Plant Genetic Control over Infection Thread Development during Legume-Rhizobium Symbiosis

Legumes possess by unique possibility to interact with soil proteobacteria, known as rhizobia, forming on the roots the special organs called symbiotic nodules, where nitrogen fixation takes place. To form the nodule, rhizobia should penetrate inside root tissue, where they colonize a nodule primordium, formed from the reactivated root cells. One of the ways of root infection by rhizobia occurs via formation of a transcellular tubular structure, termed infection thread (IT), which grows through the cytoplasm by apical deposition of primary cell wall material. Numerous mutants impaired in the infection thread development were obtained in different legumes. Genetic analysis has revealed that mutants belong to different complementation groups; this means the existence of precise genetic control over infection thread development. Moreover, it was suggested that infection and nodule organogenesis are regulated with independent but coordinated genetic programs. Using the model legumes, a set of plant genes, controlling infection thread development was identified. These genes encode transcriptional factors, LysM receptor kinases, E3 ubiquitin ligases, SCAR/WAVE actin regulatory complex, nitrate transporter, remorins, flotillins, proteins involved in membrane biogenesis and traffic, and some other components. In this review, we briefly summarized our current knowledge about genetic control over developmental processes associated with infection thread.