A new mode of NPR1 action via an NB-ARC-NPR1 fusion protein negatively regulates the defence response in wheat to stem rust pathogen

NPR1 has been found to be a key transcriptional regulator in some plant defence responses. There are nineNPR1homologues (TaNPR1) in wheat, but little research has been done to understand the function of thoseNPR1-like genes in the wheat defence response against stem rust (Puccinia graminisf. sp.tritici) pathogens. We used bioinformatics and reverse genetics approaches to study the expression and function of eachTaNPR1. We found six members ofTaNPR1located on homoeologous group 3 chromosomes (designated asTaG3NPR1) and three on homoeologous group 7 chromosomes (designated asTaG7NPR1). The group 3 NPR1 proteins regulate transcription of SA-responsivePRgenes. Downregulation of all theTaNPR1homologues via virus-induced gene co-silencing resulted in enhanced resistance to stem rust. More specifically downregulatingTaG7NPR1homeologues orTa7ANPR1expression resulted in stem rust resistance phenotype. By contrast, knocking downTaG3NPR1alone did not show visible phenotypic changes in response to the rust pathogen. Knocking outTa7ANPR1enhanced resistance to stem rust. TheTa7ANPR1locus is alternatively spliced under pathogen inoculated conditions. We discovered a new mode of NPR1 action in wheat at theTa7ANPR1locus through an NB-ARC-NPR1 fusion protein negatively regulating the defence to stem rust infection.