G. Praveen Kumar

In Vitro Screening for Abiotic Stress Tolerance in Potent Biocontrol and Plant Growth Promoting Strains of Pseudomonas and Bacillus spp.

Plant growth promoting rhizobacteria (PGPR) has been identified as a group of microbes that are used for plant growth enhancement and biocontrol for management of plant diseases. The inconsistency in performance of these bacteria from laboratory to field conditions is compounded due to the prevailing abiotic stresses in the field. Therefore, selection of bacterial strains with tolerance to abiotic stresses would benefit the end-user by successful establishment of the strain for showing desired effects. In this study we attempted to isolate and identify strains of Bacillus and Pseudomonas spp. with stress tolerance and proven ability to inhibit the growth of potential phytopathogenic fungi. Screening of bacterial strains for high temperature (50°C), salinity (7% NaCl), and drought (-1.2 MPa) showed that stress tolerance was pronounced less in Pseudomonas isolates than in Bacillus strains. The reason behind this could be the formation of endospores by Bacillus isolates. Tolerance to drought was high in Pseudomonas strains than the other two stresses. Three strains, P8, P20 and P21 showed both salinity and temperature tolerance. P59 strain possessed promising antagonistic activity and drought tolerance. The magnitude of antagonism shown by Bacillus isolates was also higher when compared to Pseudomonas strains. To conclude, identification of microbial candidate strains with stress tolerance and other added characteristic features would help the end-user obtain the desired beneficial effects.

Characterization of rhizobial isolates nodulating Millettia pinnata in India

Millettia pinnata (Synonym Pongamia pinnata) is a viable source of oil for the mushrooming biofuel industry, source for agroforestry, urban landscaping, and the bio-amelioration of degraded lands. It also helps in maintaining soil fertility through symbiotic nitrogen fixation. However, not much work is reported on classification and characterization of the rhizobia associated with this plant. In the present study, an attempt was made to isolate rhizobial strains nodulating Millettia from soils collected from southern regions of India. The isolates were characterized using numerical taxonomy, 16S rRNA gene sequencing, and cross nodulation ability. The results showed high phenotypic and genetic diversity among the rhizobia symbiotic with Millattia pinnata. The isolates formed five clusters at similarity level of 0.82 based on the results of numerical taxonomy. Results on 16S rRNA gene sequence analysis revealed that most microsymbionts of M. pinnata belonged to Rhizobium and Bradyrhizobium, which are closely related to Rhizobium sp., B. elkanii and B. yuanmingense. Among these isolates, some isolates could grow in a pH range of 4.0-10.0, some could tolerate a high salt concentration (3% NaCl) and could grow at a maximum temperature between 35 and 45 °C. M. pinnata formed nodules with diverse rhizobia in Indian soils. These results offered the first systematic information about the microsymbionts of M. pinnata grown in the soils from southern part of India.

Exploiting Plant Growth Promoting Rhizomicroorganisms for Enhanced Crop Productivity

The increasing pressure on land resources has made it imperative for vertical growth through enhanced crop intensity and productivity. To meet this challenge, appropriate integrated nutrient and pest management packages must be configured for different agro-ecological conditions. By 2050, the crop nitrogen demand is expected to reach 40–45 million tonnes. To meet such enormous nitrogen requirements through chemical fertilizers, would not only be expensive but also could severely degrade soil health. Similar is the situation with other macro- and micro-nutrients. The rhizosphere environment, at the interface between root and soil, is a major habitat for soil processes. Rhizosphere biology is approaching a century of investigations, wherein growth-promoting rhizomicroorganisms such as Rhizobium, Azotobacter, Pseudomonas, Bacillus, Azospirillum, Frankia and mycorrhizal fungi have attracted special attention on account of their beneficial activities. Plant growth promoting rhizomicroorganisms (PGPR) include diverse microbes that influence plant health by colonizing roots, enhancing plant growth, reducing plant pathogen populations and activating plant defenses against biotic stresses. PGPRs promote plant growth in different ways such as influencing plant hormonal balance, antagonistic to pathogens through various modes, stimulation of plant resistance/defense mechanisms, effects nutrient uptake by secretion of organic acids or protons to solubilize nutrients, atmospheric N2 fixation and by modifying rhizospheric soil environment by exo-polysaccharides production. Though research was going on in isolation in the above areas, with the advent of a core group for PGPR research, the pace in this direction has significantly increased. The primary emphasis on exploiting the vast biodiversity of microorganisms to identify the beneficial strains has yielded very good results. However, most of the research is yet to reach the end-users. For effective transfer of these technologies, there is a need for functional networking of research, industry and extension systems. In this paper, we describe the recent advances in PGPR research and the future needs to strengthen PGPR research and development that will transfer the benefits to the end-users for enhanced and sustainable farm productivity hence contributing towards food security challenges.

Exploiting PGPR and AMF Biodiversity for Plant Health Management

Indian subcontinent is one of the mega hotspots for biodiversity including microbes. So far, only very little microbial diversity has been harnessed for human and animal welfare. The importance of soil microorganisms in plant health management is well known. The interaction between bacteria and plant roots may be beneficial, harmful, or neutral for the plant, and sometimes the effect of a particular organism may vary as the soil conditions change. Among the diverse range of plant growth-promoting rhizobacteria (PGPR) identified, bacterial species such as Pseudomonas and Bacillus spp. have a wide distribution. The mechanisms by which PGPR enhance plant growth include plant growth promoters, resistance inducers, biochemicals, etc. The nitrogen-fixing bacteria are known to enhance growth of plants by means of symbiotic or free-living association with plants. Arbuscular mycorrhizal fungi (AMF) enhancing plant growth has been reported by several workers. These microbes could be inoculated either singly or in combinations to deliver maximum benefits to the plants. For instance, combined inoculation of AMF with other PGPR exerted positive effects on the growth of several crop plants. By exploiting the microbial biodiversity, the input cost in agricultural production systems could be reduced considerably and thereby make agriculture a sustainable venture especially for small and marginal farmers whose resources are limited. Microbes can supplement nutrients to the plants, induce resistance against biotic and abiotic stresses, protect from insect pests and plant pathogens, manage weeds and nematodes, etc. In this chapter, the rich microbial biodiversity, its systematic characterization, cataloguing, and evaluation for improving agricultural production in an economical and eco-friendly way have been discussed.

Seed Bacterization with Fluorescent Pseudomonas spp. Enhances Nutrient Uptake and Growth of Cajanus cajan L.

Among plant-growth-promoting rhizobacteria (PGPR), fluorescent Pseudomonas spp. are an important group affecting plant growth. Pigeon pea is an important pulse crop and most of the studies were aimed at using Pseudomonas spp. for pest management in pigeon pea. Seventy-five fluorescent Pseudomonas spp. were isolated from diverse agroecosystems of India and evaluated for their plant-growth-promoting ability, primarily by the paper cup method. Seventeen selected isolates were further evaluated by short-term pot assay for plant growth promotion. Seeds treated with bacteria showed greater nutrient concentration and growth than the control. Isolate P17 showed significant growth promotion in terms of root length (54.5 cm), dry mass (323 mg), chlorophyll (24 spad units), carbohydrates (21.2 percent), nitrogen (2.45 percent), calcium (1.82 percent), iron (984 ppm), and manganese (564 ppm). Pseudomonas sp. P17 strain was identified as a potential PGPR for nutrient uptake and plant growth promotion in pigeon pea, and this finding paves a way for integrated plant nutrient management in rainfed agroecosystems.