Unusual pH-dependence of diadinoxanthin de-epoxidase activation causes chlororespiratory induced accumulation of diatoxanthin in the diatom Phaeodactylum tricornutum

Based on our recent findings that in the diatom Phaeodactylum tricornutum, chlororespiration in periods of prolonged darkness leads to the accumulatio n of diatoxanthin (DT), we have elaborated in detail the interdependence be tween the chlororespiratory proton gradient and the activation of diadinoxa nthin de-epoxidase (DDE). The data clearly demonstrates that activation of DDE in Phaeodactylum occurs at higher pH-values compared to activation of v iolaxanthin de-epoxidase (VDE) in higher plants. In thylakoid membranes as well as in enzyme assays with isolated DDE, the de-epoxidation of diadinoxa nthin (DD) is efficiently catalyzed at pH 7.2, In comparison, de-epoxidatio n of violaxanthin (Vx) in spinach thylakoids is observed below pH 6.5. Phae odactylum thylakoids isolated from high light grown cells, that also contai n the pigments of the violaxanthin cycle, show violaxanthin de-epoxidation at higher pH-values, thus suggesting that in Phaeodactylum, one de-epoxidas e converts both diadinoxanthin and violaxanthin. We conclude that the activ ation of DDE at higher pH-values can explain how the low rates of chlorores piratory electron flow, that lead to the build-up of a rather small proton gradient, can induce the observed accumulation of diatoxanthin in the dark. Furthermore, we show that dark activation of diadinoxanthin de-epoxidation is not restricted to Phaeodactylum tricornutum but was also found in anoth er diatom, Cyclotella meneghiana.