Functional Characterization of Ammonium Transporter LjAMT2;4 During Lotus japonicus Symbiosis with Rhizobia and Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi (AMF) and rhizobia are important symbiotic microorganisms in soil, which can symbiose with legumes to form mycorrhizal symbionts and nodules, respectively. Once a stable symbiotic relationship is established, these microorganisms have been found to enhance nitrogen absorption by legumes. Although plants can directly utilize ammonium through ammonium transporters (AMTs), there is limited research on the role of the AMT gene family in promoting ammonium transport in symbiotic relationships. <i>Lotus japonicus</i>, a common host of arbuscular mycorrhizal fungi and rhizobia, serves as a model legume plant. In this study, we examined the characteristics of the ammonium transporter <i>LjAMT2;4</i> in <i>L. japonicus</i> and found that <i>LjAMT2;4</i> is localized to the plasma membrane and is predominantly expressed in roots. The promoter region of <i>LjAMT2;4</i> contains cis-acting elements induced by arbuscular mycorrhizal fungi and rhizomes, and the expression of <i>LjAMT2;4</i> was induced by AM fungi and rhizobia. However, there was no significant difference in the mycorrhizal colonization rate of <i>ljamt2;4</i> compared to the wild type, while the absence of <i>LjAMT2;4</i> significantly increased the number of root nodules under nitrogen-starved conditions, enhancing nitrogen fixation and alleviating nitrogen stress in extremely nitrogen-starved environments, ultimately promoting plant growth. These findings suggest that manipulating the genes involved in symbiotic nitrogen fixation, such as <i>LjAMT2;4</i>, could offer new strategies for sustainable agricultural production. Given that AM and rhizobia symbiosis are critical for crop growth, our findings may inform strategies to improve agricultural management.