School of Life Sciences, Lanzhou University, Lanzhou, Gansu 730000, People's Republic of China.
J Plant Physiol. 2011 Dec 15;168(18):2161-8. doi: 10.1016/j.jplph.2011.07.004.
In the present study, the role of phytohormone gibberellins (GAs) on regulating the nitrate reductase (NR) activity was tested in Arabidopsis seedlings. The NR activity in light-grown Col-0 seedlings was reduced by exogenous GA₃ (an active form of GAs), but enhanced by exogenous paclobutrazol (PAC, a gibberellin biosynthesis inhibitor), suggesting that GAs negatively regulate the NR activity in light-grown seedlings. Light is known to influence the NR activity through both photosynthesis and phytochromes. When etiolated seedlings were transferred to white or red light, both exogenously applied GA₃ and PAC were found to function on the NR activity only in the presence of sucrose, implying that GAs are not involved in light signaling-induced but negatively regulate photoproducts-induced NR activity. NR is regulated by light mainly at two levels: transcript level and post-translational level. Our reverse transcription (RT)-PCR assays showed that GAs did not affect the transcript levels of NIA1 and NIA2, two genes that encode NR proteins. But the divalent cations (especially Mg²⁺) were required for GAs negative regulation of NR activity, in view of the importance of divalent cations during the process of post-translational regulation of NR activity, which indicates that GAs very likely regulate the NR activity at the post-translational level. In the following dark-light shift analyses, GAs were found to accelerate dark-induced decrease, but retard light-induced increase of the NR activity. Furthermore, it was observed that application of G₃ or PAC could impair diurnal variation of the NR activity. These results collectively indicate that GAs play a negative role during light regulation of NR activity in nature.
在本研究中，测试了植物激素赤霉素（GAs）在调节拟南芥幼苗硝酸还原酶（NR）活性中的作用。外源 GA₃（GA 的一种活性形式）降低了光下生长的 Col-0 幼苗中的 NR 活性，但外源多效唑（PAC，一种赤霉素生物合成抑制剂）增强了 NR 活性，表明 GA 负调节光下生长的幼苗中的 NR 活性。光照已知通过光合作用和光敏色素影响 NR 活性。当将黄化幼苗转移到白光或红光下时，发现外源 GA₃ 和 PAC 仅在存在蔗糖的情况下对 NR 活性起作用，这表明 GA 不参与光信号诱导，但负调节光产物诱导的 NR 活性。NR 主要在两个水平上受光照调节：转录水平和翻译后水平。我们的反转录（RT）-PCR 分析表明，GA 不影响编码 NR 蛋白的两个基因 NIA1 和 NIA2 的转录水平。但是，二价阳离子（特别是 Mg²⁺）是 GA 负调节 NR 活性所必需的，考虑到二价阳离子在 NR 活性翻译后调节过程中的重要性，这表明 GA 很可能在翻译后水平上调节 NR 活性。在随后的暗-光转换分析中，发现 GA 加速了黑暗诱导的 NR 活性降低，但减缓了光诱导的 NR 活性增加。此外，观察到应用 G₃ 或 PAC 会损害 NR 活性的昼夜变化。这些结果共同表明，GA 在自然条件下光调节 NR 活性中起负作用。
