## 9 Role of PPCs in the Molecular Adaptation of _Lactobacillus_ to Host Environments  
PPCs can be released by plants to enable bidirectional interfaces with the environment. Within these interactions, the microbe signaling mediated by PPCs can be crucial for plant robustness. A known example is the microbe signaling by plant isoflavoids to transcriptionally activate bacterial key genes for nodule formation that stimulate plant growth [83; 84]. Benzoxazinoids can also acts as mediators of plant health by modulating the soil microbiome [85].  
These cases illustrate the ability of some PPCs to shape the structure and function of plant host-microbial communities. This capacity may operate not only in setting up phytomicrobiomes but also to shape the microbiome structure and function across the animal kingdom given that PPCs are consumed in the diet. Because the host microbiome markedly influences the host phenotype, it is necessary to understand the direct effects of PPCs on microbes and the influence of PPCs on host-microbial interactions to establish a causal relationship between PCs exposure and effects on the host.  
Molecular details on how PPCs influence the survival and function of _Lactobacillus_ spp. in host environments remain largely unknown. This section touches on a few approaches that reveal how individual PPCs can modulate the expression of molecular traits from _Lactobacillus_ spp. reportedly involved in the adaptive response to host environments or that have documented functions in the host-_Lactobacillus_ molecular dialog.  
Regulation of Molecular Functions Involved in the Survival of Lactobacillus to Gastrointestinal (GI) Tract Stress  
Lactobacillus undergo different stress conditions during the passage through the GI tract. Interestingly, genes coding for different molecular functions involved in the survival of _L. plantarum_ to the GI tract conditions were also responsive to PPCs (Table 3).  
_L. plantarum_ WCFS1 submitted to an orogastric-intestinal (OGI) simulator [86] induced a set of proteases (_clpB_, _clpE_, _clpP_, _stH_), molecular chaperones (_groEL_, _dnak_), and heat-shock proteins (_lsp1_, _lsp2_, _lsp3_). Figure 2 shows that all these genes were also induced by _p_-CA while some of them were also responsive to RSV, HXT, and OLE.  
High osmolarity and contact with bile salts in the duodenum lead _L. plantarum_ to induce a relatively high number of genes involved in cell envelope functions [87; 88]. _L. plantarum_ triggers similar responses when it contacts with tannic acid, _p_-CA, or OLE, including the regulation of the expression of genes involved in the biosynthesis of cell envelope constituents or that are major actors in cell-wall turnover (Table 3). Multidrug transporters may also play an important role for bile tolerance in _Lactobacillus_ spp. [89; 82]. In this regard, _p_-CA and RSV markedly induced _L. plantarum_ genes encoding an efflux pump (_lp_0990-_lp_0992_) that is also induced in the small intestine and proposed to be involved in the extrusion of bile salts [90] (Table 3). To counteract the high osmolarity stress in the small intestine, _L. plantarum_ involves transporters of compatible solutes including glycine-betaine/carnitine/choline [91], which are also responsive to _p_-CA and HXT (Table 3).  
In addition to acidic and high-osmolarity conditions, bacteria face oxidative stress at the mucosal surface of the colon [92]. As mentioned above, _Lactobacillus_ activates different mechanisms to counter oxidative stress in the presence of PPCs (Table 3) which can cross-protect these bacteria in the colon.  
In addition to these molecular mechanisms, more functions related to the survival of _Lactobacillus_ in the GI tract can be individually controlled by \(p\)-CA, GA, TA, or RSV, as described in the next sections.