Age-dependent variations of local and systemic defence responses in Arabidopsis leaves towards an avirulent strain of Pseudomonas syringae

Abstract

Despite the identification of several key mechanisms underlying disease resistance, comparatively little is known about the way developmental factors affect the capability of plants to defend themselves against microbial pathogens. Here, the influence of leaf age on inducible local and systemic defence responses in the incompatible interaction between Arabidopsis thaliana and Pseudomonas syringae pv. maculicola ( avrRPM1) is investigated. Comparison of the local defence behaviour of rosette leaves differing in age revealed that younger leaves generally had to invest in more pronounced inducible defences than older (non-senescent) leaves to achieve a similar degree of resistance. For instance, younger leaves exhibited a more distinctive oxidative burst and a stronger hypersensitive cell death response than older ones. The pathogen-induced expression of the defence related genes phenylalanine ammonia lyase ( PAL) and glutathione- S-transferase ( GST) proved to be higher and faster in younger leaves, respectively, whereas gene induction of PR-1 (pathogenesis-related protein 1) was largely independent of leaf age. Similarly, upon bacterial inoculation, the defence signal salicylic acid (SA) and the phytoalexin camalexin accumulated to higher amounts in younger than in older leaves. Despite these differences in inducible defences, bacterial growth as a measure of disease resistance proved to be similar in inoculated younger and older leaves. In tissue distant from the inoculation site, systemic acquired resistance (SAR) developed most effectively in younger leaves, and this response was associated with strong accumulation of SA and PR-1 transcripts. However, older leaves still exhibited SAR to a certain degree, and this resistance developed essentially without systemic SA or PR-1 accumulation. These results suggest that mechanisms independent of SA signalling in systemic leaves contribute to realise SAR.