Can prohexadione-Ca induce antimicrobial flavonoids in rose?

Rose (Rosa x hybrida), a popular and widely spread garden plant, is infected by numerous plant diseases, which are mainly caused by fungal and even by bacterial pathogens such as Erwinia amylovora, the causal agent of fire blight. Recently, prohexadione-Ca, a 2-oxoglutarate specific inhibitor originally developed as a growth retardant, was reported to influence the flavonoid composition of apple and pear leaves and, thus, to increase resistance against fire blight and other diseases. It was supposed that this approach could also mediate resistance in roses. As the first step, the induction of the antimicrobial 3-deoxyflavonoid in rose leaves after prohexadione-Ca application was investigated in a time course experiment. Due to the presence of disturbing phenolic compounds impeding the preparation and testing of enzymes, enzyme preparation procedures and assays had to be adapted and optimised for rose leaves. Standard enzyme assays are described for phenylalanine ammonia lyase (PAL), chalcone synthase/chalcone isomerase (CHS/CHI), flavanone 3-hydroxylase (FHT), dihydroflavonol 4-reductase (DFR) and flavanone 4-reductase (FNR). Cloning and heterologous expression showed that the DFR of rose exhibits FNR activity as well. Treatment of rose leaves with prohexadione-Ca led to the inhibition of FHT and to the formation of the 3-deoxyflavonoid luteoliflavan. Likewise, induction effects on PAL, CHS and DFR/FNR activity were also observed. In general, it could be shown that rose leaves produce 3-deoxyflavonoids after prohexadione-Ca treatment and, therefore, this method can be considered as a potential protection strategy for roses that will be studied phytopathologically in a next step.