Mucigel

Abstract: SOME plants are known to remove phosphate readily from soils low in phosphate. It is likely that root excretions and microorganisms greatly modify the chemical environment in the rhizosphere and this may result in the more ready displacement of the adsorbed phosphate from soil colloids. Organic compounds found in the root exudates of several plants, reviewed by Rovira1, include many organic acids known to form strong complexes with metal ions. Examination of the root–soil interface under the electron microscope has also revealed a slimy gel between the root and the soil colloidal particles2,3, which has been called “mucigel” and is considered to consist of pectic substances, chiefly partially methylated polygalacturonic acids.

Abstract: Right, shadow picture showing intimate junction of (1) mucigel and (2) soil particle surface with texture variation of the different areas. Separation at cell wall (3) occurred after the slice was made and shows the underlying carrier film for the sample. left, magnified view of mucilaginous region at the root surface. From lower left to upper right corner: (1) large vacuole; (2) cytoplasm (dark stained); (3) plasma membrane (thin, dark line); (4) cell wall (grayish layer); (5) mucilage; and (6) black rods and larger aggregates of iron hydroxide particles in a large open macropore. One micron, as indicated by the bar on each micrograph, is equal to about 1/400,00Oth of an inch.

Abstract: The location and topography of infection sites in soybean (Glycine max (L.) Merr.) root hairs spot-inoculated with Rhizobium japonicum have been studied at the ultrastructural level. Infections commonly developed at sites created when the induced deformation of an emerging root hair caused a portion of the root-hair cell wall to press against an adjacent epidermal cell, entrapping rhizobia within the pocket between the two host cells. Infections were initiated by bacteria which became embedded in the mucigel in the enclosed groove. Infection-thread formation in soybean appears to involve degradation of mucigel material and localized disruption of the outer layer of the folded hair cell wall by one or more entrapped rhizobia. Rhizobia at the site of penetration are separated from the host cytoplasm by the host plasmalemma and by a layer of wall material that appears similar or identical to the normal inner layer of the hair cell wall. Proliferation of the bacteria results in an irregular, wall-bound sac near the site of penetration. Tubular infection threads, bounded by wall material of the same appearance as that surrounding the sac, emerge from the sac to carry rhizobia roughly single-file into the hair cell. Growing regions of the infection sac or thread are surrounded by host cytoplasm with high concentrations of organelles associated with synthesis and deposition of membrane and cell-wall material. The threads follow a highly irregular path toward the base of the hair cell. Threads commonly run along the base of the hair cell for some distance, and may branch and penetrate into subjacent cortical cells at several points in a manner analagous to the initial penetration of the root hair.

Abstract: These micrographs show greatly enlarged views of the outer edges of root cells in contact with the soil. Of particular interest is the mucigel, a jelly-like coating on the outer surface of the roots. Mucigel, produced by the roots and perhaps also by microbes living in it, conforms to the surface contour of the soil particles it touches. Thus, intimate contact is provided for the transfer of soil nutrient ions and water from the soil to the roots.