CML20 integrates phytochrome and calcium signaling pathways to optimize seed germination under abiotic stress

Plants integrate multiple environmental cues to decide the timing of seed germination. Light promotes germination primarily through photoreceptor phytochrome B (phyB), which eliminates the action of transcription factor PHYTOCHROME-INTERACTING FACTOR 1 (PIF1). Conversely, abiotic stresses typically elevate cytosolic Ca²⁺ and inhibit germination. However, the coordination of light and calcium signaling during germination under stress conditions remains poorly understood. Here, we demonstrate that salt and osmotic stresses rapidly increase cytosolic Ca²⁺ in imbibed seeds, and that the calcium sensor CALMODULIN-LIKE 20 (CML20) is crucial for germination tolerance to these stresses. CML20 directly interacts with PIF1 in a Ca²⁺-dependent manner, suppressing PIF1 self-association, oligomerization, and DNA-binding capacity. Moreover, phyB interacts with CML20 and PIF1 independently, promoting the assembly of the CML20-PIF1 complex and further reducing PIF1's transcriptional activity. Genetic analyses reveal that PIF1 functions downstream of CML20, and that CML20 and phyB operate within the same pathway. Together, our findings define a phyB-CML20-PIF1 module, providing a rapid, reversible, and non-proteolytic mechanism to optimize seed germination under stress conditions.