Volatiles as inducers and suppressors of plant defense and immunity — origins, specificity, perception and signaling

Volatiles from attacked plants, microbes and herbivores can enhance plant defenses. However, the absence of volatiles rather than their presence has sometimes been associated with enhanced defense, suggesting that volatiles may also act as defense suppressors. Recent work provides a potential mechanistic explanation for these observations by showing that volatile cues can modulate different hormonal pathways, including jasmonate (JA), salicylic acid (SA) and auxin (IAA) signaling. Many of these pathways interact with each other through crosstalk. Thus, volatiles may suppress plant defenses through negative hormonal crosstalk. Hormonal crosstalk may also allow plants to integrate different volatile cues to respond specifically and appropriately to environmental change.