DET1 REPRESSES A CHLOROPLAST BLUE LIGHT-RESPONSIVE PROMOTER IN A DEVELOPMENTAL AND TISSUE-SPECIFIC MANNER IN ARABIDOPSIS-THALIANA

The chloroplast psbDpsbC loci, which encode the D2 and CP43 subunits o f the photosystem II reaction center, respectively, are regulated by a blue light-responsive promoter (BLRP). It has recently been shown in barley seedlings that activation of psbD-psbC transcription by blue li ght involves inhibition of a protein kinase that represses the BLRP in the dark. To elucidate further the photosensory pathways regulating t he psbD BLRP, the effects of three nuclear mutations on the expression of the BLRP in chloroplasts of Arabidopsis thaliana were examined. Th e mutants used included the det1-1 and det1-6 alleles for the nuclear protein DET1, involved in repressing photomorphogenesis, and the cry1 gene for the blue light photoreceptor, cryptochrome (CRY1), involved i n hypocotyl elongation. The BLRP was not significantly expressed in co tyledons of light-grown wild-type seedlings, unlike the light-responsi ve expression of the chloroplast, psbA and rbcL, and nuclear, Lhcb and Chs, genes. Analysis of the mutants revealed that DET1 represses tran scription from the BLRP in a developmental and tissue-specific manner, which is unique from the effects that DET1 has on other light-regulat ed promoters. In addition, the cry1 mutation did not reduce the expres sion of the BLRP in response to blue light. This suggests that the BLR P is regulated by a different photosensory system relative to CRY1. A model is proposed involving blue light, DET1 and phytochrome in regula ting transcription from the psbD BLRP.