Expression of enzymes involved in chlorophyll catabolism in Arabidopsis is light controlled

We found that the levels of mRNA of two enzymes involved in chlorophyll catabolism in Arabidopsis (Arabidopsis thaliana), products of two chlorophyllase genes, AtCLH1 and AtCLH2, dramatically increase (by almost 100- and 10-fold, respectively) upon illumination with white light. The measurements of photosystem II quantum efficiency in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-inhibited leaves show that their expression is not related to photosynthesis but mediated by photoreceptors. To identify the photoreceptors involved, we used various light treatments and Arabidopsis photoreceptor mutants (cry1, cry2, cry1cry2, phot1, phot2, phot1phot2, phyA phyB, phyAphyB). In wild-type Columbia, the amount of transcripts of both genes increase after white-light irradiation but their expression profile and the extent of regulation differ considerably. Blue and red light is active in the case of AtCLH1, whereas only blue light raises the AtCLH2 mRNA level. The fundamental difference is the extent of up-regulation, higher by one order of magnitude in AtCLH1. Both blue and red light is active in the induction of AtCLH1 expression in all mutants, pointing to a complex control network and redundancy between photoreceptors. The blue-specific up-regulation of the AtCLH2 transcript is mediated by cryptochromes and modulated by phototropin1 and phytochromes. Individually darkened leaves were used to test the effects of senescence on the expression of AtCLH1 and AtCLH2. The expression profile of AtCLH1 remains similar to that found in nonsenescing leaves up to 5 d after darkening. In contrast, the light induction of AtCLH2 mRNA declines during dark treatment. These results demonstrate that the expression of enzymes involved in chlorophyll catabolism is light controlled.