MtMAP3Kδ6 Modulates the Growth and Development through Sugar Metabolism Regulation in Medicago truncatula.

Plant growth and development are intricately regulated by molecular mechanisms, with the mitogen-activated protein kinase (MAPK) signaling cascade and its associated modules being pivotal. In this study, we identified and characterized a member of the MAPKKK family, MtMAP3Kδ6, from Medicago truncatula. This gene, classified within the B3 subgroup of the MAPKKK family, was expressed across various tissues during plant growth. The knockout mutant of MtMAP3Kδ6 displayed dwarfism, characterized by reduced branching and smaller leaf size, whereas overexpression of MtMAP3Kδ6 in Medicago truncatula led to the converse phenotypes. Transcriptome analysis and subsequent validation in leaves from different strains showed that the knockout mutants of MtMAP3Kδ6 had decreased levels of starch and sucrose, along with diminished cell wall invertase (INV; EC 3.2.1.26) activity, whereas overexpression resulted in the opposite effects. Collectively, our findings suggest that MtMAP3Kδ6 plays a role in Medicago truncatula growth and development by positively modulating sugar metabolism. This research lays a theoretical groundwork for future studies on the role of MAPKKK in sugar metabolism and its implications for plant growth and development.