The members of the microRNA (miRNA) family exert essential roles in modulating plant growth and development as well as responses to diverse stresses, through negatively regulating their target genes at posttranscriptional or translational levels. In this study, we characterized taemiR9674a, a miRNA member in T. aestivum, in mediating plant responses to drought and salt stresses. Seven genes in total were predicted to act as the targets of tae-miR9674a via modulation of transcript cleavage. The transcripts of tae-miR9674a in roots and leaves were response to both stresses of drought and salt, displaying to be gradually upregulated following the progression of a 27-h regime of above stress treatments. The transgenic tobacco lines of tae-miR9674a exhibited modified growth traits under drought and salt treatments. Of these, the line with miRNA overexpression (i.e., Sen 1) improved drastically on plant biomass, leaf area, and root length, whereas that with its knockdown expression (Anti 1) significantly alleviated on above growth traits compared with wild type. The modified stress responses of tae-miR9674a were shown to be closely associated with the role of miRNA in regulating a suite of physiological parameters, of which Sen 1 displayed improved osmotic stress defensive-related traits, such as fastened stomata closing rate, increased leaf water retention capacity, enhanced osmolytes contents, and elevated antioxidant enzyme (AE) activities. The expression of NtP5CS1 involving proline biosynthesis and NtFeSOD, NtCAT1 and NtPOD4, the AE genes involved in modulating ROS homeostasis, was upregulated in Sen 1 upon drought and salt stresses, suggesting their involvement in miRNA-mediated plant drought and salt responses. Transcriptome analysis indicated that tae-miR9674a leads to modified expression of quantities of genes that functionally associate with GO terms "biological process", "cellular component", and "molecular function", which are overrepresented by the biochemical pathways of phytohormones (i.e., ethylene and jasmonic acid), salt response, salt/drought osmotic stress response through abscisic acid-dependent pathway and reactive oxygen species (ROS) homeostasis. Our investigation suggested that tae-miR9674a is an essential mediator in plant osmotic stress tolerance by positively regulating osmotic stress acclimation, cellular ROS homeostasis, and related defensive processes.
