## 3 Discussion  
The success of plants' survival is significantly based on their ability to rapidly recognize specific environmental signals and biotic attacks and promote signal transduction pathways that lead to the biosynthesis of defensive compounds [28]. Immunity and agents that increase immunity have long been investigated in humans. In plants, only recently has intensive research been performed to understand the factors controlling plant immunity, especially with regard to their defense against biotic stress. In a previous work, based on data from the transcriptome analysis, we suggested that _Phlomis purpurea_ presents an innate immunity against the oomycete _Phytophthora cinnamomi_[26]. Combining the transcriptome data with the metabolome results, here, we discuss the interplay of multi-compound metabolite classes and the corresponding genes coding for the enzymes participating in different metabolic pathways, reinforcing the assumption that _Phlomis purpurea_ present an innate immunity against the oomycete _Phytophthora cinnamomi_. The constitutive expression and rapidly increased production of imidazole-4-acetaldehyde and taurine must be emphasized, as these compounds are very well known as being responsive for human immunity. To the best of our knowledge, this is the second paper reporting on taurine production by angiosperms infected with a pathogen and the first associating this compound and imidazole-4-acetaldehyde with plants' innate immunity.  
In combining the transcriptome results with the metabolome ones presented here and comparing these findings with the available literature, we find support for considering that _Phlomis purpurea_ is a good example of plant innate immunity against the oomycete_Plytophthora cinnamomi_, opening avenues for a better understanding of plants' immunity against plant pathogens.  
A major mechanism by which plants interact and adapt to their environments is through the production of specialized metabolites [29]. Novel tools such as metabolome and transcriptome analyses deepen our knowledge of plants' defenses against biotic and abiotic stresses. Metabolomics is a powerful tool for disclosing the molecules that are involved in the response of plants to adverse environmental conditions. The integration of metabolome with transcriptome data is crucial for understanding plants' molecular mechanisms underlying the _Pihomis purpurea_ immunity against _Plytophthora cinnamomi_. Transcription factors such as WRKY, MYB, AP2, ERF, bZIP, bHLH, CYP, and NAC, known to be involved in the biosynthesis of secondary metabolites [30; 31], were up-regulated in _Pihomis purpurea_ upon a challenge with _Plytophthora cinnamomi_, and their role in the innate immunity of _Pihomis purpurea_ has been previously reported [25; 26]. A metabolome screening of _Pihomis purpurea_ leaves and roots challenged with _Plytophthora cinnamomi_ revealed a set of secondary metabolites that were differentially produced along the challenge time course. Despite the plants being challenged by the pathogen in their roots, it was in the leaves that the greatest metabolic changes occurred. These results suggest that _Pihomis purpurea_ quickly perceive long-distance signals in their leaves, where secondary metabolites, in particular those depending on chlorolastidial metabolism, increase their production. This assumption finds support from the plastidial localization of GPP synthase (GPPS), the enzyme responsible for the synthesis of GPP as a precursor for vital metabolic branches, including specialized metabolites. The participation of chloroplasts in the biosynthesis of a large number of biomolecules (fatty acids, amino acids, sugars, vitamins, terpenes, alkaloids, and phenols, among others) is well documented [32]. The compartmentation of some terpenes and terpenoids in chloroplasts is also known. Some of the classes of compounds identified are discussed, correlating the metabolome with the transcriptome data previously associated with the innate immunity of _Pihomis purpurea_ against _Plytophthora cinnamomi_.