Beneficial microbes in agriculture

M Chandrakumar Singh, Siddhartha Singh, Pramod Kumar
Microbes which are not considered to be with us are playing many vital roles in the field of agriculture since before their discovery. Even though many of them are our greatest enemies, many are of great importance to us. Beneficial microorganisms are those that can fix atmospheric nitrogen, decompose organic wastes and residues, detoxify pesticides, suppress plant diseases and soil-borne pathogens, enhance nutrient cycling and produce bioactive compounds such as vitamins, hormones and enzymes that stimulate plant growth. Microbes which are highly beneficial to us in respect of agriculture are termed as Agriculturally Important Microbes (AIM). Based on the aspect of their role in agriculture, they can be grouped into many groups like, Plant Growth Promoting Rhizobacteria (PGPR), Phosphate solubilizing microbes, Nitrogen Fixers (including both symbiotic and free living), Mycorrhizae, Bacterial insecticides, Microbial herbicides, Entomopathogenic fungi, etc. Microbial technologies have been applied to various agricultural problems with considerable success in recent years, but they have not been widely accepted by the scientific community because it is often difficult to consistently reproduce their beneficial effects. Need of application of microbial technologies by our farmer communities of north eastern region has to be increased as these technologies can at least decrease hazardous and deleterious effect on people and soil. This is possible only when they are very much acquainted with these unseen friends.
Plant Growth Promoting Rhizobacteria (PGPR)
Pseudomonas species are the most dominant rhizobacteria and are receiving greatest attention as PGPR. Species of other bacteria such as Bacillus, Azotobacter, Azospirillum, Burkholderia, Rhizobium, Bradyrhizobium, etc are also recognized as PGPR. PGPR improves plant growth and yield by either direct or indirect mechanisms. Direct mechanisms involved fixation of nitrogen that may be utilized by plants directly, solubilization of minerals like phosphorus or by direct production of plant growth regulators like Auxin, Gibberellins, Cytokinins, etc. Indirect mechanisms include growth promotion by producing siderophores (iron chelating compounds) or by production of antimicrobial compounds to suppress soil phytopathogens. Many members of PGPR have been commercialized under different trade names in supplements to chemicals for dose reduction of chemical fertilizers.
Phosphate solubilizing microbes
Many soil microbes have the capability to solubilize insoluble inorganic phosphates like tricalcium phosphates, hydroxyapatite, aluminium phosphate, rock phosphates, etc to soluble forms which can be readily used by the plants. Many members of bacteria like Bacillus subtilis, Bacillus polymyxa, Alcaligene sppl, Pseudomonas striata, Thiobacillus thiooxidans, etc are among good phosphate solubilizers. Fungi belonging to the genera Penicillium and Aspergillus possess the ability to convert insoluble phosphates to soluble forms. Actinomycetes belonging to the genera, Micromonospora, Streptomyces and Nocardia have also been reportd to have solubilized insoluble phosphates. Phosphate solubilization is done by two mechanisms, 1) solubilization by production of organic acids 2) solubilization by action of phosphatase enzyme. A carrier-based preparation of Pseudomonas striata and Bacillus ploymyxa under the name Microphos has been commercialized by IARI for our farming communities.
Nitrogen Fixers (symbiotic & free living)
Some microorganisms have the ability to fix the atmospheric nitrogen for the benefits of the plants as nitrogen is the major nutrient next to carbon. Nitrogen fixers can be of symbiotic, endophytic or free living. Species of Rhizobium, Bradyrhizobium, Azorhizobium, Mesorhizobium, burkholderia, Ralstonia, etc are nodule forming nitrogen fixers. Species of Azotobacter, Azospirillium, etc are free living nitrogen fixers. Many members of cynobacteria like Plectonema, Anabaena, Nostoc, Xenococcus, Pleurocapsa, etc are also reportd to fix atmospheric nitrogen. Their ability to fix atmospheric nitrogen lies in the enzyme nitrogenase produced by them. Commercialization of Azotobacter, Rhizobium etc like Agro-teeka, Rhizoteeka has been started s carrier based biofertilizers for use in field crops in India.
Mycorrhizae
Mycorrhizae (meaning fungus root) occur on almost all terrestrial plants. The fungal mycelium attached to the plant roots expands the ability to absorb nutrients fromn the soil. Mycorrhizal plants increase the surface area of the root system for better absorption of nutrients, especially phosphorus. The extent of symbiosis depends on fertility. Low fertility leads to greater mycorrhizal colonization and vice versa. Glomus, Gigaspora, Scutellospora, Acaulospora and Sclerocystis formed Arbuscular mycorrhizal association with many families of plants. Arbuscular mycorrhizal association with many families of plants. Arbuscular mycorrhizal biofertilizer helps plants in many ways like increased uptake of Nitrogen, Zinc, Phosphorus, disease tolerance, drought tolerance, etc.
Bacterial insecticides
Many bacterial species are pathogenic to many species of insects and act as insecticides. Among the bacterial insecticides Bacillus thuringiensis is reported to kill a wide range of insects like moths, beetles, mosquitoes, flies, aphids, etc. the insecticidal properties lie in the á-exotoxin, â-exotoxin, ä-endotoxin and louse factor produced by the bacterium. This bacterium is now widely known as Bt. Many transgenic plants have been developed using Bt. gene. Bacterial species like Bacillus subtilis, Bacillus papilliae, Pseudomonas cepacian; Serratia, Klebsiella, etc are also reported to be insect pathogen. Bt is produced commercially in the form of powder and administered to the insects by mixing it into their diet, with larvae being allowed to fed on the powder diet mixture.
Entomopathogenic fungi
A number of fungal species are pathogenic to several species of insects. These fungi are collectively called Entomopathogenic fungi. Metarhizium  anisopliae, Beauveria bassiana, Beauveria brongriartii, Verticillium lecanii, Nomuraea  rileyi, Hirsutella thompsonii, Paecilomyces fumosoroseus, etc. are the most studied entomopathogenic fungi. Productions of different types of toxins enable them to kill different species of insects. Many countries have started production of commercial formulation of entomopathogenic fungi to be used as pathogen against many species of insect species.
Microbial herbicides
Success has been made in using endemic or exotic plant pathogens to kill weeds. The microbial origin of herbicides is definitely a major contribution and alternative to chemical weedicides. Puccinia chondrillina against skeleton weed (Chondrilla juncea), Cercosporella riparia to control Ageratina riparia, Cercospora rodmanii to control water hyacinth have been established. The use of endemic microbial herbicides needs cooperation from various sources.
These potentials of our unseen friends need to be explored in such a way that their most beneficial part can be extracted consistently.
Attraction of scientific communities is required towards this aspect of using friendly microbes consistently in contribution or alternative to chemicals for overall benefits for the people of the country.

The writers are from College of Horticulture & Forestry, CAU, Pasighat
For further details contact:- Public Relation & Media Management Cell, CAU, Imphal. Email: prmmcell@gmail.com