Munees Ahemad

Plant growth promotion by phosphate solubilizing bacteria.

Most agronomic soils contain large reserves of total phosphorus [P], but the fixation and precipitation of P cause P deficiency, and in turn, restrict the growth of crops severely. Phosphorus replenishment, especially in sustainable production systems, remains a major challenge as it is mainly fertilizer-dependent. Though the use of chemical P fertilizers is obviously the best means to circumvent P deficiency in different agro-ecosystems, their use is always limited due to its spiralling cost. A greater interest has, therefore, been generated to find an alternative yet inexpensive technology that could provide sufficient P to plants while reducing the dependence on expensive chemical P fertilizers. Among the heterogeneous and naturally abundant microbes inhabiting the rhizosphere, the phosphate solubilizing microorganisms (PSM) including bacteria have provided an alternative biotechnological solution in sustainable agriculture to meet the P demands of plants. These organisms in addition to providing P to plants also facilitate plant growth by other mechanisms. Despite their different ecological niches and multiple functional properties, P-solubilizing bacteria have yet to fulfil their promise as commercial bio-inoculants. Current developments in our understanding of the functional diversity, rhizosphere colonizing ability, mode of actions and judicious application are likely to facilitate their use as reliable components in the management of sustainable agricultural systems.

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.

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.

Effect of fungicides on plant growth promoting activities of phosphate solubilizing Pseudomonas putida isolated from mustard (Brassica compestris) rhizosphere

This study was navigated to examine the effects of fungicide-stress on the activities of plant-growth-promoting rhizobacterium Pseudomonasputida with inherent phosphate solubilizing activity. The fungicide-tolerant and phosphate solubilizing P.putida strain PS9 was isolated from the mustard rhizosphere and tentatively identified following standard morphological, physiological and biochemical tests. To further consolidate the identity of the strain PS9, the 16S rDNA sequence analysis was performed. Following the BLAST program, the strain PS9 was identified as P.putida. In the presence of the varying concentrations (0-3200 μg mL(-1); at a two fold dilution interval) of four fungicides of different chemical families (tebuconazole, hexaconazole, metalaxyl and kitazin) amended in minimal salt agar medium, the P.putida strain PS9 showed a variable tolerance levels (1400-3200 μg mL(-1)) against the tested fungicides. The strain PS9 produced plant-growth-promoting (PGP) substances in significant amount in the absence of fungicides. In general, fungicides applied at the recommended, two and three times of the recommended rates, decreased the PGP attributes of P.putida the strain PS9 and affected the PGP activities in concentration-dependent manner. Fungicides at the recommended dose had minor reducing effect while the doses higher than the recommended dose significantly reduced the PGP activities (phosphate solubilization, salicylic acid, 2,3-dihydroxy benzoic acid, and indole-3-acetic acid production except exo-polysaccharides, hydrogen cyanate and ammonia production). Of the four fungicides, tebuconazole generally, showed maximum toxicity to the PGP activities of the strain PS9. This study inferred that fungicides must be examined in vitro for their possible adverse effects on soil micro flora before their application in agricultural fields. Moreover, the results also suggested the prerequisite of application of fungicide-tolerant PGPR strains as bioinoculants so that their PGP activities may not be suppressed under fungicide stress.

Evaluation of plant-growth-promoting activities of rhizobacterium Pseudomonas putida under herbicide stress

The aim of this study was to evaluate the effect of selected herbicides (quizalafop-p-ethyl, clodinafop, metribuzin and glyphosate) at one, two and three times the recommended field rates on the plant-growth-promoting (PGP) traits of Pseudomonas putida strain PS9 isolated from the mustard rhizosphere. P. putida strain PS9 was selected for its high herbicide-tolerance and its production of substantial amounts of indole acetic acid, siderophores (salicylic acid and 2,3 dihydroxy benzoic acid) and exo-polysaccharides. It also exhibits HCN and ammonia releasing ability. A progressive, herbicide-concentration-dependent, decline in PGP properties (except exo-polysaccharide production) of P. putida strain PS9 was observed when the strain was grown in the presence of increasing concentrations of each herbicide. Generally, among the selected herbicides, the maximum toxicity to PGP traits of P. putida strain PS9 was shown by quizalafop-p-ethyl at three times the recommended dose. This study concludes that herbicides should be screened before field application for their degree of toxicity to PGP traits of beneficial microorganisms.

Effect of insecticide-tolerant and plant growth-promoting Mesorhizobium on the performance of chickpea grown in insecticide stressed alluvial soils

An experiment was carried out to determine the plant growth-promoting activities of fipronil- and pyriproxyfen-tolerant Mesorhizobium isolates in the presence and absence of insecticides. The bioremediation and plant growth-promoting potentials of Mesorhizobium isolate (MRC4) was assessed using chickpea as a test crop grown under fipronil- and pyriproxyfen-stressed soils. In this study, the most promising mesorhizobial isolate (MRC4) tolerated fipronil and pyriproxyfen up to a concentration of 1600 μg ml−1 and 1400 μg ml−1, respectively. Isolate MRC4 produced a substantial amount of indole acetic acid (44.3 μg ml−1), salicylic acid (35 μg ml−1), 2,3 di-hydroxybenzoic acid (19 μg ml−1), and exo-polysaccharides (21 μg ml−1) in the absence of insecticides. The plant growth-promoting substances displayed by the isolate MRC4 declined progressively with increasing concentrations of each insecticide. The insecticide tolerant isolate MRC4 was further tested for its effect on chickpea plants grown in fipronil- and pyriproxyfen-treated soils. The insecticide-tolerant isolate MRC4 increased the dry matter accumulation progressively. A maximum increase of 80 (at 600 μg kg−1 soil of fipronil) and 118% (at 3900 μg kg−1 soil of pyriproxyfen) was recorded 135 days after sowing when compared to noninoculated plants treated with the same rates of each insecticide. Moreover, Mesorhizobium isolate MRC4 when used in fipronil- and pyriproxyfen-treated soil also increased symbiotic properties (nodulation and leghaemoglobin content), root N, shoot N, root P, shoot P, seed yield, and seed protein compared to the un-inoculated but treated solely with insecticide. The present finding suggests that the mesorhizobial isolate endowed with multiple properties could be used to facilitate the productivity of chickpea under insecticidestressed soils.

