Role of ethylene and bacterial ACC-deaminase in nodulation of legumes

Abstract

Rhizobia-legume symbiosis is a complex process involving a number of plant and bacterial genes that lead to the formation and development of root nodules. Plant hormone ethylene plays an important role in nodule development and nodule signaling networks in response to a wide range of biotic and abiotic stresses. Ethylene is known as a negative regulator of nodulation. Inoculation of rhizobia leads to a temporal stimulation of ethylene production that suppresses nodule formation. In contrast, inhibitors of ethylene synthesis or its physiological action promote nodule formation in legumes. 1-Aminocyclopropane-1-carboxylate (ACC)-deaminase is a biological inhibitor of ethylene synthesis. The rhizosphere bacteria containing ACC-deaminase can increase nodulation in legumes by degrading ACC (an immediate precursor of ethylene) and, thus, by lowering ethylene concentration in the plant. Similarly, some rhizobia also have shown ACC-deaminase activity and improvement in nodulation by regulating the concentration of ethylene in plant tissues. In this chapter, the role of ethylene and bacterial ACC-deaminase in nodulation of legumes is reviewed and discussed.