PIF3 is a phytochrome-interacting basic helix–loop–helix transcription factor that negatively regulates light responses, including hypocotyl elongation, cotyledon opening, and hypocotyl negative gravitropism. However, the role of PIF3 in chlorophyll biosynthesis has not been clearly defined. Here, we show that PIF3 also negatively regulates chlorophyll biosynthesis by repressing biosynthetic genes in the dark. Consistent with the gene expression patterns, the etiolated pif3 mutant accumulated a higher amount of protochlorophyllide and was bleached severely when transferred into light. The photobleaching phenotype of pif3 could be suppressed by the gun5 mutation and mimicked by overexpression of GUN5 . When 4 negative phytochrome-interacting protein genes ( PIF1 , PIF3 , PIF4 , and PIF5 ) were mutated, the resulting quadruple mutant seedlings displayed constitutive photomorphogenic phenotypes, including short hypocotyls, open cotyledons, and disrupted hypocotyl gravitropism in the dark. Microarray analysis further confirmed that the dark-grown quadruple mutant has a gene expression pattern similar to that of red light-grown WT. Together, our data indicate that 4 phytochrome-interacting proteins are required for skotomorphogenesis and phytochromes activate photomorphogenesis by inhibiting these factors.

PsbS is a 22-kDa Photosystem (PS) II protein involved in non-photochemical quenching (NPQ) of chlorophyll fluorescence. Rice (Oryza sativa L.) has two PsbS genes, PsbS1 and PsbS2. However, only inactivation of PsbS1, through a knockout (PsbS1-KO) or in RNAi transgenic plants, results in plants deficient in qE, the energy-dependent component of NPQ. In studies presented here, under fluctuating high light, growth of young seedlings lacking PsbS is retarded, and PSII in detached leaves of the mutants is more sensitive to photoinhibitory illumination compared with the wild type. Using both histochemical and fluorescent probes, we determined the levels of reactive oxygen species, including singlet oxygen, superoxide, and hydrogen peroxide, in leaves and thylakoids. The PsbS-deficient plants generated more superoxide and hydrogen peroxide in their chloroplasts. PSII complexes isolated from them produced more superoxide compared with the wild type, and PSII-driven superoxide production was higher in the mutants. However, we could not observe such differences either in isolated PSI complexes or through PSI-driven electron transport. Time-course experiments using isolated thylakoids showed that superoxide production was the initial event, and that production of hydrogen peroxide proceeded from that. These results indicate that at least some of the photoprotection provided by PsbS and qE is mediated by preventing production of superoxide released from PSII under conditions of excess excitation energy.

Plant cells contain several thioredoxin isoforms that are characterized by subcellular localization and substrate specificity. Here, we describe the functional characterization of a rice (Oryza sativa) thioredoxin m isoform (Ostrxm) using a reverse genetics technique. Ostrxm showed green tissue-specific and light-responsive mRNA expression. Ostrxm was localized in chloroplasts of rice mesophyll cells, and the recombinant protein showed dithiothreitol-dependent insulin β-chain reduction activity in vitro. RNA interference (RNAi) of Ostrxm resulted in rice plants with developmental defects, including semidwarfism, pale-green leaves, abnormal chloroplast structure, and reduced carotenoid and chlorophyll content. Ostrxm RNAi plants showed remarkably decreased F_v/F_m values under high irradiance conditions (1,000 μmol m⁻² s⁻¹) with delayed recovery. Two-dimensional electrophoresis and matrix-assisted laser-desorption/ionization time-of-flight analysis showed that the levels of several chloroplast proteins critical for photosynthesis and biogenesis were significantly decreased in Ostrxm RNAi plants. Furthermore, 2-Cys peroxiredoxin, a known target of thioredoxin, was present in oxidized forms, and hydrogen peroxide levels were increased in Ostrxm RNAi plants. The pleiotropic effects of Ostrxm RNAi suggest that Ostrxm plays an important role in the redox regulation of chloroplast target proteins involved in diverse physiological functions.

Dracocephalum palmatum Stephan (Lamiaceae) is a medicinal plant used by the North-Yakutian nomads. From the crude ethanolic extract of the aerial parts of this plant, 23 compounds (phenylpropanoids, coumarins, flavonoids, and triterpenes) were isolated. Among these, eight compounds (salvianolic acid B, caftaric acid, cichoric acid, umbelliferone, aesculetin, apigenin-7-O-β-d-glucuronopyranoside, isorhoifolin, and luteolin-4'-O-β-d-glucopyranoside) were detected for the first time in the genus Dracocephalum. Their structures were elucidated based on chemical and spectral data. The levels of most of the compounds detected in the cultivated sample were close to that of the wild sample, indicating the reproducibility of the biologically active compounds of D. palmatum through cultivation. Investigation into the biological activity of D. palmatum under in vitro conditions demonstrated that its extracts have a strong antioxidant effect due to the presence of high concentrations of phenolic compounds.

The zebra‐necrosis (zn) mutant of rice (Oryza sativa) produces transversely green/yellow‐striped leaves. The mutant phenotype is formed by unequal impairment of chloroplast biogenesis before emergence from the leaf sheath under alternate light/dark or high/low temperatures (restrictive), but not under constant light and temperature (permissive) conditions. Map‐based cloning revealed that ZN encodes a thylakoid‐bound protein of unknown function. Virus‐induced gene silencing of a ZN homolog in Nicotiana benthamiana causes leaf variegation with sporadic green/yellow sectors, indicating that ZN is essential for chloroplast biogenesis during early leaf development. Necrotic lesions often occur in the yellow sectors as a result of an excessive accumulation of reactive oxygen species (ROS). The phenotypic severity (leaf variegation and necrosis) and ROS levels are positively correlated with an increase in light intensity under restrictive conditions. In the mutant leaves, chlorophyll (Chl) metabolism, ROS scavenging activities, maximum quantum yield of photosystem II (PSII), and structures and functions of the photosynthetic complexes are normal in the Chl‐containing cells, suggesting that ROS are mainly generated from the defective plastids of the Chl‐free cells. The PSII activity of normal chloroplasts is hypersensitive to photoinhibition because the recovery rates of PSII are much slower. In the PSII repair, the degradation of damaged D1 is not impaired, suggesting a reduced activity of new D1 synthesis, possibly because of higher levels of ROS generated from the Chl‐free cells by excess light. Together, we propose that ZN is required for protecting developing chloroplasts, especially during the assembly of thylakoid protein complexes, from incidental light after darkness.