Phosphorylation modification reverses the transcriptional inhibitory activity of CsMYB4a in tea plants (Camellia sinensis)

The members of the fourth subgroup of R2R3-MYB (Sg4 members) are well-known inhibitors of phenylpropanoid and lignin synthesis pathways. The C2 domain is closely related to the transcriptional inhibitory activity of Sg4 members. Phosphorylation modification enhances the transcriptional inhibitory activity of Sg4 members. Here, we identified a phosphorylation site on the C2 domain of CsMYB4a from tea plants (Camellia sinensis). A mitogen-activated protein kinase (MAPK), named CsMPK3-2, phosphorylated this site on the C2 domain of CsMYB4a. Further experiments revealed that phosphorylation of CsMYB4a weakened its ability to inhibit the gene expression of PAL, C4H, and 4CL in the phenylpropanoid pathway and activated the expression of transcription factor YABBY5, maintaining the adaxial–abaxial polarity of the leaf. Knocking out NtYAB5 in CsMYB4a transgenic tobacco partially repaired the leaf wrinkling phenotype caused by CsMYB4a. The C1 domain exhibited an activation function when the C2 domain of CsMYB4a was phosphorylated by CsMPK3-2, causing this reversal phenomenon. These results enrich our understanding of the regulatory diversity of Sg4 members.