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S-adenosylmethionine decarboxylase (SAMDC) mediates the biosynthesis of polyamines (PAs) and plays a positive role in plants’ response to adversity stress tolerance. In this study, we isolated a SAMDC gene from white clover, which is located in mitochondria. It was strongly induced when white clover exposed to drought (15% PEG6000), salinity (200 mM NaCl), 20 μM spermidine, 100 μM abscisic acid, and 10 mM H2O2, especially in leaves. The INVSc1 yeast introduced with TrSAMDC1 had tolerance to drought, salt, and oxidative stress. Overexpression of TrSAMDC1 in Arabidopsis showed higher fresh weight and dry weight under drought and salt treatment and without growth inhibition under normal conditions. Leaf senescence induced by drought and saline was further delayed in transgenic plants, regardless of cultivation in 1/2 MS medium and soil. During drought and salt stress, transgenic plants exhibited a significant increase in relative water content, maximum photosynthesis efficiency (Fv/Fm), performance index on the absorption basis (PIABS), activities of antioxidant protective enzymes such as SOD, POD, CAT, and APX, and a significant decrease in accumulation of MDA and H2O2 as compared to the WT. The concentrations of total PAs, putrescine, spermidine, and spermidine in transgenic lines were higher in transgenic plants than in WT under normal and drought conditions. These results suggested that TrSAMDC1 could effectively mitigate abiotic stresses without the expense of production and be a potential candidate gene for improving the drought and salt resistance of crops.

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 upregulated TrTIP2-1, TrTIP2-2 and TrPIP2-7 expressions 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 expressions could be blocked by Ca2+ channel blockers and the inhibitor of Ca2+-dependent protein kinase in leaves of white clover. TrSAMS in relation to Spm biosynthesis was first cloned from white clover and the TrSAMS was located in the 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 indicate that Spm regulates water balance via an enhancement in OA, WUE and water transport related to Ca2+-dependent AQP expression in plants under water stress.