Evidence for Volatile Memory in Plants: Boosting Defence Priming through the Recurrent Application of Plant Volatiles

Plant defence responses to various biotic stresses via systemic acquired resistance (SAR) are induced by avirulent pathogens and chemical compounds, including certain plant hormones in volatile form, such as methyl salicylate and methyl jasmonate. SAR refers to the observation that, when a local part of a plant is exposed to elicitors, the entire plant exhibits a resistance response. In the natural environment, plants are continuously exposed to avirulent pathogens that induce SAR and volatile emissions affecting neighbouring plants as well as the plant itself. However, the underlying mechanism has not been intensively studied. In this study, we evaluated whether plants “memorise” the previous activation of plant immunity when exposed repeatedly to plant defensive volatiles such as methyl salicylate and methyl jasmonate. We hypothesised that stronger SAR responses would occur in plants treated with repeated applications of the volatile plant defence compound MeSA than in those exposed to a single or no treatment. Nicotiana benthamiana seedlings subjected to repeated applications of MeSA exhibited greater protection against Pseudomonas syringae pv. tabaci and Pectobacterium carotovorum subsp. carotovorum than the control. The increase in SAR capacity in response to repeated MeSA treatment was confirmed by analysing the defence priming of the expression of N. benthamiana Pathogenesis-Related 1a (NbPR1a) and NbPR2 by quantitative reverse-transcription PCR compared with the control. We propose the concept of plant memory of plant defence volatiles and suggest that SAR is strengthened by the repeated perception of volatile compounds in plants.