Minisymposium 10: Photosynthesis: Light Adaptation

Abs # 22003: The nuclear-encoded factors, CRR2 and CRR4, regulate the expression of chloroplast ndh genes.

Presenter:	Shikanai, Toshiharu , shikanai@bs.aist-nara.ac.jp
Authors	Shikanai, Toshiharu  (A)   Hashimoto, Mihoko  (A)   Endo, Tsuyoshi  (B)   Peltier, Gilles  (C)   Tasaka, Masao  (A)
Affiliations:	(A): Nara Institute of Science and Technology (B): Kyoto University (C): CEA Cadarache

Cyclic electron flow around photosystem I is essential both in photoprotection and photosynthesis. In higher plants the electron flow consists of redundant pathways mediated by FQR (ferredoxin-quinone reductase, PGR5-dependent) and NDH. Chloroplastic NDH complex functions in chlororespiration, as well as cyclic electron flow around photosystem I. Genes for eleven subunits of the complex ( ndhA-K ) are encoded in chloroplast genome in higher plants. The knockout of tobacco ndhB gene using chloroplast transformation revealed that the transient increase in chlorophyll fluorescence after illumination is ascribed to the NDH activity. We focused on this subtle change in fluorescence depending on the chlororespiratory reduction of plastoquinone to screen Arabidopsis mutants affected in the expression of ndh genes using the imaging system of chlorophyll fluorescence. Isolation of NDH-specific mutants in Arabidopsis facilitates the genetic approach to understand the physiological function of the electron flow and the regulation of related gene expression. crr2-1 and crr2-2 ( chlororespiratory reduction ), and crr4-1-crr4-4 are the recessive mutant alleles in which the accumulation of the NDH complex is impaired. Except for the defect in the NDH activity, photosynthetic electron transport was unaffected. We consider that the NDH function is complemented by the FQR function in the mutants. The severe defect is appeared only when both pathways of cyclic electron flow are impaired (Munekage et al., a poster in this meeting). Both CRR2 and CRR4 encode members of AtPCMP family containing more than 200 genes in Arabidopsis (1% of total genes). We found that AtPCMP family members are structurally and functionally related with PPR proteins, some of which are related with the maturation steps of chloroplast or mitochondrial RNAs. Northern analysis of eleven ndh genes clarified that CRR2 functions in the intergenic processing of chloroplast RNA between rps7 and ndhB , which is possibly essential for the ndhB translation. Although transcripts were not affected in any ndh gene in crr4 , the RNA editing to create the initiation codon of ndhD was completely impaired. We conclude that both AtPCMP family members are involved in the maturation step of specific RNA, facilitating the post-transcriptional regulation of ndhB and ndhD , respectively.