The metabolites involved in phenylpropanoid biosynthesis increase the susceptibility of octoploid strawberry to crown rot caused by Colletotrichum siamense

Crown rot caused by Colletotrichum siamense is a serious disease affecting strawberry that leads to large-scale losses, both in their quality and yield, in the Yangtze River Basin, China. Secondary metabolites are known to play pivotal roles in defense responses against invading pathogens, and identifying metabolites involved in crown rot is necessary for understanding induced resistance and developing improved genetic resistance against strawberry crown rot. To explore this research topic, the infection process was verified via observation of the phenotypes and histopathology of susceptible ‘Benihoppe’ strawberry inoculated with C. siamense. Following this, metabolomic analysis was used that identified phenolic acids, flavonoids, and amino acids as the metabolites accumulated in the rotten crown caused by the C. siamense infection process. Furthermore, the transcripts encoding enzymes involved in the ‘phenylpropanoid biosynthesis’ of phenolic acids, trans-cinnamate 4-monooxygenase, caffeoylshikimate esterase, and caffeic acid 3-O-methyltransferase were all upregulated in the rotten crown, as shown by transcriptomic analysis and qRT-PCR. In addition, the metabolites p-coumaric acid, caffeic acid, and ferulic acid in the ‘phenylpropanoid biosynthesis’ pathway all enhanced C. siamense infection, as shown by an exogenous validation experiment. Together, the results demonstrate that the ‘phenylpropanoid biosynthesis’ pathway was induced during crown rot and thus highlight the potentially crucial regulatory role of this pathway in plant immunity.