Targeted Editing of SlMAPK6 Using CRISPR/Cas9 Technology to Promote the Development of Axillary Buds in Tomato Plants

The mitogen-activated protein kinase (MAPK) cascade signaling system has been relatively conserved throughout the evolution of eukaryotes and is involved in the regulation of growth and development and metabolism. In this study, dwarf tomato plants were used as the research material. First, the tissue-specific expression of SlMAPK6 was measured in wild-type plants by quantitative RT-PCR. The results showed that SlMAPK6 was highly expressed in the tissues of the stems, leaves and flowers but was expressed at low levels in the tissues of the roots, sepals and fruits. Second, SlMAPK6-knockout lines CRISPR-3 and CRISPR-7 were obtained by CRISPR-Cas9 technology and Agrobacterium-mediated transformation. Compared with wild-type, the mutant lines CRISPR-3 and CRISPR-7 showed significant phenotypic characteristics, such as increased numbers of axillary buds and true leaves, thickened stems, and longer leaflets. In addition, to explore the molecular mechanism by which MAPK regulates axillary bud growth, we also showed that SlMAPK6 positively regulates the strigolactone synthesis genes SlCCD7 and SlCCD8 and the gibberellin (GA) synthesis genes GA20ox3 and GA3ox1 and negatively regulates the axillary bud development-related genes Ls, BL and BRC1b/TCP8 and the GA synthesis inhibitory gene GAI. Therefore, SlMAPK6 appears to regulate the synthesis of strigolactone and GA to induce the growth and development of tomato axillary buds.