The LbMUG3-LbSTI regulatory module mediates salt gland development and salinity tolerance in recretohalophyte Limonium bicolor

BACKGROUND AND AIMS: Soil salinization severely limits crop productivity worldwide. Halophytes provide important biological resources for identifying genes that confer salinity tolerance. Salt glands, specialized epidermal structures in recretohalophytes like Limonium bicolor, excrete excess Na+, yet their developmental and functional regulation remains poorly understood. METHODS: We focused on LbSTICHEL (LbSTI), a homologue of the Arabidopsis trichome regulator STI, and investigated its function and upstream regulation in L. bicolor. KEY RESULTS: LbSTI was found to be specifically expressed in the salt glands of L. bicolor, as determined by RNA in situ hybridization and GUS histochemical staining driven by the LbSTI promoter. The function of LbSTI was further investigated in L. bicolor. In L. bicolor, plants overexpressing LbSTI produced more salt glands, exhibited increased salt secretion and greater tolerance to salinity stress compared with non-transformed controls. Conversely, LbSTI-knockdown plants generated by virus-induced gene silencing showed opposite phenotypes. Meanwhile, LbSTI increased trichome development and salinity tolerance in wild-type plants and an sti mutant of Arabidopsis, which indicated conservation across species. Furthermore, the upstream regulatory transcription factor LbMUSTANG3 (LbMUG3) was identified to bind to the LbSTI promoter and inhibit the transcription of LbSTI using a yeast one-hybrid and dual-luciferase reporter assay. Further functional validation revealed that LbMUG3 delayed salt gland development in L. bicolor. Additionally, the expression of LbSTI was significantly upregulated in LbMUG3-silenced lines. CONCLUSIONS: The current LbMUG3-LbSTI module may be a useful tool to cultivate plants with salinity tolerance.