Although iodine is not an essential nutrient for higher plants, their roots take up and transport the element. However, the exact mechanisms involved in iodine uptake and metabolism in higher plants have yet to be elucidated. In this study, we compared two cultivars differing in iodine tolerance (“Nipponbare” and “Gohyakumangoku”) to increasing levels of I⁻ and IO⁻₃ in the root solutions of water-cultured rice (Oryza sativa L.). We found that IO⁻₃ added to the root solutions was converted to I⁻ in the presence of roots. Iodate reduction occurred over the course of several hours. Furthermore, the iodate reduction activity of “Nipponbare” (iodine-sensitive) and “Gohyakumangoku” (iodine-tolerant) roots increased after adding IO⁻₃ or I⁻. The roots of barley and soybean also showed iodate reduction activity and the activity responded to iodine treatment either with IO⁻₃ and I⁻. This study suggests that plant roots biologically reduce iodate to iodide and indicates that the iodate reduction activity of roots responds to external iodine conditions.

Salt ions promote heterogeneous localization of charged molecules toward the interface of aqueous microdroplets in oil.

Photosynthetic organisms utilize carotenoids for photoprotection as well as light harvesting. Our previous study revealed that high-intensity light increases the expression of the gene for phytoene synthase (EgcrtB) in Euglena gracilis (a unicellular phytoflagellate), the encoded enzyme catalyzes the first committed step of the carotenoid biosynthesis pathway. To examine carotenoid synthesis of E. gracilis in response to light stress, we analyzed carotenoid species and content in cells grown under various light intensities. In addition, we investigated the effect of suppressing EgcrtB with RNA interference (RNAi) on growth and carotenoid content. After cultivation for 7 days under continuous light at 920 μmol m−2 s−1, β-carotene, diadinoxanthin (Ddx), and diatoxanthin (Dtx) content in cells was significantly increased compared with standard light intensity (55 μmol m−2 s−1). The high-intensity light (920 μmol m−2 s−1) increased the pool size of diadinoxanthin cycle pigments (i.e., Ddx + Dtx) by 1.2-fold and the Dtx/Ddx ratio from 0.05 (control) to 0.09. In contrast, the higher-intensity light treatment caused a 58% decrease in chlorophyll (a + b) content and diminished the number of thylakoid membranes in chloroplasts by approximately half compared with control cells, suggesting that the high-intensity light-induced accumulation of carotenoids is associated with an increase in both the number and size of lipid globules in chloroplasts and the cytoplasm. Transient suppression of EgcrtB in this alga by RNAi resulted in significant decreases in cell number, chlorophyll, and total major carotenoid content by 82, 82 and 86%, respectively, relative to non-electroporated cells. Furthermore, suppression of EgcrtB decreased the number of chloroplasts and thylakoid membranes and increased the Dtx/Ddx ratio by 1.6-fold under continuous illumination even at the standard light intensity, indicating that blocking carotenoid synthesis increased the susceptibility of cells to light stress. Our results indicate that suppression of EgcrtB causes a significant decrease in carotenoid and chlorophyll content in E. gracilis accompanied by changes in intracellular structures, suggesting that Dtx (de-epoxidized form of diadinoxanthin cycle pigments) contributes to photoprotection of this alga during the long-term acclimation to light-induced stress.

Euglena gracilis, a unicellular phytoflagellate within Euglenida, has attracted much attention as a potential feedstock for renewable energy production. In outdoor open-pond cultivation for biofuel production, excess direct sunlight can inhibit photosynthesis in this alga and decrease its productivity. Carotenoids play important roles in light harvesting during photosynthesis and offer photoprotection for certain non-photosynthetic and photosynthetic organisms including cyanobacteria, algae, and higher plants. Although, Euglenida contains β-carotene and xanthophylls (such as zeaxanthin, diatoxanthin, diadinoxanthin and 9′-cis neoxanthin), the pathway of carotenoid biosynthesis has not been elucidated. To clarify the carotenoid biosynthetic pathway in E. gracilis, we searched for the putative E. gracilis geranylgeranyl pyrophosphate (GGPP) synthase gene (crtE) and phytoene synthase gene (crtB) by tblastn searches from RNA-seq data and obtained their cDNAs. Complementation experiments in Escherichia coli with carotenoid biosynthetic genes of Pantoea ananatis showed that E. gracilis crtE (EgcrtE) and EgcrtB cDNAs encode GGPP synthase and phytoene synthase, respectively. Phylogenetic analyses indicated that the predicted proteins of EgcrtE and EgcrtB belong to a clade distinct from a group of GGPP synthase and phytoene synthase proteins, respectively, of algae and higher plants. In addition, we investigated the effects of light stress on the expression of crtE and crtB in E. gracilis. Continuous illumination at 460 or 920 μmol m−2 s−1 at 25 °C decreased the E. gracilis cell concentration by 28–40 % and 13–91 %, respectively, relative to the control light intensity (55 μmol m−2 s−1). When grown under continuous light at 920 μmol m−2 s−1, the algal cells turned reddish-orange and showed a 1.3-fold increase in the crtB expression. In contrast, EgcrtE expression was not significantly affected by the light-stress treatments examined. We identified genes encoding CrtE and CrtB in E. gracilis and found that their protein products catalyze the early steps of carotenoid biosynthesis. Further, we found that the response of the carotenoid biosynthetic pathway to light stress in E. gracilis is controlled, at least in part, by the level of crtB transcription. This is the first functional analysis of crtE and crtB in Euglena.

Carotenoids participate in photosynthesis and photoprotection in oxygenic phototrophs. Euglena gracilis, a eukaryotic phytoflagellate, synthesizes several carotenoids: β-carotene, neoxanthin, diadinoxanthin and diatoxanthin. Temperature is one of the most striking external stimuli altering carotenoid production. In the present study, to elucidate the regulation of carotenoid synthesis of E. gracilis in response to environmental stimuli, we functionally identified phytoene desaturase genes (crtP1 and crtP2) and the ζ-carotene desaturase gene (crtQ) of this alga and analyzed expression of those genes and the composition of major carotenoids in cells grown under cold (20°C) and high-intensity light (HL; 240 µmol photon m⁻² s⁻¹) conditions. 20°C–HL treatment increased the transcriptional level of the phytoene synthase gene (crtB), and crtP1 and crtP2, whose products catalyze the early steps of carotenoid biosynthesis in this alga. Cultivation at 20°C under illumination at 55 µmol photon m⁻² s⁻¹ (low-intensity light; LL) decreased the cell concentration, Chl and total major carotenoid content by 61, 75 and 50%, respectively, relative to control (25°C–LL) cells. When grown at 20°C–HL, the cells showed a greater decrease in cell concentration and photosynthetic pigment contents than those in 20°C–LL. β-Carotene, neoxanthin and diadinoxanthin contents were decreased by more than half in 20°C–LL and 20°C–HL treatments. On the other hand, when subjected to 20°C–LL and 20°C–HL, the cells retained a diatoxanthin content comparable with control cells. Our findings suggested that diatoxanthin plays crucial roles in the acclimation to cold and intense light condition. To the best of our knowledge, this is the first report on a photosynthetic organism possessing dual crtP genes.