## 3 Discussion
### Terpenoids  
Terpenoids represent the largest class of chemicals among those produced by plants [33]. The role of terpenoids in plant resistance to fungal pathogens has long been investigated. They play a critical role in the defense of plants against biotic stress [34; 35]. Mixtures of mono- and sesquiterpenes are associated with an increased resistance against fungal pathogens [36]. _Pihomis purpurea_ responds very quickly (6 hpc) to the presence of _Pytophthora cinnamomi_ by increasing the constitutive production of terpenoids in its leaves, which decreases after 24 hpc. According to [PERSON] et al. [37], emissions of biogenic volatile organic compounds (BVOCs) occur at the leaf, root, and flower levels and may be a constitutive characteristic or induced by a combination of biotic and abiotic stresses. [PERSON] et al. [24] have identified a nortriterpenoid (phlomispurpentaolone) exudated by the roots that inhibits _P. cinnamomi_ growth. Nonvolatile terpenoids can be exuded from the roots into the rhizosphere and the surrounding soil environment, where they are involved in different defense responses [38], which is in accordance with our data on _P. purpurea_. The comparative transcriptome analysis of the rice variety Digu, resistant to _Magnaporthe orgae_, revealed that the biosynthesis of terpenes is activated very soon after a challenge with the pathogen [39]. The role of terpenoids in the resistance of _Vitis vinifera_ against downy mildew has also been demonstrated [40]. The transcriptome analysis of _Pihomis purpurea_ challenged with _Plytophthora cinnamomi_ revealed the up- regulation of several transcripts related to the production of terpenoids, namely the monoterpenes, limonene, terpineol, myrcene, geraniol, and linalool, as well as of sesquiterpenes and their precursor germacrene. Their over-expression value decreased from 24 hpc [25] in a similar way to the metabolome profile of the terpenoids in leaves, opposite to the terpenoids profile in the roots. These data suggest their translocation from the leaves to the roots, from where they are released into the soil infested by _P. cinnamomi_, inhibiting its growth. In parallel, the transcriptomeof _Phlomis purpurea_ challenged with _Phytophthora cinnamomi_ revealed a high up-regulation of jasmonate (JA) [25]. [PERSON] et al. [41] reported on the role of JA in terpen biosynthesis. [PERSON] et al. [42] correlated its increased production with strawberry resistance to _Potrytis cinerea_ infection. Regarding salicate (SA), the transcriptome analysis revealed its high up-regulation at 12 hpc, which decreased thereafter [26]. This up-regulation may also be correlated with the metabolite profile of terpenoids in the leaves. In _Salvia macrosiphon_ under stress conditions, SA may change the secondary metabolites and their pathways through changes in the plastid's chlorophyll level [43]. [PERSON] et al. [44] reported on the enhanced production of primary and secondary metabolites after the application of SA to the Lamiaceae _Salvia kluzistanica_. [PERSON] et al. [45] pointed out the potential role of methyl salicate (MeSA) in plant stress signal transduction through influencing the secondary metabolism and signaling pathways, including the shikimate/phenylpropanol (PAL) pathway. According to the same authors, MeSA elicits emissions of benzenoids (BZ), monoterepnes (MT), and fatty-acid-derived compounds (LOX products). [PERSON] et al. (2019), through a transcriptome integrated analysis, reported on the coordinate defense response of poplar upon a fungal infection. These authors uncovered the possible roles of JA/SA in regulating the balance between growth and defense responses by integrating multiple hormone signaling pathways. They pointed out that the highest number of representative transcription factors include ERF, MYB/MYB-related genes, and bHLH. This is in agreement with our previous report on a transcriptome analysis [25; 26], which revealed a high expression of ERF, MYB/MYB-related genes, and bHLH, among others.