Herbaceous peony tryptophan decarboxylase confers drought and salt stresses tolerance

Herbaceous peony (Paeonia lactiflora Pall.) is known as the king of herbaceous flowers. Drought stress seriously restricts P. lactiflora growth and reduces its ornamental value, but little is known about its underlying mechanism. In this study, one tryptophan decarboxylase gene (TDC), which is the first enzyme gene for melatonin biosynthesis, was obtained, and its expression level was found to be positively related to melatonin production during drought stress. In P. lactifiora, the first TDC that was isolated was comprised of 1849 bp with no intron, and was designated as PlTDC (KY765554). It encoded a 502-amino acid protein with a molecular weight of 56 kDa, which was localized in the cytoplasm of cells and catalyzed the reaction converting tryptophan into tryptamine. When PlTDC was transferred into tobacco, the transgenic plants produced approximately 1.67-fold higher levels of melatonin than that of wild-type control. Moreover, the transgenic plants had enhanced tolerance to drought and salt stresses, potentially due to the reduction in hydrogen peroxide (H2O2) and superoxide anion free radical (O-2(center dot-)) accumulation. In addition, the enhanced H2O2 and O-2(center dot-) scavenging under drought stress led to decreased cell membrane damage, increased potential photosynthetic capacity, and delayed plant senescence. RNA-Seq data correlated well with the protective role of melatonine because it was found an increase of gene expression involved in the oxidation-reduction process and photosynthesis. These results together suggested that PlTDC was a key factor in increasing melatonin production, to confer drought and salt stress tolerance in P. lactiflora.