Receptor-like proteins (RLPs) are cell surface receptors that typically consist of an extracellular leucine-rich repeat domain, a transmembrane domain, and a short cytoplasmatic tail. In several plant species, RLPs have been found to play a role in disease resistance, such as the tomato (Solanum lycopersicum) Cf and Ve proteins and the apple (Malus domestica) HcrVf2 protein that mediate resistance against the fungal pathogens Cladosporium fulvum, Verticillium spp., and Venturia inaequalis, respectively. In addition, RLPs play a role in plant development; Arabidopsis (Arabidopsis thaliana) TOO MANY MOUTHS (TMM) regulates stomatal distribution, while Arabidopsis CLAVATA2 (CLV2) and its functional maize (Zea mays) ortholog FASCINATED EAR2 regulate meristem maintenance. In total, 57 RLP genes have been identified in the Arabidopsis genome and a genome-wide collection of T-DNA insertion lines was assembled. This collection was functionally analyzed with respect to plant growth and development and sensitivity to various stress responses, including susceptibility toward pathogens. A number of novel developmental phenotypes were revealed for our CLV2 and TMM insertion mutants. In addition, one AtRLP gene was found to mediate abscisic acid sensitivity and another AtRLP gene was found to influence nonhost resistance toward Pseudomonas syringae pv phaseolicola. This genome-wide collection of Arabidopsis RLP gene T-DNA insertion mutants provides a tool for future investigations into the biological roles of RLPs.For decades, it was thought that the communication between plant cells occurs through the cell wall-spanning cytoplasmic bridges called plasmodesmata. However, since the identification of the first plant cell surface receptor (Walker and Zhang, 1990), it has been known that, similar to other multicellular organisms, plants can perceive extracellular signals at the plasma membrane. Since then, many plant cell surface receptors have been found to play key roles in very diverse processes ranging from growth and development, in which they perceive endogenous self signals, to recognition of other organisms, in which they perceive exogenous nonself signals (Diévart and Clark, 2004).A common structural element of many plant cell surface receptors is the extracellular Leu-rich repeat (eLRR) domain that is generally thought to mediate ligand perception (Kobe and Kajava, 2001;Kinoshita et al., 2005). These eLRRs are composed of 23 to 25 amino acids with the conserved consensus sequence LxxLxxLxLxxNxLt/sgxIpxxLG (Jones and Jones, 1997). The largest group of eLRR-containing cell surface receptors is formed by the receptor-like kinases (RLKs) that are composed of an eLRR domain, a single-pass transmembrane domain, and a cytoplasmic kinase domain, with over 200 representatives in the Arabidopsis (Arabidopsis thaliana) genome (Shiu and Bleecker, 2003). The second largest group of eLRR-containing cell surface receptors is formed by the receptor-like proteins (RLPs) that differ from RLKs in that they lack the cytoplasmic kinase domai...