Govindan Selvakumar

Bacterial Mediated Alleviation of Abiotic Stress in Crops

With the advent of climate change, global agriculture faces a multitude of challenges. The most prominent amongst these are abiotic stresses imposed by increased incidences of drought, extremes of temperature, and unseasonal flooding. Such atmospheric threats, coupled with edaphic stresses that arise mainly as a result of anthropogenic activities, pose severe challenges to food production. While several agronomic and plant breeding strategies have been proposed to overcome these phenomena, the utilization of microbes, especially bacterial species that live in close association with plant organs, is receiving increased attention at present. Though most studies conducted so far are largely explorative in nature, they have provided valuable insights into the functioning of these microbes in various situations and their stress alleviation potential. This chapter attempts to comprehend the information that has been generated on the abiotic stress alleviating mechanisms of bacteria and their abiotic stress alleviation potential.

Cold-Tolerant Agriculturally Important Microorganisms

Cold-tolerant microorganisms are endowed with the ability to grow at 0°C, though their growth optima lie in the mesophilic range. To overcome the stress induced by low temperatures they have evolved a variety of adaptive responses at the cellular and molecular levels. Multiple cell membrane modifications ensure that solute transport is not impaired at low temperatures. Other mechanisms include the synthesis of cold-shock proteins (Csps), cold acclimation proteins (Caps), cryoprotectants, ice nucleation factors, cold-adapted enzymes, and RNA degradosomes. The agricultural importance of such microbes stems from the fact that the world over temperate agro-ecosystems are characterized by low temperatures and short growing seasons that subject both plant and microbial life to cold temperature induced stress. Hence, there is a need to identify potential microbes that retain their functional traits under low temperature conditions. Such microbes can be profitably used as inoculants in agricultural production systems in the temperate regions of the world. This chapter deals with the cold tolerance/resistance mechanisms operating in microorganisms and the utility of cold-tolerant microbes in improving soil quality and productivity of agricultural crops.

Diversity Utility and Potential of Actinobacteria in the Agro-Ecosystem

Actinobacteria formerly referred to as Actinomycetes are Gram-positive saprophytic bacteria, with widespread distribution in nature. They occur in the terrestrial and aquatic environments, and play a dominant role in natural geochemical cycles. Mankind’s interest in Actinobacteria, originated in the early nineteenth century, primarily due to their abilities to decompose organic matter and produce antibiotics. Amongst Actinobacteria, the genus Streptomyces has received widespread attention due to its ability to produce biologically active compounds that have been widely exploited against infectious agents. But emerging trends in microbial architecture and taxonomy have led to the re-defining of this group of microbes, and have led to the inclusion of several known and novel bacterial genera and species, within the Phylum Actinobacteria. These are widespread in the agro-environment, especially in the rhizosphere, where they produce a wide range of biologically active metabolites and influence plant development in a myriad fashion. We attempt to capture the existing information on the diversity and utility of Actinobacteria in the agro-environment, and the interventions that are required in the future in order to fully exploit this class of microbes, for the benefit of mankind.

Organic Acids in the Rhizosphere: Their Role in Phosphate Dissolution

Phosphorus is an essential plant nutrient that is made available to plants primarily from the soil phosphorus reserves. But its limited mobility in the soil and high fixation capabilities within in the soil matrix necessitate the use of fertilizer forms of phosphorus, which are again prone to fixation, thereby reducing the availability of this crucial element for plant nutrition. Soil microbes play a crucial role in mobilizing various forms of phosphorus (inorganic and organic) and making them available for plant nutrition. Microbe-mediated phosphorus mobilizing processes involve either organic acids that solubilize the inorganic forms of phosphorus or enzymes that mobilize the organic sources of phosphorus. The organic acids that play a crucial role in the dissolution of phosphates can be of plant and microbial origins and vary in their nature and properties depending on the soil, plant, and microbial species involved. Besides playing a crucial role in P cycling, they also perform assorted functions that have a direct bearing on the plant growth and development. This chapter attempts to capture the information on the nature, properties, and functions of organic acids in the rhizosphere.

Draft Genome Sequence of Phosphate-Solubilizing Bacterium Paraburkholderia tropica Strain P-31 Isolated from Pomegranate (Punica granatum) Rhizosphere

ABSTRACT We report the 8.9 Mb draft genome sequence of phosphate-solubilizing bacterium Paraburkholderia tropica strain P-31, isolated from pomegranate (Punica granatum) rhizosphere. The draft genome sequence of Paraburkholderia tropica strain P-31 consists of 8,881,246 bp with a G+C content of 64.7%, 8,039 protein-coding genes, and 49 RNAs.

Natural mycosis of mango leafhoppers (Cicadellidae: Hemiptera) by Fusarium sp.

