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Phytosterols are membrane components or precursors for biosynthesis of brassinosteroids(BRs).Since they are not able to be long-distance transported,their homeostasis is tightly controlled by biosynthesis and metabolism.We found osamiR1848 is involved in their homeostasis regulation.Transgenic rice(Oryza sative L.)of the artificial osa-miR1848 and its target OsCYP51G3 were used to analyze the role of osa-miR1848 in mediating the phytosterol biosynthesis.osa-miR1848 directs the cleavage of OsCYP51G3 mRNA to mediate biosynthesis of phytosterols and BRs in rice.OsCYP51G3 as one of osa-miR1848 target genes is supported by inverse correlation of expression between osa-miR1848 and OsCYP51G3 in the transgenic rice plants,and cleavage sites of OsCYP51G3 mRNA.Both of osa-miR1848 up-expression and OsCYP51G3 down-expression reduced the levels of phytosterols and BRs,and caused typical phenotypic changes of deficit phytosterols and BRs,including dwarf plant,erect leaf,semi-sterile pollen,and short cell.Parallel expression analysis revealed that osa-miR1848 circadian regulates the transcript abundance of OsCYP51G3 in developing organs and in response to salt stress.We proposed that osa-miR1848 mediates OsCYP51G3 expression posttranscriptionally and affects biosynthesis of phytosterols and BRs,and they are potential for rice breeding to modulate the leaf angle,the seed size and quality.