Regulation of gene expression can occur at both transcriptional and BAY 73-4506 msds post-transcriptional levels. In recent years, the discovery of numerous microRNAs has increased interest in posttranscriptional gene expression regulation during development and other biological processes. Plant miRNA-guided gene regulation has been shown to be involved in multiple plant processes including response to environmental stresses, developmental transitions, phase switch from vegetative growth to reproductive growth, organ polarity, tissue differentiation and development, auxin signaling and RNA metabolism . Several miRNA families had been reported to be involved in root development modulation in both Arabidopsis and rice. Consistent with recent notion that numerous signaling pathways are implicated in root development, these miRNAs are implicated in auxin signaling, nutrition metabolism, or stress response and have potential role in mediating the signal interactions. Some miRNA families, such as miR160, miR164, miR167, and miR390, mediated auxin signaling in roots, and they had been demonstrated to be involved in root cap formation, lateral root development, or adventitious rooting . MiR395 had been recognized as a key regulator in sulphate metabolism in both Arabidopsis and rice . MiR398 was found to be involved in copper and zinc homeostasis through its post-transcriptional effects on CSD genes . MiR399 was a well-characterized modulator implicated in phosphate starvation response in Arabidopsis , and the juvenile-to-adult transition in Arabidopsis is mediated by sequentially operating miR156 and miR172 . In rice, miR169 g is induced by drought, and the induction is more prominent in roots than in shoots . It was also reported that miR163 was involved in secondary metabolism in Arabidopsis . Topping is an important and essential cultivating measure for tobacco, and miRNA-guided post-transcriptional regulation might be involved in the response of tobacco to topping. Therefore, the identification of miRNAs could be a critical step to facilitate our understanding of the molecular regulation mechanisms of tobacco response to topping. There have been some studies to discover miRNAs and analyze their functions in tobacco , but no studies have been reported on discovering tobacco roots miRNAs before and after topping. In the present study, two sRNA libraries were generated from tobacco roots before and after topping, and a large number of miRNAs from tobacco roots were identified. The targets of miRNAs which change markedly belong to 53 miRNA families and have different biological functions including development, response to stress, response to hormone, N metabolism, C metabolism, signal transduction, Silmitasertib nucleic acid metabolism and other metabolism. The results indicated that these differential miRNAs play vital roles in the response to topping.