Molecular Pathogens 2024, Vol.15, No.1, 30-39 http://microbescipublisher.com/index.php/mp 34 for optimizing defense responses and avoiding unnecessary energy expenditure. Additionally, other hormones such as abscisic acid, auxin, and cytokinins also interact with SA, JA, and ET pathways, further adding layers of regulation to the plant immune system (An and Mou, 2011; Sanchez et al., 2012; Yang et al., 2015). 5 Role of Secondary Metabolites 5.1 Phytoalexins and phenolics Phytoalexins and phenolic compounds are critical in the tea plant's defense against pathogens. Phenolic acids and lignin, for instance, are synthesized in response to biotic and abiotic stresses, enhancing the plant's resistance. The CsHCT genes, which regulate the phenylpropanoid and lignin pathways, have been shown to increase the content of phenolic acids and lignin, thereby improving resistance to bacterial infections and abiotic stresses (Chen et al., 2021). Additionally, phenolic acids and flavonoids were found to be major metabolites in wild tea plant resources resistant to tea gray blight disease, indicating their significant role in pathogen resistance (Zhang et al., 2022). Furthermore, the synthesis of anthocyanin-3-O-galactosides, a type of phytoalexin, is induced by anthracnose infection, contributing to the hypersensitive response and resistance in tea plants (Li et al., 2023). 5.2 Flavonoids and tannins Flavonoids and tannins are another group of secondary metabolites that contribute to the defense mechanisms of tea plants. Flavonoids, such as quercetin and kaempferol, are involved in the plant's response to pathogen infection. Overexpression of transcription factors like PalbHLH1 and PalMYB90 in poplar has been shown to increase flavonoid content and enhance resistance to fungal infections, suggesting a similar mechanism may be present in tea plants (Figure 2) (Bai et al., 2020). Figure 2 Characterization of transgenic poplar plants (Adopted from Bai et al., 2020) Image caption: (A) Construction of the MYB90/bHLH1-OE plasmid; (B) Growth of transgenic (MYB90/bHLH1-OE) and wild-type (WT) poplar plants; (C) Color of anthocyanin extraction. (D) Accumulation of total anthocyanins in the leaves of MYB90/bHLH1-OE and WT plants. (E) Accumulation of total phenolics in the leaves of MYB90/bHLH1-OE and WT plants; (F, G) Contents of quercetin and kaempferol in the leaves of MYB90/bHLH1-OE and WT poplar plants, respectively(Adopted from Bai et al., 2020)
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