Aluminum stress impairs water transport via PIP aquaporin suppression in tomato

Aluminum (Al) is the most abundant metal in the Earth's crust. In acidic soils (pH < 5.0), it is predominantly found as Al³⁺, which is toxic to most plants. In Al-sensitive species such as tomato (Solanum lycopersicum), the first and most conspicuous symptom of toxicity is root growth inhibition, where most of the Al is covalently retained. However, Al also induces indirect symptoms in the shoot, including reduced gas exchange, low stomatal conductance (g_s), and decreased mesophyll hydration, likely due to impaired water transport from roots to shoots. These symptoms have been even observed in plants grown directly in nutrient solution containing Al. Aquaporins (AQPs), especially those of the plasma membrane intrinsic protein (PIP) subfamily, are known to function primarily as water channel proteins. We hypothesize that Al exposure downregulates specific PIPs in tomato, leading to reduced water transport, as evidenced by alterations in shoot water relations, growth inhibition, and increased Al accumulation. For this, the expression of selected AQP genes, root hydraulic conductance (Lp_r), root xylem sap pH, total plant transpiration (E_plant), g_s, leaf water potential (Ψ_leaf), and relative water content (RWC) were measured. Assessments were taken from 12 h up to 7 days after Al exposure. Biometric analyses and Al quantification were performed at the beginning and end of the experiment. As expected, PIP expression decreased in the root in response to Al, with this pattern observed for all PIP1 and PIP2 isoforms, except for SlPIP2;11, from 120 h onward, coinciding with reduced leaf hydration (RWC and Ψ_leaf). Downregulation of SlPIP1;3, SlPIP1;5, SlPIP1;7, and SlPIP2;8 seems to be linked to reduced Lp_r, E_plant, and increased xylem sap pH. Despite the upregulation of SlPIP2;11, this was insufficient to maintain plant water status under toxic Al, but it could represent a target for future biotechnological initiatives on Al tolerance.