Scientists studying the wars waged between plants and infectious microbes have discovered a new plant-defense chemical. While monitoring Arabidopsis plants under attack by Pseudomonas syringae pathogenic bacteria, a team led by Jun Fan of the John Innes Centre, in Norwich, England, and Peter Doerner of the Institute of Molecular Plant Sciences at the University of Edinburgh, in Scotland, discovered that the plants produce sulforaphane (4-methylsulfinylbutyl isothiocyanate) by breaking down aliphatic glucosinolates (Science, DOI: 10.1126/science.1199707). Sulforaphane helps slow or stop the pathogen’s growth, but only when the pathogen is missing sax genes; sax stands for “survival on Arabidopsis extracts.” How the proteins coded by these genes deactivate the plant’s defense weaponry is still unknown, but one of the proteins is similar to β-lactamase, an enzyme known to deactivate antibiotics; thus, it possibly chops up sulforaphane. The team also found that pathogens of cruciferous vegetables such as cabbage and broccoli have genes that are resistant to the sulforaphane weapon, whereas noncruciferous pathogens are more vulnerable to the plant-defense chemical. Blocking the action of the sax genes could be one route to staving off pathogens of green leafy vegetables in the future, the researchers suggest.