SAR initiation provokes an increased protection against a wide range of microorganisms and is associated with an activation of signal transduction pathways and the induction of PR-genes (Uknes et al. 1992; Van Loon 1997; Durrant and Dong 2004). The onset of SAR causes an endogenous increase in salicylic acid (SA) levels in local and systemic tissues (Malamy et al. 1990), resulting in a primed defense state of the plant (Durrant and Dong 2004). Several PR-genes exhibit chitinase or glucanase activities and their synergistic action yields a strong antipathogenic potential. However, the role and molecular function of PR-1 remains elusive (Van Loon 1997).
SA is a critical molecule for the establishment of SAR, proven by experiments conducted with tobacco and A. thaliana plants carrying a transgenic NahG gene. NahG encodes a bacterial hydroxylase which converts SA to catechol and thereby depletes endogenous SA levels. The NahG plants are no longer able to accumulate SA and PR-1 transcripts after pathogen infection, resulting in an impaired SAR (Gaffney et al. 1993).
The observation that SA accumulates in phloem exudates led to the hypothesis that SA is the mobile signal produced at the site of infection and transported in systemic tissues to render the whole plant more resistant to secondary pathogen attacks (Métraux et al.
1990). Tobacco grafting experiments with NahG rootstocks and wildtypic scions revealed that SA is not important for the generation of the mobile signal, but reciprocal grafting experiment illustrated the importance of SA for perceiving the mobile signal and assuring systemic resistance (Vernooij et al. 1994). A recent study has shown that a volatile ester of SA, methyl SA (MeSA), plays a critical role as mobile SAR signal, at least in tobacco plants. MeSA is synthesized at the site of infection by SA carboxyl methyltransferase (SAMT) and cleaved in systemic tissue by SA binding protein 2 (SABP2) (Park et al. 2007). The methyl esterase activity of SABP2 is essential for the perception of the signal and initiation of SAR.
Other studies suggest a lipid-based molecule to be the decisive mobile signal in SAR.
The dir1 (DEFECTIVE IN INDUCED RESISTANCE 1) mutant carries a mutation in a gene similar to lipid transfer proteins (LTPs) and displays normal local resistance to pathogens, while the generation of SAR and induction of PR-genes in systemic tissues fails (Maldonado et al. 2002).
After pathogen attack, SA is synthesized from chorismate, derived from the shikimate pathway, by isochorismate synthase (ICS1) (Wildermuth et al. 2001). The components of the shikimate pathway are transcriptionally upregulated after infection to provide sufficient amounts of chorismate for SA biosynthesis (Truman et al. 2006). The ICS1 protein shows a high chorismate binding affinity and is localized in the stroma of chloroplasts (Strawn et al. 2007). Mutation of the ICS1 protein in sid2 (SA INDUCTION-DEFICIENT 2) causes a reduction of SA accumulation after infection to only 5-10% of the wildtypic level and a decrease in basal and systemic resistance (Wildermuth et al. 2001). Regulatory steps upstream of ICS1 are dependent on the attacking pathogen and the defense pathway elicited by it. In case of resistance mediated via the R-gene RPS4 (RESISTANCE TO PSEUDOMONAS SYRINGAE 4), key regulatory proteins upstream of ICS1 are the two lipase-like proteins EDS1 (ENHANCED DISEASE SYMPTOMS 1) and PAD4 (PHYTOALEXIN DEFICIENT 4), which function in a positive feedback loop to increase SA biosynthesis and their own expression (Feys et al. 2001). Furthermore, the initiation of SAR and the production of SA are accompanied by the accumulation of ROS. Earlier experiments showed, that SA can directly inhibit the H2O2 scavanging enzymes catalase and ascorbat peroxidase (APX) (Chen et al. 1993; Durner and Klessig 1995), while later studies suggest ROS signaling upstream of SA biosynthesis (Bi et al. 1995; Neuenschwander et al. 1995).
Application of high H2O2 concentrations to plants leads to a dose-dependent induction of SA synthesis and PR-1 expression (Leon et al. 1995). On the other hand, H2O2 is not detectable in systemic tissue (Ryals et al. 1995), making the attributed second messenger function of H2O2 questionable. Taken together, it seems that the combinatorial action of SA and ROS, produced during microburst formations, increases the defense response in systemic tissues synergistically (Du and Klessig 1997; Shirasu et al. 1997). Additionally, SA changes the intracellular redox potential, observable by alterations in absolute glutathione levels and differences in the ratio of oxidized (GSSG)
to reduced (GSH) glutathione. In the first hours after pathogen attack or SA treatment, an initial oxidative burst occurs followed by a recovery and rebounce to a reduced environment in local as well as in systemic tissues (Mou et al. 2003). Interestingly, these changes only take place when a compatible interaction is given, illustrating the importance of the variations in redox potential for SAR (Schafer and Buettner 2001).
The distinct redox states go along with the activation of different sets of target genes.
During the oxidizing phase, early SA responsive genes are induced that play a role in detoxifying oxidative stress, like glutathione-S-transferases or glucosyltransferases (Blanco et al. 2009; Uquillas et al. 2004), while under later reducing conditions PR-gene expression takes place (Dong 2004).
SA is an electrophilic compound and high concentrations can cause detrimental effects due to xenobiotic stress. Therefore plants are able to form the bioinactive SA conjugate SA 2-o-ß-D-glucoside (SAG), which can be stored in the vacuole and serves as a hydrolysable source for SA. The enzyme responsible for this conversion is the UDP-glucosyltransferase (UGT) (Dean et al. 2005). The establishment of SAR and induction of PR-genes can be elicited by exogenous application of SA or synthetic compounds like 2,6-dichloroisonicotinic acid (INA) and benzo(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) (Durner and Klessig 1995; Friedrich et al. 1996). The advantage of these SA-analogs is their less toxic effect on the plant.