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

NPR1 has been found to be a key transcriptional regulator in some plant defense responses. There are nine NPR1 homologs (TaNPR1) in wheat, but little research has been done to understand the function of those NPR1-like genes in wheat defense response against stem rust (Puccinia graminius f. sp tritici) pathogens. We used bioinformatics and reverse genetics approaches to study the expression and function of each TaNPR1. We found six members of TaNPR1 located on homeologous group 3 chromosomes (designated as TaG3NPR1) and three on homeologous group 7 chromosomes (designated as TaG7NPR1). The group 3 NPR1 proteins regulate transcription of SA-responsive PR genes. Down-regulation of all the TaNPR1 homologs via virus induced gene co-silencing resulted in enhanced resistance to stem rust. More specifically down-regulating TaG7NPR1 homeologs or Ta7ANPR1 expression resulted in stem rust resistance phenotype. In contrast, knocking down TaG3NPR1 alone did not show visible phenotypic changes in response to the rust pathogen. Knocking out Ta7ANPR1 enhanced resistance to stem rust. The Ta7ANPR1 locus alternatively spliced under pathogen inoculated conditions. We discovered a new mode of NPR1 action in wheat at the Ta7ANPR1 locus through an NB-ARC-NPR1 fusion protein negatively regulating defense response to stem rust infection.