SbCASP4 improves salt exclusion by enhancing the root apoplastic barrier

SbCASP4 improves the salt tolerance of sweet sorghum [Sorghum bicolor (L.) Mocnch] by enhancing the root apoplastic barrier and blocking the transport of sodium ions to the shoot. Sweet sorghum [Sorghum bicolor (L.) Mocnch] is a C4 crop with high biomass and tolerance to abiotic stresses such as salt, drought, and waterlogging. Sweet sorghum is widely used in bioenergy production, as a forage crop, and in liquors and beer. Root salt exclusion has been reported to underlie the salt tolerance of sweet sorghum. The Casparian strip has a key role in root salt exclusion, and the membrane domain protein (CASP) family participates in Casparian strip aggregation. However, the function and the regulatory mechanisms of SbCASP in response to salt stress in sweet sorghum are unclear. In the current study, we cloned SbCASP4 and determined that it is induced by salt stress and expressed in the endodermis cells of sweet sorghum. Histochemical staining and physiological indicators showed that heterologous expression of SbCASP4 significantly increased the tolerance to salt stress in transgenic Arabidopsis thaliana. Compared with wild type and casp5 mutants, under 50 mM NaCl treatment, SbCASP4-expression lines had the less leaf Na+, lower PI accumulation in stele, smaller oxidative damage and higher salinity threshold, longer root length and higher expression levels of the genes related to Casparian strip formation.