Photoactivation and inactivation of Arabidopsis cryptochrome 2

Turning off the blue-light response In plants, blue light is perceived by cryptochromes, which, once activated, set off signaling events that regulate gene expression, circadian rhythms, and photomorphogenesis. Wang et al. now show that in the model plant Arabidopsis , one of the functions of activated cryptochromes, which are dimers or oligomers when active, is to activate production of the protein BIC1 (blue-light inhibitor of cryptochromes 1) (see the Perspective by Fankhauser and Ulm). BIC1 then favors monomerization and thus inactivation of the cryptochromes. This feedback loop resets the system so that blue-light responses can be turned off as well as turned on. Science , this issue p. 343 ; see also p. 282