Seed Priming with Spermidine and Trehalose Enhances Chilling Tolerance of Rice via Different Mechanisms

Chilling stress is an important limiting factor for rice growth and development. To examine the effect of spermidine (Spd) and trehalose (TH) priming in response to chilling stress of rice seed, we investigated the effects of seed priming with Spd, TH, and a mixture of Spd + TH on response and resilience of rice plants to chilling stress. Priming with Spd, TH and Spd + TH resulted in better seed vigor and seedling growth than hydro-priming and non-priming under low temperature (15 degrees C). In addition, Spd-, TH-, and Spd + TH-priming notably decreased malondialdehyde content but increased the proline and soluble sugar contents and antioxidant enzymes activities. Interestingly, spermine content and the expression of spermine-synthase (SPMS) gene were improved by Spd-priming. However, lower spermine content and SPMS expression was observed in the TH-primed plants. Moreover, expressions of trehalose-6-phosphate-phosphatase (TPP) genes was upregulated by TH-priming but downregulated by Spd-priming. It could be concluded from our results that (1) both Spd- and TH-priming could increase the contents of proline and soluble sugar, which were involved in osmotic adjustment, and antioxidant enzymes activities; (2) Spd-priming promoted the accumulation of endogenous spermine by upregulating SPMS1 and SPMS2, but inhibiting the expressions of TPP1 and TPP2; and (3) TH-priming increased the expressions of TPP1 and TPP2 but downregulated the expressions of SPMS1 and SPMS2. These findings suggested that seed priming with Spd and TH might enhance the chilling tolerance of rice seedling via different mechanisms.