Abscisic acid promotes proteasome‐mediated degradation of the transcription coactivator NPR1 in Arabidopsis thaliana

Summary Proteasome‐mediated turnover of the transcription coactivator NPR1 is pivotal for efficient activation of the broad‐spectrum plant immune responses known as localized acquired resistance (LAR) and systemic acquired resistance (SAR) in adjacent and systemic tissues, respectively, and requires the CUL3‐based E3 ligase and its adaptor proteins, NPR3 and NPR4, which are receptors for the signaling molecule salicylic acid (SA). It has been shown that SA prevents NPR1 turnover under non‐inducing and LAR/SAR‐inducing conditions, but how cellular NPR1 homeostasis is maintained remains unclear. Here, we show that the phytohormone abscisic acid (ABA) and SA antagonistically influence cellular NPR1 protein levels. ABA promotes NPR1 degradation via the CUL3NPR3/NPR4 complex‐mediated proteasome pathway, whereas SA may protect NPR1 from ABA‐promoted degradation through phosphorylation. Furthermore, we demonstrate that the timing and strength of SA and ABA signaling are critical in modulating NPR1 accumulation and target gene expression. Perturbing ABA or SA signaling in adjacent tissues alters the temporal dynamic pattern of NPR1 accumulation and target gene transcription. Finally, we show that sequential SA and ABA treatment leads to dynamic changes in NPR1 protein levels and target gene expression. Our results revealed a tight correlation between sequential SA and ABA signaling and dynamic changes in NPR1 protein levels and NPR1‐dependent transcription in plant immune responses. Significance Statement Both localized and systemic acquired resistance are long‐lasting plant immune responses induced by localized primary infections. They are associated with dynamic transcription of defense genes that are regulated by a key protein called NPR1. Here, we show that the phytohormone abscisic acid (ABA) and salicylic acid antagonistically influence NPR1 protein levels and that ABA promotes proteasome‐mediated degradation of NPR1. This finding is significant because NPR1 degradation plays dual roles in regulating plant immunity.