Changes in quinic acid metabolism during fruit development in three kiwifruit species

Kiwifruit are novel in that they contain high levels of quinic acid (1-2% w/w), which contributes to the flavour, sugar/acid balance and health-giving properties of the fruit. In a study of quinic acid storage and metabolism in three kiwifruit species (Actinidia chinensis Planch. var. chinensis, Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson var. deliciosa and Actinidia arguta (Sieb. et Zucc.) Planch. ex Miq. var. arguta) quinic acid accumulation occurred principally in the early stages (<60 days after anthesis; (DAA)) of fruit development. The present study established that there are separate quinate dehydrogenase (QDH) and shikimate dehydrogenase (SDH) activities in kiwifruit, probably representing different proteins. Quinate dehydrogenase activity was at a maximum around the time of greatest quinic acid accumulation and declined markedly in late fruit development, and was also higher in the species that accumulated the largest amounts of quinic acid (A. chinensis and A. deliciosa). In contrast, SDH activity was highest in the early stages of fruit development and only declined to 30-50% at later stages of fruit development in all three species. Dehydroquinate synthase gene expression levels measured by quantitative real-time PCR showed a high level in the early season, which was sustained through the mid-season. The quantitative real-time PCR results for a kiwifruit EST that had homology to chloroplastic isoforms of SDH showed an induction in the middle to late season; therefore, the high level of SDH activity in the early season (<30 DAA) may have resulted from the expression of a cytosolic isoform of the enzyme. The results are also consistent with the relative levels of the bifunctional dehydroquinate dehydratase/SDH enzyme and QDH enzyme controlling the accumulation and utilisation of quinic acid in kiwifruit.