Mycobacterial cell wall: Structure and role in natural resistance to antibiotics

Abstract Mycobacteria show a high degree of intrinsic resistance to most antibiotics and chemotherapeutic agents. The low permeability of the mycobacterial cell wall, with its unusual structure, is now known to be a major factor in this resistance. Thus hydrophilic agents cross the cell wall slowly because the myobacterial porin is inefficient in allowing the permeation of solutes and exists in low concentration. Lipophilic agents are presumably slowed down by the lipid bilayer which is of unusually low fluidity and abnormal thickness. Nevertheless, the cell wall barrier alone cannot produce significant levels of drug resistance, which requires synergistic contribution from a second factor, such as the enzymatic inactivation of drugs.     

水平回转对水稻幼苗叶细胞的影响

对在模拟微重力装置上回转１４ 天的水稻幼苗叶细胞进行了亚显微形态、电子探针和细胞酶化学研究。发现叶细胞质膜上Ｃａ２＋ －ＡＴＰ酶活性消失,膜内钙总量上升、膜外钙总量下降,细胞骨架变得疏松,细胞壁变薄并凹凸不平。叶绿体的基粒和线粒体的内嵴亦有部分变化。其变化机制,首先是细胞质膜上Ｃａ２＋ －ＡＴＰ酶活性消失,膜上钙泵停止工作,跨膜钙浓度差减小,膜内钙浓度上升,微管、微丝聚合受阻,细胞骨架疏松,分泌泡移动失去导向,从而导致细胞壁变薄等状态     

Elevated CO2 and drought alter tissue water relations of birch (Betula populifolia Marsh.) seedlings

The effect of increasing atmospheric CO₂ concentrations on tissue water relations was examined in Betula populifolia, a common pioneer tree species of the northeastern U.S. deciduous forests. Components of tissue water relations were estimated from pressure volume curves of tree seedlings grown in either ambient (350 l l^–1) or elevated CO₂ (700 l l^–1), and both mesic and xeric water regimes. Both CO₂ and water treatment had significant effects on osmotic potential at full hydration, apoplasmic fractions, and tissue elastic moduli. Under xeric conditions and ambient CO₂ concentrations, plants showed a decrease in osmotic potentials of 0.15 MPa and an increase in tissue elastic moduli at full hydration of 1.5 MPa. The decrease in elasticity may enable plants to improve the soil-plant water potential gradient given a small change in water content, while lower osmotic potentials shift the zero turgor loss point to lower water potentials. Under elevated CO₂, plants in xeric conditions had osmotic potentials 0.2 MPa lower than mesic plants and decreased elastic moduli at full hydration. The increase in tissue elasticity at elevated CO₂ enabled the xeric plants to maintain positive turgor pressures at lower water potentials and tissue water contents. Surprisingly, the elevated CO₂ plants under mesic conditions had the most inelastic tissues. We propose that this inelasticity may enable plants to generate a favorable water potential gradient from the soil to the plant despite the low stomatal conductances observed under elevated CO₂ conditions.     

Effect of osmotic stress on the growth of epicotyls of Cicer arietinum in relation to changes in the autolytic process and glycanhydrolytic cell wall enzymes

Simulation of drought by polyethylene glycol (PEG) inhibited elongation of epicotyls of Cicer arietinum L. cv. Castellana but had no effect on growth capacity since growth was restored once the inhibitory condition had been removed. The amount of proteins in the cell wall was correlated with the elongation of the epicotyls and decreased when elongation was inhibited. PEG-induced inhibition of elongation had different effects on the various glycanhydrolytic cell wall enzymes. Only α-galactosidase (EC 3. 2. 1. 22) seemed related to the lack of elongation, increasing its activity when elongation was inhibited. The β-galactosidase (EC 3. 2. 1. 23) and β-glucosidase (EC 3. 2. 1. 21) studied did not show changes in their specific activities during the inhibition of elongation. β-Galactosidase is responsible for the autolytic process in Cicer arietinum . This enzyme hydrolyzes specified linkages in the cell wall, releasing sugar constituents. Our present results show that β-galactosidase is not directly related with elongation because no changes could be observed during inhibition of elongation. The autolytic process is related with chemical processes taking place in the cell wall and preceding elongation of the epicotyls, i. e. the loosening process. Cell wall loosening is necessary for elongation to take place but elongation does not necessarily follow loosening if the osmotic conditions are unfavorable     

Acid-induced changes in the in vivo wall-yielding properties of hypocotyl sections of Vigna unguiculata

Elongation growth of hypocotyl sections of Vigna unguiculata under xylem perfusion was significantly enhanced when acid was applied by acid-aerosol to an abraded hypocotyl surface in the air. The in vivo wall extensibility (φ) and the effective turgor (Pⁱ– Y), both of which were determined by the pressure-jump method, increased during acid-induced growth as observed in IAA-induced growth. The intracellular pressure (Pⁱ), however, decreased significantly at the beginning of acid-induced growth whereas Pⁱ scarcely changed in IAA-induced growth. This result indicates that protons increase the effective turgor by decreasing the yield threshold as IAA does. There seems to be no essential difference between proton and auxin in the effects on the in vivo mechanical properties of the surface cell wall.     

