Abstract
Rhizobia form symbiotic relationships with leguminous plants and convert atmospheric nitrogen into ammonia which can be utilized by host legume plants. Symbiotic nitrogen fixation is important not only for the production of protein-rich legumes but also to improve the fertility of soils. Nodulation in the roots of legumes is regarded as an initiating event in the onset of N-2 fixation. Inhibition of nodulation restricts the N-2-fixing ability of legumes, which may occur due to variety of reasons. For instance, ethylene, as a stress hormone, inhibits nodulation in legumes. However, its action is naturally offset by an enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, produced by rhizobia. This enzyme cleaves plant-produced ACC into ammonia and alpha-ketobutyrate thereby lowering the ethylene level. As a consequence of decreased ethylene levels, nodulation is increased. Moreover, the ACC deaminase-containing plant growth-promoting rhizobacteria including symbiotic N-2 fixers reduce the physiological damage to plants caused by other environmental factors. In this chapter, the focus is to understand how the rhizobial ACC deaminase lowers the ethylene level in legumes and overcomes the inhibitory effects of ethylene on nodulation. The strategy adopted by rhizobial species to promote nodulation by adjusting ethylene levels could be exploited as an effective tool for legume improvement in different agro-ecological niches.