Arabidopsis Adapts to Copper Deficient Conditions via SPL7, a Master Regulator for Copper Homeostasis

Copper is one of the essential micronutrients for most living organisms. In higher plants, copper is mainly utilized as a cofactor of proteins involved in photosynthesis and scavenging of reactive oxygen species. The most abundant copper protein in leaves is plastocyanin, localized to thylakoid lumen of chloroplasts, and is essential for photosynthesis in higher plants. Another major copper protein, copper/zinc superoxide dismutase, localized to cytosol and chloroplast stroma, is involved in the scavenging of reactive oxygen species. Under copper deficient conditions, the expression of copper/zinc superoxide dismutase is down-regulated and their function is compensated by iron superoxide dismutase specifically expressed in low copper conditions. Previously we demonstrated that a microRNA, miR398 was expressed in low copper conditions and was involved in this down-regulation of copper/zinc superoxide dismutase by degrading their mRNAs. Consequently, limited copper is preferentially transferred to more essential copper protein like plastocyanin. In this study, we identified SPL7 (SQUAMOSA promoter-binding protein-like 7) as a transcription activator for miR398. SPL7 is similar to Crr1 functioning in copper homeostasis in Chlamydomonas. SPL7 recognizes and directly binds to a GTAC core motif, an essential element for copper response in Chlamydomonas, in the promoter region of miR398 and activates the transcription of miR398 in copper deficient conditions. In addition, SPL7 up-regulates several genes involved in copper homeostasis. Taken together, we propose that SPL7 is a master regulator involved in copper homeostasis in Arabidopsis