The first exposure to light marks a crucial transition in plant development. This transition relies on the transcription factor HY5 controlling a complex downstream growth program. Despite its importance, its function in transcription remains unclear. Previous studies have generated lists of thousands of potential target genes and competing models of HY5 transcription regulation. In this work, we carry out detailed phenotypic and molecular analysis of constitutive activator and repressor HY5 fusion proteins. Using this strategy, we were able to filter out large numbers of genes that are unlikely to be direct targets, allowing us to eliminate several proposed models of HY5’s mechanism of action. We demonstrate that the primary activity of HY5 is promoting transcription and that this function relies on other, likely light-regulated, factors. In addition, this approach reveals a molecular feedback loop via the COP1/SPA E3 ubiquitin ligase complex, suggesting a mechanism that maintains low HY5 in the dark, primed for rapid accumulation to reprogram growth upon light exposure. Our strategy is broadly adaptable to the study of transcription factor activity. Lastly, we show that modulating this feedback loop can generate significant phenotypic diversity in both Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum).

首次暴露于光下标志着植物发育的关键转变。这种转变依赖于转录因子HY5控制复杂的下游生长程序。尽管其重要性，其转录功能仍不清楚。先前的研究已经产生了数千个潜在靶基因和HY5转录调控竞争模型的列表。在这项工作中，我们进行了组成型激活物和阻遏物HY5融合蛋白的详细表型和分子分析。使用此策略，我们能够滤除大量不太可能成为直接靶标的基因，从而使我们能够消除HY5作用机制的几种拟议模型。我们证明HY5的主要活性是促进转录，并且此功能依赖于其他可能的光调节因素。此外，这种方法揭示了通过COP1 / SPA E3泛素连接酶复合物的分子反馈回路，表明了一种在黑暗中保持低HY5的机制，该机制可在曝光后迅速积累以重新编程生长。我们的策略广泛适用于转录因子活性的研究。最后，我们证明了调节此反馈回路可以在两个拟南芥中产生显着的表型多样性（拟南芥（Arabidopsis thaliana）和番茄（Solanum lycopersicum）。