Spermine signaling in defense reaction against avirulent viral pathogen in Arabidopsis thaliana

We have proposed that the polyamine spermine functions as a signaling molecule to evoke defense reactions/cell death in N-gene bearing Nicotiana tabacum infected with tobacco mosaic virus (TMV). In this work we examined whether this spermine-based signal transduction pathway is present in other plants such as Arabidopsis thaliana. For this purpose, we used the experimental system of cucumber mosaic virus-yellow strain (CMV-Y) and A. thaliana ecotype C24 carrying the RCY-1 resistance gene, in which a hypersensitive response is triggered. Almost all spermine-responsive genes in Arabidopsis were similarly regulated as in the TMV-Nicotiana tabacum system. In addition to the formerly identified spermine-signal component genes such as alternative oxidase- and mitogen-activated protein kinase 3 (WIPK orthologue)-genes, AtbZIP60, encoding an endoplasmic reticulum (ER)-localized transcription factor, was identified as a spermine-responsive gene, indicating that the ER may be involved in a spermine-signaling pathway. Treatment with polyamine oxidase inhibitors prior to CMV inoculation suppressed the expression of the genes mentioned above and compromised the host defense against CMV. Furthermore, exogenously added spermine suppressed the multiplication of CMV in Arabidopsis as did spermidine but less efficiently, while putrescine did not show such effects. Collectively, we conclude that the spermine-signaling pathway plays an important defensive role in A. thaliana as well as in N. tabacum when those plants are attacked by incompatible viral pathogens.