Snowy cotyledon 2: the identification of a zinc finger domain protein essential for chloroplast development in cotyledons but not in true leaves

In cotyledons of etiolated seedlings light-dependent transformation of etioplasts to chloroplasts marks the transition from heterotrophic to autotrophic growth. Genetic factors required for this developmental step were identified by isolating mutants of Arabidopsis thaliana that were impaired in chloroplast development in cotyledons but not in true leaves. Several mutants with chlorophyll-deficient cotyledons were isolated and dubbed snowy cotyledon (sco). Here we describe the identification and detailed characterization of the snowy cotyledon 2 mutant. The mutated SCO2 gene was identified using a map-based cloning strategy. SCO2 was shown to encode a novel protein which contains a single DnaJ-like zinc finger domain. The SCO2 protein fused to GFP was shown to be present in chloroplasts. Inactivation of SCO2 has almost no detectable impact on the levels of transcripts encoding plastid-specific proteins but leads to a significant reduction of plastid protein levels. Even though transcripts of SCO2 have been found ubiquitously in green tissues as well as in roots phenotypic changes due to SCO2 inactivation are confined to cotyledons. The cotyledons in embryos of sco2 are unaffected in their chloroplast biogenesis. Upon precocious germination seedlings of sco2 and wild type are indistinguishable. The SCO2 mutation affects chloroplast biogenesis only at the end of dormancy during seed germination. The transition from heterotrophic to autotrophic growth is dramatically impaired in sco2 when seedlings were kept in the dark for more than 5 days prior to light exposure.