Essay: Azospirillum, Phosphorus, Potassium – plant nutrition

Azospirillum
Azospirillum is a Gram- negative firee living associative symbiotic organism that is aerobic, heterotrophic and non-nodule forming and motile. It occurs in the roots of dicots and monocot plants i.e. wheat, sorghum and they belongs to the  family Spirilaceae , group 1 of the alpha subclass of the proteobacteria (Shigueru et al., 2013). They have 15 strain under this genus like, A.amazonense, A.halopraeferens, A.brasilense , A. lipoferum , A. irakense ,A. largimobile ; A. doebereinerae ; A. Oryzae; A.melinis,  A. Canadensis, A. zeae, A. rugosum, A. palatum, A. picis and A. thiophilum) (Massena Reis et al.,2011) and each has its own biochemical and molecular characteristics. It contains poly-b-hydroxy-butyrate granules, long granules, possess long polar flagellum , measures 0.8 to 1.0 μm in diameter, 2 to 4μm in length and can also grow under anaerobic, microaerophilic and aerobic condition (Orhan et al., 2006) Azospirillum are easy to culture, they grow well on organic acids such as lactate, malate, pyruvate and succinate.
The Azospirillum form associative symbiosis with several plants especially with plant having C4-dicarboxyliac path way of photosynthesis, because they fix nitrogen on salts of organic acids such as malic, aspartic acid etc. Azospirillum was first isolated from cereals such as maize, barley, pearl millet, sorghum and initial inoculation has been on cereal crops.  Studies have shown that inoculation of Azospirillum in non-cereal species have been very successfully like when used for cereal (Mishra, 2012). Azospirillum is found to be very effective in increasing 10-15% yield of cereal crops and fixing N2 up to 20-40 Kg.  It has no preferences for any crop plants or weeds or for annual or perennial plants and are successfully applied to plants that have no previous history of Azospirillum. Azospirillum enhances plant growth by colonizing the roots surface and also penetrates into the root tissues where it lives in harmony with the plants (Santi et al., 2013). It induces remarkable changes in the morphology and the entire behavior of the plant root for example hairs close to the root tip have distinctive appearance, increase in length, diameter and dimension of both lateral and adventitious roots and density. Studies also show that it posses N2-fixing capability (~1–10 kg N/ha) and this improves root development, production of growth promoting substances and increases rate of water and mineral uptake for plants (Gangwar. and Kaur, 2009). Some species of Azospirillum enhance the growth of plants when A. brasiliense strains is inoculated into wheat seed, it causes increase in seed germination, plant growth, plumule and radical length (Malhotra and Srivastava , 2008).
Phosphorus (P) is vital macronutrients for biological growth and development. This element is present in the soil in a form that cannot be utilized by plants. Rhizobacteria offer a biological rescue system which is capable of solubilizing the insoluble inorganic Phosphorus such as tricalcium phosphate, dicalcium phosphate, hydroxyapatite, and rock phosphate present in soil (Sarker et al., 2012). The solubilization occur through their metabolic activities by exudation of organic acids, which can directly dissolve the rock phosphate, or chelating calcium ions to release P and make it available for the plants to utilize. One of the important traits of Plant growth promoting rhizobacteria is the ability to convert insoluble phosphorus (P) to an accessible form, like orthophosphate (Chen et al., 2006). The use of phosphate solubilizers increase the P uptake by releasing organic acid (Chen et al., 2006) which helped in phosphate solubilizing activity and also facilitate plant growth by other mechanisms. Most of the PSB include largely bacteria and fungi. The most efficient PGPR are Pseudomona , Bacillus, Rhizobium  for bacteria ; Penicillium and Aspergillus are the commonest fungi. Pseudomonas fluorescens,  Pseudomonas striata, Pantoea agglomerans, Pseudomonas cepacia,, Pseudomonas putida, Bacillus megatherium var. Bacillus polymyxa phosphaticum, nitrobacter spp., Escherichia freundii, Acrobacter acrogens, Serratia spp., Aeromonas vaga are the rhizobacteria  which have phosphate solubilizing ability (Kumar and Dangar, 2013). Strains of Bacillus altitudinis, Pseudomonas Mandelii and  Pseudomonas monteilii, formed associations with rice plants. (Habibi et al., 2014).
Potassium
Potassium is one of the third important components of plant nutrition after nitrogen and phosphours. It is an essential macronutrient for plant growth and plays vital roles in biological functions such as activating many different metabolic processes including photosynthesis, protein synthesis and also in resistance to diseases and insects in plant (Rehm and Schmitt, 2002).
Potassium is an abundant element present in the soil.  About 90 to 98% of Mica and feldspar are the most common soil components of potassium (McAfee, 2008) and it can be applied to the soil as natural or synthetic fertilizers for plant growth. Potassium in the soil is present in a form that is inaccessible by plants and other organisms due to its insoluble state and is bound with other minerals. Only about one to two percent of the potassium in the soil is available for plant to utilize. Some PGPR such as potassium solubilizing bacteria influence the availability of soil minerals by ion cycling and soil fertility (Bin et al., 2010). Potassium solubilizing bacteria are able to solubilize potassium, aluminum and silicon from insoluble potassium bearing minerals such as to mica, illite, feldspar. This is by secreting organic acid which dissolve potassium or  chelated silicon ion  to soluble potassium  which is accessible for plant to utilize (Biswas and Basak, 2010).  Due to this ability, they are use as biofertilizers or biocontrol agents for agriculture improvement and environmental protection.
Some examples of potassium solubilizing bacteria are B. circulans Pseudomonas, Bacillus mucilaginosus, Burkholderia, Bacillus edaphicus, Acidothiobacillus ferrooxidans, and Paenibacillus sp. has been shown to solubilize potassium in accessible form from potassium-bearing minerals in soils for plant useage (Liu et al., 2012).  Potassium solubilizing bacteria have been inoculated into several plants and it has been reported to produce beneficial effects on growth of pepper and cucumber, cotton and rape, sorghum, Sudan grass (Basak  and Biswas., 2010) and wheat (Singh et al., 2010). Studies have shown that potassium also increases the fresh plant weight, plant height, and can increase herbage and oil yield on the patchouli (Singh et al., 2010).