PlaD is a transcriptomics database for plant defense responses to pathogens. PlaD contains 2444 public gene expression samples from 94 plant pathology-related studies, covering four important plants (Arabidopsis thaliana, Oryza sativa, Triticum aestivum and Zea mays). Each sample was manually curated based on plant tissues, pathogen types and infection stages. All the samples were consistently normalized and the fold change of differential expression was calculated for 522 conditions. In addition to allowing users to query, visualize and download data, the current PlaD provides several key components to support customized data mining and in-depth functional annotation. First, it focuses on both condition-specific expression profiles and co-expression networks. Second, it allows users to search co-regulated genes under the infection of various pathogens. Third, it provides the comparison of differential expression patterns among different plants. Last but not the least, other popular plant functional genomics data, such as protein interactomes, transcription regulations and metabolic pathways were also seamlessly integrated into PlaD. Although RNA-Seq has been widely used in many studies, most of the existing plant defense responses-related transcriptomics data are inferred from the microarray technique. Thus, the current PlaD is based on microarray data. We hope PlaD can serve as a one-stop transcriptomics database to the community of plant pathology, providing abundant clues for deciphering the molecular mechanism and co-evolution of plant-pathogen interaction.