Fig. 7: Schematic model of how NIb inhibits NPR1-mediated immune responses.

Under steady-state conditions (left panel), monomeric NPR1 that is released from oligomers binds to SA and is imported into the nucleus, where it is sumoylated by SUMO3 via SIM3, is phosphorylated by a thus far unknown kinase at residues Ser11/Ser15, and then associates with TGA transcription factors to maintain basal expression of defense-related genes. Under immunity-primed conditions (middle panel), an increased level of SA triggers rapid and immense release of NPR1 monomers into the nucleus, where they active massive expression of defense-related genes, including SUMO3³³. Under TuMV infection conditions (right panel), newly synthesized NIb is rapidly imported into the nucleus, where it binds to SIM3 of NPR1, which prevents NPR1‒SUMO3 interaction and sumoylation of NPR1 by SUMO3. Meanwhile, sumoylation of NIb by SUMO3 increases the affinity of NIb for NPR1, which forms a positive feedback effect. The binding of NIb also inhibits downstream phosphorylation at Ser11/Ser15, which ultimately results in the attenuation or even shutdown of NPR1-mediated immunity. Sumoylated NIb is also exported by XPO1 to promote viral replication⁵⁹. P, phosphate, S3, SUMO3.