Interaction of Rhizobia with Non-Legume Crops for Symbiotic Nitrogen Fixation Nodulation

Abstract

We have previously demonstrated that naturally occurring rhizobia, such as those isolated from the root nodules of non-legume Parasponia species, and from stem nodules of tropical legume species, are able to enter the root systems of maize, rice and wheat by ‘crack entry’. This ‘crack entry’ of rhizobia occurs where lateral roots emerge through the root cortex, resulting in the penetration of rhizobia both between and into cells of the cortex of young emerging lateral roots which, as a result, become thicker and shortened. Recently, we have interacted oxygen tolerant Azorhizobium caulinodans ORS571, isolated from stem nodules of the tropical legume Sesbania rostrata, and also oxygen tolerant Rhizobium ORS310 isolated from stem nodules of the tropical legume Aeschynomene indica, with the root systems of rice and wheat. This interaction resulted in the development of plants with short, thickened lateral roots, containing intercellular and intracellular rhizobia in cells of the cortex, interspersed amongst normally developing lateral roots. These plants possessed significant nitrogen fixation activity as determined using ethylene production in the acetylene reduction assay. Following the interaction of rice and wheat with Nif⁻ mutants of ORS571 there was no ethylene production from acetylene, confirming the acetylene reduction assay as a measure of nitrogenase activity. Currently, we are assessing the extent to which this symbiotic nitrogen fixing interaction of Azorhizobium caulinodans ORS571 and Rhizobium ORS310 with rice and wheat will be responsive to the lipo-oligosaccharide signals controlling specificity of nodulation interactions of these rhizobia with Sesbania rostrata and Aeschynomene indica respectively. Also we are assessing the extent to which this symbiotic nitrogen fixation will be beneficial to the growth and development of rice and wheat.