Hydathodes at the forefront of plant immunity against vascular pathogens

Hydathodes are tiny plant organs that form an interface between the leaf surface and xylem vasculature. They facilitate excretion of xylem fluid under conditions when leaf transpiration is low and root pressure high-a process known as guttation. Guttation fluid facilitates the entry of (opportunistic) bacterial pathogens into hydathodes. The notorious vascular pathogens of the bacterial genera Xanthomonas and Clavibacter have evolved unique mechanisms to colonize hydathodes and gain access to xylem and then spread systemically throughout the plant causing disease. For a long time, hydathodes were overlooked as plant immune barrier. Recent studies found that plants mount a defense response in hydathodes via known plant immune signaling hubs indicating that hydathode immunity involves both cell surface and intracellular immune receptors to restrict bacterial colonization. In hydathode-adapted Xanthomonas pathovars, the type III secretion system (T3SS) is critical for hydathode colonization. Through the T3SS, bacteria inject effector proteins into plant cells, indicating a role for type III-secreted effectors of Xanthomonas in promoting hydathode colonization. In addition, the type II secretion system (T2SS) and plant cell wall degrading enzymes secreted by Xanthomonas are required for bacterial translocation from the hydathode to the xylem, which indicates the presence of a physical barrier between these tissues. Future research using advanced molecular techniques give now the opportunity to deepen our understanding of hydathode colonization and hydathode immunity in order to develop novel breeding strategies against these devastating vascular bacterial pathogens.