Hydathode

(Brockhaus and Efron Encyclopedic Dictionary)|363x363px A hydathode is a type of plant organ, commonly found in vascular plants, that secretes fluid through pores in the epidermis, usually at the tip or of a leaf. They are found in a wide variety of plants, from aquatic plants to ferns to flowering trees, where they play a primary role in guttation. Hydathodes help plants regulate fluid balance and filter nutrients, functioning somewhat like kidneys in leaves. They can also serve as entry points for pathogenic bacteria.

(Brockhaus and Efron Encyclopedic Dictionary)|363x363px]] A hydathode is a type of plant organ, commonly found in vascular plants, that secretes fluid through pores in the epidermis, usually at the tip or of a leaf. They are found in a wide variety of plants, from aquatic plants to ferns to flowering trees, where they play a primary role in guttation. Hydathodes help plants regulate fluid balance and filter nutrients, functioning somewhat like kidneys in leaves. They can also serve as entry points for pathogenic bacteria. ==Structure and function== Hydathodes are composed of a group of cells spanning from the epidermal layer to the xylem vessels. They are primarily composed of modified bundle-end cells with few or no chloroplasts called the epithem. The epithem is found directly below the epidermis and is composed of thin-walled parenchyma which connect to the plant xylem via a dense network of tracheid cells. The epithem cells open out into one or more sub-epidermal chambers. These, in turn, communicate with the exterior through an open water stoma or water pore. The water pore structurally resembles an ordinary stoma, but is usually larger and has lost the power of movement (is permanently open). Hydathodes are involved in the process of guttation, in which positive xylem pressure (due to root pressure) causes liquid to exude from the water pores. This process generally occurs at night. Guttation fluid can be taken up again by the plant as it sits on the underside of the leaf. . Guttation occurs at the hydathodes.]] Beyond their role in guttation, hydathodes also 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. Additionally, 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 like a kidney: excess water and undesirable solutes are released, while nutrients can be reclaimed, maintaining leaf osmotic balance and preventing mesophyll flooding when transpiration is low. In one outlying case, hydathodes of plants in the succulent genus Crassula are utilized for the uptake of water, rather than just the exudation of water through guttation. This phenomenon appears widespread among Crassula, but has yet to be documented outside this genus. ==Occurrence and development== ==Plant defense and pathogen entry== Because water pores provide a nearly direct path from the external environment to the plant's vascular system, hydathodes also form a natural breach in the leaf's defenses. Upon guttation or the reuptake of guttation fluid, bacterial pathogens such as Xanthomonas campestris (black-rot of cabbages), Xanthomonas oryzae (rice leaf blight) and Clavibacter michiganensis (tomato canker) can 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 of bacteria can deploy effectors to suppress these responses. Consequently, the evolutionary conservation of hydathodes represents a trade-off: they are essential for water management yet constitute a favored ingress route for vascular pathogens. In addition to lignification, browning, and secretion of antimicrobial proteins to control infections at hydathodes, plants also employ hydathodes themselves as sites of preventative immunity. Studies in Arabidopsis suggest that hydathodes act as immune barriers which confine and halt the growth of bacterial pathogens. Similar observations have also been made in cauliflower. In this context, hydathodes are the location in which the plant immune system recognizes pathogens and prevents their systemic spread. The mechanism by which the plant halts pathogen growth and prevents infection is currently unknown, however. ==References== ==External links== hydathode physiology

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