Activation of the calcium signaling, mitogen-activated protein kinase cascade and phenylpropane metabolism contributes to the induction of disease resistance in pear fruit upon phenylalanine treatment

Phenylalanine, an aromatic amino acid, serves as a primary substrate for initiating phenylpropane metabolism and acts as a potent inducer of fruit defense responses. This study investigated the potential of phenylalanine to enhance disease resistance against Alternaria rot in pear fruit, along with elucidating the underlying mechanisms involved in its action. Results demonstrated that phenylalanine effectively inhibited the lesion diameter of Alternaria alternata-inoculated pear fruit, while exhibiting no significant suppression on the colony diameter in vitro. Phenylalanine treatment improved the gene expressions and enzymatic activities of phenylalanine ammonia-lyase, 4-coumarate coenzyme A ligase, cinnamate-4-hydroxylase, peroxidase, and polyphenol oxidase. Additionally, the expressions of PcC3H1, PcCAD, PcCOMT, PcCCR, PcCCoAOMT, PcF5H, PcF3H, and PcDFR were up-regulated to promote the accumulation of endogenous phenylalanine, caffeic acid, anthocyanin, flavonoids, lignin, and total phenolic compounds in pears. Furthermore, phenylalanine enhanced the transcription levels of PcCaM, PcCDPK2/3/5/11/13/26, PcCML27, PcWRKY3/11/22/24/40/51/69/71/75, PcMAPKKK1, PcMAPKK2/3, and PcMAPK3/4/6/7 in pears. These findings suggest that phenylalanine could enhance disease resistance in pears by positively regulating phenylpropane metabolism, the mitogen-activated protein kinase cascade, calcium signaling, and WRKY transcription factors.