Melatonin enhances resistance to Botryosphaeria dothidea in pear by promoting jasmonic acid and phlorizin biosynthesis

Abstract

Ring rot, caused by Botryosphaeria dothidea, is an important fungal disease of pear fruit during postharvest storage. Melatonin, as a plant growth regulator, plays an important role in enhancing the stress resistance of pear fruits. It enhances the resistance of pear fruits to ring rot by enhancing their antioxidant capacity. However, the underlying mechanism remains unclear. In this study, we examined the effect of melatonin on the growth of B. dothidea. Results showed that melatonin did not limit the growth of B. dothidea during in vitro culture. However, metabolomics and transcriptomics analyses of 'Whangkeumbae' pear (Pyrus pyrifolia) revealed that melatonin increased the activity of antioxidant enzymes, including peroxidase (POD), superoxide dismutase (SOD), and polyphenol oxidase (PPO), in the fruit and activated the phenylpropanoid metabolic pathway to improve fruit resistance. Furthermore, melatonin treatment significantly increased the contents of jasmonic acid and phlorizin in pear fruit, both of which could improve disease resistance. Jasmonic acid regulates melatonin synthesis and can also promote phlorizin synthesis, ultimately improving the resistance of pear fruit to ring rot. In summary, the interaction between melatonin and jasmonic acid and phlorizin enhances the antioxidant defense response and phenylpropanoid metabolism pathway of pear fruit, thereby enhancing the resistance of pear fruit to ring rot disease. Our results provide new insights into the application of melatonin in the resistance to pear fruit ring rot.

Keywords: Botryosphaeria dothidea; Disease resistance; Jasmonic acid; Melatonin; Pear; Phlorizin.

Fig. 1

Melatonin has no significant effect on the growth of B. dothidea mycelium. (A) Growth of B. dothidea on PDA medium containing different concentrations of melatonin. (B) B. dothidea lesion diameter under different concentrations of melatonin. (C, D) Microscopic observation of B. dothidea mycelial structure (C) and thickness (D). (E) Transmission electron microscopy observation of B. dothidea mycelial structure. Different letters indicate significant differences, according to one-way ANOVA Duncan’s multiple range test (P < 0.05)

Fig. 2

Melatonin inhibits mycelial growth in ‘Whangkeumbae’ fruits and enhances antioxidant capacity. (A) Growth of B. dothidea disease spots on the surface of ‘Whangkeumbae’ fruit after melatonin treatment. (B) Diameter of disease lesions on the surface of melatonin-treated fruit. (C) Spread of B. dothidea hyphae in ‘Whangkeumbae’ fruit pulp in the melatonin treatment. (D–F) Activity of antioxidant enzymes including PPO (D) POD (E), and CAT (F) in control and melatonin-treated fruit. Different letters indicate significant differences, according to one-way ANOVA Duncan’s multiple range tests (P < 0.05)

Fig. 3

Analysis of metabolomics and transcriptomics data. (A) Distribution of total metabolites in pear fruits. (B) Venn diagram showing the number of DAMs identified in the CK1vsMT1 and CK3vsMT3 comparisons. (C) KEGG enrichment analysis of DAMs identified in the CK1vsMT1 comparison. Count refers to the number of DEGs annotated according to the KEGG database. (D) Venn diagram showing the number of DEGs identified in the MT1vsCK1, MT3vsCK3, and MT5vsCK5 comparisons. (E–G) KEGG pathway enrichment analysis of DEGs identified in the MT1vsCK1, MT3vsCK3, and MT5vsCK5 comparisons. Arrows indicate pathways related to disease resistance

Fig. 4

Melatonin treatment regulates the phenylpropanoid synthesis pathway. (A, B) Changes in the contents of phenylpropanoid biosynthesis pathway-related metabolites (A) and expression levels of phenylpropanoid biosynthesis genes (B) in ‘Whangkeumbae’ pear flesh after melatonin treatment

Fig. 5

Melatonin promotes the biosynthesis of JA and phlorizin. (A, B) Contents of JA biosynthesis (A) and phlorizin biosynthesis (B) related metabolites. (C, D) Expression of JA biosynthesis (C) and phlorizin biosynthesis (D) related genes

Fig. 6

Phlorzin, JA, and MT synergistically promote resistance to B. dothidea. (A, B) Condition of B. dothidea-infected pear fruits after different treatments. (C) Content of melatonin in pear fruit. (D) Expression analysis of genes related to melatonin, JA, and phlorizin synthesis. Different letters indicate significant differences, according to one-way ANOVA Duncan’s multiple range test (P < 0.05)

Fig. 7

JA and phlorizin enhance the resistance of pear fruit to B. dothidea. (A, B) Changes in lesion size on B. dothidea-infected pear fruits after different treatments. (C) Content of JA in pear fruit. (D) Expression analysis of genes related to melatonin, JA, and phlorizin synthesis. Different letters indicate significant differences, according to one-way ANOVA Duncan’s multiple range test (P < 0.05)

Fig. 8

Proposed model for the mechanism of melatonin in improving the resistance of pear fruit to ring rot disease. In the model diagram of melatonin action, melatonin has no effect on the growth of B. dothidea. Melatonin can promote the synthesis of jasmonic acid and phlorizin in the fruit, all of which can improve the resistance of the fruit to ring rot disease. Jasmonic acid can feedback regulate the synthesis of melatonin, and also promote the synthesis of phlorizin. Ultimately, the resistance of the fruit is improved through antioxidant and phenylpropanoid metabolic pathways