Out of Water: The Origin and Early Diversification of Plant <i>R</i>-Genes

Abstract

During plant-pathogen interactions, plants use intracellular proteins with nucleotide-binding site and Leu-rich repeat (NBS-LRR) domains to detect pathogens. NBS-LRR proteins represent a major class of plant disease resistance genes (<i>R</i>-genes). Whereas <i>R</i>-genes have been well characterized in angiosperms, little is known about their origin and early diversification. Here, we perform comprehensive evolutionary analyses of <i>R</i>-genes in plants and report the identification of <i>R</i>-genes in basal-branching streptophytes, including charophytes, liverworts, and mosses. Phylogenetic analyses suggest that plant <i>R</i>-genes originated in charophytes and R-proteins diversified into TIR-NBS-LRR proteins and non-TIR-NBS-LRR proteins in charophytes. Moreover, we show that plant R-proteins evolved in a modular fashion through frequent gain or loss of protein domains. Most of the <i>R</i>-genes in basal-branching streptophytes underwent adaptive evolution, indicating an ancient involvement of <i>R</i>-genes in plant-pathogen interactions. Our findings provide novel insights into the origin and evolution of <i>R</i>-genes and the mechanisms underlying colonization of terrestrial environments by plants.