EXA1, a GYF domain protein, is responsible for loss‐of‐susceptibility to plantago asiatica mosaic virus in Arabidopsis thaliana

Abstract

SUMMARYOne of the plant host resistance machineries to viruses is attributed to recessive alleles of genes encoding critical host factors for virus infection. This type of resistance, also referred to as recessive resistance, is useful for revealing plant-virus interactions and for breeding antivirus resistance in crop plants. Therefore, it is important to identify a novel host factor responsible for robust recessive resistance to plant viruses. Here, we identified a mutant from an ethylmethane sulfonate (EMS)-mutagenized Arabidopsis population which confers resistance to plantago asiatica mosaic virus (PlAMV, genus Potexvirus). Based on map-based cloning and single nucleotide polymorphism analysis, we identified a premature termination codon in a functionally unknown gene containing a GYF domain, which binds to proline-rich sequences in eukaryotes. Complementation analyses and robust resistance to PlAMV in a T-DNA mutant demonstrated that this gene, named Essential for poteXvirus Accumulation 1 (EXA1), is indispensable for PlAMV infection. EXA1 contains a GYF domain and a conserved motif for interaction with eukaryotic translation initiation factor 4E (eIF4E), and is highly conserved among monocot and dicot species. Analysis using qRT-PCR and immunoblotting revealed that EXA1 was expressed in all tissues, and was not transcriptionally responsive to PlAMV infection in Arabidopsis plants. Moreover, accumulation of PlAMV and a PlAMV-derived replicon was drastically diminished in the initially infected cells by the EXA1 deficiency. Accumulation of two other potexviruses also decreased in exa1-1 mutant plants. Our results provided a functional annotation to GYF domain-containing proteins by revealing the function of the highly conserved EXA1 gene in plant-virus interactions.

Recessive Resistance to Plant Viruses: Potential Resistance Genes Beyond Translation Initiation Factors

“…More recently, Hashimoto et al (2016) demonstrated that deficiencies in essential for potexvirus accumulation 1 ( EXA1 ) gene were present in a loss-of-susceptibility Arabidopsis mutant that did not support plantago asiatica mosaic virus (PlAMV; Potexvirus ) accumulation. EXA1 is an unannotated gene in plants, but contains a putative eIF4E-binding motif and a GYF domain, which binds to proline-rich peptides (Kofler and Freund, 2006).…”

“…EXA1 is an unannotated gene in plants, but contains a putative eIF4E-binding motif and a GYF domain, which binds to proline-rich peptides (Kofler and Freund, 2006). Based on sequence comparisons with other related genes, EXA1 homologs are encoded in rice and tomato and are structurally related to human GIGYF2 protein (Hashimoto et al, 2016). T-DNA insertion of EXA1 gene, forming exa1-1 mutant, does not affect accumulation of CMV, TCV, or YoMV, but does suppress the accumulation of two distinct potexviruses other than PlAMV (Hashimoto et al, 2016).…”

“…Based on sequence comparisons with other related genes, EXA1 homologs are encoded in rice and tomato and are structurally related to human GIGYF2 protein (Hashimoto et al, 2016). T-DNA insertion of EXA1 gene, forming exa1-1 mutant, does not affect accumulation of CMV, TCV, or YoMV, but does suppress the accumulation of two distinct potexviruses other than PlAMV (Hashimoto et al, 2016). Because human GIGYF2 regulates mRNA translation (Morita et al, 2012), it is conceivable that EXA1 might also regulate the translation of a viral protein during early infection.…”

“…The rationale for this is based on the expectation that viruses from the same genus have similar life cycles. In fact, it is known that some host factors, including eIF4Es, play a similar role in infection by different viruses from the same genus (Asano et al, 2005; Ouibrahim et al, 2014; Yang et al, 2014; Poque et al, 2015; Hashimoto et al, 2016). However, there are many exceptions that challenge this rationale (for example, see the section on eIF4Es-MEDIATED RECESSIVE RESISTANCE).…”

“…Several studies have identified loci of interest from Arabidopsis mutants using whole-genome sequencing of pooled mutant F2 populations (Schneeberger et al, 2009; Austin et al, 2011; Uchida et al, 2011). The EXA1 gene was identified successfully from a loss-of-susceptibility mutant by combining conventional map-based cloning and whole-genome sequencing of mutant plants (Hashimoto et al, 2016). Methods based on a similar concept have also been established in rice (Abe et al, 2012; Takagi et al, 2013).…”