## 4 Discussion  
Endophytic fungi represent a new source for the obtainment of RV under controlled conditions [11; 12; 13; 14]. A previous work reported on the production of _A. aureus_ isolated in India, as well as some interesting conditions involved in the production of RV [14]. In the present work, we report the same species isolated in a different geographical region, which shows similar but not identical characteristics to those of the strain isolated previously [14]. The same species was treated with potential elicitors that stimulated RV biosynthesis. According to our results, differences in the biosynthetic capacity of _A. aureus_ MaC7A were observed. Under standard conditions (PDB medium, pH 7.0, 10 g L\({}^{-1}\) peptone, 30 \({}^{\circ}\)C), the production of RV was around 127.9 \(\pm\) 4.6 mg L\({}^{-1}\) at day 7, and the maximum biomass accumulation was around 195.2 \(\pm\) 1.3 g fresh weight L\({}^{-1}\) at day 8. The maximum accumulation of biomass and RV differed to that observed for the _A. aureus_ isolated in India [14]. This fact strongly suggests a different capacity of _A. aureus_ MaC7A to produce RV. Nevertheless, the optimal temperature and pH were the same as those reported previously for the _A. aureus_ isolated in India [14]. Unlike previous studies, the amount of biomass was relatively constant during days 10-12 under our experimental conditions [14]. It is known that the degradation of biomass does not occur spontaneously. Such loss is produced gradually after the stationary phase in all microorganisms because of the lack of nutrients, which accelerates programed cell death. The fact that the biomass of _A. aureus_ MaC7A remained relatively constant during days 6-12 indicates that not all of the cells were functional at these times. Optimized batch cultures confirmed that glucose levels decreased gradually and were almost undetectable at day 8. Reasonably, the metabolism of the fungus should be focused on the maintenance of basic functions to assure survival instead of biosynthesis RV at these unfavorable times. Since peptone contains variable amounts of bioavailable PHE and TYR, which are generally low or negligible (per 100 g product), the addition of amino acid precursors to the batch cultures was investigated. Dose-response curves with PHE and TYR revealed that concentrations of 200-500 mg L\({}^{-1}\) generated an increase in RV accumulation, but the highest concentration of both amino acids (500 mg L\({}^{-1}\)) did not produce a substantial increase of RV compared with a concentration of 200 mg L\({}^{-1}\). Considering this evidence, further experimentation was performed with 200 mg L\({}^{-1}\) as the optimal concentration of amino acid precursors, which was able to increase 60% of the amount of RV compared with that from standard batch cultures. Interestingly, the addition of TYR produced about 12 mg more RV (221.8 \(\pm\) 5.2 mg L\({}^{-1}\)) than PHE (207.8 \(\pm\) 1.4 mg L\({}^{-1}\)). These differences may originate from the solubility and bioavailability of TYR in aqueous media, and probably due to the amphipathic properties of TYR [22]. According to these data, _A. aureus_ MaC7A metabolized both precursors to produce RV, and TYR was more effectively converted into RV. Kinetics of batch cultures containing 200 mg mL\({}^{-1}\) demonstrated that the maximum production of RV was maintained from day 6 to day 8 post-treatment. Interestingly, the amount of biomass was more abundant in the batch cultures treated with TYR (236 \(\pm\) 3.7 g L\({}^{-1}\)). As for plant species, fungi also produce secondary metabolites, which play a crucial ecological role as defensive chemicals [23]. The biosynthesis of these metabolites may be stimulated under the pressure of distinct elicitors, which are sensed by other microbial competitors [24]. It is known that plant volatiles exert diverse changes in the physiology of fungi, including a chemical counter-response [25]. Thus, volatiles may be used as elicitors to stimulate a chemical response in fungi [25].