4.3 CsTPS1-AS enhances the resistance of tea plants to pathogen infection by regulating geraniol formation and the expression of SA-related genes
Plant pathogens can activate SA pathways, which enhance the resistance of plants to pathogen infection (Eberl et al., 2018). The pathogenesis-related defense genes PR1 and PR2 are typical markers of the SA-mediated defense system (Zhang et al., 2020). The expression of PR1 and PR2 was significantly increased in pathogen-infected tea plants (Figure 6D and 6E). This indicates that pathogens can induce the expression of pathogenesis-related genes in the SA-mediated pathway, which enhances the resistance of plants to pathogen infection. The expression of PR1 and PR2 was significantly lower in CsTPS1-AS- silenced plants than in control plants (Figure 6D). This suggests that CsTPS1-AS mediates the response to pathogen infection by up-regulating the expression of pathogenesis-related genes.
PAD4 is known to play a key role in SAR through SA-dependent and SA-independent pathways (Cui et al., 2017; Hu et al., 2022). To further clarify the role of CsTPS1-AS in plant defense, the expression ofPAD4 was assessed after pathogen infection in tea plants. As expected, silencing ofCsTPS1-AS significantly decreased the expression of PAD4in infected tea plants (Figure 6E), suggesting that CsTPS1 -AS might enhance SAR in tea plants by activating the expression ofPAD4 . Overall, these findings indicate that CsTPS1-ASmight play a role in pathogen resistance by regulating the expression ofPR1 , PR2 , and PAD4 .
Silencing of CsTPS1 -AS also significantly decreased the content of geraniol (Figure 5B) and the amount of mycelial growth on CsTPS1-AS-silenced tea leaves was more than that on control leaves (Figure 6A and 6B). Meanwhile, geraniol shown that more strongly inhibited the mycelial growth of Neopestalotiopsis sp. andC. gloeosporioides in vitro (Figure 3). These findings indicate that geraniol plays an important role in enhancing resistance to infection by both of these fungal pathogens. Our findings are consistent with the results of a previous study showing that (E)-β-caryophyllene mediates the defense response of A. thalianaflowers to pathogen infection by directly inhibiting bacterial growth (Huang et al., 2012). Our findings indicate that the function ofCsTPS1 -AS was to enhance the resistance of tea plants to pathogen infection by up-regulating the biosynthesis of geraniol. Thus,CsTPS1 -AS enhances the resistance to pathogen infection in tea plants by regulating geraniol formation and the expression of SA-related genes. Based on these results, we propose a putative working model for the function of CsTPS1/1 -AS in pathogen infection (Figure 7).
In conclusion, we identified a key TPS gene that functions as a geraniol synthase (CsGES ) in tea plants, and both in vitroand in vivo studies indicated that this geraniol synthase is involved in regulating geraniol formation and plant defense via AS. The results of this study provide new insights into geraniol biosynthesis and clarify the role of monoterpene synthases in modulating the disease resistance in plants via AS.