Melatonin Promotes Seed Germination via Regulation of ABA Signaling Under Low Temperature Stress in Cucumber

Abscisic acid (ABA) is an important phytohormone that regulates seed germination, dormancy, and plant responses to stresses such as salt, drought, and cold. Melatonin regulates ABA biosynthesis and catabolism during seed germination under various types of stress; however, the transcriptional regulation of ABA signaling genes by melatonin during cucumber seed germination under low temperature (LT, 15 °C) stress is poorly understood. Here we report that melatonin promoted seed germination under LT stress by decreasing ABA content and increasing the gibberellic acid (GA)/ABA ratio (gibberellic acid, GA). We also examined the expression levels of the ABA receptors PYR/PYL/RCAR (PYL), as well as the negative regulator protein phosphatase type 2C (PP2C) and positive regulator subfamily 2 of SNF1-related kinase (SnRK2) by real-time PCR. The expression of most tested genes increased during seed germination, showing an opposite trend to ABA level. The transcript levels of CsPYL1, CsPYL2, CsPYL3, CsPYL8, and CsPYL10 in 10 μM melatonin-pretreated seeds were significantly increased after 6 h of imbibition as a result of feedback regulation caused by the lack of ABA, and CsPP2C3, CsPP2C5, and CsSnRK2.1 exhibited higher expression levels than CK (control) treatment. The expression of most tested genes changed markedly in the initial water uptake phase (12 h after imbibition), suggesting that this period is critical for the regulation of ABA signaling during seed germination. We speculate that melatonin reduces ABA content, preventing CsPYL (CsPYL1/2/3/8/10) binding to CsPP2C and thereby enhancing the activity of CsPP2C (CsPP2C3/5) and blocking CsSnRK2.1 activation. Phosphorylation of the downstream factors ABA-responsive element-binding factor (ABF)/ABA-responsive element-binding protein (AREB) by CsSnRK2.1 is thus abolished, leading to seed germination under LT stress.