Strigolactone-mediated DNA demethylation induces phenylpropanoid biosynthesis to alleviate salt stress in tomato

Tomato (Solanum lycopersicum L.) is an important horticultural crop, but it is seriously affected by salt stress. GR24, the synthetic analogue of Strigolactones (SLs) is used in the study as an efficient and harmless growth regulator. Therefore, further exploration of the mechanism of exogenous SLs is necessary to improve tomato salt tolerance. In this study, RNA-seq analysis, sulfite whole genome sequencing and methylation-specific PCR were used to conduct salt stress analysis on the leaf samples of the tomato variety "Micro-Tom" seedlings treated with exogenous SLs. Research shows that exogenous SLs alleviate the inhibition of the number of leaves, root surface area and root volume resulting from salt stress. Compared with the 150 mmol·L−1 NaCl treatment, 150 mmol·L−1 NaCl +15 μmol·L−1 GR24 treatment increases the leaf number, root surface area and root volume by 26.67 %, 55.76 % and 55.81 %, respectively, suggesting that exogenous SLs-mediated DNA demethylation may play an important role in the salt tolerance of the four-leaf stage tomato seedlings. RNA-sequencing and genome-wide methylation analysis show that exogenous SLs reduce DNA methylation levels to affect phenylalanine metabolism and phenylpropanoid biosynthesis under salt stress. SLs-mediated DNA demethylation increases phenethylamine (PEA), coumarin, caffeic acid, and lignin contents, but decreases l-Phenylalanine (LPA) and cinnamic acid (CA) contents. Meanwhile, the activities of l-phenylalanine ammonia-lyase (PAL), phenylalanine decarboxylase (HDC), 5-O-(4-coumaroyl)-d-quinate 3′-monooxygenase (CYP98A3), and beta-glucosidase (BGLU) are enhanced, and 6 genes related to phenylpropanoid metabolism (SlPAL5, SlHDC, SlBGLU41, SlCYP98A3, SlCYP73A4, and Sl4CLL7) in the pathway were induced. The demethylation of SlCYP98A3 at CG site in promoter, SlBGLU41 at CG site in gene body, SlPAL5 at CHG site in gene body, SlPAL at CHG site in promoter and SlHDC at CHG site in promoter may result in the transcription of the genes, activating other genes expression. These findings demonstrate that exogenous SLs may improve the salt tolerance of tomato seedlings by regulating phenylalanine metabolism and phenylpropanoid biosynthesis. The results provide a reference for in-depth analysis of the response mechanism of SLs under abiotic stress.