Mango leafhoppers that feed on inflorescences and young shoots of mango (Mangifera indica L.) were found mycotized under natural conditions in Bangalore, India. Isolation and characterisation of the etiological agents by sequencing of the Translation Elongation Factor-1α gene, revealed 99% identity with the plant pathogenic fungus Fusarium proliferatum. This is an early report on the Fusarium associated entomopathogenicity in different mango leafhopper species.

Legume Root Nodule Associated Bacteria

Root nodules have intrigued mankind ever since their role in the maintenance of soil fertility has been known. The earlier school of thought amongst microbiologists and agronomists was that root nodules are highly specialised structures rich in leghaemoglobin, which house the diazotrophic bacterium Rhizobium, whose primary role was to fix atmospheric nitrogen in association with the host plant. But several path-breaking discoveries over the past few decades have thrown light on the plethora of bacterial occupants of the root nodules and their possible role in nodulation and N fixation besides several other beneficial roles. Recent technological advances in bacterial taxonomy and microbial ecology have unearthed a wide range of microbial nodule occupants, some of which have been encompassed under the classical umbrella of rhizobia, purely based on their ability to nodulate the host and fix atmospheric nitrogen, while other closely or even distantly related bacterial genera devoid of the ability to nodulate and fix nitrogen in nodules are often referred to as endophytes or simply nodule inhabitants. This chapter attempts to capture the existing knowledge on the root nodule associated bacteria both rhizobial and non-rhizobial and their possible roles in sustaining plant growth.

Burkholderia to Paraburkholderia: The Journey of a Plant-Beneficial-Environmental Bacterium

The genus Burkholderia is a versatile member of class Proteobacteria with over a hundred validly described species. Though endowed with a vast ecological diversity and metabolic versatility, the agro-biotechnological use of members of this genus has remained highly restricted over the past few decades owing to the pathogenic nature of nearly twenty species classified as the Burkholderia cepacia complex (Bcc), B. pseudomallei the causative agent of melioidosis, B. mallei the causative agent of glanders disease in equines and a few plant pathogenic species. Despite the presence of several environmental isolates with beneficial traits, they were overshadowed by their pathogenic relatives. Though initial attempts were made to segregate the clinical and environmental isolates based on the 16S rRNA gene sequences and multilocus sequence typing (MLST), they have failed to remove the stigma associated with the genus. In order to enable the utilization of this genus in agro-biotechnological applications, attempts were made to bifurcate the genus based on phylogenetic evidence. While an earlier attempt to this effect was unsuccessful, the attempt in describing the novel genus Paraburkholderia based on the presence of conserved sequence indels and its subsequent taxonomical validation have opened up the possibilities of utilizing this genus that largely remains untainted by any pathogenic potential. But the widespread use of members of this novel genus has to follow a cautious path in order to eliminate any possibility of mammalian pathogenicity and the possible transfer of virulence genes from the members of genus Burkholderia. If such concerted steps are taken up, we shall be adding one more potential genus for agro-biotechnological applications.

Rhizocompetence of Applied Bioinoculants

Concomitant with the demand for chemical free food, the demand for bioinoculants for plant growth promotion and protection against pests and disease causing organisms has also seen a phenomenal increase. This has led to the mushrooming of several products in the market that have met with varying degrees of success. Very often it has been observed that inoculant strains that perform exceedingly well under laboratory conditions fail under field conditions. This can be primarily attributed to the utilization of non-rhizocompetent strains. Since the inoculated strain has to compete with a multitude of native microbes in the rhizospheric region, strains lacking rhizocompetence traits often fail to establish and perform in the rhizosphere. Rhizocompetence traits such as biofilm formation, siderophore production, antagonism, ability to utilize root exudates, motility, and protease activity can prove to be game changers under field conditions. This chapter attempts to highlight the importance of rhizocompetence traits in inoculant selection and development, in order to harness the benefit of applied inoculants.

Potential and Prospects of Aerobic Endospore-Forming Bacteria (AEFB) in Crop Production

Members of Bacillus and the genera derived from it are an ubiquitous and important component of the agroecosystem. The diverse roles essayed by these bacteria in crop production range from nutrient cycling to protection of crops from various biotic and abiotic stress factors. The versatility and ecological fitness of this bacterial group have been attributed to its ability to form hardy endospores that help them tide over stress conditions and confers a survival advantage in the rhizosphere and related environmental niches, during unfavorable times. This chapter attempts to briefly explore the historical evolution of this group of bacteria from a two-species genus to the present-day Bacillus and the whole gamut of Bacillus-derived genera, both of which constitute the broader umbrella term, viz., aerobic endospore-forming bacteria (AEFB). The various functional facets of AEFBs in crop production and the ways and means to exploit them as functional bio-inoculants for the future are discussed.