# A Smart and Sustainable Future for Viticulture Is Rooted in Soil: How to Face Cu Toxicity
## 2 Current Challenges
### Soil Management Considering the Root-Microorganism Interactions (Rhizosphere Management)  
Considering the ever-increasing incidence of soils contaminated with Cu, it is evident that rhizosphere dynamics must be taken into account when setting up and applying soil management practices to mitigate the Cu stress in these agricultural soils. In fact, the biogeochemical cycles of the nutrients completely differ at the soil-root interface compared to non-rooted soil (i.e., bulk soil). Indeed, the highly dynamic interactions between roots, soil and microorganisms occurring in the rhizosphere control not only the speciation and the consequent availability of the nutrients (and, in turn, the extent of their root acquisition), but also determine the level of availability/toxicity of heavy metals like Cu. For instance, in the hotspot that is the rhizosphere, pH value modifications and the release of low- and high-molecular-weight organic compounds by both roots and microorganisms are the main drivers of Cu availability. Concerning soil pH values, root-induced changes of this chemical parameter are mainly triggered by plant nutrient uptake. This process is often coupled with an active proton extrusion catalyzed by the plasma membrane H\({}^{+}\)-ATPase [28]. Furthermore, an unbalanced cation/anion uptake of nutrients might lead to either acidification (i.e., due to an excessive uptake of cations) or alkalization (i.e., due to an excessive uptake of anions) of the rhizosphere. These imbalances might occur naturally due to different plant requirements, depending on the physiological status of the plants, microbial interactionsand/or competition with neighboring plants, as well as potentially being caused by the unbalanced availability of some nutrients in the growth medium [93; 94]. Indeed, in vineyards, when cover crops are co-cultivated with other plants like vine plants, nutrient competition might induce an unbalanced uptake of elements with consequent root-induced changes of the pH. As is to be expected, this phenomenon impacts Cu mobility/availability and, consequently, its toxicity. On the other hand, excessive concentrations of metals like Cu might also be themselves responsible for an unbalanced nutrient uptake. Recent studies have highlighted that Cu stress can induce synergism and/or antagonisms with many other nutrients like P, Fe, Zn and Mn, causing further root-induced pH changes both in annual grass species and in perennial plants like grapevine [15; 95]. The complexity is further exacerbated by the fact that rhizosphere effects (like acidification and alkalization processes) are not always constant over time [96] and are strongly plant species and genotype/rootstock dependent [15; 95; 96]. As mentioned earlier, besides pH values, rhizosphere organic compounds are the main drivers shaping Cu speciation in agricultural soils. Indeed, specific compound classes such as phenolic compounds and carboxylic acids play a fundamental role in both internal and external Cu tolerance strategies of plants [97]. Recent studies have shown that perennial plants like grapevines also trigger their root exudation when exposed to excessive Cu concentrations, yet the phenomenon strongly depends on the type of the rootstock and the growing conditions [15]. Indeed, when grapevines are intercropped with annual grass species like oat, the exudation pattern is completely modified. Furthermore, the enhanced exudation reduces the Cu accumulation in grapevine plants, making this a valuable agronomic strategy for mitigating Cu stress [15]. It is important to highlight that the alleviating effect is highly plant species dependent, and thus the intercropping approach should be evaluated for each specific rootstock. In addition, the intermingling of roots in intercropping systems might induce a competition between plant species leading on one side to a modified exudation profile and on the other side to an unbalanced nutrient uptake ultimately affecting the effect of Cu stress.