New ways of capturing and representing biological knowledge are needed to enable individual researchers to remain abreast of relevant discoveries and to permit computational approaches for interpreting the large volumes of diverse data generated by modern biological research. Here, we describe a promising approach that expands the term "reaction'' to represent biological processes. We show how users can represent a wide variety of biological processes in plants in terms of the concept of a reaction and assemble the information obtained from the model plant Arabidopsis thaliana into an online knowledgebase called Arabidopsis Reactome. Its curated and imported pathways currently cover similar to 8% of the Arabidopsis proteome. Arabidopsis Reactome events have also been electronically projected onto five other predicted plant proteomes. Such a system allows the visualization and interpretation of high-throughput data, hypothesis formulation in systems biology, and is a useful learning resource. The Arabidopsis Reactome project (www.arabidopsisreactome.org) is open access, open source, and open to contributions.