Comprehensive analysis of differential genes and miRNA profiles for discovery of topping-responsive genes in flue-cured tobacco roots

Decapitation/topping is an important cultivating measure for flue-cured tobacco, and diverse biology processes are changed to respond to the topping, such as hormonal balance, root development, source-sink relationship, ability of nicotine synthesis and stress tolerance. The purpose of this study was to clarify the molecular mechanism involved in the response of flue-cured tobacco to topping. The differentially expressed genes and micro RNAs (miRNAs) before and after topping were screened with a combination of suppression subtractive hybridization (SSH) and miRNA deep sequencing. In all, 560 differently expressed clones were sequenced by SSH, and then 129 high quality expressed sequence tags were acquired. These expressed sequence tags were mainly involved in secondary metabolism (13.5%), hormone metabolism (4%), signaling/transcription (17.5%), stress/defense (20%), protein metabolism (13%), carbon metabolism (7%), other metabolism (12%) and unknown function (13%). The results contribute new data to the list of possible candidate genes involved in the response of flue-cured tobacco to topping. NAC transcription factor, a differential gene identified by SSH, had been proved to have a role in the regulation of nicotine biosynthesis. High-throughput sequencing of two small RNA libraries in combination with SSH screening revealed 15 differential miRNAs whose target genes were identical to some differential genes identified in SSH, suggesting that miRNAs play a critical role in post-transcriptional gene regulation in the response of flue-cured tobacco to decapitation. Based on the role of these miRNAs and differential genes identified from SSH in response to topping, an miRNA mediated model for flue-cured tobacco in response to topping is proposed.