Recently, Xie Fang's research group of the Center for Excellence and Innovation in Molecular Plant Science of the Chinese Academy of Sciences published a research paper entitled RinRK1 enhancements NF receivers accumulation in nanodomain like structures at root air tip in Nature Communications. This study revealed the molecular mechanism by which the RinRK1 receptor kinase of Baimai root promotes the regulation of nodule factor (NF) receptors in the nanodomain of root hair membrane to regulate rhizobial infection.
The symbiotic nitrogen fixation system between leguminous plants and rhizobia is considered one of the most efficient and valuable biological nitrogen fixation methods for agricultural applications. This symbiotic system meets the nitrogen needs of leguminous plants themselves and can provide nitrogen nutrition for non leguminous crops that rotate or intercrop with leguminous plants.
The formation of nodules with nitrogen fixation ability depends on the invasion of rhizobia that occur in epidermal cells and the formation of nodule organs in cortical cells. The above two processes are relatively independent in space, but require high coordination in time. The nodulation factors secreted by rhizobia are signaling molecules in symbiotic nitrogen fixation, regulating rhizobia infection and nodule organ formation. Previous studies have found that NF signaling is an important condition for the formation of root nodule organs. In Bai Mai Gen, NF receptors are LysM receptor kinases NFR1 and NFR5, and their intracellular kinase domains are sufficient to activate the formation of root nodule organs. The extracellular domain of NF receptors is crucial for rhizobia infection, but it is not yet clear how the extracellular domain of NF receptors recognizes rhizobia signals and regulates the infection process. The plasma membrane nanodomain is a platform for cellular signal transduction. Receptor proteins typically aggregate in the plasma membrane nanodomain, helping cells respond quickly and specifically to different environmental signals. Previous studies have identified an LRR receptor kinase RinRK1. It is induced by NF and specifically regulates the formation of infection lines. However, the molecular mechanism by which RinRK1 regulates the formation of infection lines is still unclear.
This study found that RinRK1 interacts with the extracellular domain of NF receptors. RinRK1 is located in the cytoplasmic membrane and exists in the DRM component of the cytoplasmic membrane. In addition, the NF signal secreted by rhizobia can promote the aggregation of RinRK1 in the nanodomain at the top of root hairs, and this process depends on the presence of NF receptors. Meanwhile, NF signaling promotes the aggregation of NFR1 and NFR5 in the nanodomain at the root hair tip, a process dependent on RinRK1. The bimolecular fluorescence complementary experiment showed that RinRK1 interacts with NFR1/5 on the root hair membrane, while rhizobia can enhance the interaction between RinRK1 and NFRs in the nanodomain at the top of the root hair. This indicates that the NF signal secreted by rhizobia can promote the formation of receptor complexes between RinRK1 and NFR1/5 in the nanodomain at the top of root hairs.Flotillin is a nanodomain marker protein. Research has found that Flot1 in Baimai roots is induced by rhizobia and expressed in root hairs, root nodule primordia, and vascular bundles of mature root nodules. Mutant studies have shown that the absence of Flot1 leads to a reduction in rhizobial infection events, indicating that Flot1 plays an important role in the formation of infection lines. Furthermore, research has found that NF signaling promotes Flot1 aggregation at the top of root hairs; The aggregation of RinRK1 and NFR1/5 in response to NF signals in the nanodomain at the root hair tip depends on Flot1. Meanwhile, Flot1 can interact with the intracellular domains of NFR1/5 and RinRK1. RinRK1 can promote the interaction between Flot1 and NFR1/5, and Flot1 and NFR1/5 require RinRK1 to participate in the aggregation of NF signals at the root hair tip.
In addition, research has found that NFR1 has serine/threonine kinase activity and tyrosine phosphorylation activity. The experimental results showed that the serine/threonine and tyrosine phosphorylation activity of NFR1 plays a crucial role in its response to NF signals in root hair apical aggregation and symbiotic nodulation.
This study revealed how RinRK1 interacts with NF receptors through the extracellular domain and how it interacts with the nanodomain protein Flot1 through the intracellular domain. It analyzed how NF signaling promotes the aggregation of NFR1/5-RinRK1-Flot1 complexes in the nanodomain at the root hair tip. RinRK1 and Flot1 play a crucial role in the assembly of NF receptor complexes on plasma membrane specific signaling platforms, promoting the invasion process of rhizobia. The above achievements have positive significance for exploring how receptor complexes on the plasma membrane respond to signals from rhizobia and regulate the formation of infection lines.
The research work was supported by the Chinese Academy of Sciences and Shanghai Municipality.
Assembly model for infecting specific nanodomains
