The different levels of gene regulation in plant immunity

Abstract

Altering gene expression is one of the key regulatory mechanisms for plants to respond to pathogens. This alteration can and must be achieved via various means. In this thesis I investigated three different aspects of gene regulation; chromatin remodelling, histone modifications, and post-transcriptional regulation via miRNAs. In detail, we first analysed the importance of chromatin remodelling during establishment of systemic acquired resistance (SAR). SAR describes the phenomenon that plants can globally prime immune responses after local infection. We could find first hints that nucleosome occupancies at loci involved in the establishment of SAR might be altered after treatment with SAR-inducing signals and that this alteration might be dependent on the defective in mating-type switching/sucrose nonfermenting (SWI/SNF) chromatin remodelling complex. Moreover, we found first hints that nucleosome changes might persist even when expression is at non-detectable level again, possibly representing a piece of the so-called plants’ memory function after infection. Second, we asked whether the bromodomain-containing protein, BRD5, which is able to bind acetylated histones, plays a role during pathogen infection. We were able to show, that Arabidopsis brd5 mutants are less resistant towards the necrotrophic fungus Botrytis cinerea. Even though in planta salicylic acid (SA) levels were unaltered, SA signalling, e.g. PR1 expression levels might be altered in brd5 mutants. Since SA signalling negatively regulates jasmonic acid (JA) signalling we propose that BRD5 might be important for early response towards necrotrophic pathogens and is possibly part of the crosstalk between SA and JA signalling. Third, we analysed the role of miR827 during pathogen response. We were able to show that miR827 might be important for the proper response towards Alternaria brassicicola, since miR827 mutants were less susceptible. Additionally, we could proof that miR827 is specifically up-regulated by A. brassicicola treatment and that this severe up-regulation is dependent on sufficient phosphate supply. Our results propose that miR827 and its target mRNA NLA might be part of the JA-mediated signalling response towards the necrotrophic pathogen Alternaria brassicicola. In summary, we were able to show that many more processes play a role in pathogen response than initially assumed, further refining the classical zig-zag model of plant immunity.