Knock-Out of the Genes Coding for the Rieske Protein and the ATP-Synthase δ-Subunit of Arabidopsis. Effects on Photosynthesis, Thylakoid Protein Composition, and Nuclear Chloroplast Gene Expression

In Arabidopsis, the nuclear genes PetC and AtpD code for the Rieske protein of the cytochrome b(6)/f (cyt b(6)/f) complex and the delta-subunit of the chloroplast ATP synthase (cpATPase), respectively. Knock-out alleles for each of these loci have been identified. Greenhouse-grown petc-2 and atpd-1 mutants are seedling lethal, whereas heterotrophically propagated plants display a high-chlorophyll (Chl)-fluorescence phenotype, indicating that the products of PetC and AtpD are essential for photosynthesis. Additional effects of the mutations in axenic culture include altered leaf coloration and increased photosensitivity. Lack of the Rieske protein affects the stability of cyt b(6)/f and influences the level of other thylakoid proteins, particularly those of photosystem II. In petc-2, linear electron flow is blocked, leading to an altered redox state of both the primary quinone acceptor Q(A) in photosystem II and the reaction center Chl P700 in photosystem I. Absence of cpATPase-delta destabilizes the entire cpATPase complex, whereas residual accumulation of cyt b(6)/f and of the photosystems still allows linear electron flow. In atpd-1, the increase in non-photochemical quenching of Chl fluorescence and a higher de-epoxidation state of xanthophyll cycle pigments under low light is compatible with a slower dissipation of the transthylakoid proton gradient. Further and clear differences between the two mutations are evident when mRNA expression profiles of nucleus-encoded chloroplast proteins are considered, suggesting that the physiological states conditioned by the two mutations trigger different modes of plastid signaling and nuclear response.