Ees Ahmad

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.

Soil Contamination, Nutritive Value, and Human Health Risk Assessment of Heavy Metals: An Overview

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.

Heavy Metal Toxicity to Symbiotic Nitrogen-Fixing Microorganism and Host Legumes

Legume species of the flowering family Fabaceae are well known for their ability to fix atmospheric nitrogen and enhance nitrogen pool of soil, leading to increase in crop especially legumes both in conventional or derelict soils. The interaction between Rhizobia and legumes provides nutrients to plants, increases soil fertility, facilitates plant growth and restores deranged/damaged ecosystem. These characteristics together make legume extremely interesting crop for evaluating the effect of heavy metals. Environmental pollutants like heavy metals at lower concentrations are required for various metabolic activities of microbes including Rhizobia and legume crops. The excessive metal concentrations on the other hand cause undeniable damage to Rhizobia, legumes and their symbiosis. Currently, little is, however, known about how free-living Rhizobia or the legume–Rhizobium symbiosis is affected by varying metal concentration. We focus here that how the nitrogen-fixing root nodule bacteria, the “rhizobia,” increase plant growth and highlight gaps in existing knowledge to understand the mechanistic basis of how different metals affect rhizobia–legume symbiosis which is likely to help to manage legume cultivation in metal contaminated locations.

Functional Aspect of Phosphate-Solubilizing Bacteria: Importance in Crop Production

Phosphorus (P) is one of the major plant nutrients whose deficiency results in severe losses to crop yields. To achieve optimum crop production, P is, therefore, consistently required. The use of chemical fertilizers in contrast is discouraged for two basic reasons: one, the repeated and injudicious application may alter soil fertility by adversely affecting microbial composition and functions and, second, it is expensive. To address these problems, scientists have identified soil-borne microorganisms belonging to a specific functional group generally referred to as phosphate-solubilizing microorganisms (PSM) which play many ecophysiological roles, especially in providing plants with P. They can be found in any environment from conventional to contaminated ones and are able to express their activity both in vitro and under field conditions. The solubilization of P by bacteria including even some of the strict nitrogen fixers, for example, rhizobia (symbiotic) or Azotobacter (asymbiotic), is a multifactor process. The ability to release bound P from both organic (enzymatic) and inorganic (acidification) sources by this functionally diverse group of organisms and to provide growth regulators (phytohormones) to plants or protecting plants from various diseases through other mechanisms (such as synthesizing antibiotics, siderophores, cyanogenic compounds, etc.) is indeed some of the most fascinating biological traits that have resulted in increased crop yields. Here, we highlight the functional aspects of PS bacteria especially their role in crop improvement particularly legumes and cereals grown in varied agro-ecological regions. The discussion attempted here is likely to serve as a low-cost prospective option for sustainable agriculture and also to solve economic constraint to considerable extent faced by the farming communities.

Growth stimulation and management of diseases of ornamental plants using phosphate solubilizing microorganisms: current perspective

Ornamental plants play an important role in human society since flowers are considered a vital component due to their beauty, texture, color, shape and fragrance. To produce high quality ornamentals, growers in general have intensified the use of agrochemicals without considering their deleterious impact on floral attributes. Also, the agrochemicals (including fertilizers and pesticides) used in floriculture are expensive and their excessive application results in emergence of pathogens resistant to such chemicals. It has, therefore, become imperative to develop renewable, inexpensive and eco-friendly fertilizers without producing any disturbing impact on quality of ornamentals. In this regard, phosphate solubilizing microorganisms (PSM) among plant growth promoting rhizobacteria have been identified as an efficient alternative to agrochemicals in floriculture. Even though, there are adequate reports on the effect of PSM on growth and development of numerous plants, information on the impact of PSM on production and quality of ornamental plants is, however, critically scarce. Considering these gaps and success of PSM application in floriculture achieved so far, efforts have been directed to highlight the impact of PSM on the production of ornamentals grown distinctively in different production systems. Also, the role of PSM in the management of ornamental diseases is discussed and considered. The review will conclude by identifying several PSM for future researches aiming to improve the health and quality of ornamentals grown in different production systems. Use of PSM is also likely to reduce the use of chemicals in floriculture.

