## 3 Discussion
It has been previously reported that p-coumaric acid may contribute to plant resistance in response to the pathogenic fungus Fusarium [33], and the facultative anaerble plant pathogenic bacterium _Dickeya dadantii_[13]. Indeed, we observed dramatically higher levels of p-coumaric acid in the resistant LA3952 plants compared to susceptible plants. In addition, the undetectable level of this metabolite in the CM_TPP-Lso(+) plants (Figure 3F) suggests that it may be comparatively severely down-regulated in response to Lso (+) in susceptible plants, leading us to speculate that p-coumaric acid is constitutively enhanced in resistant plants, and may be a good metabolite biomarker for resistance against both the insect TPP and its transmitted pathogen, the Lso. Instructively, the dissimilar but constitutively significant elevation of 4-hydroxybenzoic acid (lower abundance in the resistant) and p-coumaric acid suggest that the genes regulating these two compounds may be antagonistic, with 4-hydroxybenzoic acid representing susceptibility factors and p-coumaric acid, representing the constitutive presence of resistance factors. Additionally, since naringenin (Figure 3I) was significantly elevated only in the TPP-Lso(+)-inoculated CM and LA3952, while the levels were similarly low in both the TPP-Lso(\(-\)) and the Control tests for both genotypes, we suggest that naringenin is associated with induced resistance specific to TPP insects carrying the pathogen bacteria Lso in both genotypes and in conjunction with the p-coumaric acid, may be important in identifying and disambiguation of induced resistance to the pathogen Lso. Differential Constitutive, and in Some Cases Antagonistic Alternating Roles of Plant Hormones May be Implicated in the Unexpected Contrasting Abundances  
Certain hormones such as cytokinin, gibberellic acid, auxin, and abscisic acid have been known to play important regulatory roles in plant development, growth [34], and plant immune responses [35]. We determined that the differential abundance of some of these plant hormones may be indicative of their contrasting responses and roles in plants infested with the psyllid _Bactericera cockrelli_ and psyllids carrying the pathogenic bacterium _Candidatus_ liberbicacter solanacearum (Lso). Gibberellic acid (GA), for instance, has been reported to have a negative role in rice basal disease resistance to the bacterial blight disease [36], and we speculate this might explain the down-regulation of GA in the LA_TPP-Lso(+) as opposed to the LA_Control and the LA_TPP-Lso(\(-\)). In fact, there has been evidence to suggest that some bacterial and fungal pathogens produce GA, which may, in some cases, implicate this metabolite in a virulent activity [36]. In contrast, the GA levels were significantly up-regulated in the susceptible CM plants infested with TPP carrying Lso compared to the control and TPP-Lso (\(-\)) plants, suggesting that in this experiment, GA's role in resistance may be constitutively insect-specific in the resistant LA3952. Three growth and defense-associated hormones--zeatin (Figure 4A), indole-3-acetic acid (Figure 4C), and salicylic acid (Figure 4F), surprisingly remained constitutively low in the resistant LA3952 independent of the TPP-Lso infestation. Some hormones coexist in several isoforms with differential plant defense activity. For instance, zeatin, a cytokinin known to encourage lateral growth of buds [37]), has been shown to have an isomerically differential role in plant defense against microbial pathogens, where tobacco plants treated with exogenous trans-zeatin were significantly protected from damage by the pathogenic bacterium _Pseudomonas syringae_, while those treated with cis-zeatin were not [38]. We can speculate that such coexistence of isoforms in different concentrations may have led to the constitutively low abundances under certain treatments in the present study.