Targeted inactivation of the plastid ndhB gene in tobacco results in an enhanced sensitivity of photosynthesis to moderate stomatal closure

The ndh genes encoding for the subunits of NAD(P)H dehydrogenase complex re present the largest family of plastid genes without a clearly defined funct ion. Tobacco (Nicotiana tabacum) plastid transformants were produced in whi ch the ndhB gene was inactivated by replacing it with a mutant version poss essing translational stops in the coding region. Western-blot analysis indi cated that no functional NAD(P)H dehydrogenase complex can be assembled in the plastid transformants. Chlorophyll fluorescence measurements showed tha t dark reduction of the plastoquinone pool by stromal reductants was impair ed in ndhB-inactivated plants. Both the phenotype and photosynthetic perfor mance of the plastid transformants was completely normal under favorable co nditions. However, an enhanced growth retardation of ndhB-inactivated plant s was revealed under humidity stress conditions causing a moderate decline in photosynthesis via stomatal closure. Tnis distinctive phenotype was mimi cked under normal humidity by spraying plants with abscisic acid. Measureme nts of CO2 fixation demonstrated an enhanced decline in photosynthesis in t he mutant plants under humidity stress, which could be restored to wild-typ e levels by elevating the external CO2 concentration. These results suggest that the plastid NAD(P)H:plastoquinone oxidoreductase in tobacco performs a significant physiological role by facilitating photosynthesis at moderate CO2 limitation.