Pivotal Role of Organic Acid Secretion by Rhizobacteria in Plant Growth Promotion

Abstract

Organic acid secretion by rhizobacteria has been major factor ­responsible for phosphate solubilization. Efficacy as phosphate biofertilizers is determined by the nature and amount of organic acids secreted by rhizobacteria. Amongst the organic acids, gluconic and 2-ketogluconic acids are most commonly secreted by rhizobacteria. Many efforts have been made to understand the factors responsible for the variable efficacy of these bacteria as inoculants in field conditions. Nature of the soil, type and abundance of sugars available in the root exudates and nature of the available phosphate complexes contribute to the variations in efficacy of these rhizobacteria between the laboratory and field conditions in terms of the nature and amount of organic acid secretion. Phytate is a predominant organic phosphate present in many soils. The utilization of phytate as a P source appears to be dependent not only on the characteristics of the phytases but also on the organic acid secretion. Rhizobacteria are also gaining prominence in providing potassium from mineral ores to plants. Potassium solubilization by rhizobacteria could also be determined by the organic acids. Acidification of the rhizosphere by organic acid secretion could also be important in providing iron to plants specifically which are grown in alkaline soils. Citric acid is known to be siderophore in Bradyrhizobium. In addition to the plant growth promotion by enhancing nutrient availability, organic acid secretion by rhizobacteria could be helpful in the amelioration toxic effects caused by heavy metals. All these aspects are limited by the inherent properties of rhizobacteria. Alternative strategies of coupling beneficial properties of organic acid secretion with other well established plant promotion properties may have high potential in agriculture. Metabolic engineering approaches may be very effective in achieving these goals.