Induced pathogen and insect resistance in Arabidopsis: transcriptomics and specificity of defense

An important question in plant defense signaling research is: how are plants capable of integrating signals induced by pathogenic micro-organisms and herbivorous insects into defenses that are specifically active against the attacker encountered? Plant defenses against pathogens and insects are differentially regulated by cross-communicating signaling pathways in which salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) play key roles. To understand how plants integrate pathogen- and insect-induced signals into specific defense responses, we monitored the dynamics of SA, JA, and ET signaling and genome-wide transcriptome changes in Arabidopsis after attack by a set of microbial pathogens and herbivorous insects with different modes of attack. Arabidopsis plants were exposed to a pathogenic leaf bacterium ( Pseudomonas syringae pv. tomato ), a pathogenic leaf fungus ( Alternaria brassicicola ), tissue-chewing caterpillars ( Pieris rapae ), cell-content-feeding thrips ( Frankliniella occidentalis ), or phloem-feeding aphids ( Myzus persicae ). Monitoring the signal signature in each plant-attacker combination showed that the kinetics of SA, JA, and ET production varies greatly in both quantity and timing. The transcriptional alterations were predominantly attacker-specific, but the processes affected surprisingly similar. This indicates that the different attackers induce changes in similar plant processes through largely non-overlapping transcriptional alterations. Yet, infestation by M. persicae induced a transcriptional response that was opposite to those induced by the other attackers or exogenous application of MeJA. We concluded that SA, JA, and ET play a primary role in the orchestration of the plant's defense response, but other regulatory mechanisms, such as pathway cross-talk and additional attacker-induced signals, eventually shape the highly complex attacker-specific defense response.Little is known about the spectrum of effectiveness of the different types of induced resistance that are expressed upon attack by pathogens or insects. Because the signaling pathways that control induced resistance against pathogens and insects partly overlap, we decided to investigate the effectiveness of microbially induced resistance against the tissue-chewing herbivorous insects Pieris rapae and Spodoptera exigua in Arabidopsis. Two types of microbially induced resistance were studied: systemic acquired resistance (SAR), which is induced upon predisposal infection by necrotizing pathogens, and rhizobacteria-mediated induced systemic resistance (ISR), which is triggered by selected strains of non-pathogenic, root-colonizingrhizobacteria. No effect of SAR or ISR was evident on herbivore-induced attractiveness of the parasitic wasp Cotesia rubecula , indicating that neither type of induced resistance influenced indirect defense against these insects. In feeding experiments on whole plants, induction of SAR and ISR significantly reduced growth and development of the generalist herbivore S. exigua , whereas the performance of the specialist P. rapae was unaffected. The JA- and ET-responsive genes PDF1.2 and HEL , which were activated upon feeding by either of the two herbivores, showed a strongly potentiated expression pattern in SAR- and ISR-expressing plants upon feeding by S. exigua , but not upon feeding by P. rapae . This differential priming for enhanced herbivore-induced gene expression was confirmed in microarray experiments using a dedicated cDNA microarray containing 111 insect-responsive Arabidopsis genes. These results suggest that the effectiveness of microbially induced SAR and ISR against S. exigua feeding is associated with priming for enhanced defense-related gene expression.In conclusion, this thesis highlights the complexity of the defense signaling interactions between plants, pathogens and insect herbivores.