I. Studies on the regulation of photosynthetic electron transfer
(1) Studies on PGR5-dependent photosystem I cyclic electron transfer pathway
Photosystem I cyclic electron transfer was first discovered more than half a century ago, but its physiological function remains unclear. Analysis of Arabidopsis mutants revealed that there are two pathways in higher plants, one dependent on a protein of unknown function called PGR5 and the other dependent on the NDH complex, and that the PGR5-dependent pathway in particular plays an important role in photosynthesis and the response to protect chloroplasts from excess light (Fig. 1). Using genetic, biochemical and physiological techniques, we aim to elucidate the entire electronic transmission pathway. We also tries to unravel the evolutionary strategies of how plants have remade the photosynthetic apparatus during their evolution on land, using rice, and moss.
(2) Analysis of structure, function and assembly of NDH complex
The NDH complex is derived from cyanobacteria and catalyzes photosystem I cyclic electron transfer in chloroplasts. We have presented important findings on the structure, function and assembly of the NDH complex in 2010, and are now working to analyze the full picture of the complex-related electron transfer.
Fig. 1 Double mutants lacking photosystem I cyclic electron transfer do not grow normally.
Wild-type (WT) and PGR5 mutants (pgr5), NDH pathway mutants (crr2-2, crr3, crr4-2), and double mutants lacking both the PGR5 and NDH pathways (crr2-2 pgr5, crr3 pgr5, crr4-2 pgr5). Munekage et al., Nature 2004.
II. Elucidation of the regulatory mechanism of chloroplast gene expression
The chloroplast is an organelle with a unique genome, and its gene expression is regulated by nuclear-coding genes. We have used chlorophyll fluorescence imaging to isolate and analyze a number of mutant strains with abnormal chloroplast gene expression regulation (Fig. 2). RNA editing has been found both in chloroplasts and mitochondria in plants, where numerous cytidine residues are converted to uridine on RNA. We have demonstrated that PPR proteins function as RNA-binding proteins in the site recognition for RNA editing, and are working to elucidate the molecular mechanisms of RNA editing and other chloroplast RNA maturation mechanisms.
C4 plants, such as maize, produce different chloroplasts in different cells to achieve efficient photosynthesis. For this purpose, tissue-specific expression of chloroplast genes is required. We aim to elucidate its molecular mechanism.
Fig. 2 Visualization of photosystem I cyclic electron transfer activity by chlorophyll fluorescence imaging.
No NDH activity is detected in Arabidopsis crr2 mutant strains due to aberrant expression of the chloroplast ndhB gene.
III. Elucidation of the molecular mechanism of copper ion homeostasis in plants
Copper is essential for many biological reactions, including photosynthetic electron transfer, but excess copper is toxic. Plants have various strategies for maintaining copper ion homeostasis in vivo. The Arabidopsis transcription factor SPL7 plays a central role in the maintenance of this copper ion homeostasis, regulating the use of two SODs, copper-zinc and iron, and the expression of copper transporters via microRNA functions. strains disrupted by SPL7 show severe growth defects under copper deficiency.