Enacting partner specificity in legume–rhizobia symbioses

Legumes, such as peas, beans, and alfalfa, have evolved a remarkable ability to establish root nodule symbioses with nitrogen-fixing soil bacteria to fulfill their nitrogen needs. This partnership is characterized by a high degree of specificity, occurring both within and between host and bacterial species. Consequently, nodulation capacity and nitrogen-fixing efficiency vary significantly among different plant–bacteria pairs. The genetic and molecular mechanisms regulating symbiotic specificity are diverse, involving a wide array of host and bacterial genes and signals with various modes of action. Understanding the genetic basis of symbiotic specificity could enable the development of strategies to enhance nodulation capacity and nitrogen fixation efficiency. This knowledge will also help overcome the host range barrier, which is a critical step toward extending root nodule symbiosis to non-leguminous plants. In this review, we provide an update on our current understanding of the genetics and evolution of recognition specificity in root nodule symbioses, providing more comprehensive insights into the molecular signaling in plant–bacterial interactions.