Arachidonic acid inhibits Alternaria growth and reshapes phenylpropanoid–flavonoid metabolism in Pyrus betulifolia

Abstract

Arachidonic acid (AA) can elicit plant responses, but its effects on a woody pear rootstock and their relationship with disease outcomes remain insufficiently characterized. In this study, tissue-cultured seedlings of Pyrus betulifolia Bunge and an Alternaria alternata isolate were subjected to four treatments: CK (0% AA), PA (0.1% AA, v/v), PB (0.01% AA, v/v), and PC (0.001% AA, v/v). Transcriptomic and untargeted metabolomic analyses were first conducted on leaves treated with AA without pathogen inoculation, and WGCNA, O2PLS, and KEGG integration was used to identify cross-omics candidate pathways. After these analyses suggested potential AA-associated disease resistance, pathogen-related phenotypes were evaluated in independent in vitro antifungal and tissue-cultured seedling inoculation assays, and the 72-h endpoint responses of candidate pathways were assessed by qRT-PCR and targeted LC-MS/MS. The mycelial growth inhibition rates of PA, PB, and PC were 44.07%, 27.78%, and 7.78%, respectively, and the corresponding leaf disease incidences in tissue-cultured seedlings were 0%, 0%, and 30.20%, all lower than the 100% incidence in CK. Transcriptomic and metabolomic scores were highly correlated on both O2PLS joint components, and phenylpropanoid biosynthesis (KEGG 00940) and flavonoid biosynthesis (KEGG 00941) received the most coherent support across evidence levels. Of the nine genes assessed by qRT-PCR, eight showed lower mean expression than CK in all three AA treatments, whereas PbPR1 was reduced in PA and PB but remained close to CK in PC. Of the 37 targeted analytes, 27 were quantified; phenylalanine, luteolin, myricetin, kaempferide, epicatechin, and quercetin 3-O-glucoside increased in all three treatments, whereas free ferulic acid and quercetin decreased in all three treatments, indicating concentration-specific remodeling among the associated precursor, free, and glycoside-conjugated metabolite pools. In conclusion, AA-containing treatments were associated with inhibited A. alternata growth, reduced leaf disease incidence in tissue-cultured P. betulifolia seedlings, and altered phenylpropanoid–flavonoid metabolite pools; the molecular effects of AA and ethanol were not separated. These parallel observations do not establish that the host metabolic changes mediate disease protection.