The pathogenicity of grapevine geminivirus A (GGVA), a recently identified DNA virus, to grapevine plants remains largely unclear. Here, we report a new GGVA isolate (named GGVAQN) obtained from grapevine ‘Queen Nina’ plants with severe disease symptoms. The infectious clone of GGVAQN (pXT-GGVAQN) was constructed to investigate its pathogenicity. Nicotiana benthamiana plants inoculated with GGVAQN by agroinfiltration displayed upward leaf curling and chlorotic mottling symptoms. A simple, quick, and efficient method for delivering DNA clones of GGVAQN into grapevine plants was developed, by which Agrobacterium tumefaciens cells carrying pXT-GGVAQN were introduced into the roots of in vitro-grown ‘Red Globe’ grape plantlets with a syringe. By this method, all ‘Red Globe’ grape plants were systemically infected with GGVAQN, and the plants exhibited chlorotic mottling symptoms on their upper leaves and downward curling, interveinal yellowing, and leaf-margin necrosis symptoms on their lower leaves. Our results provide insights into the pathogenicity of GGVA and a simple and efficient inoculation method to deliver infectious viral clones to woody perennial plants.

Grapevine (Vitis vinifera) is severely impacted by viral diseases, with grapevine leafroll disease (GLRD) being the most economically damaging. Grapevine leafroll-associated virus 2 (GLRaV-2) is a prevalent virus associated with GLRD, yet the molecular mechanisms underlying GLRaV-2 infection remain poorly understood. Here, we demonstrate that the grapevine transcription factor (TF) VvWRKY51 positively autoregulates its transcription and directly represses salicylic acid (SA)-responsive defense genes VvPR1 (pathogenesis-related gene 1), VvPR2, and VvPR5. Through in vivo and in vitro assays, we demonstrate that GLRaV-2 RNA silencing suppressor p24 specifically interacts with VvWRKY51 to promote viral infection. p24 hijacks nuclear VvWRKY51 by translocating it to the cytoplasm, thereby enhancing its RNA silencing suppression activity via 2 mechanisms: (i) evading host degradation pathways (ubiquitin-proteasome system and autophagy), and (ii) strengthening its small interfering RNA-binding affinity. Concurrently, cytoplasmic p24 enters the nucleus by binding VvWRKY51, where it reinforces VvWRKY51-mediated transcriptional repression of VvPR1, VvPR2, and VvPR5. This occurs by enhancing VvWRKY51 binding to its own promoter and those of VvPR genes. Our findings reveal a novel viral strategy wherein a single viral suppressor of RNA silencing simultaneously suppresses RNA silencing and SA-mediated immunity by interacting with a host TF to establish a permissive environment for infection.

The pathogenicity of grapevine geminivirus A (GGVA), a recently identified DNA virus, to grapevine plants remains largely unclear. Here, we report a new GGVA isolate (named GGVAQN) obtained from grapevine ‘Queen Nina’ plants with severe disease symptoms; the infectious clone of GGVAQN (pXT-GGVAQN) was also constructed. Infection assays indicated that Nicotiana benthamiana plants inoculated with GGVAQN display upward leaf curling and chlorotic mottling symptoms. Direct injection of Agrobacterium tumefaciens cells carrying pXT-GGVAQN into roots of in vitro-grown ‘Red Globe’ grape plantlets by syringe led to systemic infection with GGVAQN, and the plants exhibited chlorotic mottling symptoms on their upper leaves, and downward curling, interveinal yellowing, and leaf-margin necrosis symptoms on their lower leaves. Our results provide insights into the pathogenicity of GGVA, and a simple and efficient inoculation method to deliver infectious viral clones to woody perennial plants.