Figure 1 Schematic structural diagrams of plant photoreceptors The N-terminus of phytochromes is a photosensory region (containing PAS, GAF, and PHY), which covalently binds chromophores. The C-terminal light regulatory domains, including PAS and HKRD, are involved in phytochrome dimer formation and downstream signaling transduction. The N-terminus of cryptochromes is a PHR domain, which non-covalently binds FAD and MTHF, and the C-terminal CCE domain is important for protein-protein interactions and signaling transduction. The two N-terminal LOV domains (LOV1 and LOV2) of phototropins bind FMN to perceive light signals and are required for interaction with other proteins. ZTL proteins contain three important functional conserved domains: the N-terminal LOV domain, the intermediate F-box motif, and the C-terminal Kelch repeat region. The LOV domain binds to FMN, and the Kelch domain mediates protein-protein interactions.

Figure 2 A schematic diagram of flowering time regulation by photoreceptor-mediated environmental signaling in Arabidopsis Six flowering regulatory pathways have been identified in plants, including photoperiodic pathway, vernalization pathway, temperature pathway, autonomous pathway, gibberellin pathway, and age pathway. Phytochromes, cryptochromes, and ZTL/FKF1/ LKP2 perceive light signals in leaves, and transmit the signals to the circadian clock. After signaling integration through multiple flowering pathways, the photoreceptors eventually directly or indirectly regulate CO, FT and FLC expression and protein stability. In the ambient temperature pathway, the phytochromes indirectly regulate the transcription of FT by FLM, SVP and PIF4 through FCA/FVE. SRR1 integrates signals from the gibberellin pathway and vernalization pathway and regulate FT transcription through GA3OX1/2 and DELLA proteins in the GA pathway. In the vernalization pathway, VOZ1/2 and phytochromes regulates FT expression through FLC, SVP, and TEM1/2. The autonomous pathway promotes flowering by repressing FLC. Photoreceptors play an important role in the photoperiodic pathway. GI, CDF1, DNF, FBH1-4, COP1, SPA1, HOS1, and NF-Ys directly or indirectly interact with photoreceptors to regulate CO transcription, CO stability, and FT transcription. The age pathway regulates FT expression through miR156/172 and SPLs. There are signal integration mechanisms between different flowering pathways. The signals are integrated into FT, SOC1, and LFY, leading to the activation of AP1 and LFY and eventually the initiation of flowering. Solid arrows indicate direct promotion; dotted arrows indicate indirect promotion; blunted lines indicate direct inhibition; red ovals indicate red/far-red light photoreceptors; blue ovals indicate blue light photoreceptors, and gray ovals indicates signaling intermediate proteins.