## 4 Discussion
### BF06 VOCs Regulates Plant Uptake of Se by Upregulation of Sulfate Transporter Genes  
Recently, some PGPR strains have been shown to promote Se accumulation in plants [40; 41], implying that the system of Se uptake in plants may be triggered by certain microbial signals. In this study, combined analyses of RNA-Seq and qRT-PCR confirmed that the release of VOCs by BF06 distinctly upregulated the expression of several sulfate transporter genes including _SULRT1;1, SULRT1;2, SULTR2;1, SULTR2;2,_ and _SULTR3;5_, which are involved in the regulation of Se uptake and translocation [51; 52; 53; 54]. For this purpose, we investigated the effects of microbial VOCs on Se absorption in _Arabidopsis_ plants. The results showed that the accumulation of Se was significantly increased in the VOCs-exposed plants compared with the controls, whereas the Se content was evidently higher in roots than in shoots. Previous studies suggested that Se in soils can be taken up by the roots through high-affinity sulfate transporters (HASTs), such as AtSULT1;1 and AtSULT1;2 transporters [67]. The transcription levels of genes encoding homologs of SULRT1;1 and SULRT1;2 markedly increases in the Se non-accumulator plant species when tissue Se concentration is elevated [68]. However,these genes are constitutively expressed at a relatively high level in the roots of Se hyperaccumulator species [69], accounting for their higher Se capacity.  
Moreover, the enhanced expression of genes encoding HASTs, particularly SULTR1;1, confers improved Se uptake in plants [70; 71]. In this study, we found that a higher level of Se accumulates in VOCs-exposed _Arabidopsis_ roots, while VOCs exposure could not induce an increase of Se in the _sulfur1;1_ mutants. Thus, the VOCs-regulated _SULRT1;1_ expression was responsible for high accumulation of Se in plants. In addition, the shoots of VOCs-exposed plants also accumulated more Se than those of the controls, indicating that microbial VOCs might activate xylem or phloem transport mechanisms, in turn facilitating Se's translocation from roots to shoots. It has previously been indicated that some sulfate transporters, including SULTR2;1, SULTR2;2, and SULTR3;5, participate in the long-distance xylem transport of Se in _Arabidopsis_[60]. The low-affinity sulfate transporters AtSULTR2;1 and AtSULTR2;2 have also been found to catalyze Se uptake into the stele cells, and the activity of AtSULTR2;1 can be positively regulated by the AtSULTR3;5 that does not directly catalyze Se transport [72]. Additionally, increased Se availability can induce the expression of _SULTR2;1_ and _SULTR2;2_ in _Arabidopsis_[60; 73; 74]. Intriguingly, the transcription levels of _SULRT2_ are greater in the roots of Se hyperaccumulator species than non-Se hyperaccumulator species, indicating that high-level expression of _SULRT2_ contributed to the increased Se flux from roots to shoots in the hyperaccumulator species [75]. In this study, the expression of _SULTR2;1, SULTR2;2, and _SULTR3;5_ was observably enhanced by exposure to BF06 VOCs. Thus, upregulation of these genes by VOCs leads to efficient translocation of Se from roots to shoots, thereby elevating shoot Se accumulation.