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

Salicylic
acid (SA) exhibits positive effects against Citrus Huanglongbing (HLB), but how SA affects citrus
resistance to HLB is currently unknown. This study conducted integrated
transcriptome and metabolome analyses on SA-treated Citrus sinensis (HLB-sensitive) and Poncirus trifoliata (HLB-tolerant). The results indicated
that the syntheses of flavones and flavonols were induced by SA, while
the expression levels of associated genes and the contents of corresponding
metabolites varied significantly between the two species after SA
treatment or HLB infection. These differences may underpin the enhanced
HLB management through SA treatment and the inherent HLB tolerance
of P. trifoliata. Furthermore, two
insertions of miniature inverted-repeat transposable element (MITE)
were identified within the promoter of PtrF3′H in P. trifoliata, whereas none were
found in the promoter of CsF3′H in C. sinensis. These MITE insertions notably enhanced
the promoter activity of PtrF3′H in an SA-dependent
manner. Our findings deepen the understanding of the correlation between
SA response and HLB tolerance in Citrus.