Infection Assay for Xanthomonas campestris pv. campestris in Arabidopsis thaliana Mimicking Natural Entry via Hydathodes

Xanthomonas campestris pv. campestris (Xcc) causes the devastating disease Black rot in Brassicaceae. Typically Xcc enters the plant through specialized organs on the leaf margin, called hydathodes, and spreads from there through the vasculature. In order to mimic natural entry as closely as possible, we here describe a "hydathode guttation"-based entry assay for Xcc in Arabidopsis. This disease assay combines spray inoculation with the induction of guttation and allows reabsorption of guttation droplets by the plant. Moreover, our assay relies on a bioluminescent reporter strain of Xcc to allow direct visualization of both entry and subsequent spreading of Xcc in its host. The assay allows the routine infection from one to two hydathodes per Arabidopsis leaf. Infections are scored 14 days post inoculation, just before the infection goes systemic.