Extracellular Antagonists: Offense and Counter-Defense in the Apoplast

Extracellular interactions are pivotal in the plant–pathogen arms race. Fungal, bacterial, and oomycete pathogens manipulate the apoplast to control pH, water soaking, sugar levels, and nutrient availability. Plants respond to infection with an oxidative burst and secrete toxic metabolites and harmful hydrolases, but adapted pathogens have evolved intricate ways to disarm reactive and toxic plant metabolites and inhibit or degrade hydrolases. Pathogens also avoid their recognition by hiding, degrading, or modifying their elicitors and disabling cell surface receptors in various ways. In this review, we summarize our current knowledge on fascinating offense and counter-defense mechanisms in extracellular plant–pathogen interactions.