Ying cells in O3exposed rcd1 exhibited quite a few in the typical morphological qualities on the hypersensitive response and PCD. Doublemutant analyses indicated a requirement for salicylic acid plus the function on the cyclic nucleotidegated ion channel AtCNGC2 in cell death. In addition, a requirement for ATPases, kinases, transcription, Ca21 flux, caspaselike proteolytic activity, and also a single or extra phenylmethylsulfonyl fluoridesensitive protease activities was shown for the improvement of cell death lesions in rcd1. Furthermore, mitogenactivated protein kinases showed differential activation patterns in rcd1 and Columbia. Taken together, these outcomes straight demonstrate the induction of PCD by O3.Ozone (O3) is an atmospheric pollutant that’s phytotoxic via its breakdown within the apoplast to kind reactive oxygen species (ROS). Quick, highconcentration peaks, socalled acute O3, result in visible damage in sensitive plants (Wohlgemuth et al., 2002). Even though accumulating proof has deepened our understanding of oxidative pressure and antioxidant defenses in O3 responses (Kangasjarvi et al., 1994; Sandermann et al., 1998; Overmyer et al., 2003), the mechanisms involved in (S)-(-)-Limonene Biological Activity O3induced cell death are still fairly unknown. As a consequence of the powerful chemical reactivity of O3, its toxicity has previously been attributed to an ability to type toxic ROS that directly harm membranes (for re1 This work was supported by the Academy of Finland (grant nos. 43671 and 37995), by the Finnish Centre of Excellence Programme (2000005), and by an Academy of Finland/German Academic Exchange Service grant (SA10256/313 F PP z). R.P. was supported by the Finnish Graduate program in Environmental Physiology, Molecular Biology, and Ecotechnology, the University of Kuopio, and the Finnish Graduate School in Environmental Science and Technologies, Abo Akademi. two Present address: Biology Division, CB No. 3280, University of North Carolina, Chapel Hill, NC 27599280. three Present address: A.I. Virtanen Institute, University of Kuopio, FIN0211 Kuopio, Finland. four Present address: Umea Plant Science Centre, Division of Plant Physiology, Umea University, SE0187 Umea, Sweden. 5 Present address: Department of Biology, University of Joensuu, PO Box 111, FIN0101 Joensuu, Finland. Corresponding author; email [email protected]; fax 358919552. Article, publication date, and citation data is often identified at www.plantphysiol.org/cgi/doi/10.1104/pp.104.055681.view, see Heath and Taylor, 1997). Nonetheless, the view of O3 has recently shifted, where it really is now regarded in many cases not as a toxin but rather as an elicitor of cell death (Sandermann et al., 1998). O3induced plant responses resemble on numerous levels the hypersensitive response (HR), usually 5-Acetylsalicylic acid web observed as the outcome of challenge by an avirulent pathogen (for overview, see Rao and Davis, 2001; Langebartels and Kangasjarvi, 2004). Typical to these two processes would be the induction of a biphasic oxidative burst, salicylic acid (SA) accumulation, ion fluxes, the deposition of cell wallstrengthening phenolic compounds, induction of defense genes for example Phe ammonia lyase, pathogenesisrelated protein1 (PR1), and glutathione Stransferase (GST), as well as induction of neighborhood and systemic pathogen resistance. This has led to the view that O3 misfires HRlike cell death and defense applications by means of mimicry in the oxidative burst induced by avirulent pathogens. The HR is genetically regulated, in addition to a form of programmed cell death (PCD.