The development of Azospirillum as a commercial inoculant for improving crop yields

Abstract

Bacteria of the genus Azospirillum are nitrogen-fixing organisms that live in close association with plants in the rhizosphere. The Azospirillum-plant association leads to the enhanced development and yield of different host plants under appropriate growth conditions. This increase in yield is attributed mainly to an improvement in root development, an increase in the rate of water and mineral uptake by roots, and to a lesser extent, biological N(2) fixation. Worldwide data accumulated in the field over the past 20 years indicates that Azospirillum is capable of promoting the yield of agriculturally important crops in different soils and climatic regions. A.brasilense shows both chemotaxis and chemokinesis in response to temporal gradients of different chemoeffectors, thereby increasing the chance of root-bacterial interactions. Phytohormones synthesized by Azospirillum influence the host root respiration rate, metabolism and root proliferation and hence better the mineral and water uptake in inoculated plants. Positive effects of combined inoculation with Rhizobium have been reported for different legumes and were related to the favorable influence of Azospirillum on the nodule number, plant development, dry weight, and N(2) fixation. Additionally, A. brasilense produces the reserve material polyhydroxybutyrate comprising up to 70% of the cell dry weight This substance has received much attention recently as it can be extracted and formed into a biodegradable thermoplastic.