Identifying the receptor-like cytoplasmic kinases involved in plant symbiosis

Plants rely on symbiotic relationships with microbes such as arbuscular mycorrhizal fungi (AMF) and rhizobia to access essential nutrients like nitrogen and phosphorus. While early events in plant-microbe symbiosis such as recognition of microbial molecules and receptor activation are increasingly understood, the intracellular signaling mechanisms that transmit these cues to the nucleus remain largely unresolved. Notably, similarities between immune and symbiotic signaling pathways suggest shared components may mediate both responses. This study investigates the role of Medicago truncatula receptor-like cytoplasmic kinase (RLCK) subfamily VII genes in coordinating immune and symbiotic signaling. Using loss-of-function (LOF) mutants from a Tnt1 insertion library and newly developed gain-of-function (GOF) transgenic lines, symbiotic phenotypes we assessed through nodulation and arbuscular mycorrhization, alongside immune responses via chitin-induced reactive oxygen species (ROS) production. Among tested lines, NF10222 (a BIK1 ortholog) exhibited diminished ROS production when mutated, supporting its role as a positive regulator of immunity. In contrast, NF17979 mutants showed enhanced ROS signaling and reduced nodulation, suggesting a dual function as a negative immune regulator and positive symbiosis regulator. These roles were supported by GOF mutants such as L2-35, which overexpresses non-functional NF17979 and exhibited impaired nodulation. Collectively, these findings provide genetic evidence that RLCK VII genes mediate downstream signaling from microbial recognition to functional symbiotic development. This work offers a foundation for further research on immune-symbiosis signaling and informs strategies to enhance nutrient acquisition efficiency in crops.