Azeem Khalid

Screening plant growth-promoting rhizobacteria for improving growth and yield of wheat

Aims:u2002 Plant growth promoting rhizobacteria (PGPR) are commonly used as inoculants for improving the growth and yield of agricultural crops, however screening for the selection of effective PGPR strains is very critical. This study focuses on the screening of effective PGPR strains on the basis of their potential for inu2003vitro auxin production and plant growth promoting activity under gnotobiotic conditions.

Accelerated decolorization of structurally different azo dyes by newly isolated bacterial strains

Wastewater effluents from the textile and other dye-stuff industries contain significant amounts of synthetic dyes that require treatment to prevent groundwater contamination. In research aimed at biotechnology for treatment of azo dyes, this study examined 288 strains of azo-dye degrading bacteria to identify efficient strains and determine incubation times required for decolorization. Initial enrichment cultures were carried out using a mixture of four structurally different dyes (Acid Red 88, Reactive Black 5, Direct Red 81, and Disperse Orange 3) as the sole source of C and N to isolate the bacteria from soil, activated sludge, and natural asphalt. Six strains were selected for further study based on their prolific growth and ability to rapidly decolorize the dyes individually or in mixtures. Treatment times required by the most efficient strain, AS96 (Shewanella putrefaciens) were as short as 4xa0h for complete decolorization of 100xa0mg l−1 of AR-88 and DR-81 dyes under static conditions, and 6 and 8xa0h, respectively, for complete decolorization of RB-5 and DO-3. To our knowledge, these bacterial strains are the most efficient azo-dye degrading bacteria that have been described and may have practical application for biological treatment of dye-polluted wastewater streams.

Biodegradation of α- and β-endosulfan by soil bacteria

Extensive applications of persistent organochlorine pesticides like endosulfan on cotton have led to the contamination of soil and water environments at several sites in Pakistan. Microbial degradation offers an effective approach to remove such toxicants from the environment. This study reports the isolation of highly efficient endosulfan degrading bacterial strains from soil. A total of 29 bacterial strains were isolated through enrichment technique from 15 specific sites using endosulfan as sole sulfur source. The strains differed substantially in their potential to degrade endosulfan in vitro ranging from 40 to 93% of the spiked amount (100xa0mgxa0l−1). During the initial 3xa0days of incubation, there was very little degradation but it got accelerated as the incubation period proceeded. Biodegradation of endosulfan by these bacteria also resulted in substantial decrease in pH of the broth from 8.2 to 3.7 within 14xa0days of incubation. The utilization of endosulfan was accompanied by increased optical densities (OD595) of the broth ranging from 0.511 to 0.890. High performance liquid chromatography analyses revealed that endosulfan diol and endosulfan ether were among the products of endosulfan metabolism by these bacterial strains while endosulfan sulfate, a persistent and toxic metabolite of endosulfan, was not detected in any case. The presence of endosulfan diol and endosulfan ether in the bacterial metabolites was further confirmed by GC-MS. Abiotic degradation contributed up to 21% of the spiked amount. The three bacterial strains, Pseudomonas spinosa, P. aeruginosa, and Burkholderia cepacia, were the most efficient degraders of both α- and β-endosulfan as they consumed more than 90% of the spiked amount (100xa0mgxa0l−1) in the broth within 14xa0days of incubation. Maximum biodegradation by these three selected efficient bacterial strains was observed at an initial pH of 8.0 and at an incubation temperature of 30°C. The results of this study may imply that these bacterial strains could be employed for bioremediation of endosulfan polluted soil and water environments.

Decolorization of azo dyes by Shewanella sp. under saline conditions

Wastewaters from textile processing and dye-stuff manufacture industries contain substantial amounts of salts in addition to azo dye residues. To examine salinity effects on dye-degrading bacteria, a study was carried out with four azo dyes in the presence of varying concentrations of NaCl (0–100xa0g l−1) with a previously isolated bacterium, Shewanella putrefaciens strain AS96. Under static, low oxygen conditions, the bacterium decolorized 100xa0mg dye l−1 at salt concentrations up to 60xa0g NaCl l−1. There was an inverse relationship between the velocity of the decolorization reaction and salt concentration over the range between 5 and 60xa0g NaCl l−1 and at dye concentrations between 100 and 500xa0mg l−1. The addition of either glucose (C source) or NH4NO3 (N source) to the medium strongly inhibited the decolorization process, while yeast extract (4xa0g l−1) and Ca(H2PO4)2·H2O (1xa0g l−1) both enhanced decolorization rates. High-performance liquid chromatography analysis demonstrated the presence of 1-amino-2-naphthol, sulfanilic acid and nitroaniline as the major metabolic products of the azo dyes, which could be further degraded by a shift to aerobic conditions. These findings show that Shewanella could be effective for the treatment of dye-containing industrial effluents containing high concentrations of salt.

