This study compares the receptor-like kinase (RLK) family in Arabidopsis and rice. RLKs are a large gene family involved in plant growth, development, and defense. The Arabidopsis genome contains over 600 RLK members, while rice has nearly twice as many, indicating a significant expansion in the rice lineage. Phylogenetic analysis suggests that the common ancestor of Arabidopsis and rice had over 440 RLK/Pelle members, with lineage-specific expansions occurring in both species since their divergence. The extracellular domains of RLKs have higher nonsynonymous substitution rates than intracellular domains, consistent with their role in sensing signals. Tandem duplication is the major mechanism for recent expansions in rice, while both tandem and large-scale duplications contribute to expansions in Arabidopsis. Defense-related RLKs have undergone more duplication events than developmental ones. The study also shows that the extracellular domains of RLKs are under different selective pressures than intracellular domains, with ECDs evolving faster. These findings suggest that most recent expansions of the RLK/Pelle family are related to defense and resistance functions. The RLK/Pelle family is one of the largest in Arabidopsis, with over 600 members, and is thought to have expanded significantly in the land plant lineage. The study provides insights into the evolutionary history and functional roles of RLKs in plants.This study compares the receptor-like kinase (RLK) family in Arabidopsis and rice. RLKs are a large gene family involved in plant growth, development, and defense. The Arabidopsis genome contains over 600 RLK members, while rice has nearly twice as many, indicating a significant expansion in the rice lineage. Phylogenetic analysis suggests that the common ancestor of Arabidopsis and rice had over 440 RLK/Pelle members, with lineage-specific expansions occurring in both species since their divergence. The extracellular domains of RLKs have higher nonsynonymous substitution rates than intracellular domains, consistent with their role in sensing signals. Tandem duplication is the major mechanism for recent expansions in rice, while both tandem and large-scale duplications contribute to expansions in Arabidopsis. Defense-related RLKs have undergone more duplication events than developmental ones. The study also shows that the extracellular domains of RLKs are under different selective pressures than intracellular domains, with ECDs evolving faster. These findings suggest that most recent expansions of the RLK/Pelle family are related to defense and resistance functions. The RLK/Pelle family is one of the largest in Arabidopsis, with over 600 members, and is thought to have expanded significantly in the land plant lineage. The study provides insights into the evolutionary history and functional roles of RLKs in plants.