Volatile organic compounds and host-plant specialization in European corn borer E and Z pheromone races

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Maize headspace VOC

From maize headspace VOC collections, 21 components were detected and identified based on comparing the RI and mass spectra to authentic samples or appropriate data bases (Table 1a). Nineteen components out of 21 had previously been identified. Two VOCs were newly identified as maize VOCs: p-cymene, and a compound tentatively identified as selina-3,7 (11) diene (SQT).
The maize scent was found to be a mixture of three GLVs, six MTs, two homoterpenes (HTs), and 12 SQTs (Table 1a). The amounts of each component varied considerably throughout the 24-h cycle. The relative ratios of MT and SQT changed from day to night (Fig. 2). The peak SQT emission occurred during the day. In contrast, the peak MT emission was observed at night and dawn. HT was always present in low quantities, and the quantity did not change substantially over time. The diel GLV composition was characterized by the absence of Z3-6:Ac during the day and at dawn, but the relative amount of Z3-6:OH increased at dusk.
MeSA (an induced GLV) and !-copaene, were the two main compounds in maize headspace collections; they accounted for half of all the VOCs detected (Table 1a). The relative amounts of MeSA and !-copaene varied over time. The diel variations in these two VOCs were clearly separate at dusk and dawn. The ratio of MeSA to !-copaene in the headspace varied from 0.39 at dusk to 2.32 at dawn. The ratio between day and night was less impressive; it ranged from 1 during the day to 0.68 at night (Fig. 3). During the day, the emission rates of !-copaene and MeSA were nearly equal. At dusk, however, individual maize plants emitted about 2.5 times more !-copaene than MeSA. At night, they tended to be emitted at similar levels At dawn, the emission rates were the reverse of those at dusk, and maize plants emitted about 2.3 times more MeSA than !-copaene.
The three PCs calculated from the measured amounts of VOCs to compare time periods explained 35, 21, and 14 % of the variance, respectively. A MANOVA performed on the three PC values revealed significant differences in the ratios of maize VOCs among the four time periods (Wilk’s lambda = 0.071, p = 0.0015). When the three PC’s were analysed separately, only the scores of PC2 differed significantly between time periods (two-way ANOVA, with date as an additional factor: F 3,11 = 15.7, p = 0.0002) and captured the time-related variance of the VOC ratio (Fig. 4). Pairwise comparisons of PC2 scores among the four periods showed no significant differences between the maize VOCs ratios for day and dusk, or night and dawn. All other comparisons showed a significant difference (Tukey test: p<0.02).
Separate analyses were performed on the headspace VOC data for different classes of compounds. We found that the SQT profiles differed among time periods (Wilk’s lambda = 0.060, p = 0.006). Again, the profiles for night and dawn, and for day and dusk were similar; but pairwise comparisons among other time points attained significance. In contrast, no among-period differences could be shown for the other compound classes (Wilk’s lambda = 0.27, p = 0.15); i.e., GLV, HT, MT.
The among-period differences for individual VOCs was analysed with the Kruskal-Wallis test (Table 1a). In the individual maize static headspace collections, the diel relative amounts of three VOCs out of 21 changed significantly (Z3-6:Ac [df=3, N=16, Z=8.45, p=0.038], trans-!-bergamotene [df=3, N=16, Z=10.25, p=0.017], and !-cadinene [df=3, N=16, Z=12.91, p=0.005]). Pairwise comparisons for the GLV, Z3-6:Ac, showed no significant changes in the relative ratio over time (p0.05). However, Z3-6:Ac was detected only at dusk and night in maize VOC collections. Trans-!-bergamotene and !-cadinene were not detected at night or dawn; they were present only during the photophase, with a significantly higher amount at dusk (p=0.038, Z=8; p=0.013, Z= 8.500 respectively).

Maize field atmosphere VOCs

In the maize field atmosphere, a total of 13 VOCs were detected and identified (Table 1b). The VOCs profile was dominated by MeSA and a complex of p-cymene with limonene. There was also a constant low level of dimethyl nonatriene (DMNT). The ratio of MeSA to the complex of p-cymene-limonene did not change with diel periods; !-pinene, 3-carene, linalool, !-copaene, « -farnesene, and trans-nerolidol were detected in random amounts in the atmosphere without a clear diel pattern.
None of the 13 VOCs detected in the maize field significantly changed in amount over time. Furthermore, when the 13 VOCs were grouped into chemical classes, no significant diel variation was observed for the relative amounts (Kruskal Wallis test, p0.05 for all cases).
There were 12 VOCs in common between samples from the maize field atmosphere and the maize headspace. Out of the 13 VOCs detected in the maize field atmosphere, the MT alcohol, linalool, although repeatedly detected in the atmosphere, was the only VOC that was not found in the maize headspace. The VOC profiles from maize headspaces were considerably richer than the profiles from the field air samples. The headspace maize VOC blend mainly comprised SQT; in contrast, the maize field atmosphere mainly comprised MT. In general, the diel pattern changes in the VOC composition for individual maize plants did not match the patterns observed in the maize field atmosphere. Only MeSA (GLV) had the same diel pattern of emission in the field and the headspace collections; the emission peaked while it was light over the 24-h cycle, and it decreased at dusk and night.

