Integrated Transcriptome and Metabolome Analysis Reveals\nInsights into Flavone and Flavonol Biosynthesis in Salicylic Acid-Induced Citrus Huanglongbing Tolerance

Salicylic\nacid (SA) exhibits positive effects against Citrus Huanglongbing (HLB), but how SA affects citrus\nresistance to HLB is currently unknown. This study conducted integrated\ntranscriptome and metabolome analyses on SA-treated Citrus sinensis (HLB-sensitive) and Poncirus trifoliata (HLB-tolerant). The results indicated\nthat the syntheses of flavones and flavonols were induced by SA, while\nthe expression levels of associated genes and the contents of corresponding\nmetabolites varied significantly between the two species after SA\ntreatment or HLB infection. These differences may underpin the enhanced\nHLB management through SA treatment and the inherent HLB tolerance\nof P. trifoliata. Furthermore, two\ninsertions of miniature inverted-repeat transposable element (MITE)\nwere identified within the promoter of <i>PtrF3′H</i> in P. trifoliata, whereas none were\nfound in the promoter of <i>CsF3′H</i> in C. sinensis. These MITE insertions notably enhanced\nthe promoter activity of <i>PtrF3′H</i> in an SA-dependent\nmanner. Our findings deepen the understanding of the correlation between\nSA response and HLB tolerance in Citrus.