Biochemical and molecular biological investigations with respect to induction of fire blight resistance in apple and pear by transiently altering the flavanoid metabolism with specific enzyme inhibitors

The application of prohexadione-Ca on apple and pear trees leads to enhanced resistance against fire blight, and scab in case of apple, which is accompanied by the accumulation of unusual 3-deoxyflavonoids and other phenolic compounds in the leaves. An alternative pathway leading to the different 3-deoxyflavoids is hypothesised. A resistance effect of 3-deoxyanthocyanidins against certain fungi is known in Sorghum species. The enormous economic damage caused by fire blight leads to increased interest in the resistance mechanism. The identification of the responsible compound(s) is ongoing. The effect of prohexadione-Ca is studied at the biochemical and molecular biological levels. Standard, assays were optimised for the enzymes involved in flavonoid metabolism. The respective genes were cloned from apple and pear. Studies were performed with heterologously-expressed dihydroflavonol 4-reductase (DFR) and flavanone 3-hydroxylase (FHT) enzymes. It was shown that prohexadione-Ca has an inhibitory effect on apple and pear FHT in vitro. This supports the assumption that the formation of common flavonoids is blocked by prohexadione-Ca and that the accumulating flavanones serve as precursors of the resulting 3-deoxyflavonoids. The key step in the formation of 3-deoxyflavonoids is the enzymatic reduction of flavanones to flavan 4-ols. It has been shown that the DFR enzymes of apple and pear are able to perform this reaction. Further work will investigate if this approach is also suitable for the control of other plant diseases.