Fig. 3

Model depicting the possible roles of miRNAs and phased secondary siRNAs (phasiRNA) in the regulation of R-genes. A Under normal conditions, when the host plant is not challenged, constitutive and unregulated expression of R-genes results in high fitness costs; B In the direct targeting pathway of R-genes by miRNA, the MIR loci produce miRNA transcripts that are processed to mature miRNAs. Subsequently, mature miRNA is complexed with AGO1/7 and directly binds to R-gene transcript followed by cleavage, resulting in basal resistance response and concomitant effects on fitness cost (box with dashed line). In the indirect targeting pathway of R-genes by miRNAs, the mature miRNA is produced from MIR loci and interacts with AGO1. The AGO1-miRNA complex binds to PHAS transcripts produced from the coding region of PHAS loci and cleaves the PHAS transcripts in a sequence-specific manner. SGS3 and RDR6 convert the single-stranded RNA to long double-stranded RNA which is processed by DCL4 to phased siRNA (phasiRNA). AGO1/7-phasiRNA complex cleaves R- gene transcripts and maintain the basal level of R-gene expression to achieve optimized and well-balanced resource usage for defense and maintenance of plant fitness by robust growth and development. R genes-resistance genes; AGO1-ARGONAUTE 1; SGS3-SUPPRESSOR OF GENE SILENCING 3; RDR6- RNA-DEPENDENT RNA POLYMERASE 6; DCL4- DICER-LIKE 4