Heavy Metal Stress. Activation of Distinct Mitogen-Activated Protein Kinase Pathways by Copper and Cadmium

Abstract

Excessive amounts of heavy metals adversely affect plant growth and development. Whereas some regions naturally contain high levels of heavy metals, anthropogenic release of heavy metals into the environment continuously increases soil contamination. The presence of elevated levels of heavy metal ions triggers a wide range of cellular responses including changes in gene expression and synthesis of metal-detoxifying peptides. To elucidate signal transduction events leading to the cellular response to heavy metal stress we analyzed protein phosphorylation induced by elevated levels of copper and cadmium ions as examples for heavy metals with different physiochemical properties and functions. Exposure of alfalfa (Medicago sativa) seedlings to excess copper or cadmium ions activated four distinct mitogen-activated protein kinases (MAPKs): SIMK, MMK2, MMK3, and SAMK. Comparison of the kinetics of MAPK activation revealed that SIMK, MMK2, MMK3, and SAMK are very rapidly activated by copper ions, while cadmium ions induced delayed MAPK activation. In protoplasts, the MAPK kinase SIMKK specifically mediated activation of SIMK and SAMK but not of MMK2 and MMK3. Moreover, SIMKK only conveyed MAPK activation by CuCl 2 but not by CdCl 2 . These results suggest that plants respond to heavy metal stress by induction of several distinct MAPK pathways and that excess amounts of copper and cadmium ions induce different cellular signaling mechanisms in roots.Heavy metal ions play essential roles in many physiological processes. In trace amounts, several of these ions are required for metabolism, growth, and development. However, problems arise when cells are confronted with an excess of these vital ions or with nonnutritional ions that lead to cellular damage (Avery, 2001;Schü tzendü bel and Polle, 2002;Gaetke and Chow, 2003;Polle and Schü tzendü bel, 2003). Heavy metal toxicity comprises inactivation of biomolecules by either blocking essential functional groups or by displacement of essential metal ions (Goyer, 1997). In addition, autoxidation of redox-active heavy metals and production of reactive oxygen species (ROS) by the Fenton reaction causes cellular injury (Stohs and Bagchi, 1995).In response to toxic levels of heavy metals, plants synthesize Cys-rich, metal-binding peptides including phytochelatins and metallothioneins. Therefore, heavy metals can be detoxified by chelation and sequestration in the vacuole (Clemens, 2001;Cobbett and Goldsbrough, 2002), and various membrane transport systems play an important role in metal ion homeostasis and tolerance (Hall and Williams, 2003).Gene expression patterns change when plants encounter excessive amounts of heavy metals.Cadmium-and copper-responsive genes have been shown to code for signal transduction components, such as the Arabidopsis mitogen-activated protein kinase kinase kinase (MAPKKK) MEKK1, transcription factors, stress-induced proteins, proteins participating in protein folding, and sulfur and glutathione metabolism (Xiang and Oliver, 1998;Suzuki et al., 2001;Louie ...

AtOSA1, a Member of the Abc1-Like Family, as a New Factor in Cadmium and Oxidative Stress Response

“…It has been also suggested that Cd 21 can interfere in living cells with cellular redox reactions and displaces or releases other active metal ions (e.g. Zn 21 ) from various biological complexes, thereby causing a reduction of the capacity of the antioxidant system (Jonak et al, 2004).…”

“…In response to heavy metals, diverse signal transduction pathways are activated, including mitogenactivated protein kinases, transcription factors, and stress-induced proteins (Jonak et al, 2004). Our knowledge concerning components of these pathways is growing but still incomplete.…”

Role of ROS and auxin in plant response to metal-mediated stress

“…It has been documented that mitogen-activated protein kinases (MAPKs) can perceive the changes of ROS content in signal transmission for a range of stresses 23 and the distinct MAPK pathways involve specifically to excess particular metal ions. [24][25][26] ROS induced by Cu triggers SIMK and SAMK activation via SIMKK, whereas Cd activates MMK2, MMK3 besides SIMK and SAMK. 24 Recently, MPK3 and MPK6 are also evidenced to be activated by Cd-induced ROS accumulation in Arabidopsis plants.…”

“…[24][25][26] ROS induced by Cu triggers SIMK and SAMK activation via SIMKK, whereas Cd activates MMK2, MMK3 besides SIMK and SAMK. 24 Recently, MPK3 and MPK6 are also evidenced to be activated by Cd-induced ROS accumulation in Arabidopsis plants. 25 A similar effect is also shown in plant response to Al stress, in which a MAPKlike protein is modulated by toxic concentrations of aluminum.…”

Characterization of a Novel Plant Promoter Specifically Induced by Heavy Metal and Identification of the Promoter Regions Conferring Heavy Metal Responsiveness

“…Recently, the effect of cadmium on plant MAPKs in rice suggests that a MAPK cascade may function in the cadmium-signaling pathway (Yeh et al, 2004). Copper and cadmium stress activate two distinct MAPK pathways in alfalfa (Medicago sativa) seedlings (Jonak et al, 2004). The regulation of PvSR2 expression by heavy metal represents a novel metalloregulatory system for investigation, particularly because it is one of the very few, if not the only, that focuses on specific responsiveness to heavy metal.…”