Plants are continuously exposed to a large variety of pathogenic microbes, including bacteria, fungi, oomycetes, and viruses. To defend themselves, plants have evolved a sophisticated innate immune system that provides specificity, self-tolerance, and immune memory. These features are largely orchestrated by the immune hormone salicylic acid (SA), a derivative of the better known aspirin. Upon pathogen infection, SA accumulates at the site of infection where it is required for local resistance responses, such as programmed cell death to isolate the invading pathogen, and production of antimicrobial compounds. In addition, SA accumulates in distal tissues where it is involved in establishing a memory of the initial attack, providing broad-spectrum, long-lasting immunity to subsequent pathogen attack. Because of the importance of SA in establishing disease resistance, agribusinesses have previously developed SA mimics (e.g. Actigard® or Bion®) as well as compounds that activate SA signalling (e.g. probenazole and tiadinil), many of which are still heavily used as plant activators or fungicides in agriculture today. To establish immunity, SA and SA mimics induce dramatic changes in the expression of thousands of genes. These changes are largely managed by the activator protein NPR1, a regulator of ~10% of genes in the genome of many plant species. Mutation of NPR1 results in severe susceptibility to a large variety of pathogens, demonstrating its importance in SA-mediated plant immunity. We showed previously that while NPR1 is an activator of immunity, unexpectedly SA triggers its degradation. So why does SA signal for the degradation of NPR1? Surprisingly, we found that degradation of NPR1 is paradoxically necessary for the activation of its target genes. We suggested a model in which activation of target gene expression marks NPR1 as a ‘spent’ activator. Removal of ‘spent’ NPR1 may be necessary to allow ‘fresh’ NPR1 activator to reinitiate the expression of target genes (Fig. 1). This novel mode of gene expression by unstable activatorshas now been widely found in diverse eukaryotic organisms including humans, indicating its general importance to regulation of eukaryotic gene expression. Our contributions to understanding the perception of SA and downstream activation of SA-responsive immune genes represent major leaps forward in our understanding of plant immune responses. Nonetheless, it remains unknown what defines a ‘spent’ NPR1 activator and how quickly it is rendered in this inactive state. Moreover, SA regulates diverse E3 ligases that target NPR1 and other substrates for degradation, but the regulatory mechanisms and identity of these substrates remains elusive.
Objective 1: We aim to understand what defines the 'spent' state of NPR1, as this could open up new avenues to NPR1-based crop protection strategies.
Objective 2: Identify and characterise additional SA-induced ubiquitin E3 ligases and their substrates. This could reveal new targets for chemical or genetic crop innovation.
Objective 3: Elucidate how ubiquitin E3 ligases are regulated by the SA signal. Understanding how SA and active SA mimicking molecules regulate the activities of ubiquitin E3 ligases could offer the possibility to design improved agrichemicals.
