Making sense out of signaling during plant defense

Plants intimately communicate with their environment. They read signals coming from the “outside”, such as signals produced upon feeding by herbivorous insects or infection by microbial pathogens, and translated them to the “inside” to respond appropriately to the attacker encountered. The efficiency of responding to different signals determines the survival of the plant under attack. For my thesis research, I tried to understand how plants defend themselves against a wide variety of pathogens and insects, with special emphasis on understanding how plants finetune their defense response upon attack by multiple threats through a mechanism called cross-talk. The current knowledge on hormonal pathways and cross-talk is reviewed in Chapter 1. During my PhD research we focused on the molecular mechanism of antagonism between salicylic acid (SA) and jasmonic acid (JA). In the model plant species Arabidopsis thaliana, the SA response is effective against pathogens with a biotrophic life style, whereas the JA response is more effective against necrotrophic pathogens and insect herbivores. In Chapter 2, we showed the JA response in Arabidopsis is highly sensitive to suppression by SA, indicating that the SA pathway can be prioritized over the JA pathway. This SA-JA cross-talk appeared to be highly conserved among Arabidopsis accessions that were collected from very different geographic locations. In addition we showed that the kinetics of signal production is very important for the final outcome of the defense response. In Chapter 3, we demonstrated that ethylene (ET), another plant hormone, plays an important role in the modulation of SA-mediated suppression of the JA response. In the absence of ET, SA-mediated suppression of JA signaling is mediated via the defense regulatory protein NPR1. However, production of ET, such as upon infection by the necrotrophic pathogen Alternaria brassicicola, rendered SA-JA crosstalk independent of NPR1. This finding uncovered yet another layer of complexity in signaling during the plant immune response. In Chapter 4, we demonstrated that ET can make plants insensitive to SA-mediated suppression of JA signaling. When the JA and ET pathways are simultaneously induced prior to activation of the SA signaling, then Arabidopsis plants were insensitive to SA-JA cross-talk. We discovered that the ERF/AP2 transcription factor ORA59, is responsible for blocking SA-mediated suppression of JA responses. Together, we provide evidence that the final outcome of the interaction between the defense-related signals SA and JA is dependent on the kinetics of their production and the context in which their signaling pathways are activated. In addition ET can have an important role in shaping the final outcome of the plant defense signaling network that is activated upon pathogen or insect attack. In Chapters 5 and 6 we investigated the site of action of SA on the JA signaling pathway. In Chapter 5, we demonstrated that JA biosynthesis is not a major target of SA in the suppression of JA signaling. In Chapter 6 we demonstrated that SA targets the JA signaling pathway downstream of the SCFCOI1-complex and the JAZ repressor proteins. Using a molecular genetic approach, we provided evidence that SA suppresses the JA response at the level of gene transcription and that the GCC-box motif in JA-responsive promoters is sufficient for SA-mediated suppression. Collectively, this work provides novel insight into how plants regulate their defense response upon attack by multiple attackers. It appeared that the context in which the defense response is triggered plays an important role in the final outcome of the defense response. Moreover, we made important progress in uncovering the molecular basis of SA-JA cross-talk. Hence, the results presented in this thesis may be valuable for the development of novel strategies for crop protection.