The role of root suberin in protecting barley plants from copper stress

Abstract

Copper (Cu) contamination in agricultural soil significantly threatens crop growth, yield, and overall agricultural productivity. Although barley (<em>Hordeum vulgare</em> L.) exhibits a relatively high tolerance to metal stress, the mechanisms underlying its tolerance to Cu stress, particularly the specific role of suberin, remain unclear. Therefore, this study investigated the morphological, physiological, biochemical, and transcriptomic responses of barley (cv. Scarlett) seedlings exposed to 50 µM and 100 µM Cu for six days. <br/> The results of this study indicated that Cu exposure significantly inhibited barley growth, with roots exhibiting greater sensitivity than shoots. Despite growth inhibition, photosynthetic performance in leaves remained unaffected. Cu stress induced oxidative stress, as evidenced by increased hydrogen peroxide (H₂O₂) accumulation in roots, superoxide radical (O₂•^-) level in leaves, and enhanced lipid peroxidation in both tissues, reflected by increased malondialdehyde (MDA) content. In response, barley plants accumulated proline in both roots and leaves, likely mitigating oxidative stress. Histochemical staining and chemical quantification revealed enhanced suberin deposition in roots under Cu stress, particularly through increased synthesis of aliphatic suberin monomers. <br/> Additionally, Cu stress disrupted nutrient homeostasis by affecting the uptake of essential mineral nutrients. Histochemical analysis of Cu distribution in root cross-sections and mineral nutrition analysis further indicated that suberin functioned as an effective endodermis barrier, limiting the xylem loading of Cu ions. As a result, Cu primarily accumulated in root apoplastic regions, reducing its translocation to shoots and alleviating shoot toxicity. Transcriptomic analysis identified extensive transcriptional reprogramming involving cell wall modifications, antioxidant defense, detoxification processes, Cu transporters, and plant hormone signaling transduction pathways in response to Cu stress. Furthermore, suberin-defective mutants (lacking very long chain C22-C26 ω-hydroxy acids) exhibited increased Cu sensitivity, enhanced oxidative damage, and greater Cu translocation to shoots, underscoring the importance of suberin monomer composition in root barrier function. <br/> In summary, this study provides a comprehensive understanding of Cu-induced phytotoxicity in barley and highlights suberin's critical function as an endodermal barrier limiting Cu uptake and translocation. The findings of this study provide novel insights for genetic improvement strategies aimed at enhancing Cu tolerance in barley and other crops and contribute to phytoremediation in Cu-contaminated soil.