The CdFBX1-CdMYB26-CdMYB5 module regulates ion homeostasis under salt stress by fine-tuning <i>CdSOS1</i> expression in bermudagrass

The plasma membrane Na+/H+ antiporter Salt Overly Sensitive 1 (SOS1) is essential for maintaining ion homeostasis during salt stress. However, the transcriptional regulatory mechanism governing SOS1 expression in bermudagrass (Cynodon dactylon) remained uncharacterized. Here, we showed that two natural variants of the F-box protein CdFBX1 (CdFBX1.1/1.2) governed CdSOS1 expression by regulating the CdMYB26-CdMYB5 transcriptional module. CdSOS1 conferred salt tolerance in bermudagrass, and CdMYB5 acted as a positive regulator of salt tolerance by directly binding to and activating CdSOS1 expression under salt stress. Further screening identified CdMYB26 as a negative regulator of CdMYB5, and CdFBX1 as an interacting protein partner of CdMYB26. In addition, we identified two natural CdFBX1 variants, CdFBX1.1 and CdFBX1.2. CdFBX1.2 contains a 46-bp deletion that introduces a premature stop codon; the encoded CdFBX1.2 protein conferred stronger salt tolerance than CdFBX1.1 due to its enhanced promotion of CdMYB26 ubiquitination and degradation. In salt-tolerant germplasms, CdFBX1.2 facilitated CdSOS1 expression by degrading CdMYB26, thereby relieving repression of CdMYB5, and subsequently enhanced CdSOS1 expression. Collectively, our study reveals the CdFBX1-CdMYB26-CdMYB5-CdSOS1 regulatory module orchestrating salt tolerance in bermudagrass. The elucidated molecular mechanism, along with the identification of elite CdFBX1.2 variants, provides valuable molecular tools and markers for molecular breeding of salt-tolerant bermudagrass varieties.