RD21-like proteases: key effector hubs in plant–pathogen interactions

Over the past decades, numerous studies have demonstrated that proteases serve as a crucial regulatory mechanism in controlling plant immunity. In this review, we specifically focus on the role of one subfamily of RD21-like papain-like cysteine proteases that carry a C-terminal granulin domain. These proteases share high homology but have been described under very different names in different plant species. We provide a comprehensive overview of the background, endogenous regulation, and subcellular localization of RD21-like proteases in plants. Notably, RD21-like proteases act in immunity against various pathogens and they are targeted by many unrelated pathogen-secreted effectors that inactivate, mislocalize, or degrade RD21-like proteases. We highlight open questions and strategies to use this knowledge to develop innovative approaches for crop protection.