Differential Quantitative Requirements for NPR1 Between Basal Immunity and Systemic Acquired Resistance inArabidopsis thaliana

Non-expressor of pathogenesis-related (PR) genes1 (NPR1) is a key transcription coactivator of plant basal immunity and systemic acquired resistance (SAR). Two mutant alleles,npr1-1andnpr1-3, have been extensively used for dissecting the role of NPR1 in various signaling pathways. However, it is unknown whethernpr1-1andnpr1-3are null mutants. Moreover, theNPR1transcript levels are induced two- to threefold upon pathogen infection or salicylic acid (SA) treatment, but the biological relevance of the induction is unclear. Here, we used molecular and biochemical approaches including quantitative PCR, immunoblot analysis, site-directed mutagenesis, and CRISPR/Cas9-mediated gene editing to address these questions. We show thatnpr1-3is a potential null mutant, whereasnpr1-1is not. We also demonstrated that a truncated npr1 protein longer than the hypothesized npr1-3 protein is not active in SA signaling. Furthermore, we revealed that TGACG-binding (TGA) factors are required forNPR1induction, but the reverse TGA box in the 5'UTR ofNPR1is dispensable for the induction. Finally, we show that full induction ofNPR1is required for basal immunity, but not for SAR, whereas sufficient basal transcription is essential for full-scale establishment of SAR. Our results indicate that induced transcript accumulation may be differentially required for different functions of a specific gene. Moreover, asnpr1-1is not a null mutant, we recommend that future research should usenpr1-3and potential null T-DNA insertion mutants for dissecting NPR1's function in various physiopathological processes.