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Effect of Rhizosphere Microbial Population on Plants

Effect of Rhizosphere Microbial
Population on Plants
The microorganisms have a marked influence on the growth of plants. The plant growth may be impaired due to the absence of appropriate rhizosphere microflora. The microbial population affect the plant growth in various ways:

Promotion of growth
This is brought about by the release of growth factors like auxins and gibberellins that promote plant growth. The organisms which release these growth factor include Arthrobacter, Pseudomonas and Agrobacterium. The production of indole acetic acid (IAA), a plant growth hormone by certain group of microorganisms increases the rate of seed germination and development of root hairs. This is seen in wheat seedlings.

Neutralisation of toxic substances This is seen in the case of plants that grow in flooded sediments, e.g. rice plants and other partially submerged plants. In this case, there is production of hydrogen sulphide generated by the sulphate reduction pathway. This hydrogen sulphide is toxic to the plant roots, and this is neutralised by the bacteria Beggiatoa. This is a microaerophilic, catalase negative, sulphide oxidising filamentous bacterium. This acquires the oxygen and catalase enzyme from the rice plant and aids in the oxidation of toxic H2S to harmless sulphur or sulphate, thus protecting the rice roots

Allelopathic effect Some substances being released by the microbes can have an antagonistic effect. This may allow plants to enter in amensalic relationship with other plants. Some substances or extracellular products of certain microorganisms lead to the growth of other kinds of microorganisms that can provide a better rhizosphere microflora. These extracellular products can also inhibit the growth of pathogens, thus protecting the plant roots from getting damaged.

Nutritional recycling The nutrients in the soil are made available to the plants by mobilisation of the nutrients, by fixing it in soil in proper way. Sometimes, the nutrients are made unavailable by immobilisation. For example, microorganisms produce extracellular amino acids, vitamins, etc. using the nutrients and nitrogen fixation process. Thus they make nitrogen available to plants as nitrates or other inorganic forms, e.g. Rhizobium and Azotobacter. Similarly, sulphur oxidisers make sulphur available as sulphates, e.g. Desulfovibrio. Phosphorous is made available as phosphates by the production of acids by the microflora. Siderophore production is another important characteristic feature of rhizosphere microflora

Siderophore production Many microorganisms respond to a fall in the availability of iron in soil by producing extracellular low molecular weight iron transporting agents known as siderophores. These siderophores selectively complex with iron and supply the element to the living cell. They also act as growth factors or antibiotics. For example, Pseudomonas fluorescence (one strain produced a siderophore compound pseudobactin) inhibits the growth of a pathogen Eewinia carotovora by chelating iron from the vicinity of the pathogen and thus reducing the disease severity.

Thus microorganisms increase the recycling and solubilisation of mineral nutrients and making it available to plants. The abundant growth of microbial population in the rhizosphere can sometimes create a deficiency of required minerals for the plants, e.g. bacterial immobilisation of zinc and oxidation of manganese cause the plant diseases 'little leaf' of fruit trees and' gray speck' of oats. Nitrogen is immobilised in the form of microbial protein and some may be lost to the atmosphere by denitrification

 
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