BPG4 regulates chloroplast development and homeostasis by suppressing GLK transcription factors and involving light and brassinosteroid signaling

Chloroplast development adapts to the environment for performing suitable photosynthesis. Brassinosteroids (BRs), plant steroid hormones, have crucial effects on not only plant growth but also chloroplast development. However, the detailed molecular mechanisms of BR signaling in chloroplast development remain unclear. Here, we identify a regulator of chloroplast development, BPG4, involved in light and BR signaling. BPG4 interacts with GOLDEN2-LIKE (GLK) transcription factors that promote the expression of photosynthesis-associated nuclear genes (PhANGs), and suppresses their activities, thereby causing a decrease in the amounts of chlorophylls and the size of light-harvesting complexes. BPG4 expression is induced by BR deficiency and light, and is regulated by the circadian rhythm. BPG4 deficiency causes increased reactive oxygen species (ROS) generation and damage to photosynthetic activity under excessive high-light conditions. Our findings suggest that BPG4 acts as a chloroplast homeostasis factor by fine-tuning the expression of PhANGs, optimizing chloroplast development, and avoiding ROS generation.

Increased conversion ratio of tryptophan to niacin by the administration of clofibrate, a hypolipidemic drug, to rats

The effect of clofibrate, a hypolipidemic drug and also known as a peroxisomal proliferator, on the conversion ratio of tryptophan to niacin was investigated by using rats. The rats were fed with a nicotinic acid-free, 20% casein diet (control group) or the same diet + 0.25% clofibrate group) for 19 days. The conversion ratio gradually increased with increasing number of days. Around day 8, the ratio was about 10-times higher in the clofibrate group than in the control group, and the value remained almost constant after that day. The content of liver total nicotinamide was higher in the clofibrate group than in the control group. Among the enzymes involved in the conversion of tryptophan to niacin, the aminocarboxymuconate-semialdehyde decarboxylase (ACMSDase) activity, which is critical in the conversion, was lower in the clofibrate group than in the control group. As the change in ACMSDase activity took several days, there is a possibility that clofibrate decreased the biosynthesis of ACMSDase protein and/or mRNA. To learn whether the increase in the conversion ratio by clofibrate would be nutritionally meaningful or not, the growth-promoting activity of clofibrate was determined by using weanling rats fed with a nicotinic acid-free, tryptophan-limiting diet (basal diet). As a result, the body weight gain was higher in the clofibrate group than in the basal group. This result shows that clofibrate enhanced the conversion ratio without any side-effects under the conditions used and supports again the claim that the activity of ACMSDase exerts a critical influence on the tryptophan-NAD conversion.

In vivo inhibition of kynurenine aminotransferase activity by isonicotinic acid hydrazide in rats

It is known that the anti-tuberculosis drug, isonicotinic acid hydrazide (INH), causes pellagra, a niacin deficiency syndrome, and peripheral neuritis in humans. We investigated the effects of INH on the metabolism of tryptophan to niacin in rats fed on a niacin-free diet. The activity of kynurenine aminotransferase was significantly inhibited by feeding a diet containing INH and by an injection of INH, and the urinary excretion of xanthurenic acid, the side-reaction product of the conversion pathway of tryptophan to niacin, was below the limit of detection. The inhibition of kynurenine aminotransferase and the resulting decreased formation of xanthurenic acid generally mean a higher conversion ratio of tryptophan to niacin. However, the conversion ratio was no different between the control and INH groups.