Bio-conversion of organic wastes for their recycling in agriculture : an overview of perspectives and prospects

Largely accessible organic wastes can be turned into valuable compost product for raising crops organically on one hand, and get them disposed off safely at the other end. Straight use of organic wastes has tribulations like transportation and handling, wider C:N ratio, high application rates, nutrient overloading, weed seeds, pathogens, and metal toxicities. Composting bestows a tactic for coping high volumes of organic wastes in environmentally sound and desirable manners. Composted materials are remarkably regarded for their ability to improve soil health and plant growth, and suppress pathogens and plant diseases. Currently several composting systems have become available; ranging from a crude and slow windrows method, to the most speedy and computer monitored in-vessel system. Scientific investigations of this biological cum chemical process have reached to molecular level. Value addition of compost through beneficial microorganisms, mineral materials and fertilisers is also being considered. The nature and composition of materials put into composting is imperative for its quality rationale. On the whole, principles and processes governing composting are not so straightforward that ordinary enterprises could develop efficient composting facilities for the treatment of organic wastes. In this scenario, accessibility of comprehensive information to the scientific community as well as environmental protection agencies is imperative. This review article brings together the current information necessary for effective composting of organic wastes from different origins with diversified characteristics under various situations. It also covers the schematic description of well known composting systems, and various factors controlling the process.

Relative efficiency of rhizobacteria for auxin biosynthesis in rhizosphere and non-rhizosphere soils

Biosynthesis of auxins in the rhizosphere of different crops may vary because of quantitative and qualitative variations in microbial population and root exudation. A laboratory study was conducted to assess in vitro auxin biosynthesis, and biosynthesis in rhizosphere and non-rhizosphere soils of different crops (maize, sorghum, mungbean, cotton). Soils were inoculated with selected rhizobacteria with and without the auxin precursor L-tryptophan (L-TRP). Auxins were detected by colourimetry as indole acetic acid equivalents and confirmed by high performance liquid chromatography. Results revealed that 83% of the 60 rhizobacteria were capable of producing auxins in the absence of L-TRP. Auxin biosynthesis by the 8 most efficient rhizobacteria ranged from 5.0 to 12.1 mg/L broth medium. A comparison of rhizosphere v. non-rhizosphere soils indicated a greater accumulation of auxins in the rhizosphere soils than non-rhizosphere soils. Overall, inoculation of rhizosphere soils with selected rhizobacteria resulted in greater production of auxin (up to 10.4 mg/kg soil) than in inoculated non-rhizosphere soils (up to 5.76 mg/kg). Moreover, efficiency of these rhizobacteria for auxin biosynthesis in both rhizosphere and non-rhizosphere soils differed with crop and bacterial strain. Some rhizobacterial strains exhibited superiority over the indigenous microflora for auxin biosynthesis in soil. Application of L-TRP promoted auxin biosynthesis in both rhizosphere and non-rhizosphere soils. These findings imply that inoculation with suitable strains and/or amendment with L-TRP could promote auxin synthesis in the rhizosphere soil of a given crop, which may have consequences for better plant/crop growth.

Plant Growth Promoting Rhizobacteria and Sustainable Agriculture

The diverse groups of bacteria in close association with roots and capable of stimulating plant growth by any mechanism(s) of action are referred to as plant growth-promoting rhizobacteria (PGPR). They affect plant growth and development directly or indirectly either by releasing plant growth regulators (PGRs) or other biologically active substances, altering endogenous levels of PGRs, enhancing availability and uptake of nutrients through fixation and mobilization, reducing harmful effects of pathogenic microorganisms on plants and/or by employing multiple mechanisms of action. Recently, PGPR have received more attention for use as a biofertilizer for the sustainability of agro-ecosystems. Selection of efficient PGPR strains based on well-defined mechanism(s) for the formulation of biofertilizers is vital for achieving consistent and reproducible results under field conditions. Numerous studies have suggested that PGPR-based biofertilizers could be used as effective supplements to chemical fertilizers to promote crop yields on sustainable basis. Various aspects of PGPR biotechnology are reviewed and discussed.

Effectiveness of Organic-/Bio-Fertilizer Supplemented with Chemical Fertilizers for Improving Soil Water Retention, Aggregate Stability, Growth and Nutrient Uptake of Maize (Zea mays L.)

