## 5 Conclusions  
Cues originating from mutualistic ecological interactions often contain highly specific information. Recent studies have demonstrated that VOCs emitted by PGPR can be used as a novel agroecological strategy to improve plant growth and yields, as well as to improve resistance to abiotic and biotic stresses. PGPR are members of diverse bacterial genera, and differences in plant protective capabilities can be linked to intrinsic biochemical activities. Advances made in analytical instrumentation have made it possible to resolve analytes present in a complex mixture in a short period of time. Here, SMPE-GC-MS, in combination with chemometrics, were successfully used to profile the VOCs secreted from strains of _Ps. koreensis_, _Ps. fluorescens_, _L. sphaericus_ and _Pa. alvei_. Our results indicate that the VOC profiles of rhizobacteria can be quite specific and that even strains of the same species may present unique profiles. These observable differences in the VOC blends can be due to the relative activities of specific metabolic pathways active in these strains. The main groups of VOCs secreted by PGPR include ketones, alkanes, alkenes and alcohols, and these were all identified in the present study. Of the investigated strains, the 'volatilome' of _Pa. alvei_ (T22) exhibited more unique features in comparison to the others. These annotated molecules could be further investigated as biomarkers for the classification of an organism as a PGPR and their selection for agricultural use. Currently, efforts to elucidate the mode of action of a specific compound or mixture regulating the plant physiological processes leading to growth, higher yield and inducible protection are underway. Thus, these results support the view that diverse PGPR may affect plant biochemical pathways and processes with different mechanism(s) of action. This might even be applicable if the PGPR belong to the same species, emphasizing the importance of characteristic strain-specific features. In addition, the number of annotated VOCs indicates that there could be more bioactives/volatiles than have been reported thus far. Furthermore, this research highlights the power of high-resolution mass spectrometry for metabolomic application in order to gain deeper insight into the chemical signaling pathways in the rhizosphere and the mechanism(s) of action of plant-protective bacteria. Metabolomic profiling thus offers another tool to functionally dissect the role(s) of VOCs in PGPR priming. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/metabol12080763/s1](https://www.mdpi.com/article/10.3390/metabol12080763/s1)]([https://www.mdpi.com/article/10.3390/metabol12080763/s1](https://www.mdpi.com/article/10.3390/metabol12080763/s1)), Figure S1: Action mechanisms of PGPR-derived VOCs; Figure S2: Representative GC-TOF-MS chromatograms of PGPR volatiles sampled by headspace SPME. Figure S3: OPLS-DA modeling and variable/feature selection of _Pseudomonas fluorescens_ (N04) data acquired using GC-TOF-MS. Figure S4: OPLS-DA modeling and variable/feature selection of _Pseudomonas koreensis_ (N19) data acquired using GC-TOF-MS. Figure S5: OPLS-DA modeling and variable/feature selection of _Paenibacillus alvei_ (T19) data acquired using GC-TOF-MS. Table S1: VOC profiles of PGPR strains: _Pseudomonas fluorescens_ (N04), Pseudomonas koreensis (N19), _Paenibacillus alvei_ (T19) and _Lysinibacillus sphaericus_ (T22). **Author Contributions:** Conceptualization, M.I.M. and I.A.D. ; formal analysis, M.I.M. ; investigation, M.I.M. ; methodology, M.I.M. ; resources, I.A.D. ; supervision, L.A.P. and I.A.D. ; data curation, M.I.M. ; validation, M.I.M. ; writing--original draft, M.I.M. ; writing--review and editing, L.A.P. and I.A.D. All authors have read and agreed to the published version of the manuscript. **Funding:** Funding from the South African National Research Foundation (NRF) to I.A.D. (grant number 95818) supported the research.