Ethylene regulates miRNA-mediated lignin biosynthesis and leaf serration in Arabidopsis thaliana

microRNAs (miRNAs) regulate target gene expression by pairing to target mRNAs, leading to mRNA degradation or translation inhibition. Out of several miRNAs in Arabidopsis, miR397b and miR857 regulate secondary growth by modulating lignin polymerization and deposition in secondary xylem cells by targeting laccases. Interestingly, the phytohormone ethylene is also suggested to have a role in lignin biosynthesis in tension wood formation. Despite this information, it is not known whether ethylene has any role in controlling secondary growth via miRNAs-mediated pathways. In this study, we elucidate that ethylene acts upstream to the miR397b/miR857-laccases module and negatively regulates lignin biosynthesis by directly activating the expression of both the miRNAs. The binding of EIN3 to the promoter of miR397b is further validated by yeast one-hybrid assay. In addition to its role in lignification, ethylene also regulates leaf serration by directly regulating the expression of NAC transcription factors, like CUP-SHAPED COTYLEDON2 (CUC2) and CUC3. Together, our study suggests a novel mechanism involving ethylene and miRNAs in lignin biosynthesis and leaf serration in Arabidopsis thaliana.