Overexpression of two cold-responsive ATAF-like NAC transcription factors from fine-stem stylo ( Stylosanthes guianensis var. intermedia ) enhances cold tolerance in tobacco plants

Stylosanthes (stylo) species are economically important tropical and subtropical forage legumes. They are vulnerable to chilling and frost, and little is known about the genetic and molecular mechanisms of their responses or adaptations to low temperature stress. Two cold-responsive NAC genes, SgNAC1 and SgNAC2 , were selected from a whole transcriptome profiling study of fine-stem stylo ( S. guianensis var. intermedia ) and further investigated for their roles in cold stress tolerance. Bioinformatic analysis indicated that SgNAC1 and SgNAC2 belonged to ATAF subgroup of NAC family, and shared highly conserved N-terminal A–E NAC subdomains and a EVQS[E/x]PK[W/I] motif with ATAF-like NAC transcription factors. Expression profiling, subcellular location and transactivation assay revealed that SgNAC1 and SgNAC2 encode nucleus-localized polypeptides with transactivation activities and responds promptly to cold stress. Phenotypic and physiological changes indicated that the transgenic tobacco plants overexpressing SgNAC1 and SgNAC2 were more tolerant to cold stress than the wild-type plants. Meanwhile, the expression of SgNAC1 and SgNAC2 were significantly enhanced at the early stages of cold treatment, indicating that overexpression of SgNAC1 and SgNAC2 is sufficient to confer cold tolerance to tobacco plants. These results suggest that SgNAC1 and SgNAC2 are promising candidate genes for cold tolerance improvement strategies in stylo.