Annual variations of the biochemical composition of Gelidium latifolium (Greville) Thuret et Bornet

Agar, floridean starch, protein, ash and water content in Gelidium latifolium from nature were followed monthly over one year. Comparison of variations in these contents, algal growth and cytological observations enable us to establish a cycle for agar synthesis. In autumn, after reproduction of alga, there is an active algal growth period with agar synthesis and utilization of floridean starch. In winter, synthesis is shifted toward protein accumulation while there is a partial agar hydrolysis. In spring another active growth period of the alga occurs with accumulation of floridean starch and synthesis of agar. In summer and during reproduction, there is a depletion of thalli nitrogen content while the agar synthesis pauses.     

Mannuronan C-5 epimerase activity in protoplasts of Laminaria digitata

Biosynthesis of alginate in algae may be studied by following the cell wall regeneration of brown seaweed protoplasts in culture. The enzyme mannuronan C-5 epimerase will control the composition of the alginate being synthetized.Freshly isolated protoplasts from the thallus of young Laminaria digitata plants showed only low expression of this enzyme. However, after prolonged periods in culture, this activity increased 15-fold. The synthesis of C-5 epimerase by the protoplasts is probably essential for the formation of a new cell wall.After cellular disruption by osmotic shock and centrifugation, most of the epimerase activity resided in the pellet fraction. This may indicate that the enzyme is membrane associated.     

Cellular localization and cytochemical probing of resistance reactions to arbuscular mycorrhizal fungi in a ‘locus a’ myc− mutant of Pisum sativum L.

Pisum sativum L. myc^– mutants which fail to form arbuscular mycorrhiza have recently been identified amongst nod^– mutants (Duc et al., 1989, Plant Sci. 60, 215–222). The reason for this resistance to symbiotic fungi has been investigated in the case of a locus a mutant (P2) inoculated with Glomus mosseae (Nicol. and Gerd.) Gerd, and Trappe. The fungal symbiont formed viable appressoria in contact with the root surface but its development was stopped at the root epidermis. Abundant material was deposited on the inner face of root cell walls adjacent to the appressoria in the P2 mutant, but not in the wild-genotype parent cultivar (Frisson) forming a symbiotic mycorrhizal infection. Fluorescence, histochemical, cytochemical and immunocytological approaches were used to characterize the paramural deposits in epidermal and hypodermal cells of the mutant. Strong fluorescence under blue light indicated the accumulation of phenolic compounds although polymers like lignin or suberin were not localized. Proteins and glycoproteins were homogeneously distributed within the paramural deposits. In the latter, the periodic acid-thiocarbohydrazide-silver proteinate (PATAg) reaction for 1,4-polysaccharide detection showed a heterogeneous composition with electron-dense points surrounded by non-reactive material, but cytological tests for cellulose and pectin gave weak responses as compared to epidermal and hypodermal walls of the wild genotype. -1,3-Glucans indicative of callose were detected by in-situ immunolocalization in the paramural deposits below appressoria on mutant roots, but not in walls of the wild genotype. Thus, appressorium formation by G. mosseae on roots of the locus a P. sativum mutant elicits wall modifications usually associated with activation of defence responses to pathogens. It is proposed that this locus must be involved in a key event in symbiotic infection processes in P. sativum, and the possible role of complex regulatory interactions between symbiosis and defence genes in endomycorrhiza development is discussed.Abbreviations DAPI 4,6-diamino-2-phenylindole - FDA fluo-rescein diacetate - PATAg periodic acid-thiocarbohydrazide-silver proteinate The authors are grateful to C. Arnould for technical assistance, K. Niehaus for the purified Sirofluor, K. Roberts for the AFRC JIM5 antibody and J. Lherminier (INRA, Dijon, France), for useful discussion. This collaborative research programme was financially supported by MRT, INRA, EPR-Bourgogne (grant to A.G., Contrat de Plan project 3060A), EEC COST ACTION 8.10 (Endomycorrhizas) and the National Research Council of Italy, Special Project RAISA, Sub-project N.2, Paper N. 801     

Cell wall synthesis during growth and maturation of Nitella internodal cells

An improved ¹³C-density-labeling method was used to study cell wall synthesis in rapidly expanding, slowly expanding and recently mature internodes of Nitella translucens var axillaris (A.Br.) R.D.W. As cells matured, the rate of wall synthesis slowed and the deposition of cellulose microfibrils changed from a predominantly transverse direction in the primary wall of rapidly expanding internodes to a helicoidal array in the secondary wall of mature internodes. The secondary wall was characterized by relatively higher rates of cellulose synthesis and lower rates of pectin synthesis than the primary wall. The synthesis of xyloglucan also decreased markedly at the transition to secondary wall synthesis, while the synthesis of mannose-rich hemicellulose increased. Even though structural differences were striking between the primary and secondary walls of Nitella, compositional differences between the two types of wall were quantitative rather than qualitative. The authors appreciate the assistance of Martin Yousef with the electron microscopy.