It has been known that leaves exposed to high light contain more L-ascorbic acid (AsA) than those in the shade. However, the mechanism of the light regulation of the AsA pool size in plants is largely unknown. In this work, the relationship between gene expression levels related to AsA biosynthesis and photosynthesis have been studied. When 2-week-old Arabidopsis plants grown under a 16 h daily photoperiod were moved into the dark, the AsA level in the leaves was decreased by 91% in 72 h, whereas it increased by 171% in the leaves of plants exposed to continuous light during the same period. Among the several enzymes of the AsA biosynthesis pathway, the transcript levels of GDP-D-mannose pyrophosphorylase, L-galactose 1-P phosphatase, L-galactono-1,4-lactone dehydrogenase, and the VTC2 gene were down-regulated in the dark. Treatment with inhibitors of photosynthesis, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine, arrested a rise in the AsA pool size accompanying the decrease in the transcript levels of the genes of the above enzyme in the leaves. When the plants were transferred to a medium containing 0.5% (w/v) sucrose, the photosynthesis activities and the leaf AsA levels were lowered even under exposure to light compared with those in plants on the medium without sucrose. In contrast, the AsA level in leaves of the sugar-insensitive Arabidopsis mutant abi4/sun6 was unaffected by external sucrose. No significant difference in the expression profiles for AsA biosynthesis enzymes was observed between the wild-type and mutant plants by sucrose feeding. The results suggest that photosynthetic electron transport of chloroplasts is closely related to AsA pool size regulation in leaves.

SummaryTo evaluate the physiological importance of thylakoid membrane-bound ascorbate peroxidase (tAPX) in the active oxygen species-scavenging system of chloroplasts, the level of tAPX in tobacco plants was altered by expression of the tAPX cDNA in both sense and antisense orientation. The tobacco plants transformed with constructs of antisense tAPXs from spinach and tobacco could not be obtained, suggesting that the suppression of tAPX in higher plants had a severe effect on the growth even under normal conditions. In contrast, the transgenic tobacco plants (TpTAP-12) overexpressing tAPX, which had approximately 37-fold higher activity than that of the wild-type plants, were generated. The TpTAP-12 plants showed increased tolerance to oxidative stress caused by application of methylviologen (MV, 50 mM) under light intensity (300 and 1600 lE m À2 sec À1 ) and by chilling stress with high light intensity (48C, 1000 lE m À2 sec À1 ). At 24 h after the MV treatment under illumination at 300 lE m À2 sec À1 , destruction of chlorophyll was observed in the wild-type plants, but not in the TpTAP-12 plants. The activities of thiol-modulated enzymes in the Calvin cycle, the level and redox status of ascorbate (AsA), and the activity of tAPX in the wild-type plants significantly decreased, while those in the TpTAP-12 plants were hardly changed. These observations suggest that tAPX is a limiting factor of antioxidative systems under photo-oxidative stress in chloroplasts, and that the enhanced activity of tAPX functions to maintain the AsA content and the redox status of AsA under stress conditions.

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To establish guidelines for avoiding the side eŠects of mycophenolate mofetil (MMF) in renal transplant recipients with tacrolimus (TAC)-based immunosuppression, the relationship between the daily dose of MMF and the occurrence of side eŠects was analyzed in this study. The frequency of side eŠects was investigated retrospectively in 28 renal transplant recipients treated with immunosuppression (men 14 : women 14, age: 33.0 ± 12.4 years, weight: 50.9 ± 10.7 kg). Cytomegalovirus (CMV) infection and diarrhea were the most frequent side eŠects in the early transplant phase (from transplantation to 3-month biopsy) in the recipients. In 18 recipients, excluding the recipients with risk factors for CMV infection (ABO-incompatible transplantation, donor (＋)/recipient (-) CMV serostatus, etc.), no signiˆcant correlation was shown between the daily dose of MMF and the occurrence of CMV infection in the two-sample t-test. On the other hand, the daily dose in the diarrhea group (33.2 ± 4.3 mg/kg/day, n ＝ 5) was signiˆcantly higher than that in the no-diarrhea group at 30 days (28.4 ± 3.7 mg/kg/day, n ＝ 23, p ＜ 0.05) and 90 days (25.7 ± 4.4 mg/kg/day, n ＝ 21, p ＜ 0.005) after transplantation, respectively. The receiver-operating characteristic (ROC) curve also revealed that the risk of diarrhea increased with a daily MMF dose higher than 30 mg/kg/day. In conclusion, to decrease the risk of diarrhea in the early transplant phase in renal transplant recipients with TAC-based immunosuppression, the daily dose of MMF should not be more than 30 mg/kg/day.