ACC deaminase producing Pseudomonas putida strain PSE3 and Rhizobium leguminosarum strain RP2 in synergism improves growth, nodulation and yield of pea grown in alluvial soils

Plant growth promoting rhizobacteria affects the overall performance of plants by one or combination of mechanisms. However, little information is available on how ACC deaminase secreting bacteria enhance crop production. The present study aimed at identifying ACC deaminase producing and phosphate solubilizing bacterial strains and to assess their plant growth promoting activities. Additionally, the effect of two ACC deaminase positive bacterial strains Pseudomonas putida and Rhizobium leguminosarum on pea plants was determined to find a novel and compatible bacterial pairing for developing efficient inoculants for enhancing legume production and reducing dependence on chemical fertilizers. The isolated bacterial cultures were characterized biochemically and by 16S rRNA sequence analysis. The plant growth promoting activities was determined using standard microbiological methods. The impact of P. putida and R. leguminosarum, on pea plants was determined both in pots and in field environments. Of the total 40 bacterial strains, strain PSE3 isolated from Mentha arvenss rhizosphere and RP2 strain from pea nodules produced ACC deaminase, solubilized insoluble phosphate, synthesized indole acetic acid, ammonia, cyanogenic compounds, exopolysaccharides and had antifungal activity. The dual inoculation of P. putida strain PSE3 and R. leguminosarum strain RP2 had largest positive effect and markedly increased the growth, symbiotic characteristics, nutrient pool and quantity and quality of pea seeds. The measured parameters were further augmented when inoculated pea plants were grown in soils treated with urea or DAP. A significant variation in the measured parameters of pea plants was observed under both pot and field trials following microbial inoculation but the bacterial cultures did not differ significantly in growth promoting activities. The results suggest that ACC deaminase positive bacterial cultures endowed with multiple potential can be targeted to develop mixed inoculants for enhancing pea production and hence, to reduce dependence on synthetic fertilizers.

Role of Phosphate-Solubilizing Bacteria in Legume Improvement

Heterogeneously distributed microbial communities belonging to different genera enhance the growth and development of many crop plants including legumes. Among the various microbial populations inhabiting different habitats, the heterotrophic organisms endowed with natural phosphate-solubilizing activity and quite often called as phosphate-solubilizing microorganisms (PSM) supply one of the major plant nutrients, phosphorus, to plants and facilitate the growth of legumes. Phosphate-solubilizing microorganisms convert the complex or locked insoluble phosphorus to soluble phosphates by various mechanisms such as acidification, chelation, exchange reactions and polymeric substances formation and make it available to plants. Apart from supplying P to legumes, PSM also promote the growth of legumes by other mechanisms. Therefore, the widespread use of PSM in legume production helps both to reduce the spiralling cost of phosphatic fertilizers and to make soil free from chemical hazards. Considering these, the application of PSM endowed with multiple growth-promoting activities holds greater promise for increasing the productivity of legumes. Symbiotic/associative nitrogen-fixing bacteria are yet another important group of beneficial microbiota which is known to supply exclusively nitrogen to legumes, but they can also promote legume growth by other direct or indirect mechanisms. The co-inoculation of functionally different microflora such as N2-fixers, phosphate solubilizers and mycorrhizal fungi has, however, been found more effective than single inoculation of either organism for legume plants under nutrient-deficient soils. Basic and advance aspect of phosphate solubilization, mechanism of plant growth promotion and impact of single or synergistic association of phosphate solubilizers with other beneficial microflora on legumes growing in different regions are reviewed and discussed. The literatures surveyed in this chapter are likely to help better understand the functional role of PSM in sustainable production of legumes while reducing dependence on use of phosphatic fertilizers in legume production systems.

