by phosphate solubilizing fungi – current perspective

Phosphorus is abundant in soils in both organic and inorganic forms; nevertheless, it is unavailable to plants. Accordingly, soil becomes phosphorus (P)-deficient, making P one of the most important nutrient elements limiting crop productivity. To circumvent the P deficiency, phosphate-solubilizing microorganisms could play an important role in making P available for plants by dissolving insoluble P. The dissolution of inorganic P by microbial communities including fungi is though common under in vitro conditions; the performance of phosphate-solubilizing microbes in situ has been contradictory. Fungi exhibit traits such as mineral solubilization, biological control, and production of secondary metabolites. As such, their potential to enhance plant growth when present in association with the roots is clear. The challenge is how to make use of such biological resources to maintain soil health while increasing the crop productivity by providing P to plants through the application of phosphate-solubilizing fungi. The present review focuses on the mechanisms of phosphate solubilization, development and mode of fungal inoculants application and mechanisms of growth promotion by phosphate-solubilizing fungi for crop productivity under a wide range of agro-ecosystems, and the understanding and management of P nutrition of plants through the application of phosphate-solubilizing fungi will be addressed and discussed.

Microbial strategies for crop improvement

The Use of Microorganisms to Facilitate the Growth of Plants in Saline Soils.- Recent Advances in Plant Growth Promotion by Phosphate-Solubilizing Microbes.- Developing Beneficial Microbial Biofilms on Roots of Non legumes: A Novel Biofertilizing Technique.- Role of 1-Aminocyclopropane-1-carboxylate deaminase in Rhizobium-Legume Symbiosis.- Strategies for Crop Improvement in Contaminated Soils Using Metal-Tolerant Bioinoculants.- Functional Diversity Among Plant Growth-Promoting Rhizobacteria: Current Status.- Plant Growth Promoting Rhizobacteria and Sustainable Agriculture.- Soil Health - A Precondition for Crop Production.- Recent Advances in Biopesticides.- Benefits of Arbuscular Mycorrhizal Fungi to Sustainable Crop Production.- Enhancement of Rhizobia-Legumes Symbioses and Nitrogen Fixation for Crops Productivity Improvement.- Monitoring the Development of Nurse Plant Species to Improve the Performances of Reforestation Programs in Mediterranean Areas.- Pea Cultivation in Saline Soils: Influence of Nitrogen Nutrition.- Plant Growth-Promoting Diazotrophs and Productivity of Wheat on the Canadian Prairies.- Factors Affecting the Variation of Microbial Communities in Different Agro-Ecosystems.- Strategies for Utilizing Arbuscular Mycorrhizal Fungi and Phosphate-Solubilizing Microorganisms for Enhanced Phosphate Uptake and Growth of Plants in the Soils of the Tropics.

Recent Advances in Plant Growth Promotion by Phosphate-Solubilizing Microbes

Most soils contain large reserves of total phosphorus (P), but its fixation and precipitation with soil constituents cause a major P-deficiency and severely restrict the growth and yield of plants. The use of chemical P-fertilizers is obviously the best means to circumvent P-deficiency, but their use is always limited due to its spiraling cost. In order to increase the availability of P and to reduce the use of chemical fertilizers, solubilization of insoluble P by phosphate-solubilizing microorganisms has provided an alternative to chemical phosphatic fertilizer. Besides P, these organisms promote the growth of plants by N2 fixation, enhancement of other plant nutrients, synthesizing phytohormones, suppressing plant diseases (bio-control) and reducing the toxicity of ethylene through 1-aminocyclopropane-1carboxylate (ACC) deaminase. In this chapter, attention is paid to understanding the fundamental and molecular basis as to how precisely these microbes, notably bacteria and fungi, help plants to grow better in P-deficient soils. Effective use of such microbes is likely to result in an ideal cropping system with a lesser impact on the environment through decreased application of chemical fertilizers.

