The transition of plants from sea to land sparked an arms race with pathogens. The increased susceptibility of land plants is largely thought to be due to their dependence on micro-organisms for nutrients; the ensuing co-evolution has shaped the plant immune system. By profiling the immune receptors across flowering plants, we identified species with low numbers of NLR immune receptors. Interestingly, four of these species represent distinct lineages of monocots and dicots that returned to the aquatic lifestyle. Both aquatic monocot and dicot species lost the same well-known downstream immune signalling complex (EDS1-PAD4). This observation inspired us to look for other genes with a similar loss pattern and allowed us to predict putative new components of plant immunity. Gene expression analyses confirmed that a group of these genes was differentially expressed under pathogen infection. Excitingly, another subset of these genes was differentially expressed upon drought. Collectively, our study reveals the minimal plant immune system required for life under water, and highlights additional components required for the life of land plants.
Plant resistance to pathogens is commonly mediated by a complex gene family, known as NLRs. Upon pathogen infection, changes in the cellular environment trigger NLR activation and subsequent defence responses. Despite the dependence of agricultural practices on NLR genes to control pathogen load, relatively little is known about this gene family outside of model crop species. In this study, we identified a convergent reduction in the NLR gene family among two lineages of aquatic plants. Furthermore, we established that NLR reduction occurred in conjunction with the loss of a common immune signalling pathway. Subsequently, we identified other genes convergently lost in aquatic species and propose these as candidate components of the plant immune signalling pathway. In addition, we revealed components of the agronomically important drought response to be lost in aquatic plants. This study adds to our understanding of the complex interactions between environment and response to biotic stress, widely known as the disease triangle. The pathways identified in this study shed further light on the link between responses to drought and disease.