Role of Phosphate-Solubilizing Actinomycetes in Plant Growth Promotion: Current Perspective

Phosphorus (P), an essential plant nutrient, is a nonrenewable resource whose availability depends exclusively on mined rock phosphates. Deficiency of P in soil results in reduction in food production since all plants require an adequate supply of P for its growth and development. Even though synthetic phosphatic fertilizer has played some major roles in enhancing crop production, its excessive use has also dark sides to it where it has been found to damage the environment, destruct soil fertility, and, via food chain, seriously affect the human health. Considering the nuisance of overuse of P, there is an urgent demand by the agriculture practitioners to find nonhazardous strategy that can overcome/reduce the use of agrochemicals in agricultural practices and, hence, may preserve the very integrity of soil ecosystems. In this context, actinobacteria, a group of Gram-positive bacteria, ubiquitous in soils, are likely to play some important roles in supplying soluble P to plants by solubilizing/mineralizing complex P resources of soils. Additionally, the extracellular metabolites produced by actinomycetes may inhibit phytopathogens and, sometimes such metabolic compounds may also act as plant growth regulators. These qualities, among others, make actinobacteria an ideal candidate for developing as microbial inoculants for ultimate use in agriculture production system. The potential roles of actinomycetes as phosphate solubilizers in enhancing crop production are discussed.

Role of Phosphate-Solubilizing Microbes in the Management of Plant Diseases

Soilborne phytopathogens are one of the major problems in sustainable crop production world over. To alleviate the damaging impact of pathogens on crop yields, huge quantities of toxic chemicals especially pesticides are used in modern agronomic practices, which, however, are extremely destructive to the environment. The non-desirability of applying huge quantities of pesticides to soil due in part to residue problems, emergence of resistance among soil phytopathogens, and lack of pathogen-resistant crop varieties has forced researchers to find solutions to the increasing pesticides problems. To this end, biological control measures consisting of microbial preparations are considered a promising option to the use of expensive and environment disruptive pesticides. Microorganisms including plant growth-promoting rhizobacteria (PGPR) in general have been found to synthesize a wide array of metabolites with significant fungicidal and bactericidal capabilities. The use of phosphate-solubilizing (PS) microorganisms among PGPR has produced both direct and indirect effects on growth and development of plants. The PS microbes endowed with biocontrol activity manage the pathogens by one or simultaneous mechanisms of antibiosis, lysis, competition, and myco-parasitism and prevent the yield losses. Even though the literature on the physiological role of PS microorganisms in crop enhancement via P supply is adequately available, the information on the ability of such organisms in the control of phytopathogens is scarce. Here, different mechanisms utilized by PS organisms for plant disease suppression are discussed. It is envisioned that the PS bacteria in the near future are expected to reduce, if not completely eliminate, the use of pesticides in insect-pests management strategies.

Response of PSM Inoculation to Certain Legumes and Cereal Crops

Phosphate-solubilizing microorganisms (PSM) including bacteria, fungi, and actinomycetes dwelling in soil or other environment, for example, rhizosphere, do play some vital roles in facilitating growth and development of legumes and cereal plants via one or simultaneous mechanisms. Phosphate-solubilizing microbes when applied in agricultural practices provide one of the major plant nutrients, phosphorus, to plants by transforming insoluble P into soluble and plant available forms. This practice of applying PSM for enhancing legumes and cereal production has been found inexpensive and in many cases a successful strategy of reducing fertilizer input in intensive agricultural practices. The advent of such an eco-friendly option in farming system holds greater promise for increasing the productivity of legumes and cereal crops. Here, an attempt is made in this chapter to highlight the role of PSM involving different microbial groups, used either alone or in combination, in the promotion of growth and yield of legumes and cereal crops in different production systems.