An Arabidopsis cotyledon-specific albino locus: a possible role in 16S rRNA maturation

We report here the isolation and characterization of a cotyledon-specific a lbino locus of Arabidopsis, WHITE COTYLEDONS (WCO). This recessive mutation in the WCO locus, located on the top of Chromosome 1, results in albino co tyledons but green true leaves. An accumulation profile of chlorophylls and ultrastructure of chloroplasts indicate that WCO is necessary for developm ent of functional chloroplasts in cotyledons but is dispensable in true lea ves. This was further supported by the fact that the mutants request feedin g of sucrose for their survival at the early seedling stage where true leav es have not emerged, but the mutants which have developed true leaves are a ble to grow autotrophically without sucrose supplementation. The wco mutant s accumulate low levels of chloroplast mRNA encoding photosynthesis-related proteins and have a specific defect in 16S rRNA maturation in a cotyledon- specific manner. Although wco mutants exhibited abnormal chloroplasts and c hloroplast gene expression in cotyledons, nuclear genes for photosynthetic components are expressed at similar levels to those found in wild-type sibl ings. This lack of suppression of the nuclear genes is not due to a defect in the signaling of the so-called "plastid factor" to the nucleus since nor mal suppression of the nuclear genes was observed in response to the photo- oxidative damage due to norflurazon application.