Inhibition of flavonoid biosynthetic gene expression coincides with loss of astringency in pollination-constant, non-astringent (PCNA)type persimmon fruit

Pollination-constant and non-astringent (PCNA)-type persimmon fruit lose their astringency as the fruit develops on the tree, while pollination-constant and astringent (PCA)-type persimmon fruit remain astringent even during fruit maturation. The main reason for the loss of astringency in PCNA-type fruit is sudden termination of tannin accumulation in the fruit at an early stage of fruit development. Astringency of persimmon fruit is due to condensed tannins (CTs) synthesised via the flavonoid biosynthetic pathway. We investigated seasonal patterns of gene expression involved in flavonoid biosynthesis in PCNA- and PCA-type persimmon fruit as a first step to elucidate the mechanism for the sudden termination of tannin accumulation in PCNA-type fruit. Partial DNA sequences of nine structural genes in the flavonoid biosynthetic pathway (PAL, C4H, 4CL, CHS, CHI, F3H, F3'H, F35'H, and DFR) were determined using PCR products amplified with degenerate primers for these genes. These sequences were then used as probes for northern blot analysis of the seasonal expression patterns of these genes in the PCNA cvs.'Suruga' and 'Hanagosho', and the PCA cvs.'Kuramitsu' and 'Yokono'. In the early stages of fruit development, when both types of fruit show high astringency, all these genes were expressed at high levels in both types of fruit. However, as fruit developed, expression of all nine genes declined and became undetectable in PCNA-type cvs. 'Suruga' and 'Hanagosho', coincident with the termination of tannin accumulation. By contrast, in PCA-type cvs.'Kuramitsu' and 'Yokono', all nine genes were expressed at high levels until a late stage of fruit development, coincident with high tannin accumulation in the fruit. PCNA-type persimmon appears to be defective in a primary or regulatory step in flavonoid biosynthesis, so that expression of all nine genes involved in biosynthesis became undetectable at an early stage of fruit development.