Post-illumination reduction of plastoquinone pool in ∆ndh tobacco mutants

Abstract

Nicotiana tabacum chloroplast genome contains 11 genes encoding subunits of the NAD(P)H dehydrogenase complex (NDH) . NDH is homologous to the eubacterial and mitochondrial NADH-ubiquinone oxidoreductase (Complex I) in the respiratory chain and has putative NAD(P)H-plastoquinone oxidoreductase activity, but the physiological function in the photosynthetic electron flow is uncertain. Using plastid transformation technique, we have newly established tobacco (Nicotiana tabacum cv. Xanthi) mutants, ?ndhKJ and ?ndhCKJ, in which ndhK,J and ndhC,K,J were disrupted respectively. These mutants, as well as ?ndhB created previously, lacked the transient increase in chlorophyll fluorescence after termination of actinic light illumination (50 µmol quonta m-2 s-1), an indication of NDH activity. No significant difference was found between the phenotype of mutants and wild type tobacco under the normal growth condition. On the other hand, after the exposure to extremely strong light (>1000 µmol quonta m-2 s-1), all the mutants were susceptible to photoinhibition and photobleached. We also found that apparent Fo of the mutants increased more than that of with wild type after strong light illumination (800 µmol quonta m-2 s-1). This phenomenon suggested that plastoquinone in mutants was more reduced after strong light illumination than wild type. This plastoquinone reduction was more enhanced under water-stress conditions. The physiolosical function of NDH and plastoquinone reduction are discussed.