## 3 Discussion  
The main objective of the present study was to gain insight into the metabolite profiles of insect-susceptible and resistant tomato plants in response to the infestation with tomato-potato psyllids (TPP), carrying or not _Candidatus_ Liberibacter solanacearum (Lso). Tomato Phenolic Composition Is Differentially Regulated in Susceptible and Resistant Plants in Response to TPP-Lso Infestation  
The phenolic pathways, including those involving the nine compounds in the present study, have been previously implicated in the plant defensive mechanism against TPP carrying the Lso in Solanaceae crops [16; 20]. However, most of the phenolic compounds in our study did not profile differentially enough to unambiguously distinguish the expected resistance responses between the LA3952 and the insect susceptible CastleMart (CM). On this basis, the compounds could broadly be categorized into three. In the first category are those that did not show significant differences between treatments [TPP-Lso(-), TPP-Lso(+) and the Mock as control], and between the two genotypes (insect-susceptible CM and resistant LA3952). Compounds such as protocatechuic acid, phthalic acid, and ferulic acid (Figure 3B,C,G, respectively) were not significantly different between the two genotypes (CM and LA3952) or between the TPP-Lso treatments, although they have previously been implicated in defensive signaling in other plant-psyllid interaction systems. For example, a study on the effects of the carrot psyllid, _Trioza apicalis_, on phenolics reported that the change in ferulic acid levels could be affected by experimental conditions, such as insect density and plant growth stage [21]. They did not detect the presence of any phytoplasma associated with Lso in laboratory psyllid colonies or in the carrot seedling leaves. However, one of their earlier studies had established an association between _T. apicalis_ and Lso from both field and laboratory raised samples [25]. The second category includes compounds that showed significant treatment effects, and thus suggests the influence of TPP feeding and Lso on the phenolic profiles, but without significant differences that we expected between LA3952 and the susceptible CM. For instance, gallic acid, chlorogenic acid, rutin, and naringenin Figure 3A,E,H,I, respectively) fell in this category. Gallic acid, which showed increased abundance in the insect-inoculated plants of both genotypes, has previously been reported to be involved in the plant defense system, due to its toxicity to the development and growth of melon fruit fly (_Bactrocera cucurbite_) [26]. Similarly, chlorogenic acid has been shown to inhibit the growth of a wide spectrum of bacteria [27] and elevated levels in leaves of _Solanum lycopersicum_ offer resistance against the bacterial pathogen _Pseudomonas syringae_ pathovar Tomato [28] and against _Spodoptera litura_ has previously pointed to its anti-herbivore activity [29]. In our experiment, chlorogenic acid showed significantly elevated levels in the CM_TPP-Lso(+) and inoculated LA treatments regardless of insect resistance or susceptibility. In fact, compounds like naringenin (Figure 3I) showed very significantly elevated levels in the TPP-Lso(\(-\)) plants compared to the control plants in both the LA3952 and CM, suggesting their involvement in induced basal resistance response. The third category shows clear genotypic differences in the phenolic profiles that suggest either treatment-induced or constitutive resistance difference between LA3952, and the susceptible CM. Variation of metabolite levels with respect to genotype and treatment were observed among studied nine phenolic metabolites, and p-coumaric acid may confer the resistance in RIL LA3952 by associated plant defense mechanism [30; 31; 32]. In this group, only two compounds, 4-hydroxybenzoic acid (Figure 3C), and p-coumaric acid (Figure 3F), seem to significantly demarcate the difference between the resistance of the two genotypes.