## 4 Discussion
Secondary metabolites including phenolics are part of an array of multifaceted defense mechanisms that plants activate in response to microbial pathogens and to insect herbivores [41; 54]. Levels of either total or specific phenolics have been shown to increase following defense elicitation and/or necrotrophic pathogen infections, and are considered a part of plants' defenses against _Pectobacterium_[25; 55; 56]. Here, the effects of phenolics containing extracts were tested on virulence factors and expression of virulence genes in both _Pectobacterium_ strains. Exoenzyme activity (Pel, Peh and Prt) in both strains was assessed in the presence of non-inhibitory concentration of host phenolic extracts. Again, the unique nature of the interaction of each host with the different pathogen was clearly presented. The extracts inhibited exoenzyme activity in both strains in a reciprocal manner, with calls lily extract inhibiting the activity of Pel, Peh and Prt in WPP14 and cabbage extract reducing the activity of the same enzymes in PC16. Biofilm formation another important trait for successful colonization of hosts by plant-pathogenic bacteria [57; 58], also increased in a differential manner with each strain responding to the extract from its favored host. Increased biofilm formation was seen in _Pseudomonas aeruginosa_ and _Agrobacteriumium tumefaciens_ in the presence of non-lethal concentrations of phenolic compounds [59]. Differential biofilm formation of the two _Pectobacterium_ strains may be another factor affecting host specificity. Finally, the effects of the host extracts on the expression of bacterial virulence genes was evaluated. The genes _expl_ and _expR_ major components of the AI-1 quorum-sensing (QS) circuit in _Pectobacterium_[60], largely control virulence by regulating PCWDEs, biofilm formation and T3 SS [43; 61]. Differential expression of these genes was observed for both strains in response to the application of the extracts from their respective host. A similar pattern was observed for _pel_ gene in both strains. Similar results were observed in _Pseudomonas syringae_ pv. _phaseolicola_ NPS3121 and _Xanthomonas campstris_ pv. _campstris_ following exposure to extracts from their own hosts common bean (_Phaseolus vulgaris_) and cabbage, respectively [62; 63]. The expression of _pel_ was quite congruent with the differences in enzymatic activities of both strains. The varying expression of _Pel_ suggests that this enzyme is also regulated by other factors, as was suggested for the low-cell-density environment that exists mainly at earlier stages of infection [64]. Other genes like _hrpL_ and _araF_ were significantly upregulated by cabbage phenolics in both strains. The calls lily extract affected the expression levels of these genes only in PC16. HrpL protein, a product of the _hrpL_ gene, is an alternative sigma factor that regulates the expression of structural components of T3 SS and other genes that have been associated with _Ervinia amylovora_ biofilm formation [57]. Finally, although PCWDEs, biofilm formation and the expression of virulence genes in _Pectobacterium_ spp. were shown not to depend on the growth medium, we also examined some of these traits in MM, since it was suggested to be a better mimic of the host environment [65]. The differential effect of the plant host extracts was conserved for both strains in MM. Overall, the results support host specialization by certain pectobacteria.