Systemic Potato virus X infection induces defence gene expression and accumulation of β-phenylethylamine-alkaloids in potato

A better understanding of defence responses elicited during compatible plant-virus interactions is a current goal in plant pathology. We analysed defence responses during infection of Solanum tuberosum L. cv. Desiree with Potato virus X (PVX) at the transcript and metabolite level. A mostly unchanged primary metabolism reflects the compatible nature of this plant-virus interaction. Salicylic acid biosynthesis and expression of several defence genes including PR-1 and glutathione-S-transferase, which are involved in ethylene and reactive oxygen species dependent signalling, were highly up-regulated in upper-uninoculated (systemic) leaves of PVX-infected potato plants compared with mock-inoculated controls. Moreover, the β-phenylethylamine-alkaloids tyramine, octopamine, dopamine and norepinephrine were highly induced upon infection. β-phenylethylamine-alkaloids can contribute to active plant defence responses by forming hydroxycinnamic acid amides (HCAA), which are thought to increase cell wall stability by extracellular peroxidative polymerisation. Expression of tyramine-hydroxycinnamoyl transferase (THT) and apoplastic peroxidase (POD) was highly induced upon PVX infection in systemic leaves, which suggests synthesis and extracellular polymerisation of HCAA. Since cell-wall-bound ion concentrations could contribute to this process, we measured cell-wall-bound and total ion concentrations in PVX-infected and mock-inoculated leaves. The observed metabolic and transcriptional changes might represent a systemic acquired resistance response against subsequent pathogen challenge.