Molecular sensors for plant immunity; pattern recognition receptors and race-specific resistance proteins

Plants have to molecularly sense invasions from pathogenic microbes to activate their built-in immune responses. There are two different types of sensor proteins, called immune receptors. They are the indispensible molecular instruments to perceive non-self molecules derived from microbes. A genetic defect of the immune receptors fails to activate immune responses, consequently resulting in disease susceptibility. In general, membrane-bound immune receptors, known to be pattern recognition receptors and exposed on the outside of the cell, recognize microbe-associated molecular patterns from pathogens. Intracellular immune receptors, also called plant disease resistance proteins, directly perceive pathogen-derived effectors or indirectly recognize the effector-mediated modification of host proteins inside the cells. In this review, we introduce the classes and functions of pattern recognition receptors that were molecularly identified so far. Additionally, we summarize recent progresses in structural functions and molecular dynamics of the plant disease resistance proteins.