Development of the Casparian strip is delayed by blue light in pea stems

To understand the regulatory mechanisms involved in tissue development by light, the kinetics of regulation of Casparian strip (CS) development in garden pea stems was studied. We found that short-term irradiation with white light delayed the development of the CS and used this delay to assess the quantitative eVect of light on CS development. We examined the eVect of the duration and Xuence rates of white light treatment on CS development and observed a signiWcant relationship between Xuence and the delay in CS development indicating that the Bunsen–Roscoe law of reciprocity holds for this response. The eVect of white light irradiation was not inhibited in the presence of a photosynthetic inhibitor, DCMU, or a carotenoid biosynthesis inhibitor, NorXurazon, indicating that the delay in CS development by light is a photomorphogenetic response rather than a subsidiary eVect mediated by photosynthetic activity. An action spectrum for the response displayed a major peak in the blue-light region, suggesting a dominant role for blue-light receptors. A minor peak in the red-light region also suggested the possible involvement of phytochromes. Although phytochromes are known to contribute to blue-light responses, phytochromedeWcient mutants showed a normal delay of CS development in response to blue light, indicating that the response is not mediated by phytochrome and suggesting a role for one or more speciWc blue-light receptors.