Effect of prohexadione-Ca on various fruit crops: Flavonoid composition and substrate specificity of their dihydroflavonol 4-reductases

In pome fruit trees, the growth retardant prohexadione-Ca induces the formation of 3-deoxycatechins, which correlates with an increased resistance against fire blight and also against some fungal diseases. The formation of 3-deoxycatechins is due to the channelling of intermediates unusual for pome fruits, which requires the presence of a flavanone 4-reductase (FNR) activity. Here, it is investigated, whether the crop plants strawberry, grapevine, cranberry, cherry, peach, plum, elder, and kiwi fruit react in the same way to prohexadione-Ca application. The presence of FNR activity is shown by enzymatic investigations and the formation of a 3-deoxycatechin after prohexadione-Ca application in planta. Furthermore, by cloning of the corresponding cDNAs and studies with the heterologously expressed enzymes it is shown that the DFR enzymes of grapevine, cranberry and strawberry posses also FNR activity. Conclusively, all the crop plants studied have the potential to form 3-deoxyflavonoids. However, the changes in the flavonoid composition observed after prohexadione-Ca treatment were not exactly the same in all species studied here. In kiwi fruit leaves, a strong accumulation of flavanones rather than the formation of large amounts of 3-deoxycatechins is observed.