Differential expression of plastome-encoded ndh genes in mesophyll and bundle-sheath chloroplasts of the C4 plant Sorghum bicolor indicates that the complex I-homologous NAD(P)H-plastoquinone oxidoreductase is involved in cyclic electron transport

Abstract

Cyanobacteria and plastids harbor a putative NAD(P)H- or ferredoxin-plastoquinone oxidoreductase that is homologous to the NADH-ubiquinone oxidoreductase (complex I) of mitochondria and eubacteria. The enzyme is a multimeric protein complex that consists of at least 11 subunits (NDH-A-K) and is localized in the stroma lamellae of the thylakoid membrane system. We investigated the expression of the different subunits of the enzyme in mesophyll and bundle-sheath chloroplasts of Sorghum bicolor [L.] Moench, a C4 plant of the NADP-malic enzyme type. The relative amounts of the subunits NDH-H, -J and -K were strongly increased in bundle-sheath plastids as compared to mesophyll plastids. This increase was accompanied by enhanced transcript levels for all subunits except NDH-I. Because the main function of the protein complexes in the thylakoid membranes of bundle-sheath chloroplasts (photosystem I, cytochrome b6/f-complex and ATPase) is the generation of ATP for CO2 fixation via cyclic electron transport, we conclude that the NAD(P)H/ferredoxin-plastoquinone oxidoreductase is an essential component of the cyclic electron-transport pathway in chloroplasts.