Abnormal Regulation of Photosynthetic Electron Transport in a Chloroplast ycf9 Inactivation Mutant

Abstract

The ycf9 (orf62) gene of the plastid genome encodes a 6.6-kDa protein (ORF62) of thylakoid membranes. To elucidate the role of the ORF62 protein, the coding region of the gene was disrupted with an aadA cassette, yielding mutant plants that were nearly (more than 95%) homoplasmic for ycf9 inactivation. The ycf9 mutant had no altered phenotype under standard growth conditions, but its growth rate was severely reduced under suboptimal irradiances. On the other hand, it was less susceptible to photodamage than the wild type. ycf9 inactivation resulted in a clear reduction in protein amounts of CP26, the NAD(P)H dehydrogenase complex, and the plastid terminal oxidase. Furthermore, depletion of ORF62 led to a faster flow of electrons to photosystem I without a change in the maximum electron transfer capacity of photosystem II. Despite the reduction of CP26 in the mutant thylakoids, no differences in PSII oxygen evolution rates were evident even at low light intensities. On the other hand, the ycf9 mutant presented deficiencies in the capacity for PSII-independent electron transport (ferredoxin-dependent cyclic electron transport and NAD(P)H dehydrogenasemediated plastoquinone reduction). Altogether, it is shown that depletion of ORF62 leads to anomalies in the photosynthetic electron transfer chain and in the regulation of electron partitioning among the different routes of electron transport.

Comparative Analysis of Chloroplast Genomes: Functional Annotation, Genome-Based Phylogeny, and Deduced Evolutionary Patterns

“…In one case, the µ-score is below the threshold (µ-score = 2.2), but the function attributed to ycf9 has been recently confirmed by experimental data indicating that encodes the photosystem II core subunit protein PsbZ (Swiatek et al 2001). Other investigators have detected a down-regulation of photosynthetic activity when this gene was inactivated, consistent with an alteration of a photosystem II subunit (Baena-Gonzalez et al 2001). We detect the presence of the ycf9 gene in Synechocystis and in all other cpDNA studied, excluding the nonphotosynthetic parasites Epifagus virginiana and Toxoplasma gondii.…”

Physiological and molecular changes in plants grown at low temperatures

“…In cold-acclimated winter annuals, photosynthetic activity is maintained by increases in the abundance and activity of several Calvin cycle enzymes (Goulas et al 2006). This recovery is associated with elevated levels of thylakoid plastoquinone A and a concomitant rise in the apparent size of the intersystem electron donor pool to PSI (Baena-Gonzalez et al 2001). Consequently, non-photochemical quenching increases in cold-stressed leaves in parallel with increased zeaxanthin levels to compensate for the reduced electron consumption by photosynthesis.…”

Structure of spinach photosystem II–LHCII supercomplex at 3.2 Å resolution

“…Thereby, PsbZ reinforces the association of CP26 with CP43. When psbZ is knocked out, the content of CP26 protein decreases markedly and the formation of the PSII-LHCII supercomplex is deficient [34][35][36] .…”