The second face of a known player: Arabidopsis silencing suppressor AtXRN4 acts organ‐specifically

Abstract

Summary Plant viruses exploit the symplastic transport pathway provided by plasmodesmata by encoding for specialized movement proteins, which interact with host factors to enable viral intracellular and intercellular spread. Stable expression of the Potato leaf roll virus movement protein MP17 in Arabidopsis results in a carbohydrate export block and stunted growth. To identify host factors essential for viral infection, we screened a progeny population of EMS (ethyl methanesulfonate)‐mutagenized Arabidopsis expressing a MP17:GFP fusion for suppressor mutants with restored wild type‐like phenotype. Two suppressor mutants showed decreased susceptibility against Turnip mosaic virus and post‐transcriptional silencing of MP17:GFP RNA in source leaves. Map based cloning identified in both lines mutations in XRN4 (Exoribonuclease 4), which was previously described as a suppressor of transgene silencing in source leaves. Importantly, silencing of MP17:GFP was not present in cotyledons and roots of the two suppressor mutants, which was confirmed in a third xrn4 T‐DNA knock out line. Subsequent analysis of MP17:GFP transcript stability in xrn2 and xrn3 mutants indicated an essential role of AtXRN2 for silencing suppression in roots/cotyledons while AtXRN3 appears to act similar to AtXRN4 in source leaves, only. Overall, these findings point towards an organ‐specific regulation of gene silencing in Arabidopsis.

The Arabidopsis DCP2 gene is required for proper mRNA turnover and prevents transgene silencing in Arabidopsis

“…In order to identify host factors essential for viral infection, seeds of Col‐16 were EMS‐mutagenized and M 2 individuals were phenotyped for reduction or loss of MP17:GFP fluorescence (see also File S1). A similar approach lead to identification of EXORIBONUCLEASE 4 (XRN4) (Vogel et al. , 2011), which was previously described as a suppressor of gene silencing in Arabidopsis (Gazzani et al.…”

“…As it has been reported that uncapped mRNA accumulates in xrn4 mutants (Gazzani et al. , 2004), we also tested the previously described 12‐1‐1/xrn4/MP17:GFP mutant (Vogel et al. , 2011).…”

Evidence that XRN4, an Arabidopsis homolog of exoribonuclease XRN1, preferentially impacts transcripts with certain sequences or in particular functional categories

“…One of the phenotypes of the xrn4 mutants is higher levels of RNA silencing (Gazzani et al 2004). Indeed, xrn4 mutants in Arabidopsis are less susceptible to infection by cauliflower mosaic virus and turnip mosaic virus due to gene silencing of viral genes (Gy et al 2007;Vogel et al 2010). One possibility is that the down set is decreased in xrn4 due to the production of secondary siRNA generated from their uncapped transcripts and subsequent siRNA-mediated decay.…”

The mRNA decay factor PAT 1 functions in a pathway including MAP kinase 4 and immune receptor SUMM 2

“…In plants, the decapping machinery includes the decapping enzyme DCP2 and its activators DCP1, DCP5, VARICOSE (VCS) (Deyholos et al , ; Xu et al , ; Goeres et al , ; Brodersen et al , ; Xu & Chua, , ; Motomura et al , ) as well as the exoribonuclease XRN4 (Olmedo et al , ; Potuschak et al , ; Gregory et al , ; Rymarquis et al , ; Vogel et al , ). It is thought that DCP1, DHH1 and DCP5 form mRNPs for translational repression of target mRNA, which are then subject to decapping by recruitment of DCP2 and VCS and digestion by XRN4 (Xu & Chua, , ).…”