Dirk Lynnette M A, Kumar Santosh, Majee Manoj, Downie A Bruce
a Department of Horticulture, Seed Biology Group , University of Kentucky , Lexington , KY , USA.
b Department of Biochemistry, 243 Christopher S. Bond Life Sciences Center , University of Missouri , Columbia , MO , USA.
Plant Signal Behav. 2018;13(10):e1525999. doi: 10.1080/15592324.2018.1525999. Epub 2018 Oct 8.
In Arabidopsis thaliana, the basic Helix Loop Helix transcription factor, PHYTOCHROME INTERACTING FACTOR1 (PIF1) is known to orchestrate the seed transcriptome such that, ultimately, proteins repressing the completion of germination are produced in darkness. While PIF1-mediated control of abscisic acid (ABA) and gibberellic acid (GA) anabolism/catabolism is indirect, PIF1 action favors ABA while discriminating against GA, firmly establishing ABA's repressive influence on the completion of germination. The result is tissue that is more sensitive to and producing more ABA; and is less responsive to and deficient in GA. Illumination of the appropriate wavelength activates phytochrome which enters the nucleus, and binds to PIF1, initiating PIF1's phosphorylation by diverse kinases, subsequent polyubiquitination, and hydrolysis. One mechanism by which phosphorylated PIF1 is eliminated from the cells of the seed upon illumination involves an F-BOX protein, COLD TEMPERATURE GERMINATING10 (CTG10). Discovered in an unbiased screen of activation tagged lines hastening the completion of seed germination at 10°C, one indirect consequence of CTG10 action in reducing PIF1 titer, should be to enhance the transcription of genes whose products work to increase bioactive GA titer, shifting the intracellular milieu from one that is repressive to, toward one conducive to, the completion of seed germination. We have tested this hypothesis using a variety of Arabidopsis lines altered in CTG10 amounts. Here we demonstrate using bimolecular fluorescence complementation that PIF1 interacts with CTG10 and show that, in light exposed seeds, PIF1 is more persistent in ctg10 relative to WT seeds while it is less stable in seeds over-expressing CTG10. These results are congruent with the relative transcript abundance from three genes whose products are involved in bioactive GA accumulation. We put forth a model of how PIF1 interactions in imbibed seeds change during germination and how a permissive light signal influences these changes, leading to the completion of germination of these positively photoblastic propagules.
在拟南芥中，已知基本螺旋-环-螺旋转录因子——光敏色素相互作用因子1（PIF1）可调控种子转录组，从而最终在黑暗中产生抑制种子萌发完成的蛋白质。虽然PIF1对脱落酸（ABA）和赤霉素（GA）合成/分解代谢的调控是间接的，但PIF1的作用有利于ABA，同时抑制GA，从而坚定地确立了ABA对种子萌发完成的抑制作用。结果是组织对ABA更敏感且产生更多ABA，而对GA反应性更低且GA含量不足。适当波长的光照会激活进入细胞核并与PIF1结合的光敏色素，引发PIF1被多种激酶磷酸化，随后发生多聚泛素化和水解。光照后，磷酸化的PIF1从种子细胞中被清除的一种机制涉及一种F-盒蛋白——低温萌发10（CTG10）。在对10℃下加速种子萌发的激活标签系进行的无偏筛选中发现，CTG10作用降低PIF1水平的一个间接后果应该是增强那些其产物可提高生物活性GA水平的基因的转录，将细胞内环境从抑制性环境转变为有利于种子萌发完成的环境。我们使用了多种CTG10含量改变的拟南芥品系来验证这一假设。在此，我们通过双分子荧光互补证明PIF1与CTG10相互作用，并表明在光照处理的种子中，相对于野生型种子，PIF1在ctg10突变体种子中更持久，而在过表达CTG10的种子中则不太稳定。这些结果与三个其产物参与生物活性GA积累的基因的相对转录丰度一致。我们提出了一个模型，说明在吸胀种子中PIF1的相互作用在萌发过程中如何变化，以及允许的光信号如何影响这些变化，从而导致这些正性光敏感繁殖体的萌发完成。
