ROLE OF ZEAXANTHIN IN BLUE-LIGHT PHOTORECEPTION AND THE MODULATION OFLIGHT-CO2 INTERACTIONS IN GUARD-CELLS

The stomatal response to blue light is an intrinsic component of the s ensory transducing processes mediating light-stimulated stomatal movem ents. Guard cell chloroplasts have a specific blue light response with an action spectrum that resembles the action spectrum for blue light- stimulated stomatal opening, suggesting a role of guard cell chloropla sts in the sensory transduction of blue light. The xanthophyll, zeaxan thin has recently been identified as a blue light photoreceptor in gua rd cells. The inhibitor of zeaxanthin formation, dithiothreitol, inhib its zeaxanthin formation and the stomatal response to blue light in a concentration-dependent fashion. In greenhouse-grown leaves, guard cel l zeaxanthin content closely tracks incident radiation and it is posit ively correlated with stomatal apertures. The sensitivity of guard cel ls to blue light co-varies with guard cell zeaxanthin content. A zeaxa nthin-less mutant of Arabidopsis is devoid of a typical stomatal respo nse to blue light. At constant light and temperature, changes in ambie nt [CO2] in a growth chamber caused large changes in stomata aperture and in guard cell zeaxanthin. The aperture-zeaxanthin changes were lin early related over a wide range of [CO2]. Experiments with detached ep idermis showed a similar relation among [CO2], stomatal apertures and guard cell zeaxanthin, and DTT inhibited the CO2 response in the light without altering the CO2 response in the dark. These results indicate that blue light sensing by guard cell zeaxanthin has a regulatory rol e in the light response of stomata. Zeaxanthin also appears to mediate light-CO2 interactions in guard cells.