## 3 Discussion
### Taurine  
Taurine, an organic osmolyte, has received the most attention for its production and role in human diseases. It has been largely assumed that taurine is produced by animals. According to [PERSON] and [PERSON] [62], taurine is produced in Rhodophyta, Phaeophyta, Chlorophyta, Fungi, Bryophyta, and Pteridophyta, but in Spermat, it has not been detected at all. The same authors stated that taurine contents are high in lower plants and low in higher plants. Taurine synthesis is highly inducible in freshwater algae and largely constitutive in marine algae, which is consistent with the hypothesis that this compound is involved in osmoregulation [63]. In _Triticum aestivum_, exogenous taurine substantially improved growth, photosynthetic pigments, and nutrient uptake by regulating ROS scavenging, secondary metabolism, and ions homeostasis. The plant was protected from the detrimental effects of boron and chromium through up-regulating of nitric oxide, hydrogen sulfide, glutathione, and phenolic compound production [64]. In tomato in vitro cultures, taurine may counteract the effect of stress caused by the stress factors (NaCl and hydrogen peroxide) and can serve as an effective anti-stress agent in plant cells [65]. According to the author [65], taurine can be used as an anti-stress agent not only for animal systems, but also for plants in general. [PERSON] et al. [66] reported that, in olive trees growing in managed soils, among other compounds, the up-regulation of taurine may play a pivotal role in protecting plants from biotic and abiotic stress. In a recent paper [67], the potential role of taurine in modulating the defense system of _Trifolium alexandrinum_ plants under Mn toxicity has been reported. According to these authors, taurine circumvents Mn-induced oxidative stress by up-regulating the activities of antioxidant enzymes (ascorbate peroxidase, peroxidase, catalase, glutathione- reductase, glutathione-S-transferase, and superoxide dismutase) and the levels of ascorbic acid, proline, anthocyanins, phenolics, flavonoids, and glutathione. In the symbiotic algae _Symbiodinium_ sp., taurine might affect its metabolic pathways by altering the permeability of the algal cell membrane, diverting photosynthetically fixed carbon from storage compounds to translocated compounds [68]. Taurine protects biological cells, maintains the stability of cell membranes, and protects the activity of antioxidant enzyme systems [69]. It has been reported that taurine accumulation in rice-resistant cultivars (IR56) notably increases during brown planthopper (BPH) infestation and may work toward reducing the ROS-induced oxidative pressure caused by BPH feeding [70]. In _Astragalus membrananccus_ var. _mongholicus_, [PERSON] et al. [71] considered taurine to be a resistance-related (RR) metabolite against _Fusarium_ root rot. Few reports are available on the function of taurine in plant defense responses against pathogens and the data reported mostly concern its exogenous application. In _P. purpurea_, a high fold change increase is observed at 6 and 12 hpc in its leaves, decreasing thereafter to negative values at 72 hpc. Being the biotic stress responsible for oxidative stress and taking into consideration the _P. purpurea_ transcriptome analysis and results reported by [PERSON] et al. [67] and [PERSON] et al. [64], we are tempted to suggest that taurine may enhance the catalase and peroxidase expression, as well as that of proline, glutathione, and ascorbate, which present high up-regulation levels at 12 hpc, decreasing thereafter, as previously reported [25]. An NMR-based metabolomics study by [PERSON] et al. [71] showed taurine production by _Astragalus membrananccus_ var. _mongholicus_ against root rot. The transcriptome analysis of _P. purpurea_ also revealed very high expression levels of alanine, arginine, aspartate, glutamate, glutamine, phenylalanine, and threonine at 12 hpc.