## 1 Introduction  
Being sessile organisms, plants live in soils for growth, development, and completing the life cycle. A variety of soil-borne microorganisms naturally occur in the plant rhizosphere, many of which can assist with plant growth and adaptation to adverse conditions, especially nutrient limitation [1; 2; 3]. Beneficial soil microbes are usually called plant growth-promoting rhizobacteria (PGPR). It has previously been indicated that plant roots can secrete several organic compounds that are utilized by soil bacteria as an important nutrient source [4]. Root exudates can also inhibit soil-borne plant pathogens, and shape the composition and diversity of local soil microbiota [5]. Certain PGPR strains can release some important substances such as non-volatile (phytohormones,antibiotics, siderophores, etc.) and volatile organic compounds (VOCs), which offer great benefits to plants [1; 6; 7; 8; 9; 10]. Since their discovery in the late 18 th century, soil inoculation with PGPR has been widely applied in agriculture [11; 12; 13]. Emerging evidence indicates that application of PGPR remarkably enhances plant uptake of macro- and micro-elements by increasing the availability of soil nutrients and activating the mechanisms of nutrient acquisition in host plants [14; 15; 16]. Hence, PGPR can be exploited as biofertilizers to effectively improve nutrient assimilation by plants. Previous studies have shown that PGPR can mediate many physiological processes of host plants without being in physical contact with the roots by release of VOCs [9; 10]. Microbial VOCs (mVOCs) have recently been considered as a novel type of regulators in plant-microbe interactions [17]. In spite of different types of VOCs being emitted by a wide array of bacterial species and strains, a common characteristic is that VOCs can be released by various soil microbes [18; 19; 20]. Intriguingly, mVOCs do not contain any well-known plant growth hormones (such as auxin and cytokine) and iron-chelators (siderophores), whereas endogenous hormones and iron (Fe) uptake in plants can be positively regulated by differential mVOCs emission [21; 22; 23]. mVOCs can function as signal molecules for root-microbe communication, and mediate plant growth and resistance to phytopathogens [18; 19; 20; 21; 22; 23]. Hitherto, a large number of bacterial species such as _Paenibacillus_[14], _Bacillus_[22], and _Pseudomonas_[24] have been known to produce VOCs, which play diverse roles during plant-microbe interactions. 2,3-butanediol is a major VOC released by _Bacillus subtilis_ GB03 [9] and _Pseudomonas chlororapalis_ O6 [25], and can regulate plant growth and stress tolerance by modulation of various metabolic pathways. Plants inoculated with _Bacillus_ strains and plants treated with exogenous butanediol exhibit induced systemic resistance (ISR). In addition, a sulfur (S)-containing volatile compound, dimethyl disulfide (DMDS), from _Bacillus_ sp. strain B5 obviously recovers plant growth and enhances S assimilation under S deprivation [26]. Recently, the release of VOCs by some PGPR strains has been shown to promote Fe absorption in plants, indicating that mVOCs are involved in regulating plant Fe deficiency responses [22; 27]. Fe is an essential micro-element for plant growth and development, and Fe deficiency has increasingly become one of the wide-ranging abiotic stresses that severely affect diverse processes such as chlorophyll formation, respiration, and photosynthesis [28]. Despite a high abundance of Fe in the earth's crust, it is not easily utilized by plants due to its low availability in alkaline or calcareous soils [29]. Plants often display typical symptoms of chlorotic leaves under Fe-limited conditions [30]. To cope with Fe deficiency, the strategy I plants, dicots and monocots, can acidify the rhizosphere for increasing the solubility of Fe (III) by the _Arabidopsis_ plasma membrane H+-ATPase isoform 2 (AHA2) [31], and the reduction of Fe (III) to Fe (II) involves the ferric chelate reductase (FRO2), encoding the Fe (III) chelate reductase [32].