Deciphering the roles of Triticum aestivum TaNACL-D1 transcription factor in resistance to Fusarium head blight disease

Transcription factors are master regulators of cellular events that coordinate responses to endogenous and exogenous signalling cues including pathogen derived stimuli. Plant specific NAC transcription factor family has been known to play a significant role in responses to pathogens. This study proposed a new TaNAC nomenclature and identified 146 pathogen-responsive TaNACs based on analysis of RNA-seq datasets of wheat tissue infected with major fungal pathogens. Fifty-two pathogen-responsive TaNACs were orthologous with functionally characterised defence-associated NACs from barley, rice, and Arabidopsis, as deduced via phylogenetic analysis. Three TaNAC subfamilies (‘a’, ‘e’ and ‘f’) were significantly enriched in pathogen-responsive TaNACs. The wheat NAM domain significantly diverged in sequence in subfamilies ‘f’, ‘g’ and ‘h’, based on HMMER and motif analysis. New protein motifs were identified in both the N- and C- terminal parts of TaNACs. Three of those identified in C-terminal part were linked to the pathogen-responsiveness of TaNACs and two were linked to expression in grain tissue. TaNACL-D1 is a Triticeae-specific NAC encoding an evolutionary divergent C-terminal subdomain and is a member of a structurally divergent subfamily ‘h’. The NAC domain of TaNACL-D1, situated in the N-terminal part of the protein, lacked some of the conserved subdomains typical of these proteins (subdomains B, C and D), as deduced via motif analysis. TaNACL-D1 is transcriptionally responsive to Fusarium graminearum and deoxynivalenol (DON), and its interacting partner is TaFROG proven to enhance resistance to Fusarium head blight (FHB) disease and enhance resistance to the mycotoxin DON. Thus, this study explored the divergent nature of TaNACL-D1 by assessing which part of TaNACL-D1 could transactivate in yeast-one hybrid system and which part of the protein could interact with TaFROG in planta. Bimolecular fluorescence complementation assay studies demonstrated that TaNACL-D1, its NAC domain and its C-terminal region could interact with TaFROG in planta, as could its subgenome A TaNACL-A1 but not its subgenome B TaNACL-B1 homoeolog. A transactivation assay showed that, unlike TaNACL-A1 and TaNACL-D1, TaNACL-B1 did not have transactivation activity, and the transactivation activity of TaNACL-D1 was situated within the divergent NAC domain. Further, the potential of TaNACL-D1 to enhance FHB resistance in wheat was explored. Four transgenic wheat lines overexpressing TaNACL-D1 showed a significant decrease in the spread of the disease in wheat heads, as compared to the wild type plants. Transcriptomic profiling was conducted to analyse the response to F. graminearum in wheat cv. Fielder overexpressing TaNACL-D1, as compared to wild type plants (at one day post-inoculation). Gene ontology and KEGG enrichment analysis showed that the transgenic line was uniquely or more enriched in pathogen-up-regulated transcripts associated with hormone pathways, detoxification, phenylpropanoid pathway, oxidative stress, and the immune response, as compared to wild type plants. Processes that were either uniquely or pathogen-down-regulated in the TaNACL-D1 overexpressor as compared to the wild type were related to growth, cell cycle, DNA repair, cytoskeleton, and development. Thirty-six transcripts were pathogen-up or down-regulated only in the transgenic line and were present at significantly higher and lower levels in the pathogen-treated transgenic line compared to the wild type line. Also, twenty transcripts were constitutively present at significantly higher or lower levels in the transgenic line compared to the wild type line. The putative function of the transcripts suggests that TaNACL-D1 constitutively up-regulates development and pathogen defence processes, up- and down-regulates brassinosteroid signalling, and down-regulates transport and primary metabolism.