Cauliflower mosaic virus Protein P6 Inhibits Signaling Responses to Salicylic Acid and Regulates Innate Immunity

Abstract

Cauliflower mosaic virus (CaMV) encodes a multifunctional protein P6 that is required for translation of the 35S RNA and also acts as a suppressor of RNA silencing. Here we demonstrate that P6 additionally acts as a pathogenicity effector of an unique and novel type, modifying NPR1 (a key regulator of salicylic acid (SA)- and jasmonic acid (JA)-dependent signaling) and inhibiting SA-dependent defence responses We find that that transgene-mediated expression of P6 in Arabidopsis and transient expression in Nicotiana benthamiana has profound effects on defence signaling, suppressing expression of representative SA-responsive genes and increasing expression of representative JA-responsive genes. Relative to wild-type Arabidopsis P6-expressing transgenics had greatly reduced expression of PR-1 following SA-treatment, infection by CaMV or inoculation with an avirulent bacterial pathogen Pseudomonas syringae pv tomato (Pst). Similarly transient expression in Nicotiana benthamiana of P6 (including a mutant form defective in translational transactivation activity) suppressed PR-1a transcript accumulation in response to Agrobacterium infiltration and following SA-treatment. As well as suppressing the expression of representative SA-regulated genes, P6-transgenic Arabidopsis showed greatly enhanced susceptibility to both virulent and avirulent Pst (titres elevated 10 to 30-fold compared to non-transgenic controls) but reduced susceptibility to the necrotrophic fungus Botrytis cinerea. Necrosis following SA-treatment or inoculation with avirulent Pst was reduced and delayed in P6-transgenics. NPR1 an important regulator of SA/JA crosstalk, was more highly expressed in the presence of P6 and introduction of the P6 transgene into a transgenic line expressing an NPR1:GFP fusion resulted in greatly increased fluorescence in nuclei even in the absence of SA. Thus in the presence of P6 an inactive form of NPR1 is mislocalized in the nucleus even in uninduced plants. These results demonstrate that P6 is a new type of pathogenicity effector protein that enhances susceptibility to biotrophic pathogens by suppressing SA- but enhancing JA-signaling responses.

Intervention of Phytohormone Pathways by Pathogen Effectors

“…Victorin, a toxin produced by the necrotrophic fungus C. victoriae (the causative agent of Victoria blight disease) indirectly targets NPR1 by binding to TRX-h5, which is required for the nuclear localization of NPR1 (Tada et al, 2008). The 2b protein of CMV targets NPR1 to exploit SA-JA antagonism, while the CMV P6 protein interferes with the nuclear localization of NPR1 to suppress SA and activate JA signaling (Lewsey et al, 2010;Love et al, 2012). Syringolin A, a toxin and proteasome inhibitor produced by the bacterial pathogen P. syringae pv syringae, has been proposed to inhibit the degradation of NPR1 (Schellenberg et al, 2010), which is required for the activation of SA-mediated defenses (Spoel et al, 2009).…”

“…P6 is thought to act by altering the expression and localization of NPR1 ( Figure 3) as P6 expressing plants accumulate high levels of NPR1 in the nucleus even in unelicited plants. Thus, it was proposed that an inactive form of NPR1 disrupts NPR1-mediated SA defense gene transcription in the presence of a pathogen that activates SA signaling (Love et al, 2012).…”

Plant Immune Responses Against Viruses: How Does a Virus Cause Disease?

“…The BSCTV-encoded C2 protein is a silencing suppressor protein that also usurps host UPS processes to promote susceptibility (Zhang et al, 2011). The CaMV-encoded P6 protein suppresses SA-mediated defenses, as well as RNA silencing defenses (Love et al, 2007(Love et al, , 2012. Oftentimes, these viral proteins are recognized by R genes to trigger immune responses.…”

Modulation of host plant immunity by Tobamovirus proteins

“…However, the authors could not detect enhanced susceptibility to ORMV in npr1 mutants compared with wild-type plants (Huang et al, 2005). More recent findings demonstrated that Cauliflower mosaic virus (CaMV) P6 alters the expression and sub-cellular localization of NPR1 and inhibits the SAdependent defence responses (Love et al, 2012). Moreover, another recent report showed that SA-induced RDR1 expression is dependent on NPR1 (Hunter et al, 2013).…”