Table 3.

Overview of some heavy metal triggered signals in comparison to other environmental stresses

Heavy metal	Signal	Other stress conditions	Cellular responses	References
Copper	Calcium fluxes	Cold, drought, salinity	Phosphoprotein cascades, 2nd signalling molecules	Nielson et al. Nielsen et al. 2003reviewed by Sanders et al. 1999Knight 2000
Cadmium, chromium	Mitogen activated protein kinase (MAPK) pathways	Osmotic stress, pathogen contact	Activation of transcription factors and stress-responsive genes	Liu et al. 2010Ding et al. 2009, reviewed in Xiong et al. 2002
Iron	pH shifts	Pathogen contact	Induction of secondary metabolism	;Marschner and Römheld 1983Viehweger et al. 2006
Cobalt, zinc	Plant hormones like abscisic acid or ethylene	Cold, drought, salinity	Calcium signalling, guard cell regulation (water balance)	Zengin 2006reviewed in Zhu 2002
Cadmium, copper	Jasmonic acid	Pathogen contact, sugar, drought, salinity	Defence/stress response, development, induction of secondary metabolism	Agrawal et al. 2003Chen et al. 2006Maksymiec 2011reviewed in Howe and Schilmiller 2002Turner et al. 2002
Redox-active metals like iron, copper; almost all heavy metals at higher concentrations	Reactive oxygen species	Pathogen contact, cold, drought, salinity, high light intensity	Phosphoprotein cascades, activation of transcription factors and stress-responsive genes, activation of antioxidative defence	Reviewed in Foyer and Noctor 2005aAhmad et al. 2008

It is not comprehensive but gives an insight in the complex cellular signalling network and the resultant metabolic reactions.