Alternative pathways in the plant respiratory chain -transgenic analyses of type II NAD(P)H dehydrogenases

Abstract: During oxidative phosphorylation, the production of ATP is driven by the activity of several proton-pumping respiratory protein complexes. In plants, non-proton pumping type II NAD(P)H dehydrogenases are also present in the respiratory chain to oxidize cytosolic and matrix NAD(P)H. So far, it has not been fully clear what roles these enzymes play in the cellular metabolism. In Arabidopsis thaliana, there are seven type II NAD(P)H dehydrogenases: NDA1-2, NDB1 and NDC1. Through the use of RNAi, A. thaliana plants suppressing both NDA1 and NDA2 (NDA1,2) or NDB1 were produced. NDA1,2-suppressing plants had a decreased biomass compared to WT plants. Metabolic profiling suggested an inhibition of the citric acid cycle and a shift at the level of pyruvate, towards fermentation. Suppression of NDB1 in A. thaliana led to a decreased growth on soil, and on a glucose-containing medium. A number of metabolites, including sugars, were also decreased in the transgenic plants. This suggests a specific connection between NDB1 and sugar metabolism, which needs to be further investigated. Analyses of previously produced Nicotiana sylvestris plants, overexpressing or suppressing the NDB1 gene, revealed a change in bolting time compared to the WT in high light. The transgenic plants displayed changes in the stem NADPH/NADP+ ratio in high light that correlated to changes in the levels of several metabolites, the expression of flowering time-associated genes, and the bolting time. The change in bolting time was not followed by a change in biomass. Under conditions of normal light, NDC1 T-DNA insertion mutants also displayed a change in bolting time, but not in biomass. This indicates additional connections between NAD(P)H dehydrogenases and development. Furthermore, differential phenotypes of the mutants indicate a diversity of functions for the NDC1 gene.