The mitochondrial and chloroplast dual targeting of a multifunctional plant viral protein modulates chloroplast‐to‐nucleus communication, RNA silencing suppressor activity, encapsidation, pathogenesis and tissue tropism

• Published in: The Plant journal : for cell and molecular biology 2021-10, Vol.108 (1), p.197-218
• Main Authors: Navarro, Jose A., Saiz‐Bonilla, Maria, Sanchez‐Navarro, Jesus A., Pallas, Vicente
• Format: Article
• Language: English
• Subjects: Accumulation; Capsid Proteins - genetics; Capsid Proteins - metabolism; Cell Nucleus - metabolism; Chloroplasts; Chloroplasts - metabolism; Coat protein; Cucurbitaceae - genetics; Cucurbitaceae - physiology; Cucurbitaceae - virology; Defense mechanisms; Domains; dual targeting; Encapsidation; Genes, Reporter; Genomes; hypersensitive response; Imports; melon necrotic spot virus; Mitochondria; Mitochondria - metabolism; Mitochondrial Membranes - metabolism; Mutation; Necrosis; Nicotiana - genetics; Nicotiana - physiology; Oxidative Stress; Pathogenesis; Plant Diseases - virology; Plant Leaves - genetics; Plant Leaves - physiology; Plant Leaves - virology; Protein Transport; Proteins; Replicase; Replication; RNA Interference; RNA-mediated interference; silencing; Spot; Stroma; Tombusviridae - genetics; Tombusviridae - pathogenicity; Tombusviridae - physiology; Tropism; Viral infections; Viral Tropism; Virus Replication
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/tpj.15435

SUMMARY Plant defense against melon necrotic spot virus (MNSV) is triggered by the viral auxiliary replicase p29 that is targeted to mitochondrial membranes causing morphological alterations, oxidative burst and necrosis. Here we show that MNSV coat protein (CP) was also targeted to mitochondria and mitochondrial‐derived replication complexes [viral replication factories or complex (VRC)], in close association with p29, in addition to chloroplasts. CP import resulted in the cleavage of the R/arm domain previously implicated in genome binding during encapsidation and RNA silencing suppression (RSS). We also show that CP organelle import inhibition enhanced RSS activity, CP accumulation and VRC biogenesis but resulted in inhibition of systemic spreading, indicating that MNSV whole‐plant infection requires CP organelle import. We hypothesize that to alleviate the p29 impact on host physiology, MNSV could moderate its replication and p29 accumulation by regulating CP RSS activity through organelle targeting and, consequently, eluding early‐triggered antiviral response. Cellular and molecular events also suggested that S/P domains, which correspond to processed CP in chloroplast stroma or mitochondrion matrix, could mitigate host response inhibiting p29‐induced necrosis. S/P deletion mainly resulted in a precarious balance between defense and counter‐defense responses, generating either cytopathic alterations and MNSV cell‐to‐cell movement restriction or some degree of local movement. In addition, local necrosis and defense responses were dampened when RSS activity but not S/P organelle targeting was affected. Based on a robust biochemical and cellular analysis, we established that the mitochondrial and chloroplast dual targeting of MNSV CP profoundly impacts the viral infection cycle. Significance Statement The importance of mitochondria and chloroplasts as stress recognition initial sites and signaling after cellular dysfunction is evident, although its involvement in plant–virus interactions remains unclear. Here, we show that a viral protein is organelle‐targeted to modulate chloroplast‐to‐nucleus communication, RNA silencing suppression, encapsidation, pathogenesis and tissue tropism, making coat protein a remarkable multifunctional factor. Our results illuminate how plant viruses have evolved to evade plant defense and proliferate using a few proteins.