Light is essential to plant survival and elicits a wide range of plant developmental and physiological responses under different light conditions. A low red-to-far red (R/FR) light ratio induces shade-avoidance responses, including decreased anthocyanin accumulation, whereas a high R/FR light ratio promotes anthocyanin biosynthesis. However, the detailed molecular mechanism underpinning how different R/FR light ratios regulate anthocyanin homeostasis remains elusive, especially in non-model species. Here, we demonstrate that a low R/FR light ratio induced the expression of CmMYB4, which suppressed the anthocyanin activator complex CmMYB6-CmbHLH2, leading to the reduction of anthocyanin accumulation in Chrysanthemum (Chrysanthemum morifolium) petals. Specifically, CmMYB4 recruited the corepressor CmTPL (TOPLESS) to directly bind the CmbHLH2 promoter and suppressed its transcription by impairing histone H3 acetylation. Moreover, the low R/FR light ratio inhibited the PHYTOCHROME INTERACTING FACTOR family transcription factor CmbHLH16, which can competitively bind to CmMYB4 and destabilize the CmMYB4-CmTPL protein complex. Under the high R/FR light ratio, CmbHLH16 was upregulated, which impeded the formation of the CmMYB4-CmTPL complex and released the suppression of CmbHLH2, thus promoting anthocyanin accumulation in Chrysanthemum petals. Our findings reveal a mechanism by which different R/FR light ratios fine-tune anthocyanin homeostasis in flower petals.

中文翻译：

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转录因子CmbHLH16在不同光照下调节菊花花瓣花青素稳态。

光对于植物的生存至关重要，并在不同的光照条件下引发广泛的植物发育和生理反应。低红光与远红光 (R/FR) 光比会诱发避荫反应，包括减少花青素积累，而高 R/FR 光比则促进花青素生物合成。然而，不同 R/FR 光比如何调节花青素稳态的详细分子机制仍然难以捉摸，特别是在非模型物种中。在这里，我们证明低 R/FR 光比诱导 CmMYB4 的表达，从而抑制花青素激活剂复合物 CmMYB6-CmbHLH2，导致菊花花瓣中花青素的积累减少。具体来说，CmMYB4 招募辅阻遏物 CmTPL (TOPLESS) 直接结合 CmbHLH2 启动子，并通过损害组蛋白 H3 乙酰化来抑制其转录。此外，低R/FR光比抑制了PHYTOCHROME INTERACTING FACTOR家族转录因子CmbHLH16，该因子可以竞争性地结合CmMYB4并破坏CmMYB4-CmTPL蛋白复合物的稳定性。高R/FR光比下，CmbHLH16表达上调，阻碍CmMYB4-CmTPL复合物的形成，并解除对CmbHLH2的抑制，从而促进菊花花瓣中花青素的积累。我们的研究结果揭示了不同的 R/FR 光比微调花瓣中花青素稳态的机制。