Heavy metal stress mitigation in plants by metal-resistant fungi: molecular and biotechnological insights.

Heavy Metals (HMs) are persistent, non-biodegradable metallic elements that accumulate into soils due to weathering processes and anthropogenic activities, such as mining, smelting and industrial discharges. They can be classified as essential (Cu, Zn, Ni, Fe, Mn), which are toxic in excessive amounts, and non-essential (Cd, Pb, As, Hg), which are dangerous even in small amounts. HMs affect plant metabolism causing production of Reactive Oxygen Species (ROS), growth inhibition and nutrient deficiency, Arabidopsis thaliana seedlings germination was lowered to 55% and 21% at concentrations of 50 and 100 mg L⁻1 CdCl₂. Phytochelatins, metallothioneins and vacuole sequestration protect plants, but become ineffective at high concentrations of HMs. Metal-resistant fungi help to cope with stress conditions using mechanisms of biosorption, bioaccumulation, biomineralization, bioleaching and biotransformation, adapted Aspergillus niger managed to leach 78.5% Ni and 82.3% Mo during 30 days, while Penicillium chrysogenum eliminated 63% Cd in soil. Partners of fungi, such as Trichoderma and arbuscular mycorrhizal fungi, regulate production of ROS, activation of MAPKs and ABA/JA/SA/ethylene/auxin signaling pathway and expression of MT, PCS, GST, ABC, NRAMP and HMA genes under control of WRKY, MYB, NAC and bZIP transcription factors.