New insights in plant immunity signaling activation

► Effector-mediated enzymatic activity can be required in both direct and indirect R/Avr recognition systems. ► Some NB-LRRs can activate defense signaling from the cytosolic compartment. ► Crystal structures of two functional NB-LRRs signaling domains have been determined for the first time. ► Receptor oligomerization is a prerequisite for signaling. Plant disease resistance can be triggered by specific recognition of microbial effectors by plant nucleotide binding-leucine rich repeat (NB-LRR) receptors. Over the last few years, many efforts have greatly improved the understanding of effector and NB-LRR function, but have left a lot of questions as to how effector perception activates NB-LRR induction of defense signaling. This review describes exciting new findings showing similarities and differences in function of diverse plant NB-LRR proteins in terms of pathogen recognition and where and how resistance proteins are activated. Localization studies have shown that some NB-LRRs can activate signaling from the cytosol while others act in the nucleus. Also, the structural determination of two NB-LRR signaling domains demonstrated that receptor oligomerization is fundamental for the activation of resistance signaling.