Antibacterial and Cytotoxic Efficacy of Extracellular Silver Nanoparticles Biofabricated from Chromium Reducing Novel OS4 Strain of Stenotrophomonas maltophilia

Biofabricated metal nanoparticles are generally biocompatible, inexpensive, and ecofriendly, therefore, are used preferably in industries, medical and material science research. Considering the importance of biofabricated materials, we isolated, characterized and identified a novel bacterial strain OS4 of Stenotrophomonas maltophilia (GenBank: JN247637.1). At neutral pH, this Gram negative bacterial strain significantly reduced hexavalent chromium, an important heavy metal contaminant found in the tannery effluents and minings. Subsequently, even at room temperature the supernatant of log phase grown culture of strain OS4 also reduced silver nitrate (AgNO3) to generate nanoparticles (AgNPs). These AgNPs were further characterized by UV–visible, Nanophox particle size analyzer, XRD, SEM and FTIR. As evident from the FTIR data, plausibly the protein components of supernatant caused the reduction of AgNO3. The cuboid and homogenous AgNPs showed a characteristic UV-visible peak at 428 nm with average size of ∼93 nm. The XRD spectra exhibited the characteristic Bragg peaks of 111, 200, 220 and 311 facets of the face centred cubic symmetry of nanoparticles suggesting that these nanoparticles were crystalline in nature. From the nanoparticle release kinetics data, the rapid release of AgNPs was correlated with the particle size and increasing surface area of the nanoparticles. A highly significant antimicrobial activity against medically important bacteria by the biofabricated AgNPs was also revealed as decline in growth of Staphylococcus aureus (91%), Escherichia coli (69%) and Serratia marcescens (66%) substantially. Additionally, different cytotoxic assays showed no toxicity of AgNPs to liver function, RBCs, splenocytes and HeLa cells, hence these particles were safe to use. Therefore, this novel bacterial strain OS4 is likely to provide broad spectrum benefits for curing chromium polluted sites, for biofabrication of AgNPs and ultimately in the nanoparticle based drug formulation for the treatment of infectious diseases.

Impact of heavy metal toxicity on plant growth, symbiosis, seed yield and nitrogen and metal uptake in chickpea

Experiments were conducted to investigate the phytotoxic effects of heavy metals on chickpea, grown in unsterilised soils. Cadmium at 23 mg/kg soil, when used alone or in combination with other metals, was found to be the most toxic and significantly (P ≤ 0.05) reduced the plant growth, nodulation, chlorophyll content, and root and shoot N contents. Cadmium (23 mg/kg soil) and lead (390 mg/kg soil) reduced the number of nodules by 69.2 and 13.7%, respectively. Cadmium at 5.75 and 11.5 mg/kg soil decreased the seed yield by 14 and 19%, respectively, compared with the control. In contrast, lead at 97.5 and 195 mg/kg soil increased the seed yield by 12.3 and 8.8%, respectively, above the control. Generally, the chlorophyll content decreased with increasing rates of each metal. The root and shoot N content decreased by 33.3 and 30.7% at 23 mg/kg of cadmium, whereas lead at 390 mg/kg soil increased the root and shoot N content by 10 and 3%, respectively, above the control. The grain protein decreased gradually with increasing rates of each metal. An average maximum reduction (27%) in grain protein was observed with mixtures of 23 mg cadmium + 135 mg chromium + 580.2 mg nickel per kg soil. Flowering in chickpea plants was delayed following metal application. The degree of toxicity of heavy metals on the measured parameters decreased in the following order: cadmium, zinc, nickel, copper, chromium, then lead. Accumulation of heavy metals was higher in the roots relative to the shoots of chickpea and was significantly correlated with the concentration of the metals added to the soil.

Microbes for Legume Improvement

1. Velazques E., et al.: Bacteria involved in nitrogen-fixing legume symbiosis: Current taxonomic perspective 2. Skorupska E., et al.: Enhancing Rhizobium-legume symbiosis using signaling factors 3. Peix A., et al.: Key molecules involved in beneficial infection process in rhizobia-legume symbiosis 4. Mussarat J., et al.: Recent advances in Rhizobium-legume interactions: A proteomic approach 5. Arshad M., et al.: Role of ethylene and bacterial ACC-deaminase in nodulation of legumes 6. Seneviratne G., et al.: Microbial biofilms: How effective in Rhizobium-legume symbiosis? 7. Kayser Vargas L., et al.: Potential of Rhizobia as plant growth promoting rhizobacteria 8. Gattupalli A.: Engineering nodulation competitiveness of rhizobial bioinoculants in soils 9. Sindhu S. S., et al.: Growth promotion of legumes by inoculation of rhizosphere bacteria 10. Azcon R., Barea J-M.: Mycorrhizosphere interactions for legume improvement 11. Zaidi A., et al.: Role of phosphate solubilizing bacteria in legume improvement 12. Muleta D.: Legume responses to arbuscular mycorrhizal fungi inoculation in sustainable agriculture 13. Rinaudi L. V., Giordano W.: Bacterial biofilms: Role in Rhizobium-legume simbiosi 14. Oves M., et al.: Role of metal tolerant microbes in legume improvement 15. Zahran H. H.: Legumes-microbes interactions under stressed environments 16. Saikia S. P., et al.: Role of Azospirillum in the improvement of legumes 17. Javaid A.: Role of arbuscular mycorrhizal fungi in nitrogen fixation in legumes 18. Vieira R. F., et al.: Symbiotic nitrogen fixation in tropical food grain legumes: Current status 19. Medeot D. B., et al.: Plant growth promoting rhizobacteria improving the legume-rhizobia symbiosis 20. Mabrouk Y., Belhadj O.: The potential use of Rhizobium-legume symbiosis for enhancing plant growth and management of plantdiseases 21. Patil C. R., Alagawadi A. R.: Microbial inoculants for sustainable legume production

