Exploring a Role for the Arabidopsis TIR-X Gene (TIRP) in the Defense Against Pathogenic Fungi or Insect Herbivory Attack

Plants are challenged regularly with multiple types of biotic stress factors, such as pathogens or insect herbivores, in their environment. To detect and defend against pathogens, plants have evolved an innate immune system in which intracellular receptors in the so-called effector-triggered immunity play a vital role. In <i>Arabidopsis thaliana</i> the Toll/interleukin-1 receptors (TIRs) domain is related to intracellular immunity receptors, for example in TIR-NBS-LRR (TNL) proteins. Among the TIR domain carrying proteins, very little is known about the function of the TIR-X proteins. Here, we focus on the recently described TIR-X (TIRP; At5g44900) to analyze its role in phytohormone-mediated plant defense through gene expression and phytohormone quantification. Therefore, we employed two fungal pathogens, the necrotrophic <i>Alternaria brassicicola</i> and the hemibiotrophic <i>Verticillium dahliae</i>, to infect <i>A. thaliana</i> WT (Col-0), TIRP knock-out, and TIRP overexpressing lines for comparative analyses. Furthermore, we included the insect herbivore <i>Spodoptera littoralis</i> and a treatment with <i>S. littoralis</i> egg extract on the plants to analyze any role of TIRP during these attacks. We found that both <i>A. brassicicola</i> and <i>V. dahliae</i> infections increased TIRP gene expression systemically. The salicylic acid content was higher in the TIRP overexpressing line, corresponding to a better <i>S. littoralis</i> larval growth performance in feeding assays. However, since we never observed clear infection-related differences in jasmonate or salicylic acid levels between the wild type and the two transgenic <i>Arabidopsis</i> lines, our results rule out the possibility that TIRP acts via the regulation of phytohormone synthesis and accumulation.