Effects of the silencing of CmMET1 by RNA interference in chrysanthemum (Chrysanthemum morifolium)

DNA methylation is an epigenetic modification involving many biological processes. It is known that epigenetic mechanisms such as cytosine methylation play a pivotal role in regulating plant development. In this current study, a conservative domain-spliced hairpin RNA (ihpRNA) plant expression vector aimed at gene of DNA METHYLTRANSFERASE1 (CmMET1) has been constructed. Transgenic chrysanthemum materials (Zijingling) were obtained by Agrobacterium-mediated transformation with expression vectors. Transgenic plants were used as rootstock, grafted onto non-transgenic the scions [Guoqinghong (GQH) and Huanshuijinqiu (HSJQ)], which silencing CmMET1 gene, and exhibited the early flower phenotypes. A high-performance liquid chromatography analysis indicated that the decrease of CmMET1 expression decreased the methylation level of genomic DNA. Similarly, a quantitative real-time polymerase chain reaction analysis revealed that CmMET1 expression levels decreased in transgenic chrysanthemum plants and in the scions grafted onto transgenic plants. This decrease of CmMET1 expression upregulated the expression of the methyltransferase gene, METHYLTRANSFERASE2 (CmDRM2), but downregulated the expression of the demethylating enzyme gene, DEMETER (CmDME), while the CHROMOMETHYLASEA (CmCMT3) expression level remained low and could be almost undetectable. Among the CmFT-likes genes that affect flowering time, CmFTL1 expression was downregulated, as well as CmFTL2 and CmFTL3 expression levels which were upregulated. Our data indicated that silencing CmMET1 could decrease plant height, change the phenotype of chrysanthemum, and promote earlier flowering. The transgenic plants bloomed 8 days earlier. GQH and HSJQ scions grafted onto transgenic plants were 12 and 9 days earlier than the scions grafted onto non-transgenic plants, respectively. Overall, the results have some meanings for promoting the flowering of chrysanthemum scion varieties using genetic modified rootstock.