圖11.種子萌發(fā)過程中ABI5和PRR蛋白相互作用調(diào)節(jié)ABA信號的簡化模型

進(jìn)一步的機(jī)制研究表明 PRR5激活 ABI5的轉(zhuǎn)錄功能而不影響其穩(wěn)定性?？偟膩碚f，我們的研究結(jié)果表明，這些 PRR 蛋白與 ABI5協(xié)同作用，激活種子萌發(fā)過程中的 ABA 反應(yīng)，從而提供了 ABA 信號和晝夜節(jié)律時鐘如何通過涉及 ABI5和中樞生物鐘成分的轉(zhuǎn)錄復(fù)合物直接整合的機(jī)制性理解。

Seed germination and postgerminative growth require the precise coordination of multiple intrinsic and environmental signals. The phytohormone abscisic acid (ABA) suppresses these processes in Arabidopsis thaliana and the circadian clock contributes to the regulation of ABA signaling. However, the molecular mechanism underlying circadian clock-mediated ABA signaling remains largely unknown. Here, we found that the core circadian clock proteins PSEUDO-RESPONSE REGULATOR5 (PRR5) and PRR7 physically associate with ABSCISIC ACID INSENSITIVE5 (ABI5), a crucial transcription factor of ABA signaling. PRR5 and PRR7 positively modulate ABA signaling redundantly during seed germination. Disrupting PRR5 and PRR7 simultaneously rendered germinating seeds hyposensitive to ABA, whereas overexpression of PRR5 enhanced ABA signaling to inhibit seed germination. Consistent with this, the expression of several ABA-responsive genes is upregulated by PRR proteins. Genetic analysis demonstrated that PRR5 promotes ABA signaling mainly dependently on ABI5. Further mechanistic investigation revealed that PRR5 stimulates the transcriptional function of ABI5 without affecting its stability. Collectively, our results indicate that these PRR proteins function synergistically with ABI5 to activate ABA responses during seed germination, thus providing a mechanistic understanding of how ABA signaling and the circadian clock are directly integrated through a transcriptional complex involving ABI5 and central circadian clock components.