Alleviation of fungicide-induced phytotoxicity in greengram [Vigna radiata (L.) Wilczek] using fungicide-tolerant and plant growth promoting Pseudomonas strain

This study was designed to explore beneficial plant-associated rhizobacteria exhibiting substantial tolerance against fungicide tebuconazole vis-à-vis synthesizing plant growth regulators under fungicide stressed soils and to evaluate further these multifaceted rhizobacteria for protection and growth promotion of greengram [Vigna radiata (L.) Wilczek] plants against phytotoxicity of tebuconazole. Tebuconazole-tolerant and plant growth promoting bacterial strain PS1 was isolated from mustard (Brassica compestris) rhizosphere and identified as Pseudomonas aeruginosa following 16S rRNA gene sequencing. The P. aeruginosa strain PS1 solubilized phosphate significantly and produced indole acetic acid, siderophores, exo-polysaccharides, hydrogen cyanide and ammonia even under tebuconazole stress. Generally, tebuconazole at the recommended, two and three times the recommended field rate adversely affected the growth, symbiosis, grain yield and nutrient uptake in greengram in a concentration dependent manner. In contrast, the P. aeruginosa strain PS1 along with tebuconazole significantly, increased the growth parameters of the greengram plants. The inoculant strain PS1 increased appreciably root nitrogen, shoot nitrogen, root phosphorus, shoot phosphorus, and seed yield of greengram plants at all tested concentrations of tebuconazole when compared to the uninoculated plants treated with tebuconazole. The results suggested that the P. aeruginosa strain PS1, exhibiting novel plant growth regulating physiological features, can be applied as an eco-friendly and plant growth catalyzing bio-inoculant to ameliorate the performance of greengram in fungicide stressed soils.

Phosphate-solubilizing bacteria-assisted phytoremediation of metalliferous soils: a review

Heavy metal pollution of soils is of great concern. The presence of the toxic metal species above critical concentration not only harmfully affects human health but also the environment. Among existing strategies to remediate metal contaminates in soils, phytoremediation approach using metal accumulating plants is much convincing in terms of metal removal efficiency, but it has many limitations because of slow plant growth and decreased biomass owing to metal-induced stress. In addition, constrain of metal bioavailability in soils is the prime factor to restrict its applicability. Phytoremediation of metals in association with phosphate-solubilizing bacteria (PSB) considerably overcomes the practical drawbacks imposed by metal stress on plants. This review is an effort to describe mechanism of PSB in supporting and intensifying phytoremediation of heavy metals in soils and to address the developmental status of the current trend in application of PSB in this context.

Functional Diversity Among Plant Growth-Promoting Rhizobacteria: Current Status

Root-colonizing bacteria (rhizobacteria) that exert beneficial effects on plant development via direct or indirect mechanisms have been defined as plant growth-promoting rhizobacteria (PGPR). These natural bioresources provide essential nutrients to plants and improve growth, competitiveness, and responses to external stress factors by an array of mechanisms under different agro-ecosystems. The PGPR facilitate plant growth by synthesizing or altering: the concentration of phytohormones: asymbiotic and symbiotic N2 fixation, antagonism against phytopathogenic microorganisms by producing siderophores, antibiotics and cyanide; solubilization of mineral phosphates and other nutrients; and by synthesizing 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which helps to reduce the inhibitory effects of ethylene on plants. And hence, use of such PGPR may be a viable alternative to chemical fertilizers for increasing the productivity of various crops. However, despite their proven ability of growth promotion, PGPR have yet to fulfil their promise and potential as commercial bioinoculants. Understanding functional diversity of PGPR is vital for low-input sustainable production. Recent progress focusing on the principles and mechanisms of action of PGPR is reviewed and discussed.

Bacterial mechanisms for Cr(VI) resistance and reduction: an overview and recent advances

Chromium pollution is increasing incessantly due to continuing industrialization. Of various oxidation states, Cr6+ is very toxic due to its carcinogenic and mutagenic nature. It also has deleterious effects on different microorganisms as well as on plants. Many species of bacteria thriving in the Cr6+-contaminated environments have evolved novel strategies to cope with Cr6+ toxicity. Generally, decreased uptake or exclusion of Cr6+ compounds through the membranes, biosorption, and the upregulation of genes associated with oxidative stress response are some of the resistance mechanisms in bacterial cells to overcome the Cr6+ stress. In addition, bacterial Cr6+ reduction into Cr3+ is also a mechanism of specific significance as it transforms toxic and mobile chromium derivatives into reduced species which are innocuous and immobile. Ecologically, the bacterial trait of reductive immobilization of Cr6+ derivatives is of great advantage in bioremediation. The present review is an effort to underline the bacterial resistance and reducing mechanisms to Cr6+ compounds with recent development in order to garner a broad perspective.