Mohd. Saghir Khan

Heavy metal induced oxidative damage and root morphology alterations of maize (Zea mays L.) plants and stress mitigation by metal tolerant nitrogen fixing Azotobacter chroococcum

Heavy metals are one of the major abiotic stresses that adversely affect the quantity and nutritive value of maize. Microbial management involving the use of plant growth promoting rhizobacteria (PGPR) is a promising inexpensive strategy for metal clean up from polluted soils. Considering these, metal tolerant plant growth promoting nitrogen fixing rhizobacterial strain CAZ3 identified by 16SrRNA gene sequence analysis as Azotobacter chroococcum was recovered from metal polluted chilli rhizosphere. When exposed to varying levels of metals, A. chroococcum survived up to 1400 and 2000 µg mL-1 of Cu and Pb, respectively and expressed numerous plant growth promoting activities even under metal stress. Strain CAZ3 secreted 65.5 and 60.8 µg mL-1 IAA at 400 µg mL-1 each of Cu and Pb, respectively and produced siderophores, ammonia and ACC deaminase under metal pressure. The melanin extracted from A. chroococcum revealed metal chelating ability under EDX. Following application, strain CAZ3 enhanced growth and yield of maize grown both in the presence of Cu and Pb. The dry biomass of roots of inoculated plants grown with 2007 mg Cu kg-1 and 585 mg Pb kg-1 was increased by 28% and 20%, respectively. At 585 mg Pb kg-1, the bioinoculant also increased the kernel attributes. At 2007 mg Cu kg-1 strain CAZ3 enhanced the number, yield and protein of kernels by 10%, 45% and 6%, respectively. Interestingly, strain CAZ3 significantly reduced the levels of proline, malondialdehyde and antioxidant enzymes in foliage. The roots of inoculated plants accumulated greatest amounts of metals compared to other organs. In kernels, the concentration of Pb was more as compared to Cu. The metal concentrations in roots, shoots and kernels, however, declined following CAZ3 inoculation. Copper and lead had substantial distortive impact on root and leaf morphology while cell death were visible under CLSM and SEM. Conclusively, A. chroococcum CAZ3 could be a most suitable and promising option to increase maize production in metal polluted soils despite the soils being contaminated with heavy metals.

Perspectives of Plant Growth Promoting Rhizobacteria in Growth Enhancement and Sustainable Production of Tomato

Tomato is an important horticultural product with a high content of bioactive compounds such as folate, ascorbate, polyphenols, and carotenoids and many other essential nutrients. Due to these, tomatoes are considered extremely valuable to human health. To optimize tomato production, chemical fertilizers and pesticides are frequently used. These chemicals are however, destructive for both crops and soil ecosystems. A reduction of these detrimental practices is therefore urgently required to protect both tomato and environments from damaging effects of agrochemicals. In this context, microbial inoculation especially those consisting of plant growth-promoting rhizobacteria (PGPR) could be used to replace chemical fertilizers/pesticides. Also, PGPR can be integrated with such chemical practices to reduce their application in tomato cultivation. Plant growth-promoting rhizobacteria that naturally inhabit the rhizosphere stimulate the growth and development of tomato plants directly or indirectly via availability of many essential plant nutrients, phytohormones, or through suppression/destruction of plant diseases. A better understanding of the plant growth-promotion activity of these bacterial strains is likely to enhance the production of safe, fresh, and high-quality tomatoes while reducing chemical inputs in different agronomic setups.

Fungicide tolerant Bradyrhizobium japonicum mitigate toxicity and enhance greengram production under hexaconazole stress

Bacterial strain RV9 recovered from greengram nodules tolerated 2400μg/mL of hexaconazole and was identified by 16S rDNA sequence analysis as Bradyrhizobium japonicum (KY940048). Strain RV9 produced IAA (61.6μg/mL), ACC deaminase (51.7mg/(protein·hr)), solubilized TCP (105μg/mL), secreted 337.6μg/mL EPS, and produced SA (52.2μg/mL) and 2,3-DHBA (28.3μg/mL). Exopolysaccharides produced by strain RV9 was quantified and characterized by SEM, AFM, EDX and FTIR. Beyond tolerance limit, hexaconazole caused cellular impairment and reduced the viability of strain RV9 revealed by SEM and CLSM. Hexaconazole distorted the root tips and altered nodule structure leading thereby to reduction in the performance of greengram. Also, the level of antioxidant enzymes, proline, TBARS, ROS and cell death was increased in hexaconazole treated plants. CLSM images revealed a concentration dependent increase in the characteristic green and blue fluorescence of hexaconazole treated roots. The application of B. japonicum strain RV9 alleviated the fungicide toxicity and improved the measured plant characteristics. Also, rhizobial cells were localized inside tissues as revealed by CLSM. Colonization of B. japonicum strain RV9 decreased the levels of CAT, POD, APX, GPX and TBARS by 80%, 5%, 13%, 13% and 19%, respectively over plants grown at 80μg/(hexaconazole·kg) soil. The ability to detoxify hexaconazole, colonize plant tissues, secrete PGP bioactive molecules even under fungicide pressure and its unique ability to diminish oxidative stress make B. japonicum an attractive choice for remediation of fungicide polluted soils and to concurrently enhance greengram production under stressed environment.

