The hub of salicylic acid-mediated antiviral immunity in phalaenopsis orchid

Orchid is one of the most important flower crops in Taiwan, however viral disease causes great economic loss in the orchid industry. To develop antiviral strategies, we focus on the study of how orchid defend virus infection. Although the plant hormone salicylic acid (SA)-mediated immunity is important for virus resistance, the downstream pathway of SA-mediated antiviral immune response is still not well understood. We performed the highthroughput cymbidium mosaic virus-based virus induced gene silencing (CymMV-VIGS) system and identified an orchid protein, Pha13, containing A20 and AN1 zinc finger domains. Pha13 is early induced by SA treatment and serves as an important hub to mediate SA-responsive genes including the master regulator gene, orchid NPR1, PhaNPR1, as well as PhaNPR1-independent virus defense genes. Overexpression of Pha13 also enhances antiviral immunity in both monocot orchid and dicot Arabidopsis against different viruses, suggesting that the Pha13-mediated downstream antiviral immune response is conserved. In addition, we also found that the Arabidopsis homolog of Pha13, AtSAP5, functions similarly to Pha13 in the SA-mediated immunity. Biochemical assay on Pha13 indicated that Pha13 confers multiple activities including the self-E3 ligase and ubiquitin chain binding activities. Our findings revealed that A20/AN1 proteins confer diverse biochemical activities and serve as an important regulatory hub in the SA-mediated antiviral immunity among plants. © 2021 by World Scientific Publishing Co. Pte. Ltd.