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

Abstract

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.

Modified Clp Protease Complex in the ClpP3 Null Mutant and Consequences for Chloroplast Development and Function in Arabidopsis

“…Once this block is broken, by the addition of either Glc or Suc to the growth medium, cotyledons unfold and true leaves are formed. These CLP alleles do not have white cotyledons, as observed for a subset of plastid mutants, the locus white cotyledon1 (Yamamoto et al, 2000), the cyo1 stromal elongation factor G (SCO1; Albrecht et al, 2006;Ruppel and Hangarter, 2007), or the thylakoid protein disulfide isomerase CYO1/SCO2 (Shimada et al, 2007;Albrecht et al, 2008;Tanz et al, 2012; for review, see Pogson and Albrecht, 2011). These sco mutants are not affected in chloroplast development during embryogenesis in the developing silique.…”

Genetic Dissection of Chloroplast Biogenesis and Development: An Overview

“…That is, chloroplast mutants have been described that have the phenotype restricted to one leaf organ, such as chlorotic true leaves, but green cotyledons as in the variegated (var) and immutans mutants (Liu et al, 2010). Conversely, the snowy cotyledon (sco) mutant group has chlorotic or bleached cotyledons but green true leaves (Albrecht et al, 2006(Albrecht et al, , 2008(Albrecht et al, , 2010Shimada et al, 2007). The sco and cyo1 (shiyo-u means cotyledon in Japanese) mutants are able to develop almost normally on soil, but their vitality is reduced compared with wild type as measured by reduced seed set.…”

“…Since all of the SCO genes identified to date are unique and not only expressed in cotyledons but through the entire plant, we have to assume that there is no redundancy with other members of the gene family. Furthermore, the different SCO genes encode proteins involved in complete different processes such as chloroplast protein translation (SCO1), chloroplast protein folding (SCO2), or being associated with microtubules and the peroxisome (SCO3; Albrecht et al, 2006Albrecht et al, , 2008Albrecht et al, , 2010. Thus, the observed impairment in chloroplast development in cotyledons seems to be due to a differential chloroplast development process between the plant organs rather than redundancy of proteins in the true leaves.…”

“…Chloroplast protein folding is mediated by chaperones, such as HSP70 and Cpn60, with the hsp70-1 mutation leading to variegated cotyledons (Latijnhouwers et al, 2010). Also, protein disulfide isomerases have been shown to be required for protein folding, with, for example, sco2/cyo1 lesions in a protein disulfide isomerase, resulting in a cotyledon-restricted pale-green phenotype (Shimada et al, 2007;Albrecht et al, 2008).…”

Genetic suppression of plant development and chloroplast biogenesis via the Snowy Cotyledon 3 and Phytochrome B pathways

“…We have identified a group of genes whose impact on chloroplast biogenesis is greater in cotyledons than leaves. These genes have been named Snowy Cotyledon (SCO) (Albrecht et al 2006;Albrecht et al 2008;Pogson and Albrecht 2011;Tanz et al 2012;Albrecht-Borth et al 2013). The snowy cotyledon 3 (sco3) mutant was of particular interest, encoding a protein of unknown function that is not located in the chloroplasts but in the periphery of peroxisomes.…”