Mechanism of Phosphate Solubilization and Physiological Functions of Phosphate-Solubilizing Microorganisms

Phosphorus (P) is the second important key plant nutrient after nitrogen. An adequate supply of P is therefore required for proper functioning and various metabolisms of plants. Majority of P in soils is fixed, and hence, plant available P is scarcely available despite the abundance of both inorganic and organic P forms in soils. A group of soil microorganisms capable of transforming insoluble P into soluble and plant accessible forms across different genera, collectively called phosphate-solubilizing microorganisms (PSM), have been found as best eco-friendly option for providing inexpensive P to plants. These organisms in addition to supplying soluble P to plants also facilitate the growth of plants by several other mechanisms, for instance, improving the uptake of nutrients and stimulating the production of some phytohormones. Even though several bacterial, fungal and actinomycetal strains have been identified as PSM, the mechanism by which they make P available to plants is poorly understood. This chapter focuses on the mechanism of P-solubilization and physiological functions of phosphate solubilizers in order to better understand the ecophysiology of PSM and consequently to gather knowledge for managing a sustainable environmental system. Conclusively, PSM are likely to serve as an efficient bio-fertilizer especially in areas deficient in P to increase the overall performance of crops.

Toxicity of Heavy Metals to Legumes and Bioremediation

Globally, rapidly increasing industrialization and urbanization have resulted in the accumulation of higher concentrations of heavy metals in soils. The highly contaminated soil has therefore become unsuitable for cultivation probably because of the deleterious metal effects on the fertility of soils among various other soil characteristics. In addition, the uptake of heavy metals by agronomic crops and later on consumption of contaminated agri-foods have caused a serious threat to vulnerable human health. Considering these, a genuine attempt is made to address various aspects of metal contamination of soils. In addition, the nutritive value of some metals for bacteria and plants is briefly discussed. Here, we have also tried to understand how heavy metals risk to human health could be identified. These pertinent and highly demanding discussions are likely help to strategize the management options by policy makers/public for metal toxicity caused to various agro-ecosystems and for human health program.

Cadmium, chromium and copper in greengram plants

Soils contaminated with heavy metals including cadmium, chromium and copper present a major concern for sustainable agriculture. We studied the effects of cadmium, chromium and copper used both separately and as mixtures, on plant growth, nodulation, leghaemoglobin, seed yield and grain protein in seeds, in greengram inoculated with Bradyrhizobium sp. (Vigna). Cadmium at 24 mg kg−1 of soil reduced the dry matter accumulation and number of nodules by 27 and 38%, respectively. Chromium at 136 mg kg−1 of soil increased the dry phytomass and nodule numbers by 133 and 100%, respectively. The average maximum increase of 74% in seed yield occurred at 136 mg Cr kg−1 of soil. Cadmium and copper at 24 and 1338 mg kg−1 soil decreased the seed yield by 40 and 26%, respectively. Chromium at 136 kg−1 of soil increased the root and shoot N and leghaemoglobin content by 42, 31% and 50%, respectively. In contrast, the root and shoot N decreased by 22% at 24 mg Cd kg−1 of soil, while a maximum decrease of 50% in leghaemoglobin content occurred at 12 and 669 and 24 and 1338 mg Cd with Cu kg−1 of soil, relative to the control. The average maximum grain protein (283 mg g−1) was observed at 136 mg Cr kg−1 of soil, while minimum grain protein (231 mg g−1) was recorded at 24 and 1338 mg kg−1 of cadmium with copper. The metal accumulation in roots and shoots at 50 days after sowing and in grains 80 days after seeding differed among treatments. The degree of toxicity of heavy metals to the measured parameters decreased in the order Cd > Cu > Cr.