This study compares the receptor-like kinase (RLK) family in *Arabidopsis thaliana* and *Oryza sativa*, focusing on their evolutionary history and functional diversification. The researchers found that rice has nearly twice as many RLK/Pelle members as *Arabidopsis*, indicating a significant expansion in the rice lineage. They estimated that the common ancestor of *Arabidopsis* and rice had over 440 RLK/Pelles, and subsequent expansions have occurred in both lineages. The extracellular domains of RLKs show higher nonsynonymous substitution rates than intracellular domains, suggesting their role in sensing diverse signals. Lineage-specific expansions in *Arabidopsis* are attributed to both tandem and large-scale duplications, while tandem duplication is the primary mechanism in rice. Interestingly, RLKs involved in defense/disease resistance have undergone many duplication events, whereas those involved in development have rarely been duplicated. These findings suggest that recent expansions of the RLK/Pelle family have involved genes related to defense and resistance. The study also highlights the conservation of domain composition and organization in the RLK/Pelle family, with some novel domain configurations specific to either *Arabidopsis* or rice. The preferential expansion of defense/resistance-related RLK/Pelles may be due to strong selection pressure for recognizing pathogens.This study compares the receptor-like kinase (RLK) family in *Arabidopsis thaliana* and *Oryza sativa*, focusing on their evolutionary history and functional diversification. The researchers found that rice has nearly twice as many RLK/Pelle members as *Arabidopsis*, indicating a significant expansion in the rice lineage. They estimated that the common ancestor of *Arabidopsis* and rice had over 440 RLK/Pelles, and subsequent expansions have occurred in both lineages. The extracellular domains of RLKs show higher nonsynonymous substitution rates than intracellular domains, suggesting their role in sensing diverse signals. Lineage-specific expansions in *Arabidopsis* are attributed to both tandem and large-scale duplications, while tandem duplication is the primary mechanism in rice. Interestingly, RLKs involved in defense/disease resistance have undergone many duplication events, whereas those involved in development have rarely been duplicated. These findings suggest that recent expansions of the RLK/Pelle family have involved genes related to defense and resistance. The study also highlights the conservation of domain composition and organization in the RLK/Pelle family, with some novel domain configurations specific to either *Arabidopsis* or rice. The preferential expansion of defense/resistance-related RLK/Pelles may be due to strong selection pressure for recognizing pathogens.