Breaking New Ground! CRY2, the Blue Light Receptor, Also Functions in Darkness

Light is crucial not only as the energy source for photosynthesis but also as a key environmental signal that regulates plant growth. While seedlings grow with elongated hypocotyls and short roots in darkness, they exhibit shorter hypocotyls and longer roots under light. This difference arises because plant photoreceptors, acting as the "eyes" of plants, sense light and modulate development. Cryptochromes (CRYs), the blue light receptors, mediate light-induced processes like hypocotyl elongation, flowering, and the circadian clock. Most previous research has focused on how CRYs are activated by blue light, but does CRY2 have a regulatory role in the dark or in its“non-photoexcited”state?

On November 15, 2024,Cellpublished a groundbreaking study from Professor Liu Hongtao’s lab at Shenzhen University titled“The Arabidopsis blue-light photoreceptor CRY2 is active in darkness to inhibit root growth.”This study challenges traditional research paradigms by revealing that CRY2 has a distinct function in the dark, demonstrating that blue light not only“activates”but also“inactivates”CRY2.

The research found that CRY2 regulates key processes such as cell division and photosynthesis in darkness by influencing the expression of related genes. Under dark conditions, CRY2 suppresses root elongation by inhibiting cell division in the root apical meristem. Interestingly, blue light alleviates this inhibition.The study also identified two proteins from the FORKED-LIKE (FL) family, FL1 and FL3, that bind to CRY2 in the dark, with this interaction being specifically suppressed by blue light. Further investigation revealed that FL proteins bind only to monomeric CRY2 in the dark, not to its polymerized form after blue light exposure. FL1 and FL3 are cell-division-promoting factors that facilitate root growth by promoting gene expression. In darkness, CRY2 inhibits FL proteins’activity, suppressing root elongation. Under blue light, CRY2 forms a polymer and releases FL proteins, promoting root growth. (Figure 1)

Figure 1: CRY2-FL regulation of primary root elongation in Arabidopsis

This study not only answers a critical question about CRY2’s activity in its non-photoexcited state but also opens new avenues for research on photoreceptor functions in the dark. These findings highlight that CRYs are not only“turned on”by blue light but also“turned off”by it, reshaping our understanding of light signaling.

Understanding CRY2’s role in darkness provides new insights into how plants utilize both light and darknesssignals to regulate growth.For instance, in dark conditions, etiolated seedlings prioritize elongating their hypocotyls while limiting root growth, allowing the seedling to break through the soil and reach light. This strategy is crucial for plants that germinate deep underground. In contrast, seedlings under light grow with short hypocotyls and long roots, as blue light activates CRY2 to balance growth above and below ground, optimizing root development for nutrient uptake. This discovery provides a fresh perspective on how plants coordinate growth and development in response to light and darkness. (Figure 2)

Figure 2: CRY2’s coordination of plant growth under different light conditions and developmental stages

Dr. Zeng Desheng, a postdoctoral researcher at Shenzhen University, is the first author of this study, with Professor Liu Hongtao as the corresponding author. (Figure 2)PhD students Lv Junqing and Professor Li Xu also contributed to the study. The research was supported by the National Natural Science Foundation of China.

Figure 3: Professor Liu Hongtao (left) with Dr. Zeng Desheng