Excess Copper-Induced Alterations of Protein Profiles and Related Physiological Parameters in Citrus Leaves

This present study examined excess copper (Cu) effects on seedling growth, leaf Cu concentration, gas exchange, and protein profiles identified by a two-dimensional electrophoresis (2-DE) based mass spectrometry (MS) approach after <i>Citrus</i> <i>sinensis</i> and <i>Citrus</i> <i>grandis</i> seedlings were treated for six months with 0.5 (control), 200, 300, or 400 μM CuCl₂. Forty-one and 37 differentially abundant protein (DAP) spots were identified in Cu-treated <i>C.</i> <i>grandis</i> and <i>C.</i> <i>sinensis</i> leaves, respectively, including some novel DAPs that were not reported in leaves and/or roots. Most of these DAPs were identified only in <i>C.</i> <i>grandis</i> or <i>C.</i> <i>sinensis</i> leaves. More DAPs increased in abundances than DAPs decreased in abundances were observed in Cu-treated <i>C.</i> <i>grandis</i> leaves, but the opposite was true in Cu-treated <i>C.</i> <i>sinensis</i> leaves. Over 50% of DAPs were associated with photosynthesis, carbohydrate, and energy metabolism. Cu-toxicity-induced reduction in leaf CO₂ assimilation might be caused by decreased abundances of proteins related to photosynthetic electron transport chain (PETC) and CO₂ assimilation. Cu-effects on PETC were more pronounced in <i>C.</i> <i>sinensis</i> leaves than in <i>C.</i> <i>grandis</i> leaves. DAPs related to antioxidation and detoxification, protein folding and assembly (viz., chaperones and folding catalysts), and signal transduction might be involved in <i>Citrus</i> Cu-toxicity and Cu-tolerance.