Loss-of-function mutation in SCY1 triggers chloroplast-to-nucleus retrograde signaling in Arabidopsis thaliana

The secretory (Sec) pathway is one of the most important systems for transporting proteins across the thylakoid membrane into the lumen. Two Arabidopsis genes encoding SecY translocon proteins, designated SCY1 and SCY2, were characterized in this study. Semi-quantitative RT-PCR and histochemical staining beta-glucuronidase (GUS) activity reveal that both SCY1 and SCY2 promoters were active in germinating seeds, etiolated cotyledons, and flowers, but not in roots. In particular, the expression of GUS gene driven by the SCY1 promoter was almost undetectable in green leaves, whereas GUS staining controlled by the SCY2 promoter was clearly detected. Moreover, homozygous scy1-1 plants could grow heterotrophically but appeared sensitive to radiation. Further studies show that chloroplasts of scy1-1 were arrested in early developmental stages with fewer thylakoid membranes. Real-time quantitative RT-PCR reveals that a number of nuclear-encoded genes involved in chlorophyll biosynthesis and photosynthesis were substantially down-regulated in the scy1-1 mutant. All these results indicate that the SCY1/2 genes were regulated developmentally and spatially, and a loss-of-function mutation in SCY1 triggered chloroplast-to-nucleus retrograde signaling in Arabidopsis thaliana.