Photoprotection of Photosystem II: Reaction Center Quenching Versus Antenna Quenching

Singlet Oxygen in Plants: Generation, Detection, and Signaling Roles

Singlet oxygen (1O2) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, 1O2 is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, 1O2 destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, 1O2 also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, 1O2 can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, 1O2-activated signaling pathways depend not only on the levels but also on the sites of 1O2 production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. 1O2 can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of 1O2 generation in plants, on 1O2-activated pathways of retrograde- and cellular signaling, and on the methods to study 1O2 production in plants.

Chlorella vulgaris integrates photoperiod and chloroplast redox signals in response to growth at high light

Photoacclimation to variable light and photoperiod regimes in C. vulgaris represents a complex interplay between "biogenic" phytochrome-mediated sensing and "operational" redox sensing signaling pathways. Chlorella vulgaris Beijerinck UTEX 265 exhibits a yellow-green phenotype when grown under high light (HL) in contrast to a dark green phenotype when grown at low light (LL). The redox state of the photosynthetic electron transport chain (PETC) as estimated by excitation pressure has been proposed to govern this phenotypic response. We hypothesized that if the redox state of the PETC was the sole regulator of the HL phenotype, C. vulgaris should photoacclimate in response to the steady-state excitation pressure during the light period regardless of the length of the photoperiod. As expected, LL-grown cells exhibited a dark green phenotype, low excitation pressure (1 - qP = 0.22 ± 0.02), high chlorophyll (Chl) content (375 ± 77 fg Chl/cell), low Chl a/b ratio (2.97 ± 0.18) as well as high photosynthetic efficiency and photosynthetic capacity regardless of the photoperiod. In contrast, C. vulgaris grown under continuous HL developed a yellow-green phenotype characterized by high excitation pressure (1 - qP = 0.68 ± 0.01), a relatively low Chl content (180 ± 53 fg Chl/cell), high Chl a/b ratio (6.36 ± 0.54) with concomitantly reduced light-harvesting polypeptide abundance, as well as low photosynthetic capacity and efficiency measured on a per cell basis. Although cells grown under HL and an 18 h photoperiod developed a typical yellow-green phenotype, cells grown at HL but a 12 h photoperiod exhibited a dark green phenotype comparable to LL-grown cells despite exhibiting growth under high excitation pressure (1 - qP = 0.80 ± 0.04). The apparent uncoupling of excitation pressure and phenotype in HL-grown cells and a 12 h photoperiod indicates that chloroplast redox status cannot be the sole regulator of photoacclimation in C. vulgaris. We conclude that photoacclimation in C. vulgaris to HL is dependent upon growth history and reflects a complex interaction of endogenous systems that sense changes in photoperiod as well as photosynthetic redox balance.

Novel Insights into the Influence of Seed Sarcotesta Photosynthesis on Accumulation of Seed Dry Matter and Oil Content in Torreya grandis cv. "Merrillii"

Seed oil content is an important trait of nut seeds, and it is affected by the import of carbon from photosynthetic sources. Although green leaves are the main photosynthetic organs, seed sarcotesta photosynthesis also supplies assimilates to seed development. Understanding the relationship between seed photosynthesis and seed development has theoretical and practical significance in the cultivation of Torreya grandis cv. "Merrillii." To assess the role of seed sarcotesta photosynthesis on the seed development, anatomical and physiological traits of sarcotesta were measured during two growing seasons in the field. Compared with the attached current-year leaves, the sarcotesta had higher gross photosynthetic rate at the first stage of seed development. At the late second stage of seed development, sarcotesta showed down-regulation of PSII activity, as indicated by significant decrease in the following chlorophyll fluorescence parameters: the maximum PSII efficiency (Fv/Fm ), the PSII quantum yield (Φ PSII ), and the photosynthetic quenching coefficient (qP). The ribulose 1, 5-bisphosphate carboxylase (Rubisco) activity, the total chlorophyll content (Chl(a+b)) and nitrogen content in the sarcotesta were also significantly decreased during that period. Treatment with DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] preventing seed photosynthesis decreased the seed dry weight and the oil content by 25.4 and 25.5%, respectively. We conclude that seed photosynthesis plays an important role in the dry matter accumulation at the first growth stage. Our results also suggest that down-regulation of seed photosynthesis is a plant response to re-balance the source-sink ratio at the second growth stage. These results suggest that seed photosynthesis is important for biomass accumulation and oil synthesis of the Torreya seeds. The results will facilitate achieving higher yields and oil contents in nut trees by selection for higher seed photosynthesis cultivars.