NRC4 gene cluster is not essential for bacterial flagellin-triggered immunity

Plants utilise cell surface pattern recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat containing receptors (NLRs) to fend off invading pathogens (Dodds and Rathjen, 2010; Win et al., 2012). Both types of immune receptors detect pathogen molecules directly or indirectly to activate complex immune responses and disease resistance (Kourelis and van der Hoorn, 2018). Although PRR- and NLR-triggered immunity are generally thought to activate distinct pathways, they can induce similar outputs such as production of reactive oxygen species (ROS) and hypersensitive cell death (Peng et al., 2018). Both PRR-and NLR-activated pathways involve calcium-dependent protein kinases, mitogen-activate protein kinases (MAPKs), phytohormone signalling, and transcriptional reprogramming (Peng et al., 2018). However, whether these two pathways converge at some point to potentiate and strengthen the immune response remains unclear. A recent study suggested that the tomato NLR helper NRC4 positively regulates the ROS burst induced by the bacterial flagellin peptide flg22 (Leibman-Markus et al. 2018b). We took advantage of the CRISPR/Cas9 system to knock out multiple NRC genes in tomato and Nicotiana benthamiana. Although these mutants failed to respond to the NRC-dependent NLRs, they remained unaltered in flg22-induced responses. We conclude that the NRC genes are not essential for flg22-induced responses in tomato and N. benthamiana.