Abstract
The βC1 protein of tomato yellow leaf curl China β-satellite functions as a pathogenicity determinant. To better understand the molecular basis of βC1 in pathogenicity, a yeast two-hybrid screen of a tomato (Solanum lycopersicum) cDNA library was carried out using βC1 as bait. βC1 interacted with a tomato SUCROSE-NONFERMENTING1-related kinase designated as SlSnRK1. Their interaction was confirmed using a bimolecular fluorescence complementation assay in Nicotiana benthamiana cells. Plants overexpressing SnRK1 were delayed for symptom appearance and contained lower levels of viral and satellite DNA, while plants silenced for SnRK1 expression developed symptoms earlier and accumulated higher levels of viral DNA. In vitro kinase assays showed that βC1 is phosphorylated by SlSnRK1 mainly on serine at position 33 and threonine at position 78. Plants infected with βC1 mutants containing phosphorylation-mimic aspartate residues in place of serine-33 and/or threonine-78 displayed delayed and attenuated symptoms and accumulated lower levels of viral DNA, while plants infected with phosphorylation-negative alanine mutants contained higher levels of viral DNA. These results suggested that the SlSnRK1 protein attenuates geminivirus infection by interacting with and phosphorylating the βC1 protein.
Highlights
In nature, plants are continuously exposed to attacks by a variety of microbial pathogens including viruses
Previous studies showed that βC1 interacts with ASYMMETRIC LEAVES 1 (AS1) to alter leaf development and suppress selected jasmonic acid responses (Yang et al, 2008). βC1 interacts with a host ubiquitin-conjugating enzyme SlUBC3 and modifies the host ubiquitination system (Eini et al, 2009)
We demonstrate that SlSnRK1 phosphorylates the βC1 protein in vitro and that mutations in the primary phosopho-residues impact the pathogenicity function of βC1
Summary
Plants are continuously exposed to attacks by a variety of microbial pathogens including viruses. Geminiviruses are a group of plant DNA viruses characterized by single-stranded circular genomes encapsidated in twinned icosahedral particles that range in size from 18 to 30 nm (Hanley-Bowdoin et al, 2000; Rojas et al, 2005). They can be divided into four genera (Mastrevirus, Topocuvirus, Curtovirus and Begomovirus) based on genome structure, insect vectors, and host range (Fauquet and Stanley, 2005). We demonstrate that SlSnRK1 phosphorylates the βC1 protein in vitro and that mutations in the primary phosopho-residues impact the pathogenicity function of βC1
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