Plants are able to protect themselves against pathogen attack in many different ways. One key mechanism involved is the rapid production of reactive oxygen species (ROS; Alvarez et al., 1998) and nitric oxide (NO; Delledonne et al., 1998). It has been shown that Ca2+ signals, as well as different protein kinase pathways, trigger the generation of ROS by phosphorylation and activation of the plasma membrane NADPH oxidase RBOH (respiratory burst oxidase homolog; Kobayashi et al., 2007; Asai et al., 2008). In this issue of the New Phytologist, Kobayashi et al. (pp. 223–237) show that the calcium-dependent protein kinase StCDPK5 triggers the production of ROS in response to pathogen infection in potato (Solanum tuberosum), which results in resistance to the near-obligate hemibiotrophic pathogen Phytophtera infestans but increases the susceptibility to the necrotrophic pathogen Alternaria solani. It has long been suggested that contrasting molecular mechanisms of defense are employed against biotrophic and necrotrophic pathogens (Glazebrook, 2005), acting mostly antagonistically (Robert-Seilaniantz et al., 2011). The rapid production of ROS, accompanied by the hypersensitive response (HR), a type of programmed cell death (PCD), and also by salicylic acid (SA)-dependent defenses, is most effective against biotrophic pathogens, whereas ethylene (ET)- and/or jasmonate (JA)-dependent processes seem to act mostly against necrotrophic pathogens. The current study of Kobayashi et al. is based on the conditional expression of a constitutive active variant of the potato calcium-dependent protein kinase (CDPK) StCDPK5, which is composed of the N-terminal variable domain and the serine/threonine protein kinase domain (VK). Owing to the lack of the regulatory C-terminal junction and calmodulin-like domain, this kinase shows constitutive activity in the absence of Ca2+ (Cheng et al., 2002; Harper et al., 2004). Similarly, ectopic expression of a truncated NtCDPK2VK variant in Nicotiana benthamiana has been used before to demonstrate the role of CDPKs in pathogen response (Ludwig et al., 2005) and revealed that NtCDPK2VK triggered ROS production and PCD and also JA and ET signaling, but not SA-dependent responses. In 2007, Kobayashi et al. (2007) identified that Ser-82 and Ser-97 in the N-terminus of the potato NADPH oxidase StRBOHB are phosphorylated in a Ca2+-dependent manner by StCDPK4 and StCDPK5 and that ectopic expression of StCDPK5VK triggered ROS production in N. benthamiana leaves, suggesting that StCDPK5 induces the phosphorylation of StRBOHB and regulates the oxidative burst. However, further analysis of this CDPK-mediated signaling pathway in planta was hampered by the fact that the expression of such constitutively active kinase variants leads to PCD.
‘… further analysis of this CDPK-mediated signaling pathway in planta was hampered by the fact that the expression of such constitutively active kinase variants leads to PCD.’
In their current study, Kobayashi et al. have continued their research along these lines and demonstrate that co-infiltration of StCDPK5VK with StRBOHs leads to ROS production in N. benthamiana leaves (Fig. 1a). The direct interaction at the plasma membrane is supported by targeting of StCDPK5 and StCDPK5VK to the plasma membrane as shown by GFP-fusion proteins and further underpinned by molecular fluorescence complementation (BiFC) of YFP-fusion proteins of StCDPK5 and StRBOHC. Similar to many other CDPKs (Cheng et al., 2002; Dammann et al., 2003; Benetka et al., 2008), the membrane targeting of StCDPK5 is most likely due to N-terminal myristoylation, thereby promoting phosphorylation of membrane-localized targets (Mehlmer et al., 2010). Next, the authors started to assess the role of StCDPK5 during pathogen infection in planta. To this end, they used the pathogen-inducible potato vetispiradiene synthase 3 (PVS3) promoter to control expression of StCDPK5VK in transgenic potato plants. Indeed, the PVS3 promoter turned out to be correctly activated in response to infection with pathogens specifically at the infection sites as shown for P. infestans in PVS3::GUS plants (Fig. 1b). Therefore PVS3::StCDPK5VK constructs (Fig. 1c) were used to generate transgenic potato lines to test the function of StCDPK5 during pathogen response. If these plants were infected with P. infestans, they showed a high resistance to this hemibiotrophic pathogen compared with wild-type plants (Fig. 1d,e). This was accompanied by HR-like cell death as indicated by trypan blue staining of the cells around the infection sites (Fig. 1f,g), which is remarkable because the Agrobacterium-mediated transient expression of StCDPK5VK did not induce cell death. By contrast, infections with the necrotrophic pathogen Alternaria solani revealed a higher susceptibility of the transgenic potato lines compared with the wild type and massive ROS production at the penetration sites.
All together, these data support the view of a complex regulatory network of different signaling pathways in regulating ROS production and responses of plants to pathogen attack (Fig. 2). The production of ROS seems synergistically triggered by Ca2+ binding and phosphorylation by different kinase pathways. Here, the expression of StCDPK5VK induced ROS production, but not NO production, which has been shown to be MAP kinase dependent (Asai et al., 2008). The synergistic action of NO and ROS seems to be required to induce cell death and for defense against necrotrophic pathogens. In this context, one important feature of the involved kinases is their subcellular localization, supporting different modes of action, regulating either preferentially membrane-localized targets, or soluble targets in the cytosol and/or the nucleus (Wurzinger et al., 2011), which will lead to different forms of cross-talk between these pathways (Boudsocq et al., 2010).