Abstract In this study, an organic-fertilizer was prepared by composting fruit and vegetable wastes in a locally fabricated unit and enriching it with N applied at the rate of 147 g kg−1 compost. This “organic-fertilizer” was also used as a carrier for PGPR strain, Pseudomonas fluorescens biotype G (N3) containing ACC-deaminase to formulate a bio-fertilizer. The organic- and/or bio-fertilizers were applied at 300 kg ha−1 to maize in pots/plots supplemented with 0, 88 or 132 kg ha−1 urea-N. A basal dose of P and K (100 and 50 kg ha−1, respectively) was applied to all pots/plots. Results of two-years pot and field trials revealed that the organic-fertilizer supplemented with 88 kg ha−1 N was equally effective compared to full dose of N-fertilizer (175 kg ha−1) in improving root weight, fresh biomass, and ear and grain yields of maize. However, bio-fertilizer supplemented either with 88 or 132 kg N ha −1 significantly increased the growth and yield of maize over full dose of N-fertilizer and exhibited superiority over organic-fertilizer. Organic-/bio-fertilizer application significantly enhanced N and P uptakes while substantially reducing the rate of water loss from the soil and increased aggregate stability. Results may imply that organic waste could be composted into value-added soil amendment by enriching/blending it with N and PGPR containing ACC-deaminase activity. The novelty of this approach is that the organic- or bio-fertilizer was used at a low rate (just 300 kg ha−1). Moreover, this strategy could also be useful to protect our environments against threat posed by organic wastes.

Effect of substrate-dependent microbial ethylene production on plant growth

Various compounds have been identified as precursors/substrates for the synthesis of ethylene (C2H4) in soil. This study was designed to compare the efficiency of four substrates, namely L-methionine (L-MET), 2-keto-4-methylthiobutyric acid (KMBA), 1-aminocyclopropane-1-carboxylic acid (ACC), and calcium carbide (CaC2), for ethylene biosynthesis in a sandy clay loam soil by gas chromatography. The classic “triple” response in etiolated pea seedling was employed as a bioassay to demonstrate the effect of substrate-dependent microbial production of ethylene on plant growth. Results revealed that an amendment with L-MET, KMBA, ACC (up to 0.10 g/kg soil) and CaC2 (0.20 g/kg soil) significantly stimulated ethylene biosynthesis in soil. Overall, ACC proved to be the most effective substrate for ethylene production (1434 nmol/kg soil), followed by KMBA, L-MET, and CaC2 in descending order. Results further revealed that ethylene accumulation in soil from these substrates caused a classic “triple” response in etiolated pea seedlings with different degrees of efficacy. A more obvious classic “triple” response was observed at 0.15, 0.10, and 0.20 g/kg soil of L-MET, KMBA/ACC, and CaC2, respectively. Similarly, direct exposure of etiolated pea seedlings to commercial ethylene gas also modified the growth pattern in the same way. A significant direct correlation (r = 0.86 to 0.97) between substrate-derived C2H4 and the classic triple response in etiolated pea seedlings was observed. This study demonstrated that the presence of substrate(s) in soil may lead to increased ethylene concentration in the air of the soil, which may affect plant growth in a desired direction.

Effect of compost enriched with N and L-tryptophan on soil and maize

Composting provides an excellent way to manage the huge volume of organic waste and convert it into a useful soil amendment. The effectiveness of composted organic waste can be further improved by enriching and blending it with nutrients and biologically active substances. The resulting value-added composts can be used at substantially low rates such as a few hundred kg per ha compared with conventional use of organic wastes in tons per ha. This approach could have practical significance in reducing the use of chemical fertilizer for sustainable agriculture and the environment. L-tryptophan is a precursor of the growth hormone indole acetic acid and is known to stimulate plant growth at extremely low concentrations. Here, we studied the effect of composted fruit and vegetable wastes, enriched with N at 133 g kg−1 compost, with or without L-tryptophan at 10 mg kg−1 compost, on soil and maize crops. The enriched compost was applied at 300 kg ha−1 to a sandy clay loam soil either by mixing with the top 15-cm soil layer in pots or as a band placement along the maize plants grown in the field. The compost was applied alone and in combination with 40 or 80 kg ha−1 urea N and compared with a treatment containing 160 kg N ha−1, a full dose of N fertilizer alone, while P and K fertilizers were applied in all the treatments. Our results show that application of the enriched compost to soil increased aggregate stability by up to 24.8% and water retention by up to 43.1% compared with untreated control. A gradual increase in the concentration of indole acetic acid in compost, ranging from 1.02 to 3.34 mg kg−1, was observed when compost was treated with its precursor L-tryptophan. The results of pot and field experiments revealed that compost enriched with N and L-tryptophan in the presence of 80 kg N fertilizer significantly increased cob and grain yields, by up to 19.8 and 21.4%, respectively, compared with a full dose of N fertilizer. These findings suggest that enrichment of composted organic wastes with N and L-tryptophan can change them into a value-added organic product that could be used as a soil amendment at rates as low as 300 kg ha−1 to increase crop production on a sustainable basis.