Periodic Reporting for period 4 - IMMUNE-EXPRESS (Proteasome-Mediated Gene Expression in Plant Immunity)
Periodo di rendicontazione: 2020-09-01 al 2021-08-31
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.
Objective 2: While progress has been made in understanding substrate ubiquitination during plant immunity, how these substrates are processed upon arrival at the proteasome remains unclear. We discovered that specific members of the HECT domain-containing family of ubiquitin E3 ligases play important roles in proteasomal substrate processing during plant immunity. These HECT-type ligases were found to physically interact with the proteasome, enabling proteasomes to form ubiquitin chains. Moreover, we discovered that unknown ubiquitin ligase relays that terminate with proteasome-associated HECT-type ligases may be a universal mechanism for processive degradation of proteasome-targeted TAs and other substrates. Thus, we identified HECT-type ligases and the proteasome as potential new targets for improvement of broad-spectrum crop immunity.
Objective 3:
So how does SA regulate the activity of E3 ligases that are essential for activation of plant immunity? We reported previously that SA induces changes in the cellular redox environment, resulting in oxidation and reduction reactions of proteins. We now discovered that the specific substrate adaptors of immune-related E3 ligases are modified by redox changes. Substrate adaptors ensure that E3 ligases recruit specific substrates for ubiquitination and subsequent degradation. We found that these substrate adaptors can be specifically modified by oxidative molecules such as nitric oxide (NO), which results in rapid multimerization. Moreover, deubiquitinases (DUBs) that trim ubiquitin chains of substrates, are also subject to inhibitory oxidation. Conversely, we demonstrate that antioxidant enzymes of the Thioredoxin (TRX) family reverse this process, thereby facilitating the active states of ubiquitin modifying enzymes, indicating that TRX enzymes play a key role in regulating ubiquitin signalling in plant immunity. Thus, TRX enzymes represent new chemical and genetic targets from crop improvement.