Cryptic blues: Mechanism in sight!

Several plant responses are mediated by blue-light as also UV-A radiation. Until a few years ago, the identity of the blue/UV-A sensing photoreceptors was equivocal. In the past few years, however, the molecular genetic analy sis of Arabidopsis mutants has greatly aided in the identification and char acterization of blue/UV-A sensing cryptochrome 1 (cry1), cryptochrome 2 (cr y2) and phototropin (JK224/ nph1), The cry1 and cry2 receptors, like DNA ph otolyases, are dual chromophore proteins and may harbour both a flavin (FAD ) and a pterin for primarily regulating hypocotyl growth inhibition, cotyle don expansion and flowering time, besides sensing light for entraining endo genous clocks. In contrast, JK224/nph1 anchors another flavin, FMN, and reg ulates phototropism, In addition to flavins and pterin, evidence is mountin g in favour of a carotenoid, zeaxanthin, for regulating blue-light-induced stomatal opening. There is little information on the signalling components acting downstream to blue sensory receptors,. but nph1 has an intrinsic kin ase domain that on photoexcitation initiates a phosphorelay through interac tion with phosphoproteins like NPH3. Although the primary mechanism of acti on of cry1 and cry2 is unknown, their localization in the nucleus and prote in-protein interaction with phytochromes indicates that they may directly r egulate changes in gene expression, The occurrence of cryptochromes in both plants and animals indicates their ubiquitous nature and a prominent role in regulating diverse responses.