Functional characterization of two NAC transcription factors HfNAP1 and HfNAC090 associated with flower programmed cell death in daylily (Hemerocallis fulva)

Daylily (Hemerocallis fulva) is one of the most widely used perennial flowers, but its ornamental and economic value is greatly limited due to its ephemeral flowering period. In general, the flower senescence is regulated by the developmental signals and considered as an irreversible process of programmed cell death (PCD). However, the molecular mechanism of flower PCD in daylily still remains unclear. In this study, two NAC transcription factors, namely HfNAP1 and HfNAC090, are first identified and found to be upregulated significantly in both the age-induced and the ABA-induced flower PCD processes in daylily. Then, the functions of HfNAP1 and HfNAC090 in regulating the flower PCD are investigated through transgenic phenotypes analysis. The results demonstrate that the ectopic and transient overexpression of these two genes can effectively regulate the flower PCD in tobacco and daylily. While the overexpression of HfNAP1 accelerates the flower PCD process, the overexpression of HfNAC090 significantly delays that. Furthermore, the yeast two-hybrid assay is performed to discover potential interactions related to these two genes, and the results demonstrate that HfNAP1 and HfNAC090 can interact with each other, or interact with other flower aging-related genes. Additionally, the yeast one-hybrid assay suggests that HfNAP1 and HfNAC090 can bind directly to the promoters of downstream senescence-associated genes HfSAG39 and HfSAG15. Taken overall, this study provides sufficient evidences to confirm that HfNAP1 and HfNAC090 play dominant roles in regulating the flower PCD in daylily, supporting the development of new strategies to prolong the longevity of daylily flowers.