Subcellular Localization of Chitinase and of Its Potential Substrate in Tomato Root Tissues Infected by Fusarium oxysporum f. sp. radicis-lycopersici

Ethylene-induced chitinase and β-1,3-glucanase accumulate specifically in the lower epidermis and along vascular strands of bean leaves

We have studied the spatial pattern of accumulation of chitinase (EC 3.2.1.14) and β-1,3-glucanase (EC 3.2.1.39) in ethylene-treated leaves of bean (Phaseolus vulgaris L.). Electron-microscopical examination of chemically fixed tissue demonstrated the presence of large electron-dense aggregates in the vacuoles of ethylene-treated leaf cells. No such vacuolar structures were observed in untreated control cells. Immunogold labelling with antisera directed against the basic forms of chitinase and β-1,3-glucanase indicated that the vacuolar aggregates were the major site of accumulation of chitinase and β-1,3-glucanase. The chitinase- and β-1,3-glucanase-containing vacuolar aggregates were not randomly distributed within the leaf tissue but were restricted to the lower epidermal cells and to parenchyma cells adjacent to vascular strands. In addition, heavy β-1,3-glucanase labelling was observed over spongy plugs of expanded middle-lamella material that appear to occlude the transition regions between the airspaces underlying the stomata and those throughout the rest of the leaf. Some labelling was also seen to extend along the surface layer of the cell walls lining all of the airspaces. Protein analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting as well as enzyme-activity measurements showed that the peeled lower epidermis of the ethylene-treated leaves contained on a protein and on a per-weight basis several times more chitinase and β-1,3-glucanase than the remainder of the leaf.

Transgenic Plants with Enhanced Resistance to the Fungal Pathogen Rhizoctonia solani

The production of enzymes capable of degrading the cell walls of invading phytopathogenic fungi is an important component of the defense response of plants. The timing of this natural host defense mechanism was modified to produce fungal-resistant plants. Transgenic tobacco seedlings constitutively expressing a bean chitinase gene under control of the cauliflower mosaic virus 35S promoter showed an increased ability to survive in soil infested with the fungal pathogen Rhizoctonia solani and delayed development of disease symptoms.

Accumulation of beta-Fructosidase in the Cell Walls of Tomato Roots following Infection by a Fungal Wilt Pathogen

Active defense in plants is associated with marked metabolic alterations, but little is known about the exact role of the reported changes in specific activity of several enzymes in infected plant tissues. beta-Fructosidase (invertase), the enzyme that converts sucrose into glucose and fructose, increases upon infection by fungi and bacteria. To understand the relationship between fungal growth and beta-fructosidase accumulation, we used an antiserum raised against a purified deglycosylated carrot cell wall beta-fructosidase to study by immunogold labeling the spatial and temporal distribution of the enzyme in susceptible and resistant tomato (Lycopersicon esculentum) root tissues infected with the necrotrophic fungus, Fusarium oxysporum f. sp. racidis-lycopersici. In susceptible plants, the enzyme started to accumulate in host cell walls about 72 hours after inoculation. Accumulation occurred only in colonized cells and was mainly restricted to areas where the walls of both partners contacted each other. In resistant plants, accumulation of beta-fructosidase was noticeable as soon as 48 hours after inoculation and appeared to reach an optimum by 72 hours after inoculation. Increase in wall-bound beta-fructosidase was not restricted to infected cells but occurred also, to a large extent, in tissues that remained uncolonized during the infection process. The enzyme also accumulated in wall appositions (papillae) and intercellular spaces. This pattern of enzyme distribution suggests that induction of beta-fructosidase upon fungal infection is part of the plant's defense response. The possible physiological role(s) of this enzyme in infected tomato plants is discussed in relation to the high demand in energy and carbon sources during pathogenesis.

Time-course study of the accumulation of hydroxyproline-rich glycoproteins in root cells of susceptible and resistant tomato plants infected by Fusarium oxysporum f. sp. radicis-lycopersici

The accumulation of hydroxyproline-rich glycoproteins (HRGPs) in cell walls of dicotyledonous plants is thought to be involved in the defense response to pathogens. An antiserum raised against deglycosylated HRGPs from melon was used for studying the subcellular localization of these glycoproteins in susceptible and resistant tomato (Lycopersicon esculentum Mill.) root tissues infected by Fusarium oxysporum f.sp. radicis-lycopersici. A time-course of HRGP accumulation revealed that these glycoproteins increased earlier and to a higher extent in resistant than in susceptible cultivars. In the compatible interaction, increase in HRGPs was largely correlated with pathogen invasion and appeared to occur as a result of wall damage. In the incompatible interaction, HRGPs accumulated in the walls of uninvaded cells, thus indicating a possible role in the protection against fungal penetration. The occurrence of substantial amounts of HRGPs in papillae, known to be physical barriers formed in response to infection, and in intercellular spaces provides additional support to the concept that such glycoproteins play an important role in disease resistance.

Induction of Arabidopsis defense genes by virulent and avirulent Pseudomonas syringae strains and by a cloned avirulence gene

We developed a model system to study the signal transduction pathways leading to the activation of Arabidopsis thaliana genes involved in the defense against pathogen attack. Here we describe the identification and characterization of virulent and avirulent Pseudomonas syringae strains that elicit disease or resistance symptoms when infiltrated into Arabidopsis leaves. The virulent and avirulent strains were characterized by determining growth of the pathogen in Arabidopsis leaves and by measuring accumulation of mRNA corresponding to Arabidopsis phenylalanine ammonia-lyase (PAL), beta-1,3-glucanase (BG), and chalcone synthase (CHS) genes in infected leaves. The virulent strain, P. syringae pv maculicola ES4326, multiplied 10(5)-fold in Arabidopsis leaves and strongly elicited BG1, BG2, and BG3 mRNA accumulation but had only a modest effect on PAL mRNA accumulation. In contrast, the avirulent strain, P. syringae pv tomato MM1065, multiplied less than 10-fold in leaves and had only a minimal effect on BG1, BG2, and BG3 mRNA accumulation, but it induced PAL mRNA accumulation. No accumulation of CHS mRNA was found with either ES4326 or MM1065. We also describe the cloning of a putative avirulence (avr) gene from the avirulent strain MM1065 that caused the virulent strain ES4326 to grow less well in leaves and to strongly elicit PAL but not BG1 and BG3 mRNA accumulation. These results suggest that the Arabidopsis PAL and BG genes may be activated by distinct signal transduction pathways and show that differences in plant gene induction by virulent and avirulent strains can be attributed to a cloned presumptive avr gene.

Subcellular localization of acidic and basic PR proteins in tobacco mosaic virus-infected tobacco

Infection of Samsum NN tobacco with tobacco mosaic virus (TMV) results in the induction of the synthesis of acidic and basic isoforms of many pathogenesis-related (PR) proteins. By immunogold-electromicroscopy we have shown that PR proteins accumulate mainly in cells around the necrotic spots of TMV-induced lesions. The acidic chitinases, beta-(1,3)-glucanases and thaumatin-like proteins were found to accumulate in extracellular "pocket-like" vesicles while the basic chitinases were found in electron dense inclusion bodies in the vacuoles. These structures were not detectable in PR-containing leaves devoid of virus nor in healthy plants.