Nodulation: Rhizobial signals and regulatory factors

Extensive research on the nitrogen fixation mechanism of rhizobia has advanced our understanding of the complex interactions between plant hosts and microorganisms. Furthermore, rhizobial diversity and the genetic, physiological and environmental factors that influence their survival and nodulation ability have also been widely studied. Optimizing symbiotic nitrogen fixation can significantly enhance legume productivity while reducing fertilizer dependence and improving soil health. To achieve these benefits, various rhizobia inoculants have been successfully developed and applied to legume crops such as soybeans and peanuts. However, there are still limitations in terms of the effectiveness and stability of these inoculants under field conditions. The advancement of sequencing and new technologies has led to the discovery of increasingly more novel functions and mechanisms, providing a wealth of valuable information for the improved application of rhizobial inoculants. In this review, we explore the diversity of rhizobia and molecular signals involved in rhizobium-legume symbiosis, and discuss the role of hormones produced by rhizobia in symbiotic nitrogen fixation. Furthermore, we have synthesized recent progress in understanding the regulatory roles of the plant host, the rhizosphere microbiota, and root exudates on rhizobial nodulation. Finally, we propose ways to utilize the latest discoveries to enhance symbiotic nitrogen fixation and achieve more advanced applications. • The genetic diversity of Bradyrhizobium offer a rich pool for advancing symbiotic nitrogen fixation studies. • Rhizobial synthesis or degradation of plant hormones plays a key role in symbiosis. • Rhizosphere microbes and root metabolites collectively regulate rhizobial distribution and nodulation.