Mulberry (Morus alba) MmSK gene enhances tolerance to drought stress in transgenic mulberry

Shaggy-like protein kinase (SK) plays important roles in the plant growth development, signal transduction, abiotic stress and biotic stress and substance metabolism regulation. However, the exact function of the response to drought stress in mulberry with SK remains unclear. In this study, a new SK gene that was designated as MmSK (GenBank accession NO: KY348867) was isolated and cloned from mulberry (Morus alba). MmSK contains two SK conservation domains of ATP domain and Serine/Threonine protein kinases active-site signature, and belonged to GSK3/shaggy protein kinase family. The expression of MmSK in mulberry was up-regulated under various abiotic stress treatments. Meanwhile, we observed higher expression levels in the phloem contrasted with other tissues. Mulberry MmSK gene was successfully silenced by virus induced gene silencing (VIGS), and after MmSK was silenced, the expression of MmSK in pTRV2-MmSK-VIGS plant (transgenic mulberry) dropped to 34.02% compared with the negative control inoculated with empty vector pTRV2-00 (CK). Under drought stress, the soluble protein content, proline content, superoxide dismutase (SOD) and peroxidase (POD) activities in transgenic mulberry decreased in different degree compared with the CK. In contrast, the accumulation of malondialdehyde (MDA) increased significantly in transgenic mulberry. With the extension of drought stress treatment time, the soluble protein content, proline content and MDA content gradually increased. The SOD activity and POD activity under drought stress gradually rose to the maximum on the fifth day and then decreased, which consistent with the change trend of MmSK gene expression. These results suggested that MmSK gene could function as a positive regulator of drought stress in mulberry.