Etr1

Ethylene receptor aReactive oxigen intermediates.

function as oxidases and they are either located at the chloroplast or at the extracellular space (Table 1). For instance, apoplastic laccase and diamino oxidases are associated to cell walls and xylem lignification (5,6) and ascorbate oxidase is involved in cell-wall reorganization processes that occur during cell expansion at the initial stages of fruit development (7,8). Moreover, copper participates in the plant hormone ethylene signaling pathway, because the copper ion acts as a cofactor in the Arabidopsis thaliana ethylene receptor ETR1, providing a chemical environment where the metal binds the gaseous hormone with unusual stability (9).

2.2. Copper Phytotoxicity

2.2.1. Oxidative Stress and Cellular Damage

Despite being necessary, high copper levels induce oxidative stress that leads to accelerated plant senescence processes. The copper-catalyzed Haber-Weiss reaction takes place primarily at the chloroplast in plant cells (10) and results in the oxidative attack of chlorophylls, proteins, and thylakoid lipids, mainly affecting electronic transport at the level of photosystem II (11,12). In this sense, oxidative modifications that make ribulose-1,5-bisphosphate carboxylase/oxygenase prone to degradation have been described as one of the first detectable symptoms after copper treatment (13).

Among the earliest plant physiological responses observed after copper exposure, there is also a K+ efflux from roots. This leakage has been interpreted as a symptom of the oxidative damage of membrane polyunsaturated fatty acids (14). Lipid peroxidation at the plasma membrane occurs, as well, in other processes such as natural senescence or after pathogen attack, where it becomes associated to an increase in lipoxygenase (LOX) activity. Nevertheless, no correlation has been found between lipid peroxidation and LOX activity during copper exposure; therefore, it has been suggested that the reaction could be directly initiated by copper ions (15) or indirectly mediated by free-radical production (14).

2.2.2. Defense Against Oxidative Stress: Antioxidant and Repair Systems

Because of their extra photoelectronic transport chain, plants are specially prepared to deal with oxidative stress-induced processes through a battery of both enzymatic and nonenzymatic mechanisms. Effectively, plants are enriched in antioxidant molecules such as ascorbic acid, glu-tathione, a-tocopherol, and carotenoids. Moreover, different enzymes, including catalases, peroxidases, and superoxide dismutases, protect plant cells against oxidative damage (16).

Fig. 1. Plant cell copper homeostasis scheme. Model based on the components identified in Arabidopsis thaliana. Copper uptake is performed by the plasma membrane transporters COPT1 and COPT2. Once inside the cell, different metallochaperones bind and distribute specifically copper to the cytosolic Cu/Zn-SOD (aCCS) and to the secretory pathway (CCH). Other uncovered chaperones (?) may deliver copper to mitochondria and chloroplasts. RAN1 is a post-Golgi vesicle transporter that accepts copper from CCH and mediates its incorporation to copper-requiring proteins at the endomembrane system, such as the ethylene receptor ETR1. The copper excess is chelated in the cytosol by metallothioneins (MT1 and MT2) and/or sequestered as low-molecular-weight phytochelatin complexes (LMW-PC) and subsequently stored in the vacuole as high-molecular-weight phytochelatin complexes (HMW-PC).

Fig. 1. Plant cell copper homeostasis scheme. Model based on the components identified in Arabidopsis thaliana. Copper uptake is performed by the plasma membrane transporters COPT1 and COPT2. Once inside the cell, different metallochaperones bind and distribute specifically copper to the cytosolic Cu/Zn-SOD (aCCS) and to the secretory pathway (CCH). Other uncovered chaperones (?) may deliver copper to mitochondria and chloroplasts. RAN1 is a post-Golgi vesicle transporter that accepts copper from CCH and mediates its incorporation to copper-requiring proteins at the endomembrane system, such as the ethylene receptor ETR1. The copper excess is chelated in the cytosol by metallothioneins (MT1 and MT2) and/or sequestered as low-molecular-weight phytochelatin complexes (LMW-PC) and subsequently stored in the vacuole as high-molecular-weight phytochelatin complexes (HMW-PC).

On the other hand, injured cell structures that escape antioxidative barriers have to be repaired to efficiently protect cells. Recent studies have shown that proteins involved in lipid metabolism accumulate in copper-treated Arabidopsis plants and their absence results in enhanced copper sensitivity (4). In addition, copper tolerance in certain ecotypes has been related to a decrease in the susceptibility of the cell membrane to copper-induced oxidative damage (17,18). Moreover, posttranslational modified metallothionein 1 (MT1) accumulates after copper exposure. It has been suggested that MT1 becomes prenylated and directed to the membrane in order to participate in copper scavenging and, therefore, increase the efficiency of the repair mechanisms (19). Once again, these observations point to the maintenance of plasma membrane integrity as an important resistance mechanism against copper toxicity.

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