Ontogenetic variation of catechin biosynthesis as basis for infection and quiescence of Botrytis cinerea in developing strawberry fruits Translated title: Entwicklungsabhängige Catechin-Biosynthese als Basis für Infektion und Quieszenz von Botrytis cinerea in Erdbeerfrüchten

The biosynthesis of flavonoids and proanthocyanidins was studied in cultivated strawberry (Fragaria xananassa) by combining biochemical and molecular approaches. Chemical analyses showed that ripe strawberries accumulate high amounts of pelargonidin-derived anthocyanins, and a larger pool of 3',4'-hydroxylated proanthocyanidins. Activities and properties of major recombinant enzymes were demonstrated by means of in vitro assays, with special emphasis on specificity for the biologically relevant 4'- and 3',4'-hydroxylated compounds. Only leucoanthocyanidin reductase showed a strict specificity for the 3',4'-hydroxylated leucocyanidin, while other enzymes accepted either hydroxylated substrate with different relative activity rates. The structure of late flavonoid pathway genes, leading to the synthesis of major compounds in ripe fruits, was elucidated. Complex developmental and spatial expression patterns were shown for phenylpropanoid and flavonoid genes in fruits throughout ripening as well as in leaves, petals and roots. Presented results elucidate key steps in the biosynthesis of strawberry flavonoid end products.

Treatment with the dioxygenase inhibitor prohexadione-Ca leads to major changes in the flavonoid metabolism of apple (Malus domestica) and pear (Pyrus communis) leaves. Accumulation of unusual 3-deoxyflavonoids is observed, which have been linked to an enhanced resistance toward fire blight. The committed step in this pathway is the reduction of flavanones. Crude extracts from leaves are able to perform this reaction. There was previous evidence that DFR enzymes of certain plants possess additional flavanone 4-reductase (FNR) activity. Such an FNR activity of DFR enzymes is proved here by heterologous expression of the enzymes. The heterologously expressed DFR/FNR enzymes of Malus and Pyrus possess distinct differences in substrate specificities despite only minor differences of the amino acid sequences. Kinetic studies showed that dihydroflavonols generally are the preferred substrates. However, with the observed substrate specificities the occurrence of 3-deoxyflavonoids in vivo after application of prohexadione-Ca can be explained.