For many decades soil bacteria are very important in bio and chemical cycles and have been used for crop production.
For the health of plant and soil fertility interactions plant bacterial interaction in the rhizosphere are the epitope. Soil beneficial bacteria to plant growth are usually called as plant growth-promoting rhizobacteria (PGPR) are capable of promoting plant growth by colonizing the plant root.
For plant growth interaction these are associated with the rhizosphere are which is an important soil ecological environment. PGPR can contribute to sustainable for plant growth advancement.
There are three main functions for PGPR are; synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil, and lessening are preventing the plants from diseases. Plant growth development can be facilitated by directly and indirectly.
Indirect plant growth developments include the prevention of injurious of the phytopathogenic organism. Direct plant growth promotions include symbiotic and a symbiotic. PGPR play functions through plant hormones such as auxin, cytokinins gibberellin ethylene, and abscisic acid. PGPR keeps more soil organic N, and other nutrients in the plant-soil system, thus decreasing the need for fertilizers N and P and increasing the release of nutrients.
For many centuries soil bacteria are used in crop production. The main function of these bacteria are
- To stimulate the plant growth, e.g. ( through the production of plant hormones apply nutrients to crop)
- To supply nutrients to crop
- To control or inhibit the activity of plant pathogen
- To improve soil structure
- Bio accumulation or microbial leaching of inorganics
Transformation, mobilization, solubilization etc. plays a key role in the microbial population in this era of crop production. Nowadays the use of biological approaches is becoming more popular as an addictive to chemical fertilizer for enhancing the crop yield in an integrated plant nutrient management system.
In this regard, the use of PGPR has found a key role in developing a sustainable system in agriculture. Free-living soil bacteria are useful for plant growth are usually referred to as plant growth-promoting rhizobacteria, capable of promoting plant growth by colonizing the plant root.PGPR is also termed as health-promoting rhizobacteria (PHPR) or nodule promoting rhizobacteria(NPR) and is associated with rhizosphere which is an integral soil ecological environment for plant-microbe interaction.
According to their relationship with plants, PGPR can be categorized into two groups; symbiotic bacteria and free-living rhizobacteria. PGPR can also be divided into two groups according to their residing sites: i.e. symbiotic bacteria which lives inside the plant cell, produced nodules and localized inside the specialized structure and PGPR i.e. free-living rhizobacteria, which lives the outside the plant cell and do not produce nodules, but still promote plant growth.PGPR can contribute to the development of a sustainable agricultural system. Usually, PGPR functions in three different ways: synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the soil and lessening or preventing the plant from diseases.
Symbiotic N2-fixing bacteria
Nitrogen is required for cellular creation of enzymes, protein chlorophyll, DNA and RNA, and is therefore necessary for plant growth production of food and feed. Important biochemical of biological nitrogen fixation occurs mainly through the symbiotic association of N2 fixing microorganisms with legumes that convert atmospheric elemental nitrogen into ammonia. Rhizobia(species of Rhizobium, Mesobizohium, Brady rhizobium, Allorhizobium) form an intimate symbiotic relationship with legumes by responding chemo tactically to flavonoid molecules produce a signal by the legume host. These plant compounds induce the expression of Nodulation genes in Rhizobia, which is, in turn, produce lipo-chitooligosaccharides (LCO) signal that triggers mitotic cell division roots, leading to nodule formation.
There are numerous of factor which affects the modulation on legumes roots including host-micro symbiont compatibility, physiochemical conditions of the soil and the presence of both known and unknown biomolecules such as flavonoids polysaccharides and hormones. The N2 fixed by Rhizobia in legumes can also lead advantages associated with non-legumes via a direct transfer of biologically fixed nitrogen to cereals growing on intercrops or to subsequent crop rotated with symbiotic legumes. Due to addition to N2 in legumes, Rhizobia is a species of Rhizobium and Brady rhizobium produces molecules such as auxin cytokinins, abscisic acid, riboflavin, and vitamins that promote plant growth.
Non-symbiotic N2-fixing bacteria
PGPR participates in interaction with C3 and C4 plants wheat, maize, sugarcane, and cotton and significantly increase their vegetative growth and plant yield. Azotobacter is free-living heterotrophic diazotrophs that depend on an adequate supply of reduced C compounds such as sugar for their energy source. By the application of straw material their concentration could be increased in rice culture crops and as a result of microbial breakdown of cellulose into cellobiose and glucose.
The yield of rice cotton and maize could be increased by the application of Azotobacter. Beneficial effects of inoculation with Azosprillum on in wheat yield in both greenhouse and field conditions have been reported. Azosprillum species are aerobic heterotrophs that fix N2 under the microscopic condition and grow extensively in the rhizosphere of gramineous plants. The association of Aspirrilum plants leads to enhance the development and yield of different host plants. This addition in yield is attributed mainly to an improvement in root development by an increase in water and mineral uptake, and to a closer extent biological nitrogen fixation.
Phytohormone s are synthesized by Azosprillum influence the host root respiration rate metabolism and root proliferation and hence improve mineral and water uptake in inoculated plants. Soil application with Azosprillum can significantly increase both plant and rotten crops in the field. Substantial increase in N uptake by wheat plants and grain was observed in greenhouse trials with inoculation of Azosprillum. Inoculation with Azosprillum also significantly increased the N content of sugarcane leaves in the greenhouse experiment. Azosprillum capable of producing anti fungal and antibacterial compounds, growth regulators and siderophores.
Phosphorus solubilizing bacteria
Phosphorous is one of the major essential macro nutrients for plant growth and development. Phosphorus exists in two forms organic and inorganic phosphates. To convert insoluble phosphate both organic and inorganic compounds in a form accessible to the plant is an important trait for a PGPR in increasing plant yield. The concentration of soluble phosphorus in soil is very low normally at the level of 1ppm or less. The production of organic acid especially gluconic acid seems to be the most frequent agent. Soil also contains a wide range of organic substrates which can be a source of phosphorus for plant growth. To make this form of P available for plant nutrition.it must be hydrolyzed to inorganic Activity of various phosphatase in the rhizosphere of maize, barley and wheat showed that phosphate activity was considered in the major rhizosphere at 5 acidic and neutral soil pH.
Soil bacteria transform atmospheric N2 into ammonia and are central to soil and plant growth. Within the soil, they play a key role in the cycling of nutrients. The soil contains various species of bacteria many have not an important role in nutrient cycling but also protect from various diseases. Growth promoting rhizobacteria (PGPR) benefits the growth and development of plants directly and indirectly throughout several mechanisms. The production of secondary metabolites i.e. plant growth substances, changes root morphology resulting in greater root surface area for the uptake of nutrients.
The PGPR inoculants Azosprillum, Azotobacter, Bacillus, Pseudomonas, etc. are also available for a variety of crops, used alone or co-inoculating with Rhizobium spp.PGPR must propagate artificially to optimize its validity and biological activity under field application. It is also suggested that PGPR need to reinoculate every year as they will not live forever in the soil.