Citrulline was chemically isolated more than 100 years ago and is ubiquitous in animals, plants, bacteria, and fungi. Most of the research on plant citrulline metabolism and transport has been carried out in Arabidopsis thaliana and the Cucurbitaceae family, particularly in watermelon which accumulates this non-proteinogenic amino acid to very high levels. Industrially, citrulline is produced via specially optimized microbial strains; however, the amounts present in watermelon render it an economically viable source providing that other high-value compounds can be co-extracted. In this review, we provide an overview of our current understanding of citrulline biosynthesis, transport, and catabolism in plants additionally pointing out significant gaps in our knowledge which need to be closed by future experimentation. This includes the identification of further potential enzymes of citrulline metabolism as well as obtaining a far better spatial resolution of both sub-cellular and long-distance partitioning of citrulline. We further discuss what is known concerning the biological function of citrulline in plants paying particular attention to the proposed roles in scavenging of excess NH4+ and as a compatible solute.
