Major Latex Protein MdMLP423 Is a Negative Regulator to Resist Fungus Infection in Apple

Major latex proteins (MLPs) play critical roles in defense and stress responses in plants. However, the functions of MLPs from the apple (Malus × domestica) have not been clearly characterized. Here, we focused on the biological function of MdMLP423, which was previously identified as a potential pathogenesis-related gene. Phylogenetic analysis and conserved domain analysis revealed that MdMLP423 was a protein with a `Gly-rich loop' (GXGGXG) domain and belonged to the Bet v_1 subclass. Gene expression profile revealed that MdMLP423 was predominantly expressed in the flower. Additionally, the expression of MdMLP423 was significantly inhibited by Macrophoma kawatsukai and Alternaria alternata apple pathotype (AAAP) infection. To verify the function of MdMLP423, we generated its overexpressing transgenic lines in apple callus. The MdMLP423-overexpressing callus exhibited lower resistance to Macrophoma kawatsukai and AAAP infection, as evidenced by the lower expression of resistance-related genes, higher degree and faster speed of the disease than those of non-transgenic callus. RNA-seq analysis for MdMLP423-overexpressing callus and non-transgenic callus was constructed, and the expression analysis indicated that MdMLP423 regulated the expression of a series of differential expression genes (DEGs) and transcription factors, including DEGs involved in phytohormone signaling pathways, cell wall reinforcement, defense-related proteins, AP2-EREBP, WRKY, MYB, NAC, Zinc finger protein and ABI3. Taken together, our results demonstrate that MdMLP423 plays negative regulation on Macrophoma kawatsukai and AAAP resistance through inhibiting the expression of cooperating with defense- and stress-related genes and transcription factor. Overall design: mRNA profiles of transgenic callus (T)and non-transgenic callus (CK), in apple, using BGISEQ-500