Spermine regulates water balance associated with Ca2+-dependent aquaporins (TrTIP2-1, TrTIP2-2, and TrPIP2-7) expression in plants under water stress.

Spermine (Spm) regulates water balance involved in water channel proteins aquaporins (AQPs) in plants. An increase in endogenous Spm content via exogenous Spm application significantly improved cell membrane stability, photosynthesis, osmotic adjustment (OA), and water use efficiency (WUE) contributing to enhanced tolerance to water stress in white clover. Spm up-regulated TrTIP2-1, TrTIP2-2, and TrPIP2-7 expression and also increased the abundance of TIP2 and PIP2-7 proteins in white clover under water stress. Spm quickly activated intracellular Ca2+ signaling and Spm-induced TrTIP2-2 and TrPIP2-7 expression could be blocked by Ca2+ channels blockers and the inhibitor of CDPK in leaves of white clover. TrSAMS in relation to Spm biosynthesis was firstly cloned from white clover and the TrSAMS was located in nucleus. Transgenic Arabidopsis overexpressing the TrSAMS had significantly higher endogenous Spm content and improved cell membrane stability, photosynthesis, OA, WUE, and transcript levels of AtSIP1-1, AtSIP1-2, AtTIP2-1, AtTIP2-2, AtPIP1-2, AtPIP2-1, and AtNIP2-1 than wild type in response to water stress. Current findings indicates that Spm regulates water balance via enhancement in OA, WUE, and water transport related to Ca2+-dependent AQPs expression in plants under water stress.