Geminivirus Fitness via bZIP17/28 is impaired by the autophagic degradation of the betasatellite βV1 protein.

The unfolded protein response (UPR) is a vital cellular pathway that maintains endoplasmic reticulum (ER) homeostasis under conditions of ER stress, which is associated with the degradation of misfolded proteins. However, the role of ER-associated degradation in plant-microbe interactions has yet to be explored. In this study, we identified a novel viral protein βV1, encoded by tomato yellow leaf curl betasatellite (TYLCCNB), is an ER-localized protein that triggers ER aggregation. Transient expression of βV1 in Nicotiana benthamiana induces robust ER stress and activates the bZIP17/28 branch of the UPR signaling pathway. The induction of bZIP17/28 by βV1 is crucial for successful virus infection. Furthermore, we demonstrate that βV1 is unstable in N. benthamiana mesophyll cells, as it is targeted for autophagic degradation. The autophagy-related protein ATG18a, a key component of autophagosomes, participates in the degradation of βV1, thereby exerting an anti-viral role. Taken together, our results unveil a novel function of the βV1 protein and provide the first evidence of the involvement of bZIP17/28 and ATG18a in ER-associated autophagic degradation during geminivirus infection. These findings significantly expand our comprehension of the arms race dynamics between plants and viruses.