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Discussion

This work, performed in the field on healthy maize plants, has generated a new description of volatiles released by maize. This study was the first to provide insight on what a flying insect might encounter in a maize field. The diel variations observed in this study supported our hypothesis that pests, like ECB moths, are likely to be tuned to maize plant VOCs, and probably use specific volatile cues released by plants at the beginning of the night to reach an appropriate oviposition site.
At the plant level, the relevant changes were the Z3-6:Ac levels, which was released only during the dark period, and the increase in limonene and p-cymene, which were newly identified in the maize headspace. The SQTs were absent during the dark period, and only detected during the day, in contrast to the MT emission. Other studies showed that the diel variations in MT and SQT emissions from other plants were influenced by abiotic factors and photosynthesis (Duhl et al., 2008; Fall et al., 1999; Fuentes et al., 2000; Grote and Niinemets, 2008; Ibrahim et al., 2010; Sharkey et al., 2008). Interestingly, MeSA was identified as one of two main components in the headspace of healthy maize plants. The second main component, !-copaene, showed opposite fluctuations in the diel pattern. Moreover, our results showed that MeSA was not strictly an herbivore-induced component; instead, it appeared to be a constituent of the maize headspace. This result was reinforced with the results for the maize field atmosphere, where MeSA was identified in large amounts. MeSA can be considered a key compound for maize plant recognition by ECB, because it can elicit strong responses from female antennae (Bengtsson et al., 2006), and it acts on female attraction behaviour (Solé et al., 2010). The most frequently detected VOCs in maize headspace analyses were MeSA, !-pinene, p-cymene with (S)-limonene, and !-copaene. The volatile compound pattern described here differed from those of water-stressed plants (Solé et al., 2010) and herbivore-damaged plants (Turlings et al., 1995). The global changes in VOCs over time may be sensed as different signals to host-seeking moths.
Analyses of maize field atmosphere VOCs showed that the natural chemical environment of host-seeking insects is poor in volatiles and comprises mainly MeSA and MT. SQT were detected rarely and without any apparent pattern. The most consistently detected VOCs in the maize field atmosphere were MeSA, !-linalool, and « -myrcene; these were previously reported as VOCs from maize, but after herbivore damage, (D’Alessandro and Turlings, 2005; Ozawa et al., 2008; Turlings et al., 1998). Moreover, limonene, !-pinene, 3-carene were also frequently detected, but little is known about their biological activity on ECBs.
Linalool, a previously identified compound from stressed or damaged maize plants, was not detected in our VOCs collected from healthy plants in the field. Thus, this emission may be part of a chemical signal for plant stress. We speculated that the linalool identified in the field atmosphere was derived from the woods at the edges of the fields. Large amounts of this compound are generally released in deciduous forests (Ciccioli et al., 1999; Owen et al., 2001).
The apparent discrepancy between VOC profiles of maize headspaces and field atmosphere was primarily due to the different patterns of MT and SQT. In the atmosphere VOC collections, SQT were rarely detected. This distortion in the SQT:MT ratios between the atmosphere and plants was similar to that previously described (Bouvier-Brown et al., 2009; Ciccioli et al., 1999). The SQTs are highly reactive to atmospheric O3, and they are often completely destroyed in the atmosphere before they can be detected (Atkinson, 1990; Bonn and Moortgat, 2003). The average lifetime of an MT molecule in the air is about 1 h; that for SQT is only 2-4 min (Kesselmeier and Staudt, 1999). The differences might also be explained by the experimental design. For the headspace VOCs collection, we created a limited space, where the volatiles were concentrated; in contrast, for the atmospheric VOCs collection, the SPME fibres were exposed to field air with extremely low concentrations of VOCs. Indeed, similar results were obtained when maize field VOCs were compared to VOCs in a deciduous forest atmosphere (Leppik & Frérot, submitted).

Table of contents :

Chapter 1: Study subjects
European corn borer
Chemical communication in European corn borer
Taxonomy of European corn borer pherotypes
European corn borer in agriculture
European corn borer host plants
Maize
Mugwort
Hop
Chapter 2: Volatile organic compounds
Volatile organic compounds in plants
Green leaf volatiles
Terpenes
Monoterpenes
Sesquiterpenes
Homoterpenes
Inducible volatile organic compounds
Olfactory environment
Paper I Chemical landscape of maize field for host-seeking moth
Paper II Diel patterns of volatile organic compounds released by maize plants: The chemical environment of the Ostrinia nubilalis moth
Chapter 3: Host plant specialization
Host plant choice and recognition
European corn borer host plants
Paper III Volatile organic compounds and host-plant specialization in European corn borer E and Z pheromone races
Chapter 4: Assortative mating
Male scent organs
Male pheromones
Courtship behaviour
European corn borer courtship behaviour
Speciation
Sympatric speciation

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