## 4 Discussion
### Enriched Biochemical Pathways After C. pepo Eliciation by R. sachalinensis Extract  
Biochemical pathways and DEPs highlighted in the RS vs. Control comparison indicate the RS eliciting effect on _C. pepo_ plants, irrespective of pathogen infection. We have previously shown that application of RS on _C. pepo_ plants effectively triggers plant responses [30]. Four pathways (group 2) appeared uniquely enriched in the RS vs. Control comparison, with \"Biosynthesis of secondary metabolites\", \"Flavonoid biosynthesis\", and \"Starch and Sucrose metabolism\" containing highly expressed DEPs (Table 2). Plant secondary metabolites (PSMs) are a large heterogenous class of chemical compounds utilized as part of the plants' innate immune system in their defense against abiotic and biotic stressors. PSMs function as end-products for defense, or signaling molecules often associated with expression of defense genes and accumulation of related end-product metabolites [74]. Flavonoids are PSM-synthesized by plants to serve diverse physiological roles such as phytoalexins, antioxidants, antimicrobials, allelopathy, and signaling molecules [75]. A variety of different enzymes including catalases, isomerases, reductases, hydroxylases, and dioxygenases can act in and selectively divert the flavonoid biosynthesis pathway, thus leading to a different flavonoid subclass regarding the aim to be accomplished each time [76; 77]. In the RS vs. Control comparison, both pathways are significantly enriched and represented with a total of 25 DEPs (two being common) of which eight are highly expressed (>4-fold), including three dehydrogenases, two catalase, two peroxididases, two phosphatases, one phospholipase, one flavonoid synthase, and three proteins yet uncharacterized. Our previous results have shown that RS application had an impact on the production of the phenolic compounds salicylic acid and p-coumaric acid and the flavonoid caffeic acid in _C. pepo_ plants [30], supporting the eliciting role of RS in modulation of DEPs involved in PSM production. Starch degradation, besides its role as carbon source for sucrose synthesis, is closely integrated with processes of a plant's secondary metabolism [78], providing sugar metabolites as skeleton molecules for defense compounds such as flavonoids, stilbenes, lignin, and callose, or as sugar signaling molecules acting directly, or in conjunction with, redox-mediated and hormone-regulated gene expression processes [79; 80]. There is also evidencesupporting a direct role of these sugar metabolites as priming molecules, under the concepts of \"sweet priming\" and \"sweet immunity\", where they stimulate plant defenses against fungal pathogens by modulating sugar efflux transporters [81]. Sucrose synthesis is catalyzed in the cytosol by sucrose-phosphate synthase and sucrose-phosphate phosphatase, two enzymes that were found enriched in the starch and sucrose metabolism pathway. In previous research we showed that RS elicited callose formation in _C. pepo_ plants along with secondary metabolites as stated previously [30]. Callose formation may be related with the modulation of starch and sucrose metabolism by RS. However, further experimentation towards an untargeted or oriented metabolomics approach to connect RS-elicited production of callose and previously-reported secondary metabolites to starch and sucrose metabolism is required. This could unravel roles of sugars as signaling molecules, and further describe groups of secondary metabolites produced. Biochemical Pathways Enriched Due to Synergistic Action of RS-Priming Followed by P. xanthii Infection  
Elicitors also function by priming plants, setting various biochemical mechanisms in a \"stand-by\" mode to be activated in a faster and/or more robust way at the onset of a subsequent pathogen infection and onwards [82, 83]. In the RSPX vs. Control comparison, the putative synergistic effects of RS-primming and the pathogen's infection-trigger could be an anticipated plant state. A total of 11 pathways were enriched in this comparison (Table 2).