Identification and characterization of early pathway genes of ellagitannin biosynthesis in strawberry (Fragaria vesca)

Ellagitannins are polyphenolic antioxidants found in certain fruits, trees, tea and medicinal plants. In many fruits, such as strawberries, raspberries, blackberries or pomegranate, ellagitannins, besides anthocyanins, are the most abundant antioxidants. The high amount of antioxidants present in these fruits have been associated with a reduced risk of cardiovascular disease, Diabetes mellitus (type 2) or cancer and these properties, together with the pleasant taste, have made berries one of the favorite fruits on the fresh food market but also as source for nutraceuticals or functional foods. Due to the previously described bioactivities associated with ellagitannins, studies at the molecular level and identification of genes involved in this biosynthetic pathway are mandatory for further engineering strategies in planta to modulate the amount of metabolites (transgenic strawberry; breeding programmes). We show here the identification and characterization of four putative shikimate dehydrogenase (SDH) encoding genes from strawberry. SDH has recently been shown to convert 3‐dehydroshikimate (3‐DHS) to gallic acid (GA), the first intermediate in the ellagitannin biosynthetic pathway. Until this finding, SDH was mainly known to catalyze the reversible reduction of 3‐DHS to shikimic acid (SA), an essential intermediate for the production of aromatic amino acids. In higher plants the shikimate pathway is present in plastids but has been proposed to exist as a second pathway in the cytoplasm. Both tobacco (N. tabacum) and tomato (L. esculentum) have two SDH encoding genes, one localizes to the chloroplast, where it participates in the production of aromatic amino acid for protein synthesis, and one is localized in the cytoplasm, possibly involved in synthesis of natural products. However, the function of this cytoplasmic SDH is still not completely clear. We propose here that the cytoplasmic SDH is involved in GA formation in strawberry (F. vesc a) and that it catalyzes the first step in the ellagitannin biosynthetic pathway.