MICROPROPAGATION Ionic homeostasis disturbance is involved in tomato cell death induced by NaCl and salicylic acid Péter Poór & Dóra Szopkó & Irma Tari Received: 24 December 2010 /Accepted: 23 September 2011 /Published online: 17 December 2011 / Editor: J. Finer # The Society for In Vitro Biology 2011 Abstract The ability of salicylic acid and NaCl to induce programmed cell death by disturbing ionic homeostasis was investigated using tomato suspension culture cells. NaCl (300 mM) and salicylic acid (1 mM) inhibited cell growth and caused cell death within 1 wk of exposure. Treatment with NaCl increased the production of reactive oxygen species and the permeability of plasma membrane, but it also led to a reduction in the pH of the culture medium and resulted in a disturbance in ionic homeostasis of the cells. Salicylic acid-induced cell death in tomato suspension cul- ture was also accompanied by production of reactive oxygen species and increases in both electrolyte leakage and pH of the culture media. However, reactive oxygen species pro- duction was not significantly different in cultures treated with a lethal salicylic acid concentration and 100 mM NaCl, in which most of the cells survived. A decrease in the K + / Na + ratio was observed only in those cell cultures in which the salicylic acid treatment induced the death of cells. These results suggest that the decrease of the intracellular K + concentration and K + /Na + ratio is a common phenomenon in triggering programmed cell death by lethal concentrations of salicylic acid and NaCl. Keywords Ionic homeostasis . Programmed cell death . Salicylic acid . Salt stress . Tomato suspension culture Introduction Programmed cell death (PCD) plays an important role dur- ing the normal life cycle of the plant and in response to changing environments. It can be triggered by abiotic or biotic stressors and is a genetically defined process associ- ated with special morphological and biochemical changes. One key event, the breakdown of DNA into nucleosomal units (Katsuhara and Kawasaki 1996; Ito and Fukuda 2002), is not observed in all PCDs, but the activation of proteases comprising cysteine proteases is common to most PCD systems in plants (Rogers 2005). The best characterized types of PCD in plants include the hypersensitive response (HR) that occurs during biotic stress and is mediated by the signaling activity of salicylic acid (SA) (Lam et al. 2001) and PCD induced by various abiotic stressors such as high salinity (Shabala 2009; Joseph and Jini 2010). Salinity and SA may exert their effects on plant PCD through the regulation of reactive oxygen species (ROS) accumulation (Alvarez 2000). Salt stress results in the disturbance of ionic homeostasis, water status, and redox equilibrium in plant cells (Sun et al. 2010) and leads to a reduction in growth of cell suspension cultures (Gangopadhyay et al. 1997) and callus tissues (Zhang et al. 2004). The maintenance of ionic homeostasis is important to acclimation of plant cells to high salinity. Potassium is required in many physiological processes in plants, includ- ing protein and nucleic acid synthesis. Release of K + from the cytoplasm, leading to K + deficiency in the cell, may activate cysteine proteases which are the effectors of PCD (Shabala et al 2007; Shabala 2009). Hughes and Cidlowski (1999) demonstrated that the loss of intracellular K + in mam- malian cells led to the activation of the initiator caspases and nucleases during apoptosis. Under salt stress, a strong membrane depolarization caused by Na + uptake results in K + efflux via depolarization- activated outward-rectifying K + channels (KOR). At the same time, salinity elevates cytosolic Ca 2+ levels which lead to a dramatic rise in the level of reactive oxygen species by P. Poór (*) : D. Szopkó : I. Tari Department of Plant Biology, University of Szeged, H-6701 Szeged, Középfasor str. 52, Szeged, P.O. Box 654, Hungary e-mail: poorpeti@bio.u-szeged.hu In Vitro Cell.Dev.Biol.—Plant (2012) 48:377–382 DOI 10.1007/s11627-011-9419-7