Monoterpenes Support Systemic Acquired Resistance within and between Plants

Abstract

This study investigates the role of volatile organic compounds in systemic acquired resistance (SAR), a salicylic acid (SA)-associated, broad-spectrum immune response in systemic, healthy tissues of locally infected plants. Gas chromatography coupled to mass spectrometry analyses of SAR-related emissions of wild-type and non-SAR-signal-producing mutant plants associated SAR with monoterpene emissions. Headspace exposure of <i>Arabidopsis thaliana</i> to a mixture of the bicyclic monoterpenes α-pinene and β-pinene induced defense, accumulation of reactive oxygen species, and expression of SA- and SAR-related genes, including the SAR regulatory <i>AZELAIC ACID INDUCED1</i> (<i>AZI1</i>) gene and three of its paralogs. Pinene-induced resistance was dependent on SA biosynthesis and signaling and on <i>AZI1</i> Arabidopsis <i>geranylgeranyl reductase1</i> mutants with reduced monoterpene biosynthesis were SAR-defective but mounted normal local resistance and methyl salicylate-induced defense responses, suggesting that monoterpenes act in parallel with SA The volatile emissions from SAR signal-emitting plants induced defense in neighboring plants, and this was associated with the presence of α-pinene, β-pinene, and camphene in the emissions of the "sender" plants. Our data suggest that monoterpenes, particularly pinenes, promote SAR, acting through ROS and <i>AZI1</i>, and likely function as infochemicals in plant-to-plant signaling, thus allowing defense signal propagation between neighboring plants.