Characterization of the Snowy Cotyledon 1 Mutant of Arabidopsis Thaliana: The Impact of Chloroplast Elongation Factor G on Chloroplast Development and Plant Vitality

Abstract

During seedling development chloroplast formation marks the transition from heterotrophic to autotrophic growth. The development and activity of chloroplasts may differ in cotyledons that initially serve as a storage organ and true leaves whose primary function is photosynthesis. A genetic screen was used for the identification of genes that affect selectively chloroplast function in cotyledons of Arabidopsis thaliana. Several mutants exhibiting pale cotyledons and green true leaves were isolated and dubbed snowy cotyledon (sco). One of the mutants, sco1, was characterized in more detail. The mutated gene was identified using map-based cloning. The mutant contains a point mutation in a gene encoding the chloroplast elongation factor G, leading to an amino acid exchange within the predicted 70S ribosome-binding domain. The mutation results in a delay in the onset of germination. At this early developmental stage embryos still contain undifferentiated proplastids, whose proper function seems necessary for seed germination. In light-grown sco1 seedlings the greening of cotyledons is severely impaired, whereas the following true leaves develop normally as in wild-type plants. Despite this apparent similarity of chloroplast development in true leaves of mutant and wild-type plants various aspects of mature plant development are also affected by the sco1 mutation such as the onset of flowering, the growth rate, and seed production. The onset of senescence in the mutant and the wild-type plants occurs, however, at the same time, suggesting that in the mutant this particular developmental step does not seem to suffer from reduced protein translation efficiency in chloroplasts.

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.…”

Thylakoid Development from Biogenesis to Senescence, and Ruminations on Regulation

“…Whereas the VAR genes encode FtsH proteases associated with thylakoid protein degradation, Miura et al found that FUG1 encodes chloroplast prokaryotic translation initiation factor2, suggesting that it functions in chloroplast protein synthesis. A loss-of-function mutation of SCO1, which encodes chloroplast elongation factor G (Albrecht et al, 2006), was also found to suppress var1 and var2 phenotypes (see figure). Additional experiments showed that fug1 and sco1 mutants are deficient in translation of chloroplast proteins, such as D1.…”

Arabidopsis Cotyledon-Specific Chloroplast Biogenesis Factor CYO1 Is a Protein Disulfide Isomerase

“…Mutants with pigment deficiencies that are confined either to cotyledons or true leaves also suggest differences in the regulation of plastid development in these two organs. Arabidopsis thaliana plants with mutations in VAR1 and VAR2 (FtsH proteases) have normal green cotyledons (Sakamoto et al, 2003), whereas a number of other Arabidopsis mutants, including wco (unidentified gene product) (Yamamoto et al, 2000), sco1 (chloroplast elongation factor G) (Albrecht et al, 2006), and sig2 and sig6 (sigma factors) (Privat et al, 2003;Ishizaki et al, 2005), have albino or pale-green cotyledons but normal green leaves. Despite these reports, how cotyledonspecific chloroplasts develop remains largely unknown.…”