Refractory Behavior in Plant Cells—Calcium Signaling Induced by Biotic Stress

When in contact with microbes or other pathogens plants develop an induced defense response. This reaction is triggered by pathogen-derived molecules that provoke the so-called microbe-associated molecular pattern (MAMP)-triggered immunity (MTI) or pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). Recognition of a MAMP or PAMP by a pattern recognition receptor (PRR) activates rapid downstream signaling, manifested in, e.g., a rise in the cytosolic Ca2+ concentration. As a consequence, defense-related genes are expressed and antimicrobial substances are produced. There is also evidence that Ca2+-induced responses show a refractory behavior in plant cells, as the reaction to an identical stimulus applied shortly after the first one is strongly suppressed, if it can be observed at all. Subsequent elicitations over a longer period of time, on the other hand, can trigger stronger Ca2+ responses, which lead to so-called “defense priming”. Although refractory behavior has been documented in various plant cell types, its underlying function and causative mechanisms remain unclear. In this review article we give an overview of the refractory machinery, including elicitors, receptors, typical Ca2+ responses, and signal transduction pathways. We shed light on possible explanatory scenarios and address open questions.