## 4 Discussion  
Comparison and Evaluation of Volatile Organic Compounds Secreted by the Two PGPR Pseudomonas Strains  
_Pseudomonas_ spp. are one of the most dominating groups in the rhizosphere for which both the promotion of plant growth and antagonistic properties towards pathogens have been documented. Interestingly, studies have shown that bacteria of the same species can exhibit different VOC profiles and have differential perturbation effects on the physiology of plants and microorganisms [15; 53]. These have been found to lead to enhanced plant growth and induce resistance and inhibit pathogen growth [26]. In this study, we first compared the profile of two strains of plant protective _Pseudomonas_ spp. in order to identify common/unique VOCs (Figure 3). The two strains had 28 VOCs in common, 34 VOCs specific to _Ps. fluorescens_ (N04) and 18 VOCs specific to _Ps. koreensis_ (N19) (Figure 3). The shared VOCs included methyl salicate (MeSA), dimethyl trisulfide, nonadecane and 11-hexadecen-1-ol, amongst others. Interestingly, _Ps. fluorescens_ (N04) had hexyl salicate among its unique VOCs, whereas _Ps. koreensis_ had isoamyl salicate as a unique VOC (Figure 3). SA and MeSA are known plant hormones that play different roles in plant growth and development [54]. Accordingly, the ability of these strains to secrete SA derivatives could be one of the mechanisms they employ to establish mutual relationships,  
Figure 4: Venn diagram of volatile organic compounds produced by _Pseudomonas_ strains (_Pseudomonas koreensis_ (N19) and _Pseudomonas fluorescens_ (N04)), vs. _Penaibacillus alvei_ (T22) and _Lysinibacillus__sphericus_ (T19). The diagram shows overlapping and distinct VOCs, indicated by the numbers in the intersections and ellipses, respectively. enhance plant growth and induce resistance. An interesting molecule worth mentioning is the dimethyl disulfide specific to _Ps. koreensis_ (N19), which has been commercialized as a soil fumig against soil-borne pathogens and nematodes due to its antimicrobial properties and its ability to prevent nematodes from feeding on roots [25]. In addition, the 'volatilome' of these strains (common or unique) (Figure 3) consisted of the following classes of compounds: alcohols, ketones, aldehydes, alkanes and sulfides. These classes of VOCs have been reported in various PGPR strains and form part of the complex communication in the rhizosphere that leads to growth stimulation, induced resistance or pathogen growth inhibition. For example, _Pseudomonas_ strains that are associated with potato roots produce VOCs with the capacity to greatly inhibit _Phytoophthora infestans_, and SPME-GC-MS has shown that these strains secrete similar and unique VOCs that are associated with such inhibition [55]. This report further demonstrated that these strains had varying effects on the mycelial growth of _P. infestans_, thus, indicating that the VOC blend (with regards to its composition and concentration) plays an important role in pathogen inhibition. The VOCs identified in [55] included alcohols, sulfides, alkanes, ketones and aldehydes. Dimethyl disulfide was the only VOC that was found to be secreted in high concentrations. Moreover, when the inhibitory ability of these strains was tested against other plant pathogens, such as _Rhizoctonia solani, Helmitthosporium solani, Dickeya dianthicola_ and _Fusarium oxysporum_, varying inhibitory effects on the growth of the different phytopathogens were observed [55]. Similarly, VOCs from _Ps. chlororaphis_ promoted growth and salt tolerance in Arabidopsis, and 2,3-butanediol from _Ps. chlororaphis_ was identified as a VOC that contributed to the induced resistance of tobacco against _Erwinia carostova_, but not against _Pseudomonas syringae_ pv. _tabaci_[21,56]. Analogous to the cited examples, the uniqueness of the VOCs observed in this study (Figure 3) might further explain the different metabolic perturbations caused by these strains in tomato seedlings [57].