UC San Diego

Biotic stresses such as crop disease are one of the major threats to food production worldwide. Global losses due to pathogens and pests associated with wheat, rice, maize, potato and soybean reduced the output of these five major crops, which generate 50% of the global human calorie intake, by 10-40% (Savary, 2019). It has been projected that crop disease and pest incidences are expanding in a poleward direction (2.7 km annually) (Bebber, 2014). As evidence of the negative impacts of environmental shifts accumulates in combination with a market preference for pesticide-free produce, it is expected that the main strategies to combat food insecurity will come from science and technology (Cole, 2018). Thus, scientists and policy makers alike consider increasing the pathogen tolerance of crop plants through molecular and plant breeding approaches as the most attractive and viable option to sustain food production (Cole, 2018). In this regard, disease resistant crops designed to constitutively overexpress NONEXPRESSER OF PR GENES1 (NPR1)- a master immune regulator protein homologous to many plant species- have gained popularity given their phenotypic ability to resist pathogenic attacks (Silva, 2018). However, this approach has led to undesirable side effects that contest whether NPR1 overexpressing (NOX) plants are a viable solution to improve crop fitness (Silva, 2018). given their tendency towards growth retardation. The finer details governing the signaling cascades involved in NPR1-associated defense responses remain elusive, calling for further plant defense studies. The Arabidopsis thaliana-Pseudomonas syringae pathosystem has been broadly employed to understand plant-pathogen interactions (Volko, 1998). Through morning/evening infection experiments and bioluminescence imaging of NPR1 luciferase reporter lines, we discovered that NPR1 mediates the circadian regulation of defense responses, but said regulation does not involve changes in NPR1 transcription. Given P. syringae’s hemibiotrophic approach (Thaler, 2004) we investigated changes in NPR1 expression following salicylic acid (SA) and jasmonic acid (JA) treatments, tissue wounding, and P. syr infection. Remarkably, we found that while NPR1 expression increased after any of the aforementioned treatments, it expressed rhythmically after JA treatments, tissue wounding, and P. syr infection. From this we concluded that wounding triggers increase in NPR1 expression and expression oscillations.