Regulation of Copper Homeostasis by Micro-RNA in Arabidopsis

Abstract

Major copper proteins in the cytoplasm of plant cells are plastocyanin, copper/zinc superoxide dismutase, and cytochrome c oxidase. Under copper limited conditions, expression of copper/ zinc superoxide dismutase is down-regulated and the protein is replaced by iron superoxide dismutase in chloroplasts. We present evidence that a micro-RNA, miR398, mediates this regulation in Arabidopsis thaliana, by directing the degradation of copper/zinc superoxide dismutase mRNA when copper is limited. Sequence analysis indicated that the transcripts encoding cytosolic copper/zinc superoxide dismutase and COX5b-1, a subunit of the mitochondrial cytochrome c oxidase, are also targeted by miR398. This regulation via miR398 takes place in response to changes in a low range of copper levels (0.2-0.5 M), indicating that miR398 is involved in a response to copper limitation. On the other hand, another major copper protein, plastocyanin, which is involved in photosynthetic electron flow and is essential in higher plants, was not regulated via miR398. We propose that miR398 is a key factor in copper homeostasis in plants and regulates the stability of mRNAs of major copper proteins under copper-limited conditions.

How do plants respond to copper deficiency?

“…This switching between Cu/Zn SOD and Fe SOD takes place at the range of 0.1-1 mM CuSO 4 in agar-solidified medium. 10 Arabidopsis seedlings grown in the soil used in our laboratory express both Cu/Zn and Fe SODs (data not shown), suggesting that the soil contains copper ion equivalent to the range of 0.1-1 mM in the medium. Arabidopsis seedlings can perceive a change in copper concentration in the physiological range for maintaining copper homeostasis.…”

“…18 miR398 is expressed only in low copper conditions 10 and directly involved in the degradation of CSD1 and CSD2 mRNA. 10,19 Consequently, this regulation allows limited copper to be preferentially transferred to plastocyanin whose expression is independent of the regulation via miR398. Even in the absence of the functional homolog of algal cytochrome c 6 , higher plants can sustain photosynthesis in the low copper conditions.…”

“…5 MicroRNA, miR398, is involved in this downregulation of both Cu/ZnSOD genes. 10 While microRNAs extensively regulate a variety of developmental processes, 16 they are also involved in responses to the environmental stresses including deficiency in sulfur 17 and phosphate. 18 miR398 is expressed only in low copper conditions 10 and directly involved in the degradation of CSD1 and CSD2 mRNA.…”

“…10 While microRNAs extensively regulate a variety of developmental processes, 16 they are also involved in responses to the environmental stresses including deficiency in sulfur 17 and phosphate. 18 miR398 is expressed only in low copper conditions 10 and directly involved in the degradation of CSD1 and CSD2 mRNA. 10,19 Consequently, this regulation allows limited copper to be preferentially transferred to plastocyanin whose expression is independent of the regulation via miR398.…”

“…To minimize the damage by excess copper and also respond to copper deficiency, higher plants have several strategies, including the regulation of copper uptake in root cells, 3 strict copper trafficking via P-type ATPases and copper chaperones [4][5][6][7][8][9] or regulation of the levels of copper proteins in response to a change in the metal availability. 10,11 In addition plants respond to copper deficiency by expressing the alternative iron proteins which complement the function of copper proteins. 12 Among photosynthetic eukaryotes, the unicellular green alga, Chlamydomonas reinhardtii, is best studied with respect to the molecular mechanism of copper homeostasis.…”