Sustained NIK‐mediated antiviral signalling confers broad‐spectrum tolerance to begomoviruses in cultivated plants

Abstract

SummaryBegomovirus-associated epidemics currently threaten tomato production worldwide due to the emergence of highly pathogenic virus species and the proliferation of a whitefly B biotype vector that is adapted to tomato. To generate an efficient defence against begomovirus, we modulated the activity of the immune defence receptor nuclear shuttle protein (NSP)-interacting kinase (NIK) in tomato plants; NIK is a virulence target of the begomovirus NSP during infection. Mutation of T474 within the kinase activation loop promoted the constitutive activation of NIK-mediated defences, resulting in the down-regulation of translation-related genes and the suppression of global translation. Consistent with these findings, transgenic lines harbouring an activating mutation (T474D) were tolerant to the tomato-infecting begomoviruses ToYSV and ToSRV. This phenotype was associated with reduced loading of coat protein viral mRNA in actively translating polysomes, lower infection efficiency and reduced accumulation of viral DNA in systemic leaves. Our results also add some relevant insights into the mechanism underlying the NIK-mediated defence. We observed that the mock-inoculated T474D-overexpressing lines showed a constitutively infected wild-type transcriptome, indicating that the activation of the NIKmediated signalling pathway triggers a typical response to begomovirus infection. In addition, the gain-of-function mutant T474D could sustain an activated NIK-mediated antiviral response in the absence of the virus, further confirming that phosphorylation of Thr-474 is the crucial event that leads to the activation of the kinase.

Tomato leaf curl New Delhi virus: a widespread bipartite begomovirus in the territory of monopartite begomoviruses

“…Of particular interest is NIK, a leucine‐rich repeat receptor‐like kinase (LRR‐RLK), which triggers translational suppression on begomovirus infection, and has been found to show strong antiviral activity (Zorzatto et al ., ). Moreover, transgenic tomato plants with constitutively expressed NIK have been demonstrated to show high degrees of tolerance to tomato‐infecting begomoviruses (Brustolini et al ., ).…”

Immune Receptors and Co-receptors in Antiviral Innate Immunity in Plants

“…Thr-468 and Thr-474 are located within the conserved activation loop and align to the same positions as conserved BAK1 residues Thr-449 and Thr-455 and SERK1 residues Thr-462 and Thr-468, which are intramolecular targets for BAK1 and SERK1 kinase activation (Shah et al, 2001; Wang et al, 2005, 2008; Yun et al, 2009). Phosphorylation of the functional analogs NIK1 Thr-474, SERK1 Thr-468 and BAK1 Thr-455 is essential for receptor/co-receptor signaling, which may underscore a similar mechanism for activation (Shah et al, 2001; Wang et al, 2008; Santos et al, 2009; Yun et al, 2009; Brustolini et al, 2015). Nevertheless, unlike BAK1 or SERK1, phosphorylation at NIK1 Thr-474 leads to phosphorylation at Thr-469, which has an inhibitory effect, thereby providing a mechanism by which NIK1 modulates the extent of auto- and substrate phosphorylation.…”

“…Begomovirus NSP binds in vitro and in vivo with the kinase domain of NIK1 to suppress NIK1 activity (Fontes et al, 2004; Brustolini et al, 2015). The NSP binding site corresponds to an 80-amino acid stretch (positions 422–502) of NIK1 that encompasses the putative Ser/Thr kinase active site (subdomain VIb–HrDvKssNxLLD) and the activation loop (subdomain VII–DFGAk/rx, plus subdomain VIII–GtxGyiaPEY; Fontes et al, 2004).…”

“…In fact, activation of NIK1 signaling by constitutive or inducible expression of the gain-of-function T474D NIK1 mutant, which is not inhibited by viral NSP, results in a massive down-regulation of translation machinery-related genes, suppression of host global translation and enhanced broad-spectrum tolerance to begomoviruses in Arabidopsis and tomato (Brustolini et al, 2015; Zorzatto et al, 2015). T474D-mediated suppression of global translation is associated with a decrease in host and viral mRNA in actively translating polysomes.…”

Translational control in plant antiviral immunity

“…In the case of begomoviruses, it has been shown that in addition to encoding suppressors for siRNA-mediated defenses, these viruses enhance their pathogenicity in susceptible hosts by suppressing the antiviral activity of the transmembrane receptor NIK1 by the viral NSP (Fontes et al , 2004; Santos et al , 2009; Brustolini et al , 2015). …”