Differential Roles of Circular RNAs and Their Homologous Linear RNAs in Hevea brasiliensis Immunity Against Erysiphe...

Hevea brasiliensis (H. brasiliensis) is the principal source of natural rubber, but its productivity is severely threatened by powdery mildew caused by the biotrophic fungus Erysiphe quercicola (E. quercicola). Although circular RNAs (circRNAs) are emerging as key regulators in plant stress responses, their functions in H. brasiliensis immunity remain largely unexplored. Here, we aimed to systematically characterize circRNAs involved in the early immune response of H. brasiliensis to E. quercicola. Transcriptome sequencing and bioinformatic analyses identified 52 and 177 differentially expressed circRNAs (HbcircRNAs) at 1 and 3 days post-inoculation, respectively. Twelve HbcircRNAs with significant expression changes were validated, and nine were confirmed as true circRNAs. Functional assays using spray-induced gene silencing (SIGS) in H. brasiliensis leaves and heterologous overexpression in the Arabidopsis thaliana eds1 mutant, which is susceptible to E. quercicola, revealed that HbcircARF3, HbcircSCSA1, and HbcircARFGAP8, together with their homologous linear counterparts (HbLinearRNAs), exert distinct regulatory effects on disease resistance. Silencing of these HbcircRNAs enhanced host immunity, whereas overexpression increased susceptibility. Pathway analyses suggested their involvement in auxin signaling, mitochondrial energy metabolism and vesicle trafficking. Collectively, our findings uncover the differential regulatory roles of circular and linear RNAs in the H. brasiliensis-E. quercicola interaction, providing mechanistic insights and potential molecular targets for breeding disease-resistant H. brasiliensis.