Toshiharu Shikanai

The light reactions in photosynthesis convert light energy into chemical energy in the form of ATP and drive the production of NADPH from NADP+. The reactions involve two types of electron flow in the chloroplast. While linear electron transport generates both ATP and NADPH, photosystem I cyclic electron transport is exclusively involved in ATP synthesis. The physiological significance of photosystem I cyclic electron transport has been underestimated, and our knowledge of the machineries involv

To evaluate the physiological significance of cyclic electron flow around photosystem (PS) I, we used a reverse genetic approach to focus on 11 chloroplast genes that encode homologs of mitochondrial complex I subunits ( ndhA - K ). Since their discovery, the exact function of the respiratory components in plant chloroplasts has been a matter of discussion. We disrupted one of these genes ( ndhB ) in tobacco by chloroplast transformation. Analysis of the transient increase in chlorophyll fluores

Copper (Cu) is an essential trace element with important roles as a cofactor in many plant functions, including photosynthesis. However, free Cu ions can cause toxicity, necessitating precise Cu delivery systems. Relatively little is known about Cu transport in plant cells, and no components of the Cu transport machinery in chloroplasts have been identified previously. Cu transport into chloroplasts provides the cofactor for the stromal enzyme copper/zinc superoxide dismutase (Cu/ZnSOD) and for

The chloroplast NDH complex, NAD(P)H dehydrogenase, reduces the plastoquinone pool non-photochemically and is involved in cyclic electron flow around photosystem I (PSI). A transient increase in chlorophyll fluorescence after turning off actinic light is a result of NDH activity. We focused on this subtle change in chlorophyll fluorescence to isolate nuclear mutants affected in chloroplast NDH activity in Arabidopsis by using chlorophyll fluorescence imaging. crr2-1 and crr2-2 (chlororespiratory

The pentatricopeptide repeat (PPR) proteins form one of the largest families in higher plants and are believed to be involved in the posttranscriptional processes of gene expression in plant organelles. It has been shown by using a genetic approach focusing on NAD(P)H dehydrogenase (NDH) activity that a PPR protein CRR4 is essential for a specific RNA editing event in chloroplasts. Here, we discovered Arabidopsis crr21 mutants that are specifically impaired in the RNA editing of the site 2 of nd

In higher plants, RNA editing is a post-transcriptional process that converts C to U in organelle mRNAs. We have previously shown that an Arabidopsis thaliana crr4 mutant is defective with respect to RNA editing for creating the translational initial codon of the plastid ndhD gene (the ndhD-1 site). CRR4 contains 11 pentatricopeptide repeat motifs but does not contain any domains that are likely to be involved in the editing activity. The green fluorescent protein fused to the putative transit p

ClpP is a proteolytic subunit of the ATP-dependent Clp protease, which is found in chloroplasts in higher plants. Proteolytic subunits are encoded both by the chloroplast gene, clpP, and a nuclear multi gene family. We insertionally disrupted clpP by chloroplast transformation in tobacco. However, complete segregation was impossible, indicating that the chloroplast-encoded clpP gene has an indispensable function for cell survival. In the heteroplasmic clpP disruptant, the leaf surface was rough

The chloroplast NAD(P)H dehydrogenase (NDH) complex is involved in photosystem I (PSI) cyclic and chlororespiratory electron transport in higher plants. Although biochemical and genetic evidence for its subunit composition has accumulated, it is not enough to explain the complexes putative activity of NAD(P)H-dependent plastoquinone reduction. We analyzed the NDH complex by using blue native PAGE and found that it interacts with PSI to form a novel supercomplex. Mutants lacking NdhL and NdhM acc

To analyze the potential of the active oxygen-scavenging system of chloroplasts, we introduced Escherichia coli catalase into tobacco chloroplasts. Photosynthesis of transgenic plants was tolerant to high irradiance under drought conditions, while the wild plants suffered severe damage in photosynthesis under the same conditions. Irrespective of responses to the stress, ascorbate peroxidase was completely inactivated both in the transgenic and wild-type plants. These findings are contrary to the

Plastid gene expression is regulated by a variety of nuclear genes. We have isolated Arabidopsis thaliana proton gradient regulation 3 (pgr3) mutants, which display aberrant chlorophyll fluorescence because of defects in chloroplast gene expression. High chlorophyll fluorescence (HCF) because of a reduced level of the cytochrome b6/f complex was observed in two alleles, pgr3-1 and pgr3-2 but not in pgr3-3. In contrast, a transient increase in fluorescence after turning off the actinic light, whi