One characteristic feature of many phytopathogenic organisms is their ability to produce an array of enzymes capable of degrading the complex polysaccharides of the plant cell wall (Bateman and Millar, 1966; Wood, 1967; Albersheim et al., 1969; Wood, 1973) and membrane constituents (Porter, 1966; Tseng and Bateman, 1968). These enzymes usually are produced inductively. Generally, they are extracellular, highly stable and present in infected host tissues. In most plant diseases caused by microbial agents, cell walls are penetrated, tissues are colonized, and permeability of host cells is altered. A brief summary of our understanding of cell wall and membrane structure, coupled with knowledge of the enzymes capable of degrading the components of these structures, and an analysis of the association of these enzymes with diseased tissue, should enable us to make an appraisal of their involvement in pathogenesis and point the way to an objective consideration of this area of disease physiology.

Degradation of Plant Cell Walls and Membranes by Microbial Enzymes

A mixture consisting of 1% phosphotungstic acid (PTA) in 10% chromic acid (CrO3) selectively stains the plasma membrane of plant cells. Whole tissue or pelleted cell fractions are prepared for electron microscopy using conventional methods including glutaraldehyde fixation and OsO4 postfixation, dehydration in acetone and embedding in Epon. To stain the plasma membrane, thin sections are transferred with a plastic loop to the surface of a 1% aqueous solution of periodic acid for 30 min for destaining. Following transfer through 5 distilled water rinses, the sections are exposed to the PTA-CrO3 mixture for 5 min, rinsed and mounted on grids for viewing with the electron microscope. The selectivity of the stain is retained in homogenates and serves to identify the plant plasma membrane in cell fractions.

Phosphotungstic Acid-Chromic Acid as a Selective Electron-Dense Stain for Plasma Membranes of Plant Cells

Publisher Summary This chapter discusses the structure and properties of the cell surface coat. Cell surface coats have been found on all living cells that have been examined with sufficiently sensitive methods. These cells protect their fragile plasma membrane with a relatively strong, hydrophilic coat which is a permanent feature of the cell surface complex. Under special conditions, however, rarely and momentarily, these coats can be breached or even removed from certain cells to reveal the delicate, oily, liquid like layer of the plasma membrane, damage to which is lethal unless immediately repaired. These cells usually are able to replace their missing coats in a matter of minutes. Most animal cell coats appear in the electron microscope (EM) as a uniform layer at the external cell surface. The layer seems to originate at, or be a part of, the outer leaflet of the trilaminar plasma membrane, and usually is 100-200 A thick. A variety of methods has been developed to reveal the ultrastructure of cell coats, the details of which depend on the particular method employed. The strength or durability of the cell surface coat varies dramatically among various cells.

The Structure and Properties of the Cell Surface Coat

Publisher Summary This chapter presents the evidence obtained at different levels of biological organization that characterizes the plasma membrane ATPase of fungi and plants as a novel type of proton pump. It presents a parallel discussion of studies with fungal and plant cells to stress the similarities of their bioenergetic mechanisms that plant physiologists and microbial biochemists may have failed to appreciate. The membranes of bacteria, mitochondria, and chloroplasts contain a similar ATPase involved in proton transport. These enzymes are sensitive to azide and dicyclohexylcarbodiimide and, in the case of mitochondria and photosynthetic bacteria, to oligomycin. They do not form phosphorylated intermediates and they have alkaline pH optima and very complicated structures. These enzymes have a hydrophilic catalytic part with five different subunits protruding from the membrane and a hydrophobic part embedded in the membrane with three different subunits that form a proton channel blocked by dicyclohexyl-carbodiimide. A different type of ATPase is found in the microsomes and plasma membranes of animal cells. These enzymes catalyze the exchange of sodium for potassium and of protons for potassium and calcium transport. The ATPase activities are dependent on the presence of the transported cations and the pH optima are neutral.

Plasma membrane ATPase of fungi and plants as a novel type of proton pump

The fine structure of pit plugs was examined in 63 species of red algae representing 34 families. The number of plug cap layers was found to be a taxonomically reliable character when tested against a recent revision of the Rhodymeniales which established the order Palmariales. In a broader survey involving all the orders of red algae possessing pit plugs, three basic plug cap configurations were detected. These differ in having one, two, or no plug cap layers. Some taxa have plug caps in which the outer cap layer is greatly elaborated, however, this layer was shown to be positionally and cytochemically identical, thus probably homologous, to the outer cap layer of the typical two-layered plug cap. The pit plug character confirms the validity of the order Palmariales and indicates that the Nemaliales and Cryptonemiales are heterogeneous assemblages. Revision of these groups along lines indicated by the plug character is supported by significant features of morphology and reproduction. These correlations lead us to propose that Rhodophysema and Coriophyllum be transferred to the Palmariales, that the Gelidiales be resurrected, that the Bonnemaisoniales (including the Naccariaceae) is deserving of recognition, that the Batrachospermaceae be raised to a higher rank which will also receive the Lemaneaceae and the Thoreaceae, and that the Corallinaceae and Hildenbrandiaceae both be raised to ordinal status. AMONG MEMBERS of the Florideophyceae, complete cytokinetic furrowing occurs only during the formation of tetraspores. Division of all other cells is initially incomplete: cessation of furrowing produces a uniperforate septum. The septal pore is soon plugged by the deposition of a substance which is largely proteinaceous in composition (Pueschel, 1980b). Similar plugs have been demonstrated in one stage of the life history- of only two genera of the Bangiophyceae (Bourne, Conway and Cole, 1970; Lee and Fultz, 1970; Cole and Conway, 1975). We retain "pit connection" as a term useful to the morphologist when referring to apparent cellular continuity (though usually not cytoplasmic continuity) through a septal pore. In a cytological context we refer to the structure which occludes the septal pore as a "pit plug." Illustrations of the fine structure of pit plugs from over 50 species have been published.

Rhodophycean pit plugs an ultrastructural survey with taxonomic implications

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C. A. Lembi

Papers

Phosphotungstic Acid-Chromic Acid as a Selective Electron-Dense Stain for Plasma Membranes of Plant Cells

Comparisons of Isolated Plasma Membranes from Plant Stems and Rat Liver