Shikimate Kinase Plays Important Roles in Anthocyanin Synthesis in Petunia

In plants, the shikimate pathway is responsible for the production of aromatic amino acids L-tryptophan, L-phenylalanine, and L-tyrosine. L-Phenylalanine is the upstream substrate of flavonoid and anthocyanin synthesis. Shikimate kinase (SK) catalyzes the phosphorylation of the C3 hydroxyl group of shikimate to produce 3-phosphate shikimate (S3P), the fifth step of the shikimate pathway. However, whether SK participates in flavonoid and anthocyanin synthesis is unknown. This study characterized the single-copy PhSK gene in the petunia (Petunia hybrida) genome. PhSK was localized in chloroplasts. PhSK showed a high transcription level in corollas, especially in the coloring stage of flower buds. Suppression of PhSK changed flower color and shape, reduced the content of anthocyanins, and changed the flavonoid metabolome profile in petunia. Surprisingly, PhSK silencing caused a reduction in the shikimate, a substrate of PhSK. Further qPCR analysis showed that PhSK silencing resulted in a reduction in the mRNA level of PhDHQ/SDH, which encodes the protein catalyzing the third and fourth steps of the shikimate pathway, showing a feedback regulation mechanism of gene expression in the shikimate pathway.