Genome-wide analysis of UDP-glycosyltransferases family and identification of UGT genes involved in abiotic stress and flavonol biosynthesis in Nicotiana tabacum.BACKGROUND
 Uridine disphosphate (UDP) glycosyltransferases (UGTs) act upon a huge variety of highly diverse and complex substrates, such as phytohormones and specialized metabolites, to regulate plant growth, development, disease resistance, and environmental interactions.  However, a comprehensive investigation of UGT genes in tobacco has not been conducted.


 In this study, we carried out a genome-wide analysis of family-1 UDP glycosyltransferases in Nicotiana tabacum.  We predicted 276 NtUGT genes, which were classified into 18 major phylogenetic subgroups.  The NtUGT genes were invariably distributed among all the 24 chromosomes with structural diversity in exon/intron structure, conserved motifs, and cis-acting elements of promoters.  Three groups of proteins which involved in flavonoid biosynthesis, plant growth and development, transportation and modification were identified that interact with NtUGT proteins using the PPI analysis.  Expression analysis of NtUGT genes in cold stress, drought stress and different flower color using both online RNA-Seq data and the realtime PCR analysis, suggested the distinct role of NtUGT genes in resistance of cold, drought and in flavonoid biosynthesis.  The enzymatic activities of seven NtUGT proteins that potentially involved in flavonoid glycosylation were analyzed, and found that all seven exhibited activity on myricetin; six (NtUGT108, NtUGT123, NtUGT141, NtUGT155, NtUGT179, and NtUGT195) showed activity on cyanidin; and three (NtUGT108, NtUGT195, and NtUGT217) were active on the flavonol aglycones kaempferol and quercetin, which catalyzing the substrates (myricetin, cyanidin or flavonol) to form new products.  We further investigated the enzymatic products and enzymatic properties of NtUGT108, NtUGT195, and NtUGT217, suggested their diverse enzymatic activity toward flavonol, and NtUGT217 showed the highest catalyzed efficient toward quercetin.  Overexpression of NtUGT217 significantly increase the content levels of the quercetin-3-O-glucoside, quercetin-3-O-rutinoside and kaempferol-3-O-rutinoside in transgenic tobacco leaves.


 CONCLUSION
 We identified 276 UGT genes in Nicotiana tabacum.  Our study uncovered valuable information about the phylogenetic structure, distribution, genomic characters, expression patterns and enzymatic activity of NtUGT genes in tobacco.  We further identified three NtUGT genes involved in flavonoid biosynthesis, and overexpressed NtUGT217 to validate its function in catalyze quercetin.  The results provide key candidate NtUGT genes for future breeding of cold and drought resistance and for potential metabolic engineering of flavonoid compounds.
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