Identification of the NAC Family and a Functional Analysis of NoNAC36a Under Flooding Stress in Watercress (Nasturtium officinale R.Br.)

Watercress (Nasturtium officinale R.Br.) is a cruciferous aquatic vegetable that possesses significant nutritional value. The NAC family is a transcription factor family specific to plants that play an important role in regulating plant responses to abiotic stress. In order to investigate the response of NAC genes to flooding stress in watercress, we conducted a study on the NoNAC family. In this study, a total of 119 NoNAC genes were obtained through genome-wide identification. Phylogenetic analysis indicated that the NoNAC family members can be categorized into ten subgroups. The results of gene structure analysis revealed that each branch within the subgroups exhibited similar motif composition and gene structure. The heatmap analysis showed that several NoNAC genes demonstrated tissue-specific expression patterns, suggesting their potential as regulators of associated tissue development. As an aquatic plant, watercress serves as a valuable material for investigating plant resistance to flooding stress. This study found that flooding can significantly increase the watercress plant height, which is a typical escape strategy under flooding. The analysis of the expression of NoNAC genes in the stem transcriptome after flooding indicated that only NoNAC36a consistently exhibited significant differential changes and down-regulated expression at the three time points of flooding treatment. This suggests that NoNAC36a may be involved in regulating watercress plant height increases under flooding stress. The utilization of a virus-induced gene silencing assay to investigate the biological function of NoNAC36a revealed that NoNAC36a silencing caused cell elongation and expansion, thus increasing watercress plant height. The yeast one-hybrid and dual luciferase assays demonstrated that NoNAC36a binds the promoter of NoXTH33 and inhibits its expression. Subsequently, the results of yeast two-hybrid, luciferase complementary, and pull-down assays revealed the interaction between NoMOB1A and NoNAC36a in vivo and in vitro. Sequence alignment indicated that NoMOB1A and AtMOB1A share an identical amino acid sequence. RT-qPCR analysis indicated that flooding prompted the expression of NoMOB1A in stems. Thus, it is speculated that NoMOB1A may exhibit functions similar to AtMOB1A and that the up-regulation of NoMOB1A expression in stems may facilitate an increase in plant height under flooding. In summary, the NoNAC family was analyzed, and revealed a regulatory network centered on NoNAC36a that facilitates watercress resistance to flooding stress. This study enhanced the understanding of the NoNAC genes and established a theoretical foundation for investigating plant flooding tolerance.