## 1 Introduction  
Plant growth-promoting rhizobacteria (PGPR), part of the plant microbiome in the rhizosphere, is a community that is largely influenced by plant roots and soil type [1, 2, 3]. The roots secrete molecules as part of the root exudates, and these inadvertently influence the microbial consortium in the adjacent rhizosphere [4, 5, 6]. In agrocosystems, PGPR has been shown to influence plant growth by improving soil nutrients, inhibiting deleterious organisms and enhancing plant health [1, 3]. Numerous strains have been reported to interact with plants (below and above ground), resulting in either antagonistic or mutualistic symbiosis [7, 8, 9, 10]. To help plants defend against the plethora of pathogens, PGPR can elicit a form of resistance known as induced systemic resistance (ISR) [11, 12]. Symbiotic interactions have been thought to involve physical contact between plants and PGPR. Nonetheless, recent studies have reported that microbial signals (i.e., volatile organic compounds or VOCs) play an integral role in multi-trophic interactions with plants. These include functions in plant growth modulation and health, as well as enhancement of the availability of soil nutrients and interactions with other microbial communities within the rhizosphere [13; 14; 15]. PGPR coexist in dynamic communities and secrete VOCs that allow these organisms to survive in their environments [10]. As low molecular weight compounds and easily vaporized molecules, they can diffuse through complex matrixes, such as cellular membranes, water, soil and air [10; 16; 17]. VOCs are thus suitably equipped as info-chemicals produced by soil microbes that mediate both short- and long-distance signaling, as well as inter- and intra-organismic communication below and above ground [9; 10; 15; 16; 17; 18; 19]. Plant growth enhancement and physiological processes, such as ISR and abiotic stress tolerance after VOC exposure (summarized in Figure S1), are dependent on systemic changes modulated by alterations in the levels of phytohormone, such as ethylene (ET), auxin and jasmonic acid (JA). Still, other phytohormones and concomitant cross-talk cannot be excluded [19; 20; 21]. Studies suggest that the inoculation of plant roots with PGPR or exposure to VOCs involves several signaling pathways that lead to the stimulation of photosynthesis and plant growth, cell wall modification, phytohormone regulation and stress responses [22; 23]. In addition, PGPR VOCs can influence rhizosphere communities in either a positive or negative manner through bacterial-bacterial and bacterial-fungi interactions [10]. For example, these can either recruit other beneficial microbes, inhibit pathogenic microbes or attract natural enemies for feeding on soil-borne herbivores [24; 25]. Multiple signals are responsible for the successful colonization of plant roots by PGPR in a complex process controlled by molecules from the two organisms involved. In this study, four bacterial strains with PGPR activity (_Pseudomonas koreensis_ (N19), _Ps. fluorescens_ (N04), _Lyssinibacillus sphaericus_ (T19) and _Paenibacillus alvei_ (T22)) were investigated. Previously, these PGPRs have been reported to successfully colonize maize, wheat and tomato roots, and to enhance the growth of these plants [26; 27]. Some of the elucidated mechanisms by which these bacterial isolates stimulate plant growth include the production of indole-acetic acid and related hormones, phosphate solubilisation and the production of siderophores [26]. Lastly, _Pa. alvei_ (T22) was also found to be effective as a biocontrol agent against soil-borne diseases in sorghum and wheat [28; 29]. A previous study investigating the four PGPR strains revealed strain-specific defence-related metabolic reprogramming in tomato plants [30].