Microbial phosphate solubilization
Phosphorus is the second most essential major plant nutrient after nitrogen. It occurs in soils as inorganic salts like calcium phosphates, iron and aluminium phosphates, hydroxyapatite and fluorapatite; organic phosphorus as inositol hexaphosphate, phospholipids, nucleic acids, inositol phosphates and phytate in plant debris. Natural deposits of low grade rock phosphate are also abundant. Plant phosphorus nutrition has always been of great concern as it is most limiting nutrient. About 96% of phosphorus is present in the soil as insoluble phosphate; inorganic phosphorus is locked into crystal lattices of clay particles and is the main reason behind its low solubility. Thus soils worldwide are deficient in assimiable inorganic phosphate (H2PO4-) form. This low solubility of phosphatic compounds which is characteristic feature of soil phosphorus is responsible for its limiting concentration in soil solution and hence its unavailability to plants. Rock phosphate cannot be directly utilized to replenish phosphorus requirement of plants but instead utilized in the manufacture of superphosphate fertilizers by employing very costly and tedious processes. In this regard, phosphate solubilizing microorganisms (PSM) play an important function of solubilization of phosphatic compounds and making them available for plant assimilation. Phosphorus has important role to play in plant growth and nutrition. It increases biological activities like nodulation, nitrogen fixation and nutrient uptake in soil and rhizosphere environment resulting in higher yield of crops. Its deficiency results in stunted growth, lack of sufficient chlorophyll, deepening of green and red color of leaves, abnormal/arrested root development and it delays maturity of plant. Though the soils worldwide are poor in their 'available' phosphorus content, various forms of insoluble phosphates are solubilized by microorganisms and made available to plants. However microbial action is influenced by moisture content and pH of soil, weather conditions, chemical nature, particle size and degree of solubility of phosphate form in water.
In natural phosphorus cycle, phosphorus is continuously leached from the soil and similarly it is replenished with rainfall and microbial breakdown of litter and organic debris. In solution form, it is taken up by plant roots and subsequently translocated to other plant parts. Different species of plants and different parts of plant vary in their content and requirement of phosphate. It is concentrated in younger plant parts, seeds and flowers. With agricultural crops, phosphorus cycle is open and in natural vegetation it represents closed cycle. Indigenous diverse group of bacteria and fungi from soil and rhizosphere have capacity to solubilize different insoluble phosphates and they are directly involved in turnover of soil phosphorus. Major species of phosphate solubilizing bacteria (PSB) include Achromobacter, Aerobacter Acetobacter, Agrobacterium, Bacillus cereus, B. circulans, B. megaterium, B. mesentricus, B. polymyxa, B. pulvifaciens, B. pumilus, B. subtilis, Brevibacterium, Rhizobium, Azospirillum, Azotobacter, Erwinia, Pseudomonas putida, P. striata, P. liquefaciens, P. fluorescens, Serratia phosphaticum, Xanthomonas, actinomycetes belonging to genera Micromonospora, Nocardia and Streptomyces. Native and efficient phosphate solubilizing fungal species are Aspergillus awamori, A. carbonum, A. flavus, A. fumigatus, A. niger, A. terreus, A. wentii, A. nidulans, Candida, Cylindrocladium, Fusarium oxysporum, Penicillium bilaii, P. digitatum, P. lilacinum, Rhizoctonia, Torula thermophilia, Sclerotium rolfsii, Oidiodendron, Rhizopus, Pseudogymnoascusi, Pythium and Fomitopsis. Some of the phosphate solubilizing cyanobacterial genera includes Anabaena, Aulosira, Nostoc and Anacystis.
Mechanism of phosphate solubilization: Phosphorus is treated by PSM in 3 different ways namely, solubilization, mineralization and immobilization. Insoluble phosphates are dissolved by acidification, chelation or proton extrusion mechanisms. Solubilization by means of acidification of growth medium, that means the production of organic and inorganic acids is the most common mechanism found in PSM. Citric, fumaric, Ketogluconic acids, succinic, tartaric, oxalic, malic, vanilic, salicylic, benzoic, pyrrogallic, phthalic and protocatechuic acids are some of the organic acids produced by various PSM and are involved in the solubilization of insoluble phosphates. Acid production in laboratory cultures is indicated by decrease in pH of growth medium and efficiency of solubilization is dependent on nature of acids like aliphatic or phenolic rather than total acidity. Some PSM have been found to produce nitric or sulfuric acids via oxidation of ammonium or sulphur respectively to release orthophosphate ions during rock phosphate solubilization. PSM are also known to create acidity by CO2 evolution (carbonic acid); this type of acidification was observed in solubilization of calcium phosphates. Some PSM, under anaerobic conditions release H2S which reacts with insoluble ferric phosphate to yield solubilized ferrous sulphate. Chelation process is also one of the important phosphate solubilization processes. Many aerobic PSM excrete 2-ketogluconic acid which is powerful chelator of calcium and such PSM are versatile in solubilization of various form of hydroxyapatites, fluorapatites and aluminium phosphates. Humic and fulvic acids released during microbial degradation of plant debris are also good chelators of calcium, iron and aluminium present in insoluble phosphates. Organic form of phosphatic compounds is transformed into utilizable form by PSM via process of mineralization. Soil Bacillus and Streptomyces spp. are able to mineralize very complex organic phosphates by production of extracellular enzymes like phosphoesterases, phosphodiesterases, phytases and phospholipases. Immobilization of phosphorus is sometimes practiced by soil PSM. They compete with other bacteria and even plants for the assimilation of available phosphate and in result accumulate more phosphorus than plant. This stored phosphate is released into the environment under stress conditions or after death of PSM. Such released phosphorus is then taken up by plants or other microbes to fulfill their phosphorus requirement.
Fertilizer phosphate: PSM are efficient to utilize low grade phosphates and therefore they have been exploited for supplementing phosphorus nutrition of crop plants. Large amount of fertilizer phosphate in the form of superphosphate is still regularly used by farmers from all over world. It has been estimated that only 20% of phosphate in fertilizer is taken up by crops and remaining builds up as residual phosphate. On the other hand, PSM allow sustainable use of fertilizer phosphate. PSM in the form of bioinoculants have been recommended for crop nutrition after successful field trials and positive plant growth promoting effects such as increase in grain, straw and dry weight of important agricultural crops like rye, wheat and soybean. Phosphobacterin, Microphos, Nitragin and Azotobacterin are some of the commercialized phosphate solubilizing biofertilizers. PSM which could perform well under stress conditions like salinity, drought or alkalinity, utilize broad range of carbon and nitrogen compound for growth or extremophilic PSM are very promising for future development of phosphate based biofertilizers. Attempts were made to increase solubilization efficiency of PSM and also to enable non-solubilizers to solubilize phosphatic compounds via genetic modification or incorporation of PQQ synthase gene respectively. It would be injustice to describe genetics of phosphate solubilization in one sentence but I felt it mandatory to present vastness of the subject in front of you. I hope you would find my future article on genetics of phosphate solubilization very interesting.
About Author / Additional Info: