Hydathode

Hydathodes are made of a group of living cells with numerous intercellular spaces filled with water, but few or no chloroplasts, and represent modified bundle-ends. These cells (called epithem cells) open out into one or more sub-epidermal chambers. These, in turn, communicate with the exterior through an open water stoma or open pore. The water stoma structurally resembles an ordinary stoma, but is usually larger and has lost the power of movement. Hydathodes are involved in the process of guttation, in which positive xylem pressure (due to root pressure) causes liquid to exude from the pores. ==Occurrence==

Hydathodes occur across most groups of vascular plants, from ferns to flowering herbs and trees. They also occur in the leaves of submerged aquatic plants such as Ranunculus fluitans as well as herbaceous plants of drier habitats such as Campanula rotundifolia. Each comprises an epidermal water pore that remains permanently ajar and an internal mass of thin-walled, lacunate parenchyma known as the epithem; short tracheary endings fan out through this tissue, providing the sap that forms guttation droplets. Research using Arabidopsis (a common laboratory plant) has shown that hydathodes form very early as leaves develop, appearing at predetermined spots along the leaf edges where plant hormones are most concentrated. Although the water pore is built from a guard cell pair like a stomate, it lacks the machinery for full closure and displays only a limited response to abscisic acid or darkness. ==Regulatory function==

Beyond acting as simple overflow vents, hydathodes help regulate the composition of the xylem stream. Analyses of guttation fluid reveal a complex mixture of mineral ions, amino acids, sugars, vitamins and pathogenesis-related proteins, usually at lower concentrations than in the bulk sap; transporter transcripts for phosphate, potassium, chloride and various organic solutes are highly expressed in the epithem, implying active retrieval before loss. The structure therefore functions rather like a miniature kidney: excess water and undesirable solutes are released, while nutritionally valuable molecules can be reclaimed, maintaining leaf osmotic balance and preventing mesophyll flooding when transpiration is low. ==Plant defense and pathogen entry==

Because the vascular endings lie almost exposed, hydathodes also form a natural breach in the leaf's defences. Bacterial pathogens such as Xanthomonas campestris (black-rot of cabbages), X. oryzae (rice leaf blight) and Clavibacter michiganensis (tomato canker) routinely enter through water pores, establish in the epithem and then spread systemically. Plants respond with reactive oxygen bursts, lignification of the epithem and secretion of antimicrobial proteins into the guttation droplets, but virulent strains deploy type III effectors to suppress these responses. Consequently, the evolutionary conservation of hydathodes represents a trade-off: they are essential for water management yet constitute a favoured ingress route for vascular pathogens. ==References==