Multiple interactions between cryptochrome and phototropin blue-light signalling pathways in Arabidopsis thaliana

Higher plants contain two structurally unrelated flavoprotein blue-light photoreceptors, the cryptochromes and the phototropins, which mediate largely distinct response pathways. Cryptochromes regulate plant development and photomorphogenesis whereas phototropins are primarily implicated in photomovement responses such as phototropism and chloroplast relocation. In the present study we identify interactions between cryptochromes and phototropins in several photoresponses of Arabidopsis thaliana. Cryptochromes are shown to exert a positive effect on phototropic curvature under long-term irradiation conditions. Specifically, in a phot1-deficient genetic background (phot1 mutant), curvature is reduced in the absence of cryptochromes, particularly at wavelengths where cryptochromes show preferential absorption. Phototropins in turn exert a small promotive effect on such cryptochrome-mediated responses as hypocotyl elongation and anthocyanin accumulation. These effects are apparent in a cryptochrome-deficient (cry1cry2 mutant) genetic background. In addition to positive interactions between signalling pathways, we demonstrate that the cryptochromes also exert a negative regulatory effect. Levels of phot1 protein decrease in blue light as a function of cryptochrome photoreceptor activation. This negative regulation occurs in part at the level of phot1 transcription but may also involve post-transcriptional mechanisms. These two classes of photoreceptor thereby reciprocally modulate their overall responsivity to blue light through multiple forms of interaction.