Signal Transduction Under Cadmium Stress

The response to heavy metals depends on a complex signal transduction pathway within the cell which begins with the sensing of heavy metal and converges in transcription regulation of metal-responsive genes (Sing et al. 2002), although much remains to be learned about the molecular components of metal-induced signal transduction. Various transcription factors (TFs) involved in the regulation of cell response to metal stress have recently been identified ( Sect. 21.5.2). The modulation of different groups of TFs highlights the complex response of plants to Cd (DalCorso et al. 2008) . ROS and NO are important players in the regulation of plant response from signal perception to the intracellular transduction cascade, triggering the activation of genes involved in the induction of different metabolic pathways to deal with Cd toxicity. Hydrogen peroxide governs the transduction of cellular response in different abiotic stresses including those caused by heavy metals. The transduction of H2O2 signals into biologically relevant information is coordinated by a complex network of sensors and receptors, such as MAPKs, and transcription factors and is thought to be evolutionarily conserved (Vandenbroucke et al. 2008), although there are around 400 H2O2-responsive protein families in A. thaliana and may vary depending on the plant species in question (Vandenbroucke et al. 2008). There are several elements in the signal transduction pathway of ROS-sensitive plants which include the MAPK, MAPKK, MAPKKK, AtMPK3/6, AtANPl, NtNPKl, Ntp46MAPK, and calmodulin (Mittler 2002; Vanderauwera et al. 2009). The increase in H2 O2 levels induced by Cd can be perceived by oxidative protein modifications. The protein thiol groups tyrosine, tryptophan, and histidine can be oxidised by H2O2 and O2-. The redox changes in the Cys residues of transcription factors directly regulate nuclear gene expression. However, transcriptional modifications may also require additional upstream sensing and transduction of ROS and ROS-derived signals, being involved MAPKs and several protein phosphatases (Vanderauwera et al. 2009). Salicylic and jasmonic acid as well as ethylene can also participate in signal transduction under Cd stress (Rodríguez-Serrano et al. 2009; Ogawa et al. 2009). Salicylic acid (SA) acts as an important signaling element in plants and has been observed to alleviate Cd-induced growth inhibition and oxidative damage (Metwally et al. 2003). Although this mechanism is not fully understood, it has been suggested that SA may induce H2O2 signals involved in Cd tolerance, such as repair processes, Cd binding and compartmentation (Metwally et al. 2003). The Cd-induced ethyl-ene biosynthesis has been reported to occur in various plant species (Sanitá di Toppi and Gabbrielli 1999; Rodríguez-Serrano et al. 2006), although the molecular relationships between ethylene biosynthesis and Cd stress have yet to be clearly determined. Transcriptomic studies of Arabidopsis plants have detected Cd-dependent up-regulation of ACC oxidase and ACC synthase as well as the ethylene responsive factors ERF2 and ERF5 (Herbette et al. 2006). JA content increases in response to heavy metals in various plant species (Wang and Wu 2005; Rodríguez-Serrano et al. 2006, 2009). JA regulates genes involved in GSH and PCS in Arabidopsis plants under Cd treatment (Xiang and Oliver 1998). In different plant species, Ca2+, calmodulin, CDPK and an MAPK act as signaling molecules which regulates cell response to cadmium stress (Romero-Puertas et al. 2004, 2007a, b; Herbette et al. 2006; Yeh et al. 2007; Rodríguez-Serrano et al. 2009). Several studies have provided genetic evidence for the importance of NO in gene regulation. Two studies involving large-scale transcriptional analysis of A. thaliana have revealed NO-dependent regulation of genes involved in signal transduction, disease resistance, stress response, photosynthesis, and basic metabolism (Grün et al. 2006); however, the intracellular signaling pathway involved has not yet been defined. Most of the information available relates to plant defense and wounding and suggests that NO and salicylic and jasmonic acids are interrelated (Grün et al. 2006). The activity of different nuclear regulatory proteins is dramatically affected by NO. Modification by S-nitrosylation can regulate the activity and function of some regulatory proteins and transcription factors. Although no plant transcription factor has been observed to be regulated by this process, some regulatory proteins could be S-nitrosylated (Grün et al. 2006). NO can regulate cell signaling by controlling Ca2+ homeostasis. Most Ca2+ channels are regulated by NO either directly through S-nitrosylation or indirectly through cyclic ADP-ribose (cADPR) involving GMP (Courtois et al. 2008). NO-dependent activation of protein kinases, MAPK, and CDPK has been reported in various plant species. The activation of these kinases by NO is thought to be involved in defense responses and/or cell death (Courtois et al. 2008).

7 Conclusion and Future Perspectives

A hypothetical model depicting some of the players involved in ROS, NO perception, and signal transduction pathways in response to cadmium is shown in Fig. 9.3 . Cadmium promotes an increase of ROS production in different cell compartments and their accumulation could give rise to oxidative damages affecting to lipids and proteins. However, ROS can also trigger defense cellular responses indirectly acting as signaling molecules, by promoting changes in Ca2+ concentration, through GMP or by altering the redox status of several proteins, and further activating the MAPK cascade. But ROS can also directly regulate nuclear gene expression by affecting the redox state of transcription factor Cys residues.

In this scenario, NO plays an important role in the regulation of cell responses to this metal, but further works are needed to better understand the coordinated role of ROS and NO in both toxicity and regulation of cell response to cadmium. The role of some hormones, such as JA and ET, in the cell response to Cd is also an interesting point which deserves more research in order to understand the cross-talk between hormone balance/ ROS and NO in the regulation of plant defense against heavy metals. In response to Cd the up-regulation of some defense genes takes place, although some of them are also induced during pathogen attack, which suggests an overlap in the regulatory mechanisms governing these processes. However, the role of genes such as HSPs or chitinases in the mechanisms of tolerance to Cd has not been explored in depth so far, and they could be important in the development of

Cadmium Toxicity Plants

Fig. 9.3 Hypothetical model showing signal transduction of cell response to cadmium toxicity. Cd-dependent changes in H2O2 and NO levels can be perceived by changes in Ca2+ concentration, and oxidation or S-nitrosylation of proteins or transcription factors. The transcriptional response can also require more upstream transduction involving mitogen-activated protein kinases (MAPKs) and hormones such as jasmonic acid (JA) and ethylene (ET)

Fig. 9.3 Hypothetical model showing signal transduction of cell response to cadmium toxicity. Cd-dependent changes in H2O2 and NO levels can be perceived by changes in Ca2+ concentration, and oxidation or S-nitrosylation of proteins or transcription factors. The transcriptional response can also require more upstream transduction involving mitogen-activated protein kinases (MAPKs) and hormones such as jasmonic acid (JA) and ethylene (ET)

new strategies for phytoremediation. Further studies will be necessary to understand the role of the post-translational modification of proteins in the perception of metal toxicity and also in the transduction and regulation of cell response to Cd. An integrated study of all the players mentioned in this chapter at biochemical, molecular, and cellular levels is needed in order to understand the complex network involved in perception, transduction, and development of cell responses to cope with adverse conditions caused by heavy metals. This could allow the development of new and more efficient strategies for phytoremediation.

Acknowledgments This work was supported by ERDF-cofinanced grants from the Ministry of Education and Science (Grant BIO2008-040067) and Junta de Andalucía (Project P06-CVI-01820), Spain

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