How do plants achieve immunity? Defence without specialized immune cells

Key Points Unlike vertebrates, plants do not have an adaptive immune system. Nonetheless, plants can launch specific, self-tolerant immune responses and establish immune memory. To promote virulence, pathogens inject effector molecules that target conserved immune signalling hubs into the plant cell. In response, plants have evolved resistance (R) proteins that detect effector-induced perturbations in these hubs, providing the potential to specifically recognize a large number of pathogens with similar infection strategies through a smaller number of R proteins. Intraspecific and interspecific plant crosses suggest that autoimmunity can arise from self-reacting R proteins, illustrating the threat of uncontrolled R protein activity. Dynamic transcriptional and post-transcriptional regulation of R protein levels is thought to minimize the risk of autoimmunity in plants. Pathogen-infected tissues generate a mobile immune signal consisting of multiple proteins as well as lipid-derived and hormone-like molecules. These signal molecules are transported to systemic tissues, where they induce systemic acquired resistance (SAR). SAR is associated with the systemic reprogramming of thousands of genes to prioritize immune responses over routine cellular requirements. Epigenetic modifications and site-specific chromatin remodelling seem to provide a long-lasting memory of pathogen attack. They are also hypothesized to induce genome rearrangements in specific loci, which can be transmitted to subsequent generations. This Review article looks at how the features of vertebrate adaptive immunity — specificity, self-tolerance and memory — can be achieved in plants through different immune strategies. Vertebrates have evolved a sophisticated adaptive immune system that relies on an almost infinite diversity of antigen receptors that are clonally expressed by specialized immune cells that roam the circulatory system. These immune cells provide vertebrates with extraordinary antigen-specific immune capacity and memory, while minimizing self-reactivity. Plants, however, lack specialized mobile immune cells. Instead, every plant cell is thought to be capable of launching an effective immune response. So how do plants achieve specific, self-tolerant immunity and establish immune memory? Recent developments point towards a multilayered plant innate immune system comprised of self-surveillance, systemic signalling and chromosomal changes that together establish effective immunity.