Herbivore damage to specific plant organs frequently results in systemic defense responses, such as emission of volatiles from other parts of the plant  or the production of defensive proteins like protease inhibitors in adjacent organs . However, here we report that the induction of sesquiterpene volatiles from the maize inbred line B73 damaged by S. littoralis is almost completely restricted to the damaged leaf and occurs at only trace levels in adjacent leaves. Within the herbivore-damaged leaf, emission was highest at the site of herbivore damage, declining slowly towards the upper part of the leaf, but was barely detectable towards the base of the leaf (Figure 2A). These results plus measurements of biosynthetic gene expression and jasmonate signaling compounds within the leaf have interesting implications for the physiology, function and regulation of terpene emission in maize.
The limitation of volatile sesquiterpene emission to the actual leaves damaged by herbivores and not to undamaged adjacent leaves fits with previous studies . In general, when volatile emission after herbivory has been measured in a leaf-specific manner, many other plant species also show only localized release of sesquiterpenes and/or monoterpenes that is largely confined to the damaged leaves. For example, the emission of herbivore-induced sesquiterpenes from holm oaks is mostly restricted to herbivore-infested leaves . The absence of a systemic response was also observed after wounding of transgenic Arabidopsis that contained an herbivore-inducible monoterpene synthase promoter-GUS fusion . In contrast, the study of maize roots attacked by the larvae of Diabrotica virgifera virgifera did show systemic induction  with (E)-ß-caryophyllene emission being detected at the site of attack and in adjacent roots.
The emission of sesquiterpene volatiles from the site of damage and towards the leaf tip, but not towards the leaf base, will create a volatile gradient from the tip of the leaf increasing towards the damage site. For herbivore enemies that start their searching at the tips of the long leaves, this might aid in host or prey finding. If herbivore-induced sesquiterpenes function in other ways as signals to other plants or other parts of the same plant , release from the tip may also be valuable to increase the range of signal transmission. On the other hand, if volatiles act in direct defense against herbivores , their release from the damage site and apical, but not basal portions of leaves is not readily understandable unless feeding herbivores tend to move apically as they feed. One might expect increased emission from the leaf base since the presence of meristematic tissue likely makes it more valuable than the tip. However, greater emission from the tip may be an unavoidable result of a damage signaling pathway that is constrained to be directed toward the leaf apex rather than the base.
The localization of sesquiterpene formation to the inner leaf tissues of maize may reflect a link to photosynthesis. Constitutive and herbivore-induced terpene emission in plants typically follows a diurnal rhythm with an emission maximum during the day and emission minimum during the night [17, 23, 25–27]. Moreover, a previous study showed that volatile release from leaves of herbivore-damaged maize seedlings is dependent on light. In experiments with an artificial light–dark cycle comprising three repetitions of three hours light and three hours dark, Gouinguene and Turlings  demonstrated that the plants only released herbivore-induced volatiles during the light phases. In the present study, the transcripts of tps10 and tps23 were not found in the epidermis, but only in underlying leaf tissues. Given the fact that much of this underlying tissue is photosynthetic, the dependence of volatile terpene biosynthesis on light suggests that volatile formation in leaves occurs in photosynthetic cells. A finer, cell-specific localization of tps transcripts and measurements of terpene biosynthetic rate after application of specific photosynthetic inhibitors could help to understand more about the link between light and terpene emission. Sesquiterpene formation occurring in maize roots  does not appear to have a direct link to photosynthesis.
The close correlation of the site of terpene synthase expression in maize and the site of volatile terpene emission suggests that terpene products are formed de novo in the emitting tissues and are not transported from the damage site to other parts of the leaf, such as the tip. Previous experiments with radioisotopes established definitively that in cotton herbivore-induced, systemic volatiles are synthesized at the site of release . To induce terpene synthase activity in distant parts of the leaf, a mobile chemical messenger must exist that moves from the site of damage to other parts of the plant. The first such systemic signaling compound, the oligopeptide systemin, was described in species of the Solanaceae . However, the pattern of systemic damage signaling in maize is fundamentally different than that in the Solanaceae  and other dicotyledonous plants as the signal is only directed from the wounding site in an apical direction, but cannot cross the midrib. This signal path is much like that of xylem movement which conducts water and nutrients from the roots to above-ground organs, suggesting that the mobile damage signal moves through the xylem. It is notable that several plant hormones like abscisic acid, cytokinins and strigolactones are transported by the flow of the xylem .
The plant hormone JA and related jasmonates are perhaps the best known components of signal transduction pathways which trigger defense responses after herbivory, including the emission of volatiles in many plant species [16, 33–35]. These compounds are known to act at a distance from the site of herbivore damage. Radio-labeled JA applied to leaves of Nicotiana sylvestris was transported to the roots where it is thought to increase nicotine biosynthesis in these organs . Similarly, methyl jasmonate can function as a mobile signal in N. tabacum where it is transported via the phloem and the xylem to undamaged systemic leaves . In the wound response of tomato, Wasternack and coworkers  concluded that JA, rather than the peptide systemin, acts as the mobile signal. The plant hormone JA could also be involved in mobile defense signaling in maize leaves, given that its accumulation is highest at the sites of highest volatile emission (Figure 4). A similar JA accumulation pattern has recently been reported by Engelberth and coworkers [39, 40]. In both studies, herbivory on the middle part of a maize leaf was mimicked by mechanical wounding and subsequent application of either crude caterpillar regurgitant  or the elicitor N-linolenoyl-glutamine . Both treatments resulted in high JA accumulation around the wounding site and in the leaf tip, but only trace accumulation in the leaf base. Furthermore, a transcript analysis of allene oxide synthase (AOS), an enzyme considered to catalyze the key step in JA biosynthesis , revealed that AOS expression was highest in the treated part of the leaf and the distal tissues . Since the expression of AOS and other JA biosynthesis genes is known to be inducible by jasmonates, one can speculate that these compounds could also function as long-distance signals in maize, being transported via the xylem from the wounding site to apical leaf tissues where they induce their own biosynthesis as well as the expression of terpene synthase genes. However, the presence of other signaling compounds that activate JA biosynthesis locally after transport cannot be excluded at this time, and physical processes like wound-induced changes in hydraulic pressure  or an electrical signal  could also be involved. Further studies with labeled jasmonates and the chemical analysis of maize xylem sap might help to understand the complete nature of the signals that mediates terpene biosynthesis in this species.