## 4 Discussion
Here, specific VOCs were emitted, and characterized in the context of the interactions of PC16 and WPP14 with the two hosts. The results provided additional support for the specificity of the interaction, and indeed every plant with every pathogen produced a different array of VOCs. Calla lily leaves presented higher levels of VOC and a more complex VOC pattern in response to infiltration with PC16; whereas WPP14 triggered a high and complex response in cabbage leaves. Neither the opposite combinations nor the mock control led to the same plant response. Some of the volatiles produced by each compatible interface and analyzed by GC-MS and a library search, were recognized to be associated with typical defense responses, i.e., calls lily-PC16 interaction and the cabbage-WPP14 interaction. Some of the identified compounds, namely methyl- and ethyl- acetate and isopentyl alcohol, which were produced by calls lily in response to PC16, were previously suggested to be produced by the oxidation of monoterenes by pathogens in a compatible plant-pathogen interaction [37]. Terpenes have been reported to play a key role in protecting plants from bacterial pathogens as in the case of rice against bacterial rice blight [38]. Isopentyl alcohol, dimethyl disulfide and several unidentified compounds were shown to be representative of cabbage-WPP14 unique interaction. Ethanol, ethyl acetate, isopentyl alcohol, dimethyl disulfide and octane were previously reported to be involved in the interaction of _Pectobacterium_ spp. with different hosts [39, 40]. To the best of our knowledge, this is the first report to propose that different _Pectobacterium_ strains differentially modulate the VOCs production and emission by their hosts during compatible and non-compatible interactions. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are the most recognized defense pathways in plants and are associated with biotrophic and necrotic pathogens, respectively [41]. Defense mechanisms activated in response to _Pectobacterium_ infections are complex, as the pathogen deploys stealthy and brute-force modes of action depending on the prevailing environmental conditions [42, 43, 44]. Here, we assessed the expression of _lox2_, a typical ISR marker [45], that is also involved in the production of volatiles in relation to pathogen attacks [46]; the SARmarker _PR-1_[47]; _pal_, an enzyme involved in biosynthesis of salicylic acid and other phenolics [48]; and _ast_, which modifies the primary amino acid metabolism as part of defense responses in plants [49]. Significantly higher expression of _pal_ and _lox2_ was observed in calls lily when it was challenged with both strains. It is suggested that specificity may be expressed as the intensity of the gene expression of calls lily in response to PC16 or cabbage's expression of _pal_ and _lox2_ upon WPP14 infection. Synthesis of phenolics by these enzymes was previously demonstrated as part of ISR activation in calls lily and potato against pectobacteria [25; 50]. These enzymes were also shown to be involved in the increased production of VOCs [46], and the expression of the relevant genes was shown to increase following infection of tomato and _A. thaliana_ with _P. carootovum_[51; 52]. _PR-1_ expression increased in both hosts upon infection with both strains, similar to the infections caused by heterotrophic pectobacteria or the necrotrophic fungus _Alternaria solani_ in _A. thaliana_ and tomato, respectively [51; 53]. Since _PR-1_ is related to SAR and programmed cell death, this response may in some cases enhance the infection. Another defense related gene, _ast_, was also shown to affect infection (lesion development) in overexpressing lines of _Arabidopsis_ upon infection with _Botrytis cinerea_[49]. Here, upon infection of the host with its compatible strain a higher expression of _ast_ was correlated with stronger development of disease symptom in both hosts, suggesting that the higher levels of expression of this gene leads to elevated disease.