Defense and Offense Strategies: The Role of Aspartic Proteases in Plant–Pathogen Interactions

SIMPLE SUMMARY: Plants are sessile organisms that are continuously exposed to adverse environmental factors, both abiotic and biotic. Plant immunity is an intricate system that involves a remarkable array of structural, chemical, and protein-based layers of defense, aiming to stop pathogens before they cause irreversible damages. Proteases are an integral part of plant defense systems, with several hubs of action, from pathogen recognition and priming to the activation of plant hypersensitive response. Within this wide group of proteolytic enzymes, aspartic proteases have been implicated in several plant development functions and are gaining more prominence due to their involvement in plant–pathogen interactions. In this review, we summarize the current knowledge on plant and pathogenic aspartic proteases and highlight the most recent findings on their participation on plant defense, as well as in pathogen infection strategies. ABSTRACT: Plant aspartic proteases (APs; E.C.3.4.23) are a group of proteolytic enzymes widely distributed among different species characterized by the conserved sequence Asp-Gly-Thr at the active site. With a broad spectrum of biological roles, plant APs are suggested to undergo functional specialization and to be crucial in developmental processes, such as in both biotic and abiotic stress responses. Over the last decade, an increasing number of publications highlighted the APs’ involvement in plant defense responses against a diversity of stresses. In contrast, few studies regarding pathogen-secreted APs and AP inhibitors have been published so far. In this review, we provide a comprehensive picture of aspartic proteases from plant and pathogenic origins, focusing on their relevance and participation in defense and offense strategies in plant–pathogen interactions.