Recent Advances in Management Strategies of Vegetable Diseases

Vegetables are one of the most important components of human foods since they provide proteins, vitamins, carbohydrates and some other essential macro- and micronutrients required by the human body. Phytopathogenic diseases, however, cause huge losses to vegetables during cultivation, transportation and storage. To protect vegetable losses, various strategies including chemicals and biological practices are used worldwide. Pesticides among agrochemicals have however been found expensive and disruptive. Due to the negative health effects of chemical fungicides via food chain, the recent trend is shifting towards safer and more eco-friendly biological alternatives for the control of vegetable diseases. Of the various biological approaches, the use of antagonistic microorganisms is becoming more popular throughout the world due to low cost and environment safety. Numerous phytopathogenic diseases can now be controlled by microbial antagonists which employ several mechanisms such as antibiosis, direct parasitism, induced resistance, production of cell wall-lysing/cell wall-degrading enzymes, and competition for nutrients and space. The most commonly used biological control agents belong to the genera, Bacillus, Pseudomonas, Flavobacterium, Enterobacter, Azotobacter, Azospirillum and Trichoderma, and some of the commercial biocontrol products developed and registered for the use against phytopathogens are Aspire, BioSave, Shemer etc. Here, an attempt is made to highlight the mechanistic basis of vegetable disease suppression by some commonly applied microbiota. This information is likely to help vegetable growers to reduce dependence on chemicals and to produce fresh and healthy vegetables in different production systems.

Growth Improvement and Management of Vegetable Diseases by Plant Growth-Promoting Rhizobacteria

Vegetables are an important part of human dietary systems. They contain several important nutrients including vitamins, antioxidants, etc. and affect immensely the human health. Vegetables are cultivated and consumed globally on a large scale and serve as the food of choice for millions of people across the globe. During cultivation, most of the vegetable crops are, however, often attacked by various insect pests and pathogenic microorganisms, thereby causing severe diseases, leading to huge yield losses. The agricultural practitioners depend heavily on chemical fertilizers to supply nutrients to vegetables while they apply pesticides to manage insect pests and to concurrently enhance vegetable production. The injudicious application of agrochemicals including pesticides into vegetable production practices adversely affects the soil fertility and consequently the plant health, thus making it unfit for human consumption. In order to protect the crops and to minimize yield losses due to phytopathogens, an alternate and inexpensive approach involving the use of plant growth-promoting rhizobacteria (PGPR) has been introduced into the vegetable production system. The application of PGPR formulations into the vegetable production strategies has been found to protect them from various diseases leading to improved yield and quality of the vegetables. The present chapter focuses on the disease incidence among some of the popularly grown vegetables and the role of PGPR in suppression of common vegetable diseases.

Metal Toxicity to Certain Vegetables and Bioremediation of Metal-Polluted Soils

The production of quality vegetables is a crucial issue worldwide due to consistently deteriorating soil health. Plants including vegetables absorb a number of metals from soil, some of which have no biological function, but some are toxic at low concentrations, while others are required at low concentration but are toxic at higher concentrations. As vegetables constitute a major source of nutrition and are an important dietary constituent, the heavy metal uptake and bioaccumulation in vegetables is important since it disrupts production and quality of vegetables and consequently affects human health via food chain. Considering the serious threat of metals to vegetables, an attempt in this chapter is made to highlight the effects of certain metals on vegetables grown in different agroclimatic regions of the world. Also, the bioremediation strategies adopted to clean up the metal-contaminated soil is discussed. The results of different studies conducted across the globe on metal toxicity and bioremediation strategies presented in this chapter are likely to help vegetable growers to produce fresh and contaminant-free vegetables.

Metal-Legume-Microbe Interactions: Toxicity and Remediation

Heavy metals discharged from various sources accumulate within soils and disrupt ecosystems. The toxic metals are taken up by beneficial soil microbiota and growing plants and cause potential human risks via food chain. Also, heavy metals seriously affect the microbial compositions and their physiological functions. Among plant species, legumes play an important role in human dietary systems and supply nitrogen to legumes through symbiosis with rhizobia. Metals when present in legume habitat act as a devastating stress factor and restrict the growth of rhizobia, legumes, and legume-Rhizobium symbiosis. Several physical and chemical methods have been developed to remediate heavy metal-polluted soils, but these methods are unacceptable due to their high cost, and they are not environmentally friendly. Therefore, the use of metal-tolerant/metal-detoxifying microbes collectively called bioremediation offers a sustainable and low-cost option to clean up polluted soils. Besides remediation, the metal-tolerant microbes also promote plant growth by other direct or indirect means. Owing to the importance of legumes in maintaining soil fertility and human health, there is greater emphasis to identify the metal-resistant/metal-tolerant rhizobia and legume plants. The present chapter gives an in-depth insight into the impact of metals on rhizobia-legume symbiosis. Also, the role of metal-tolerant rhizobia in metal toxicity abatement is highlighted.

Inoculation Effects of Associative Plant Growth-Promoting Rhizobacteria on the Performance of Legumes

Constantly increasing human population requires that the crop production including those of legumes be enhanced rapidly to fulfill the food demands across the globe. In order to optimize pulse production, growers generally apply agrochemicals including fertilizers and pesticides. However, the excessive and uncontrolled use of such chemicals has resulted in reduced crop production besides their adverse impact on environment. In order to protect losses in soil fertility and to preserve environmental quality, the use of inexpensive and eco-friendly microbial preparations (biofertilizers) has been exploited in farming practices with remarkable success. Among various plant growth-promoting rhizobacteria (PGPR), the associative nitrogen-fixing PGPR, belonging to the genus Azospirillum, has long been employed as microbial inoculant worldwide to promote legume production. Azospirillum, when used as inoculant, increase the production of root hairs and root growth which in effect benefit plants with better absorption of water and nutrients. The inoculation of Azospirillum either alone or in combination with other beneficial PGPR has been found to increase N2 fixation and concomitantly the grain yield of legumes. Considering the importance of Azospirillum, this chapter highlights the role of Azospirillum in the production of legumes in different agronomic setup.