## 2 Results
Others such as zeatin (Figure 4A), a cytokin known to encourage lateral growth of buds and salicylic acid--SA (Figure 4F), a well-documented plant defense signaling hormone, surprisingly, were constitutively high in the susceptible CM but constitutively low in the resistant LA3952 under all treatments, suggesting that these compounds might not have played a key role in the defense against the TPP-Lso complex at 48 h after inoculation. The dramatic absence of indole-3-acetic acid, IAA, in the resistant LA3952 (Figure 4C) with no observable breach in growth was, particularly puzzling since IAA is a well-reported auxin, which is important in plant growth functions. Other plant hormones such as gibberellic acid, GA (Figure 4B) and jasmonic acid, JA (Figure 4H) showed constitutively higher levels in the resistant LA3952 than in the susceptible CM, suggesting a putative role in defensive signaling. However, the presence of Lso in TPP may inversely regulate the accumulation of these two plant hormones. For example, GA abundance in the LA_TPP-Lso(+) plants were significantly lower than those of the LA_Control, LA_Lso(-) and in CM_TPP-Lso(+). Conversely, JA levels were substantially lower in LA_TPP-Lso(-) than in the LA_control and in LA_Lso(+). Together, the data suggest that the JA is slightly suppressed by psyllid activity in the absence of Lso, while GA may be suppressed in the presence of Lso in the resistant LA genotype. Therefore, the presence or absence of psyllids and Lso may indirectly regulate defensive signaling against TPP by influencing plant hormone accumulation. Likewise, it has been previously reported that TPP mortality in resistant LA plants is reduced in LA_TPP-Lso(-) infested plants [5]. However, further analysis needs to be performed to determine if the observed changes in JA and GA changes are associated to lower mortality in the LA resistance plants. Only a Handful of Volatile Metabolites of the Fatty Acid Derivatives, Monoterpene, Norisoprenoids, Phenylpropanol-Derivatives and Sesquiterpenes Show Significantly Altered Abundance Associated with Resistance to TPP  
A total of 43 volatile metabolites (Table 1) were identified and grouped into five main chemical classes namely monoterpene, sesquiterpene, fatty acids-derived, norisoprenoids, and phenylpropanoids (Table 1, Figure 5A-E). At the class level, there were only two groups, fatty acid (FA)-derived and phenylpropanol (PA)-derived, which showed significant within-class mean differences between genotypes and between treatments. The FA-derived group showed the smallest mean values in the CM_TPP-Lso(+) compared to the CM_Control and CM_TPP-Lso(-), while in the resistant LA3952, the FA-derived group had the largest mean abundance in the LA_TPP-Lso(+) among the treatments (Figure 5C), suggesting that the genes regulating some species of VOCs derived from FA are disproportionately highly induced in the presence of the pathogen Lso. On the other hand, the mean levels of phenylpropanol (PA)-derived species were significantly larger in both the LA_TPP-Lso(-) and LA_TPP-Lso(+) plants than in the LA_Control; levels in the LA_Control showed no statistical difference compared to those of all the treatments in CM plants, suggesting that some of the PA-derived species of VOCs are disproportionately down-regulated in the presence of the TPP feeding. In fact, among the eight FA-derived VOC species, the mean differences noted above were determined mainly by (E)-2-hexenal and hexanal. (E)-2-hexenal was the most abundant, between a low of 230.63 \(\pm\) 22.94 (CM-Control), 223.99 \(\pm\) 42.87 in CM_TPP-Lso(-), 141.97 \(\pm\) 17.01 in CM_TPP-Lso(+), and in the resistant LA3952 at 492.11 \(\pm\) 51.39 (LA_TPP-Lso(+) compared to the 309.03 \(\pm\) 43.91 (\(\sim\)40% reduction) in LA_TPP-Lso(-) and the 257.62 \(\pm\) 31.03 (\(\sim\)48% lower) in the LA-Control. In the same group, hexanal was the only other FA-derived metabolite that showed a similar trend though at a much-reduced abundance (Table 1).