4. Discussion
GSB is a prevalent fungal disease not only for melon but also for most commonly cultivated cucurbits, which is also considered as a potential risk in the global movement of plant pathogens as it can be on or in seeds and transplants (Seblani et al., 2023). Several GSB resistant resources and related inheritance of resistant genes have been identified in melon (Luo et al., 2022). However, the interaction between melon and GSB and the related disease resistance mechanism are still unknown, which impedes genetic improvement of melon with GSB resistance. To address these problems, full-length transcriptome, metabolome, and histochemical staining methods were used in this study to reveal the early defense response of melon to GSB.
Plants cope with pathogen attack through the innate immune response initiated by cell surface-localized pattern-recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat containing receptors (NLRs), which trigger pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), respectively (Yuan et al., 2021). Upon recognition of pathogen or microbe-associated molecular patterns (PAMPs/MAMPs), cell surface-localized PRRs recruit co-receptors to form receptor complexes and activate downstream phosphorylate receptor-like cytoplasmic kinases (Liang & Zhou, 2018), which subsequently phosphorylate downstream components to trigger ROS burst, Ca2+ influx, MAPK activation, phytohormone production, and transcriptional reprograming. NLRs form resistosomes upon activation, which eventually leads to multiple immune responses (Yuan et al., 2021). Several studies have also shown that plant MAPK cascades play pivotal roles in signaling plant defense against pathogen attack (Zhang & Zhang, 2022). In this study, through staining experiments, we found that the growth of S. cucurbitacearum was significantly inhibited in PI511890, and O2- and H2O2 were accumulated significantly at 24 hpi. DEGs including novel genes were specifically enriched in the MAPK signaling pathway in PI511890, indicating that the MAPK signaling pathway is involved in the resistance of melon to GSB pathogen infection. Additionally, GO enrichment analysis showed that hydrogen peroxide catabolic process, cell wall organization, response to wounding, and defense response were significantly enriched in PI511890, suggesting that PI511890 inhibits S. cucurbitacearum invasion probably by catabolizing hydrogen peroxide and preventing pathogenic fungi to destroy cell wall.
The inherent advantage of full-length transcriptome makes it possible to understand the complexity of AS events at the whole genome scale. AS is an important modulator of gene expression that can increase proteome diversity (Ule & Blencowe, 2019). In this study, 11793 AS events including seven types were detected, which followed an order of A3 > A5 > RI > SE > AF > AL > MX in the number of members. Moreover, 158 differential AS events were related to carbohydrate metabolism in Payzawat after GSB pathogen infection, while 117 differential AS events were related to RNA transport in PI511890 after infection. These results demonstrated that AS events are widely involved in the early response of melon to GSB infection.
It has been reported that oxalic acid has a protective effect against pathogen attack (Palmieri et al., 2019; Schmalenberger et al., 2015). In this study, oxalic acid was positively accumulated in PI511890, while decreased in Payzawat after GSB pathogen infection. In the pathway of glyoxylate and dicarboxylate metabolism enriched by DEGs, the positively accumulated oxalic acid was derived from the glyoxylate cycle. Moreover, the pathway of chloroalkane and chloroalkane degradation, which promotes the accumulation of oxalic acid, was also enriched by DAMs in PI511890 after GSB pathogen infection. The pathways enriched by both DEGs and DAMs, as well as the significantly increased accumulation of oxalic acid induced by GSB in PI511890, demonstrated that oxalic acid can be used as a metabolite mark for GSB resistance in melon.
Lignin acts as a passive physical barrier (Lee et al., 2019; Vanholme et al., 2019), and reduction of its synthesis can relax cell wall structure to facilitate the release of DAMPs by pathogens (Savatin et al., 2014; Xiao et al., 2021), thereby initiating the immune response of plants to enhance disease resistance. In this study, lignin content in PI511890 was lower than that in Payzawat after GBS pathogen infection, suggesting that lignin is also involved in the GSB resistance in melon. Additionally, flavonoid metabolites have multifaceted roles in mediating plant-microbe interactions (Wang et al., 2022). The differentially accumulated flavonoids differed between PI511890 and Payzawat after GSB pathogen infection. Eriodictyol was specifically accumulated in infected PI511890 and apigenin was specifically accumulated in infected Payzawat. It is currently unknown how these two metabolites contribute to the GSB resistance, but the discrepancy between them must be related to the difference in GSB resistance between PI511890 and Payzawat.
Combined transcriptome and metabolome analysis provides an important approach for the mining of metabolic networks and key genes (Gong et al., 2021; Wei et al., 2016). In this study, we established the relationship between DEGs and DAMs, and found that eriodictyol and oxalic acid have the potential to be used as marker metabolites for GSB resistance in melon. However, no significant correlations were observed between accumulation patterns of DAMs and expression profiles of DEGs in the same enriched pathways. These results demonstrated that post-transcription regulation is widely involved in the defense response of melon to GSB. This study provides some insights and theoretical basis for understanding the resistance mechanism of melon to GSB. However, the specific disease resistance mechanism and relevant effectors remain to be further explored. Thus, other multi-omics studies should be incorporated to establish a more comprehensive metabolic regulatory network for understanding the resistance of melon to GSB.