WRKY8 transcription factor functions in the TMV-cg defense response by mediating both abscisic acid and ethylene signaling in Arabidopsis

WRKY transcription factors are key players in the plant immune response, but less is known about their involvement in antiviral defense than about their roles in defense against bacterial or fungi pathogens. Here, we report that Arabidopsis thaliana WRKY DNA-binding protein 8 (WRKY8) has a role in mediating the long-distance movement of crucifer-infecting tobacco mosaic virus (TMV-cg). The expression of WRKY8 was inhibited by TMV-cg infection, and mutation of WRKY8 accelerated the accumulation of TMV-cg in systemically infected leaves. Quantitative RT-PCR analysis showed that the expression of ABA insensitive 4 (ABI4) was reduced and the expression of 1-aminocyclopropane-1-carboxylic acid synthase 6 (ACS6) and ethylene response factor 104 (ERF104) was enhanced in the systemically infected leaves of wrky8. Immunoprecipitation assays demonstrated that WRKY8 could bind selectively to putative W-boxes of the ABI4, ACS6, and ERF104 promoters. Furthermore, TMV-cg infection enhanced WRKY8 binding to the ABI4 promoter but reduced the binding of WRKY8 to the ACS6 and ERF104 promoters, indicating that regulation of ABI4, ACS6, and ERF104 by WRKY8 is at least partially dependent on TMV-cg. Exogenous applications of abscisic acid (ABA) reduced the systemic accumulation of TMV-cg. Mutations in ABA deficient 1, ABA deficient 2, ABA deficient 3, or abi4 accelerated systemic TMV-cg accumulation. In contrast, exogenous application of aminocyclopropane-1-carboxylic acid enhanced the systemic accumulation of TMV-cg, but mutations in acs6, erf104, or an octuple acs mutant inhibited systemic TMV-cg accumulation. Our results demonstrate that WRKY8 is involved in the defense response against TMV-cg through the direct regulation of the expression of ABI4, ACS6, and ERF104 and may mediate the crosstalk between ABA and ethylene signaling during the TMV-cg-Arabidopsis interaction.