Physiological Aspects of Polysaccharide Formation in Plants
Resin and gum ducts develop normally in some plants, or in response to external stimuli, such as microorganisms or growth substances. Among the latter, ethylene is the most effective stimulus (Fahn, 1988). Carbohydrate mucilages and gums are synthesized by dictyosomes (also called Golgi apparatus, Golgi body or Golgi complex: this is an organelle found in most eukaryotic cells whose primary function is to process and package the macromolecules that are synthesized by the cell), but virtually every cell compartment has been suggested to play a role in the secretion of lipophilic substances (Fahn, 1988). Once eliminated from the secretory structures, the secreted materials do not normally re-enter the plant’s metabolism. Secretion may take place in specially formed cell complexes or in ordinary tissues (Fahn, 1979). e first type appears in some Prunoideae and Meliaceae species (Butler, 1911; Groom, 1926), where special groups of parenchyma cells (i.e. thin-walled ground-tissue cells that make up the bulk of most non-woody structures, although sometimes their cell walls can be lignified) are formed by the cambium (i.e. a layer or layers of tissue that are the source of cells for secondary growth), instead of the normal wood elements (Fahn, 1979). Immediately after formation of these special groups of parenchyma cells, gummosis begins in the center of these complexes and then proceeds to the periphery (Fahn, 1979). Disintegration of each cell’s walls initiates in the primary cell wall and proceeds towards the innermost lamella of the secondary cell wall (i.e. all cell walls contain two layers, the middle lamella and the primary cell wall, and many cells produce an additional layer, called the secondary wall. e middle lamella serves as a cementing layer between the primary walls of adjacent cells). e resultant cavity is filled with gum. Gummosis may also occur in the bark (i.e. the outermost layers of stems and roots of woody plants), as in the case of gum arabic of Acacia senegal and other Acacia species. In cherry, vessels (i.e. specialized cells for fluid transport) of otherwise normal wood are often filled with gum (Fahn, 1974), formed only by the lamellae of the secondary wall. When plant organs of Dianthus and Ulmus were experimentally infected with a pathogen, gum production was shown to be the result of secretion from vessel-associated parenchyma cells and not of wall lysis (Catesson et al., 1976). A particular type of traumatic duct forms kino resins. ese are found in the wood of the genus Eucalyptus (Fig. 2.1). In contrast to gums, kino contains polyphenols. Kino veins are 1.5 to 5 mm in length; they are oriented longitudinally, and occur as isolated veins or in a dense anastomosing (i.e. coalescing) network (Fahn, 1979). Kino veins form in the cambial region as an outcome of injury and develop in the zone of traumatized parenchyma. At specific foci, assemblies of polyphenol-containing cells break down and form ducts into which the contents of these kino-producing cells are released. In parallel, the cells surrounding the future kino veins divide and form peripheral “cambium”. Derivatives of these latter cells grow, accumulate polyphenols, break down and enlarge the quantity of kino already present in the ducts. e final phase includes production of a layer of derivatives by the peripheral “cambium”, which in turn become suberized in the shape of a typical periderm (i.e. the innermost area of the bark, which in older stems is a living tissue) (Fahn, 1979; Skene, 1965).