Photoreceptors and Circadian Clock Regulators Control Growth and Flowering Under Ionizing Radiation Stress

Abstract Plants have evolved mechanisms to optimize flowering time in response to adverse environments, ensuring reproductive success and survival. Although diverse regulatory networks controlling flowering have been identified, the molecular mechanisms by which environmental stresses influence flowering remain poorly understood. In particular, little is known about the impact of ionizing radiation (IR) such as γ-rays. Here, we show that photoreceptors and circadian clock components are crucial for growth and flowering regulation of Arabidopsis thaliana under IR stress. Exposure to 125 Gy of γ-radiation accelerated flowering in wild-type plants, primarily through elevated expression of the floral activator FT , rather than its upstream regulator CO . Plants lacking the blue-light photoreceptor FKF1 exhibited heightened sensitivity to IR. At the same time, the cry1 cry2 mutant, in which a different type of blue light photoreceptor is impaired, exhibited promotion of flowering by IR, although weaker than that observed in fkf1 . Unlike fkf1 and cry1 cry2 , the cry1 cry2 fkf1-2 mutant showed an IR-insensitive flowering phenotype. Similarly, the flowering time of ZTL mutant and overexpression plants was unaffected by IR. The cca1 lhy mutant lost IR-induced flowering promotion and accumulated high H 2 O 2 levels, implicating ROS regulation in IR-responsive flowering. Interestingly, the phyA-211 mutant exhibited extreme growth sensitivity, with severe retardation and anthocyanin accumulation. In contrast to phyA-211 , the phyB-9 mutant showed no significant differences in growth or anthocyanin accumulation compared with the wild type. These findings reveal distinct roles of photoreceptors and circadian clock regulators in coordinating flowering and growth under IR, providing insights into plant adaptation to radiation stress.