A Novel Proteinase, SNOWY COTYLEDON4, Is Required for Photosynthetic Acclimation to Higher Light Intensities in Arabidopsis  

Abstract

Excess light can have a negative impact on photosynthesis; thus, plants have evolved many different ways to adapt to different light conditions to both optimize energy use and avoid damage caused by excess light. Analysis of the Arabidopsis (Arabidopsis thaliana) mutant snowy cotyledon4 (sco4) revealed a mutation in a chloroplast-targeted protein that shares limited homology with CaaX-type endopeptidases. The SCO4 protein possesses an important function in photosynthesis and development, with point mutations rendering the seedlings and adult plants susceptible to photooxidative stress. The sco4 mutation impairs the acclimation of chloroplasts and their photosystems to excess light, evidenced in a reduction in photosystem I function, decreased linear electron transfer, yet increased nonphotochemical quenching. SCO4 is localized to the chloroplasts, which suggests the existence of an unreported type of protein modification within this organelle. Phylogenetic and yeast complementation analyses of SCO4-like proteins reveal that SCO4 is a member of an unknown group of higher plant-specific proteinases quite distinct from the welldescribed CaaX-type endopeptidases RAS Converting Enzyme1 (RCE1) and zinc metallopeptidase STE24 and lacks canonical CaaX activity. Therefore, we hypothesize that SCO4 is a novel endopeptidase required for critical protein modifications within chloroplasts, influencing the function of proteins involved in photosynthesis required for tolerance to excess light.

Essentials of Proteolytic Machineries in Chloroplasts

“…The conserved VIR3 protein family seems to play a role in early chloroplast biogenesis (Qi et al, 2016). Other proteases such as NANA and SCO4 are plant specific (Paparelli et al, 2012;Albrecht-Borth et al, 2013), which suggests unique proteolytic regulatory pathways in chloroplasts. Future studies on chloroplast glutamyl endopeptidase (cGEP) and M48 are expected to elucidate the functional relationships among chloroplast proteolysis, cell differentiation, and aging (van Wijk, 2015).…”

“…Coexpression profile analysis (using publicly available microarray databases) predicted that M48 is potentially involved in senescence (Lundquist et al, 2012;van Wijk, 2015). SCO4 SCO4 (SNOWY COTYLEDON4) is a novel hypothetical plantspecific metalloendopeptidase with multiple TMDs (Albrecht-Borth et al, 2013). SCO4 is suggested to be located in the thylakoid.…”

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

“…Dysfunction of this protein impairs the fine structure of the cytoskeleton (Albrecht et al 2010). In contrast, the other identified SCO proteins are required for chloroplast protein translation, as is the case for SCO1; chloroplast protein folding and targeting, as shown for the protein disulfide isomerase SCO2, or a likely novel protease activity, as is the case for SCO4 (Albrecht-Borth et al 2013;Pogson and Albrecht 2011;Tanz et al 2012). Although the effect of the sco3 mutation on chloroplast development in cotyledons is apparent, only a few genes have been found to be misregulated in the sco3 mutant (Albrecht et al 2010).…”

“…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.…”

Natural genetic variation for regulation of photosynthesis response to light in Arabidopsis thaliana

“…When the high irradiance level persists for longer time, photosynthetic acclimation will change the composition of mesophyll cells in terms of their proteins, lipids, pigments, and other cofactors involved in electron transport and reactive-oxygen species metabolism (Bailey et al, 2004;Walters, 2005). Increasingly more research is being performed to identify the genetic loci that are regulating these (sub-) processes in photosynthetic acclimation Albrecht-Borth et al, 2013;Jin et al, 2014;Van Rooijen et al, 2015). The natural allelic variants of the genes underlying such loci can be applied for breeding for photosynthetic performance (Van Rooijen et al, 2015).…”