# A Smart and Sustainable Future for Viticulture Is Rooted in Soil: How to Face Cu Toxicity
### Cu in Agricultural Soils and Crops  
Copper is a trace element in soil-plant systems. The average Cu concentration in the Earth's crust is 60 mg kg\({}^{-1}\) and in soil, it typically ranges between 2 and 50 mg kg\({}^{-1}\). Natural soils with a high content of clay minerals (e.g., _Vertisols_) or organic matter (e.g., _Spodosols_, _Histosols_) are usually characterized by a higher Cu content (up to 180 mg kg\({}^{-1}\)) [10]. Anthropogenic atmospheric depositions (traffic and industry related) and agricultural materials (inorganic fertilizers, agrochemicals, sewage sludge, livestock manure, irrigation water, compost, etc.) can dramatically increase the Cu concentration in soil. Indeed, Cu concentrations higher than 1000 mg kg\({}^{-1}\) have been recorded in polluted soils, including in agricultural soils [11]. In particular, concentrations exceeding 1000 mg kg\({}^{-1}\) have been reported for vineyard soils in France and Brazil [9] (see also the following chapter). The main form of Cu in the soil is Cu\({}^{2+}\), typically bound to inorganic and organic ligands, forming both soluble and insoluble compounds. In the solid phase, Cu has a very high affinity for organic matter and manganese oxides, as well as for iron oxides and clay minerals. Because of its high affinity for soil colloids and especially organic matter, Cu shows low mobility in near-neutral soils and is mainly concentrated in the upper soil layers. For instance, in rhizosphere soil (i.e., at the root-soil-microorganism interface), Cu is complexed by low-molecular-weight organic compounds exuded by plants and microbes like carboxylic and phenolic acids, which play an essential role in both external and internal plant tolerance mechanisms [9]. In arable soils, Cu concentration in the soil solution lies in the range 1-300 ug L\({}^{-1}\)[12]. Concerning the plant acquisition process, roots can use the free ionic forms (Cu\({}^{+}\)/Cu\({}^{2+}\)), although the direct use of Cu complexes with different organic molecules present in the rhizosphere cannot be excluded. Indeed, the identification of an uptake system of Fe-phytosideophore (i.e., root exudates with a high affinity for Fe) complexes also in di cots [13] further corroborates this hypothesis. Moreover, a reduction step (Cu\({}^{2+}\rightarrow\) Cu\({}^{+}\)) is also very likely to occur before the root uptake of the nutrient [9]. Indeed, previous works demonstrated that Fe chelate reductase enzymes and their related gene family (_FRO_) responded to both Cu deficiency and toxicity in _Arabidopsis thaliana_ and cucumber plants, respectively [14, 15]. The metal is taken up through a specific transmembrane Cu Transporter Protein 1 (COPT1) [16]. However, the involvement of other transporters like Zn/Fe Permeases (ZIPs), Natural Resistance Associated Macrophage Proteins (NRAMP) and Iron Related Transporters (IRT) has also been speculated [17, 18, 19, 20]. In this context, the extent of the Cu bioavailable fraction (i.e., the two free ionic Cu forms and the soluble metal complexes with organic ligands) strongly depends on soil properties, especially cation exchange capacity (CEC), pH values, organic matter and clay content [12]. Once taken up by the roots, Cu is loaded into the xylem vessels to be translocated towards the aboveground organs. Despite only a few pieces of information available, the Heavy Metal-transporting P-type ATPase 5 (OsHMA5) has been shown to be involved in the Cu xylem-loading in rice plants [21]. In this respect, the Cu concentration in the xylem sap and in shoots is remarkably limited when the expression of this gene is down-regulated. Moreover, in _A. thaliana_, Yellow Stripe Like 2 (YSL2) transporter was also found to play a pivotal role in the xylem loading of Cu complexed with nicotianamine [22]. Once loaded in the xylem, Cu is bound to an organic ligand, generally amino acids (like histidine and proline in _Brassica carinata_) or nicotianamine (e.g., in tomato and chicory) [23, 24, 25].