Maize seedlings emit sesquiterpenes during the day in response to insect herbivory. Parasitoids and predators use induced volatile blends to find their hosts or prey. To investigate the diurnal regulation of biosynthesis and emission of induced sesquiterpenes, we applied linolenoyl-L-glutamine (LG) to maize seedlings in the morning or evening using a cut-stem assay and tracked farnesene emission, in planta accumulation, as well as transcript levels of farnesyl pyrophosphate synthase 3 (ZmFPPS3) and terpene synthase10 (ZmTPS10) throughout the following day. Independent of time of day of LG treatment, maximum transcript levels of ZmFPPS3 and ZmTPS10 occurred within 3-4h after elicitor application. The similarity between the patterns of farnesene emission and in planta accumulation in light-exposed seedlings in both time courses suggested unobstructed emission in the light. After evening induction, farnesene biosynthesis increased dramatically during early morning hours. Contrary to light-exposed seedlings dark-kept seedlings retained the majority of the synthesized farnesene. Two treatments to reduce stomatal aperture, dark exposure at midday, and abscisic acid treatment before daybreak, resulted in significantly reduced amounts of emitted and significantly increased amounts of in planta accumulating farnesene. Our results suggest that stomata not only play an important role in gas exchange for primary metabolism but also for indirect plant defences.
The importance of volatile organic compounds in plant defense against insect herbivores and pathogens is widely recognized, but the mechanisms by which they are synthesized and released are not yet fully understood. Our detailed time course data on gene transcript levels, as well as emitted and in-planta accumulated sesquiterpene levels during light and dark periods, force the conclusion that, in maize, stomata guard cells play a critical role in regulating sesquiterpene emission. This suggests that reduction in the number and size of stomata, whether due to climate change or development of drought resistant varieties, may reduce amounts of emitted volatiles. As a result, these plants would be more vulnerable to insect herbivory, due to reduced capability to recruit natural enemies of the attacking herbivore.