Induction of CAM in Mesembryanthemum crystallinum abolishes the stomatal response to blue light and light-dependent zeaxanthin formation in guard cell chloroplasts

Facultative CAM plants such as Mesembryanthemum crystallinum (ice plant) possess C3 metabolism when unstressed but develop CAM under water or salt stress. When ice plants shift from C3 metabolism to CAM, their stomata remain closed during the day and open at night. Recent studies have shown that the stomatal response of ice plants in the C3 mode depends solely on the guard cell response to blue light. Recent evidence for a possible role of the xanthophyll, zeaxanthin in blue light photoreception of guard cells led to the question of whether changes in the regulation of the xanthophyll cycle in guard cells parallel the shift from diurnal to nocturnal stomatal opening associated with CAM induction. In the present study, light-dependent stomatal opening and the operation of the xanthophyll cycle were characterized in guard cells isolated from ice plants shifting from C3 metabolism to CAM. Stomata in epidermis detached from leaves with C3 metabolism opened in response to white light and blue light, but they did not open in response to red light. Guard cells from these leaves showed light-dependent conversion of violaxanthin to zeaxanthin. Induction of CAM by NaCl abolished both white light- and blue light-stimulated stomatal opening and light-dependent zeaxanthin formation. When guard cells isolated from leaves with CAM were treated with 100 mM ascorbate, pH 5.0 for 1 h in darkness, guard cell zeaxanthin content increased at rates equal to or higher than those stimulated by light in guard cells from leaves in the C3 mode. The ascorbate effect indicates that chloroplasts in guard cells from leaves with CAM retain their competence to operate the xanthophyll cycle, but that zeaxanthin formation does not take place in the light. The data suggest that inhibition of light-dependent zeaxanthin formation in guard cells might be one of the regulatory steps mediating the shift from diurnal to nocturnal stomatal opening typical of plants with CAM.