## 3 Results and Discussion
### Elements Typically Involved in the Stringent Response Are Responsive to HXT  
The transcriptomic analysis revealed the involvement of the stringent response (SR), a bacterial process that can be triggered by a broad variety of stress conditions [33], in the adaptation of _L. plantarum_ to HXT. Among the transcriptional alterations triggered by HXT that concern the SR were those affecting the expression of genes involved in cell proliferation. Thus, genes encompassed within the GO term \"ribosome\" were significantly enriched in the DAVID analysis (Figure 2). Among these, a set of nine rRNA genes (_lp_0011_ [rpsR], _lp_1063_ [rplQ], _lp_1078_ [rpsI], _lp_2125_ [rpsO], _lp_2126_ [rpsT], _lp_0543_ [rpsS1], _lp_1594_ [rpl27], _lp_1640_ [rpsS], _lp_1517_ [rplT],) and a ribosomal protein-serine acetyltransferase (_lp_2840_) were all downregulated. In addition, other genes coding for translation related functions including _lp_2378_ or _prfA_ (peptide chain release factor), _lp_0963_ or _rluE_ (ribosomal large subunit pseudouridine synthetase), _lp_2039_ or _rbfA_ (ribosome binding factor), _lp_2277_ or _alaRS_ (alanyl-tRNA synthetase and _lp_1027_ or _fusA2_ (elongation factor), were downregulated (Supplementary Table S2). Also, other genes related to cell proliferation that code for factors playing roles in DNA(_lp_0787_ or _rpoN; lp_0811_ (DNA-directed DNA polymerase III subunit epsilon; _lp_2454_ (DNA adenine methylase)) or RNA replication (_lp_1613_ or _rpoZ_), were downregulated. In the same vein, genes coding for cell division (_lp_2201_ or _ftsL_) or cell shape determination (_lp_2317_ or _mreD_) were downregulated in the presence of HXT. Furthermore, genes that are part of the SR with known roles in the adaptation to stress-related processes such as those coding for the alkaline shock proteins _asp1_, and _asp2_ or the molecular chaperones _groEL_ and _groES_, were up-regulated. Other transcriptional alterations elicited by HXT owing to the SR concerned the metabolism of (p)ppGpp, the nucleotide-based secondary messenger that acts as alarmone to trigger the SR. The (p)ppGpp is interlinked and modulates intracellular purine concentrations since GTP mediates in the biosynthesis of this alarmone. HXT regulated the expression of several genes involved in purine metabolism (Supplementary Table S2). The expression of genes involved in GTP biosynthetic pathways (purine biosynthesis) such as _lp_0848_ (purine transporter), _lp_2710_ (xanthine permease) and _lp_3269_ or _purB_ (adenylosuccinate lyase, AMP synthesis) were downregulated whereas _lp_2727_ or _purC_ (phosphoril aminoimidazole succinocarboxylate synthase, IMP synthesis) was upregulated. Genes involved in GTP-consuming pathways such as _lp_1869_ or _dfrA_ (dihydrofolate reductase, folate biosynthesis) and _lp_1436_ (riboflavin synthase) were upregulated upon HXT which may also interfere with the synthesis of (p)ppGpp via intracellular GTP consumption. In addition, HXT modulated the expression of genes coding for enzymes that are directly involved in the interconversion of (p)ppGpp to ppGpp such as small translational GTPases (_lp_1881_ or _engA_; _lp_1687_ or _rsgA2_). Also, a GTP pyrophosphokinase (_lp_0293_) with high homology to other (p)ppGpp synthetases (_Staphylococcus aureus relP_), was upregulated. Concomitant with this profile _lp_2114_ coding for an NTP pyrophosphohydrolase with high homology with _mazG_ (an enzyme with GTP hydrolytic activity [34]) and a phosphohydrolase encoding gene (_lp_1985_) located in the vicinity of _relA_ (_lp_1988_), were downregulated. Genes coding for the stringent response factors (enzymes) exopolynophasatase (_lp_3587_ or _prx3_) and pyrophosphokinase (_lp_3578_ or _prpk_) which control the intracellular levels of inorganic phosphate (poly-P), a key regulatory molecule in the stringent response [35], were upregulated. The regulation of genes involved in the SR, a highly conserved stress defense mechanism, indicates that HXT causes stress to _L. plantarum_ WCFS1. Interestingly the SR was also triggered in _L.