Regulation of copper transport in Arabidopsis thaliana: a biochemical oscillator?

Plants are among the most versatile higher eukaryotes in accommodating environmental copper availability to largely variable demands. In particular, copper deficiency in soils is a threat for plant survival since it mostly affects reproductive structures. One of the strategies that plant cells use to overcome this situation is to increase copper levels by expressing high-affinity copper transporters delivering the metal to the cytosol. In this minireview, we discuss recent advances in the structure, function, and regulation of the CTR/COPT family of copper transporters, and pay special attention to the Arabidopsis thaliana counterparts. These are constituted by transmembrane polypeptides, containing several copper-binding sequences of functional and/or regulatory value, and assembling as trimers. Copper deficiency activates the expression of some members of the COPT family via the interaction of the SPL7 transcription factor with reiterative GTAC motifs present in their promoters. Interestingly, the regulation of the synthesis of these transporters by copper itself constitutes a negative-feedback loop that could cause a sustained oscillation in the cytosolic copper levels. We analyze the theoretical conditions required for this hypothetical copper oscillation and the potential advantages of synchronization with other cycles. Diverse data in other organisms point to the relationship between copper homeostasis and circadian cycles.