Two recently duplicated maize NAC transcription factor paralogs are induced in response to Colletotrichum graminicola infection

BACKGROUND: NAC transcription factors belong to a large family of plant-specific transcription factors with more than 100 family members in monocot and dicot species. To date, the majority of the studied NAC proteins are involved in the response to abiotic stress, to biotic stress and in the regulation of developmental processes. Maize NAC transcription factors involved in the biotic stress response have not yet been identified. RESULTS: We have found that two NAC transcription factors, ZmNAC41 and ZmNAC100, are transcriptionally induced both during the initial biotrophic as well as the ensuing necrotrophic colonization of maize leaves by the hemibiotrophic ascomycete fungus C. graminicola. ZmNAC41 transcripts were also induced upon infection with C. graminicola mutants that are defective in host penetration, while the induction of ZmNAC100 did not occur in such interactions. While ZmNAC41 transcripts accumulated specifically in response to jasmonate (JA), ZmNAC100 transcripts were also induced by the salicylic acid analog 2,6-dichloroisonicotinic acid (INA). To assess the phylogenetic relation of ZmNAC41 and ZmNAC100, we studied the family of maize NAC transcription factors based on the recently annotated B73 genome information. We identified 116 maize NAC transcription factor genes that clustered into 12 clades. ZmNAC41 and ZmNAC100 both belong to clade G and appear to have arisen by a recent gene duplication event. Including four other defence-related NAC transcription factors of maize and functionally characterized Arabidopsis and rice NAC transcription factors, we observed an enrichment of NAC transcription factors involved in host defense regulation in clade G. In silico analyses identified putative binding elements for the defence-induced ERF, Myc2, TGA and WRKY transcription factors in the promoters of four out of the six defence-related maize NAC transcription factors, while one of the analysed maize NAC did not contain any of these potential binding sites. CONCLUSIONS: Our study provides a systematic in silico analysis of maize NAC transcription factors in which we propose a nomenclature for maize genes encoding NAC transcription factors, based on their chromosomal position. We have further identified five pathogen-responsive maize NAC transcription factors that harbour putative binding elements for other defence-associated transcription factors in the proximal promoter region, indicating an involvement of the described NACs in the maize defence network. Our phylogenetic analysis has revealed that the majority of the yet described pathogen responsive NAC proteins from all plant species belong to clade G and suggests that they are phylogenetically related.