Defining Arabidopsis NPR1 orthologues in crops for translational plant immunity

Arabidopsis has been the model dicot plant system to study a variety of processes including immunity. One type of immunity, termed systemic acquired resistance (SAR), is deployed upon detection of avirulent pathogens by a plant. SAR deployment is dependent on the small molecule salicylic acid (SA) and confers immunity against a broad range of biotrophic pathogens. In this process, SA is perceived by NPR1, a key transcriptional co-activator. Recruitment of SA by NPR1 leads to changes in the conformation of NPR1, which frees a C-terminal transactivation domain from the auto-inhibitory effect of the N-terminal BTB/POZ domain. During this unfolding process, NPR1 is itself recruited by TGA2 and together they form an enhanceosome capable of activating downstream genes, including the SAR marker gene PR1. NPR1 is conserved in sequence and function in both dicot and monocot species. As such, it is portable from one plant species to another. In Arabidopsis, NPR1 is part of a multigene family containing six members, labelled NPR1 to NPR6, which show no redundancy with respect to NPR1 function in immunity. Arabidopsis NPR1 and NPR2 share 63% amino acid sequence identity and yet NPR2 is not a functional NPR1 with respect to immune functions. When looking across plant species, the per cent amino acid identity between Arabidopsis NPR1 and potential NPR1 orthologues tends to be below the 63% shared between NPR1 and NPR2. In this paper, we will show that relying on sequence identity alone to identify NPR1 orthologues can be misleading.