Mangroves in the Leaves: Anatomy, Physiology, and Immunity of Epithemal Hydathodes

Hydathodes are organs found on aerial parts of a wide range of plant species that provide almost direct access for several pathogenic microbes to the plant vascular system. Hydathodes are better known as the site of guttation, which is the release of droplets of plant apoplastic fluid to the outer leaf surface. Because these organs are only described through sporadic allusions in the literature, this review aims to provide a comprehensive view of hydathode development, physiology, and immunity by compiling a historic and contemporary bibliography. In particular, we refine the definition of hydathodes. We illustrate their important roles in the maintenance of plant osmotic balance, nutrient retrieval, and exclusion of deleterious chemicals from the xylem sap. Finally, we present our current understanding of the infection of hydathodes by adapted vascular pathogens and the associated plant immune responses.