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.