The expression response of plasma membrane aquaporins to salt stress in tomato plants

Aquaporin isoforms have distinct functions, and show differential responses to stress conditions among different plant species. In tomato (Solanum lycopersicum), the functions of individual aquaporin isoforms under environmental stresses remain largely unknown. As a step toward understanding their functions under stress conditions, the expression response of plasma membrane intrinsic proteins (PIPs) to salt stress was investigated. Tomato seedlings were subjected to 150 mM NaCl for 1 h and 24 h, and water uptake and SlPIP expressions were investigated. The results showed that the root hydraulic conductivity (Lpr) was decreased under salt stress. Salt stress for 1 h had no effect or increased the expressions of most of the highly expressed SlPIPs in roots and leaves. Salt stress inhibited cell-to-cell water transport. Protein phosphatase and endocytosis inhibitors partially inhibited the Lpr decrease under salt stress. After a 24-h salt stress, the expressions of most SlPIPs were decreased in both roots and leaves, and a recovery treatment in control nutrient solution demonstrated a partial or full expression restoration of most previously downregulated SlPIPs. The expressions of SlPIP isoforms highly varied in both roots and leaves, and across different stress durations. SlPIP1;1, SlPIP1;3, SlPIP1;7, SlPIP2;10 and SlPIP2;12 demonstrated consistent expression responses to salt stress and recovery treatment in both roots and leaves, implying their important roles in regulating water transport. Our results suggest that SlPIPs are involved in mediating water transport in tomato, and their regulations are associated with stress duration and tissue type. At early stage of salt stress, the inhibition of Lpr in tomato might be due to the decrease of SlPIP activity; while at later stress period, the Lpr reduction might be mainly attributed to the decrease of SlPIP expression.