Plants are constantly challenged by changes in temperature, light, nutrient conditions, and exposure to pathogens and by other fluctuations in their environment. The molecular basis of how plants respond to these external factors is an active area of investigation. Plant cells often use receptors at the cell surface to sense environmental changes, and then transduce this information via activated signaling pathways to trigger adaptive responses. In Arabidopsis, the receptor-like protein kinase (RLK) gene family contains more than 600 members, many of which are likely to respond to the external challenges presented by an ever-changing environment. RLKs are involved in hormonal response pathways, cell differentiation, plant growth and development, self-incompatibility, and symbiont and pathogen recognition.