Copper Homeostasis: Regulation in Plants

Abstract

Copper (Cu) is essential for plant life because of its key role in photosynthetic electron transport, respiration, and perception of the plant hormone ethylene. The most abundant Cu protein in plants is plastocyanin, an electron carrier in the chloroplast thylakoid lumen that is essential for photoautotrophic growth of plants. Copper is also a cofactor of superoxide dismutase and a number of extracellular cell wall enzymes for which the biological function is not yet fully elucidated. Cellular uptake is accomplished by the CopT Cu(I) family of transporters. CopT1 and CopT2 are especially important at the root surface, whereas CopT6 is important for Cu uptake in green cells. CopT5 serves to release Cu from vacuolar stores. Members of the large ZIP family of divalent metal transporters might add capacity for uptake of Cu( II ). Members of the yellow stripe‐like ( YSL ) family are proposed to function in the transport of Cu( II ) complexed to the chelator nicotianamine, which might be important for mobilization of Cu from vegetative tissues to developing seeds. ATP ‐dependent P‐type ATPases of the HMA family transport Cu(I) out of the cytosol. Of these P‐type ATPases , HMA5 serves to allow Cu exit from the cell, which is required for tolerance to excess and for long‐distance root‐to‐shoot transport in the vasculature. Other HMA transporters deliver the Cu cofactor to the ethylene receptors in the endomembrane system ( HMA7 ) or to the chloroplasts ( HMA6 ). A fourth Cu‐transporting P‐Type ATPase ( HMA8 ) delivers Cu to plastocyanin in the thylakoid lumen. Cu‐specific metallochaperones have been identified in plants similarly to other eukaryotes. Under impending Cu deficiency, plants use three mechanisms to adjust their physiology. The cell‐surface‐localized CopT transporters are up‐regulated in response to low Cu availability in the cytosol via the conserved Cu‐responsive transcription factor SPL7 in order to up‐regulate assimilation. In addition, plants enter a Cu economy mode by the SPL7 ‐mediated up‐regulation of a set of small RNA molecules called the coppermicroRNAs because they target the messenger RNAs that encode for certain apparently dispensable Cu proteins. This mechanism should ensure that enough Cu is left for essential functions such as photosynthesis. Finally, the HMA8 transporter in the thylakoid membrane undergoes turnover, but low Cu availability stabilizes the transporter to ensure efficient delivery of Cu to plastocyanin. Together, these homeostatic mechanisms fine‐tune cellular and whole plant Cu distribution and allow plants to thrive on a broad range of Cu concentrations.