Oh Eunkyoo, Yamaguchi Shinjiro, Kamiya Yuji, Bae Gabyong, Chung Won-Il, Choi Giltsu
Department of Biological Sciences, KAIST, Daejeon 305-701, Korea.
Plant J. 2006 Jul;47(1):124-39. doi: 10.1111/j.1365-313X.2006.02773.x. Epub 2006 Jun 1.
Angiosperm seeds integrate various environmental signals, such as water availability and light conditions, to make a proper decision to germinate. Once the optimal conditions are sensed, gibberellin (GA) is synthesized, triggering germination. Among environmental signals, light conditions are perceived by phytochromes. However, it is not well understood how phytochromes regulate GA biosynthesis. Here we investigated whether phytochromes regulate GA biosynthesis through PIL5, a phytochrome-interacting bHLH protein, in Arabidopsis. We found that pil5 seed germination was inhibited by paclobutrazol, the ga1 mutation was epistatic to the pil5 mutation, and the inhibitory effect of PIL5 overexpression on seed germination could be rescued by exogenous GA, collectively indicating that PIL5 regulates seed germination negatively through GA. Expression analysis revealed that PIL5 repressed the expression of GA biosynthetic genes (GA3ox1 and GA3ox2), and activated the expression of a GA catabolic gene (GA2ox) in both PHYA- and PHYB-dependent germination assays. Consistent with these gene-expression patterns, the amount of bioactive GA was higher in the pil5 mutant and lower in the PIL5 overexpression line. Lastly, we showed that red and far-red light signals trigger PIL5 protein degradation through the 26S proteasome, thus releasing the inhibition of bioactive GA biosynthesis by PIL5. Taken together, our data indicate that phytochromes promote seed germination by degrading PIL5, which leads to increased GA biosynthesis and decreased GA degradation.
被子植物种子整合各种环境信号，如水的可利用性和光照条件，以做出合适的发芽决定。一旦感知到最佳条件，就会合成赤霉素（GA），从而触发发芽。在环境信号中，光照条件由光敏色素感知。然而，光敏色素如何调节GA生物合成尚不清楚。在这里，我们研究了拟南芥中光敏色素是否通过与光敏色素相互作用的bHLH蛋白PIL5来调节GA生物合成。我们发现，多效唑抑制pil5种子发芽，ga1突变对pil5突变呈上位性，外源GA可挽救PIL5过表达对种子发芽的抑制作用，这些结果共同表明PIL5通过GA负调控种子发芽。表达分析表明，在依赖PHYA和PHYB的发芽试验中，PIL5抑制GA生物合成基因（GA3ox1和GA3ox2）的表达，并激活GA分解代谢基因（GA2ox）的表达。与这些基因表达模式一致，pil5突变体中生物活性GA的含量较高，而在PIL5过表达系中较低。最后，我们表明红光和远红光信号通过26S蛋白酶体触发PIL5蛋白降解，从而解除PIL5对生物活性GA生物合成的抑制。综上所述，我们的数据表明，光敏色素通过降解PIL5促进种子发芽，这导致GA生物合成增加和GA降解减少。
