Lu Yan, Gehan Jackson P, Sharkey Thomas D
Department of Botany , University of Wisconsin, Madison, Wisconsin 53706, USA.
Plant Physiol. 2005 Aug;138(4):2280-91. doi: 10.1104/pp.105.061903. Epub 2005 Jul 29.
Transitory starch is stored during the day inside chloroplasts and broken down at night for export. Maltose is the primary form of carbon export from chloroplasts at night. We investigated the influence of daylength and circadian rhythms on starch degradation and maltose metabolism. Starch breakdown was faster in plants of Arabidopsis (Arabidopsis thaliana) ecotype Wassilewskija growing in long days. Transcript levels of genes encoding enzymes involved in starch degradation and maltose metabolism showed a strong diurnal rhythm. Under altered photoperiods, the transcript levels and the rate of starch degradation changed within one day/night cycle. However, the amount of proteins involved in starch degradation was maintained relatively constant throughout the day/night cycle. To investigate whether the diurnal cycling of the transcript levels is only a response to light or is also regulated by a circadian clock, we measured the amount of messenger RNAs in Arabidopsis leaves under continuous light and continuous darkness. The expression of genes encoding starch degradation-related enzymes was under very strong circadian control in continuous light. Under continuous light, the amount of maltose also showed a strong endogenous rhythm close to 24 h, indicating that maltose metabolism is under circadian control. Light is necessary for the cycling of transcript levels and maltose levels. Under continuous darkness, these genes were barely expressed, and no cycling of maltose levels was observed.
暂存淀粉在白天储存在叶绿体中，夜间分解并输出。麦芽糖是夜间叶绿体输出碳的主要形式。我们研究了日照长度和昼夜节律对淀粉降解和麦芽糖代谢的影响。在长日照条件下生长的拟南芥生态型瓦西列夫斯基植株中，淀粉分解速度更快。编码参与淀粉降解和麦芽糖代谢的酶的基因转录水平呈现出强烈的昼夜节律。在光周期改变的情况下，转录水平和淀粉降解速率在一个昼夜周期内发生变化。然而，参与淀粉降解的蛋白质数量在整个昼夜周期中保持相对恒定。为了研究转录水平的昼夜循环是仅仅对光的反应还是也受生物钟调节，我们在持续光照和持续黑暗条件下测量了拟南芥叶片中的信使核糖核酸量。在持续光照下，编码淀粉降解相关酶的基因表达受到很强的生物钟控制。在持续光照下，麦芽糖量也呈现出接近24小时的强烈内源性节律，表明麦芽糖代谢受生物钟控制。转录水平和麦芽糖水平的循环需要光。在持续黑暗条件下，这些基因几乎不表达，也未观察到麦芽糖水平的循环。
