Searching for a photocycle of the cryptochrome photoreceptors

The initial photochemistry of plant cryptochromes has been extensively investigated in recent years. It is hypothesized that cryptochrome photoexcitation involves a Trp-triad-dependent photoreduction. According to this hypothesis, cryptochromes in the resting state contain oxidized FAD; light triggers a sequential electron transfer from three tryptophan residues to reduce FAD to a neutral semiquinone (FADH); FADH is the presumed signaling state and it is re-oxidized to complete the photocycle. However, this photoreduction hypothesis is currently under debate. An alternative model argues that the initial photochemistry of cryptochromes involves a photolyase-like cyclic electron shuttle without a bona fide redox reaction mediated by the Trp-triad residues, leading to conformational changes, signal propagation, and physiological responses.

Research highlightsRecent experimental results of the studies of cryptochrome photochemistry are analyzed to present current opinions about the photoexcitation mechanism of this type of photoreceptors.