Regulated Nuclear Trafficking of rpL10A Mediated by NIK1 Represents a Defense Strategy of Plant Cells against Virus

Abstract

The NSP-interacting kinase (NIK) receptor-mediated defense pathway has been identified recently as a virulence target of the geminivirus nuclear shuttle protein (NSP). However, the NIK1–NSP interaction does not fit into the elicitor–receptor model of resistance, and hence the molecular mechanism that links this antiviral response to receptor activation remains obscure. Here, we identified a ribosomal protein, rpL10A, as a specific partner and substrate of NIK1 that functions as an immediate downstream effector of NIK1-mediated response. Phosphorylation of cytosolic rpL10A by NIK1 redirects the protein to the nucleus where it may act to modulate viral infection. While ectopic expression of normal NIK1 or a hyperactive NIK1 mutant promotes the accumulation of phosphorylated rpL10A within the nuclei, an inactive NIK1 mutant fails to redirect the protein to the nuclei of co-transfected cells. Likewise, a mutant rpL10A defective for NIK1 phosphorylation is not redirected to the nucleus. Furthermore, loss of rpL10A function enhances susceptibility to geminivirus infection, resembling the phenotype of nik1 null alleles. We also provide evidence that geminivirus infection directly interferes with NIK1-mediated nuclear relocalization of rpL10A as a counterdefensive measure. However, the NIK1-mediated defense signaling neither activates RNA silencing nor promotes a hypersensitive response but inhibits plant growth and development. Although the virulence function of the particular geminivirus NSP studied here overcomes this layer of defense in Arabidopsis, the NIK1-mediated signaling response may be involved in restricting the host range of other viruses.

Mimic Phosphorylation of a βC1 Protein Encoded by TYLCCNB Impairs Its Functions as a Viral Suppressor of RNA Silencing and a Symptom Determinant

“…We also investigated the infection course of Y10A associated with ␤-WT or its phosphorylation mimic mutants in wild-type N. benthamiana plants, as described previously (27,33). For Y10A/␤-WT inoculation, disease symptoms started to appear at 3 or 4 dpi, with all the plants displaying typical symptoms at 8 to 10 dpi (Fig.…”

“…The course of viral infection was monitored as described previously (27,33). Total nucleic acids were extracted from systemically infected leaves using a cetyltrimethylammonium bromide (CTAB)-based method (63).…”

New Evidence for Differential Roles of L10 Ribosomal Proteins from Arabidopsis

“…One possibility is that the increased number of nuclear proteins associated to AtRPL10 arises from increased translocation of AtRPL10B to the nucleus after the UV-B treatment. Carvalho et al (2008) previously demonstrated that, after viral infection, AtRPL10A is phosphorylated by the NSP-INTERACTING KINASE1 (NIK1), redirecting the protein to the nucleus, whereas when NIK1 is mutated, RPL10A fails to accumulate in the nucleus. Therefore, a similar situation may occur after UV-B exposure with AtRPL10B, where it may fulfill specific functions in response to this radiation.…”

Tomato SlSnRK1 Protein Interacts with and Phosphorylates βC1, a Pathogenesis Protein Encoded by a Geminivirus β-Satellite

“…To assess the biological significance of SlSnRK1-bC1 interactions in vivo, we inoculated wild-type and transgenic N. benthamiana plants carrying an Arabidopsis antisense SnRK1 expression cassette (AS-12) or an Arabidopsis sense SnRK1 expression cassette (S-5; Hao et al, 2003) with TYLCCNV/TYLCCNB and monitored infection over time (Carvalho et al, 2008b). Changes in SnRK1 expression altered the timing of symptom appearance, with overexpressing plants developing symptoms later and silenced plants showing symptoms earlier than wild-type plants (Fig.…”

“…Wild-type or transgenic N. benthamiana plants expressing Arabidopsis antisense SnRK1 (AS-12) or Arabidopsis sense SnRK1 (S-5) were agroinoculated with an overnight culture of A. tumefaciens carrying TYLCCNV and TYLCCNB (pBinPLUS-1.7A and pBinPLUS-2b; Cui et al, 2004) or TYLCCNB mutants. The course of infection was monitored as described (Carvalho et al, 2008b). DPI 50% was determined using data from three independent experiments.…”