Use of a New Tetrazolium-Based Assay to Study the Production of Superoxide Radicals by Tobacco Cell Cultures Challenged with Avirulent Zoospores of Phytophthora parasitica var nicotianae1

The relationship between the production of reactive oxygen species and the hypersensitive response (HR) of tobacco (Nicotiana tabacum L.) toward an incompatible race of the Oomycete Phytophthora parasitica var nicotianae has been investigated. A new assay for superoxide radical (O2−) production based on reduction of the tetrazolium dye sodium,3′-(1-[phenylamino-carbonyl]-3,4-tetrazolium)-bis(4-methoxy-6-nitro) benzene-sulfonic acid hydrate (XTT) has enabled the quantitative estimation of perhydroxyl/superoxide radical acid-base pair (HO2·/O2−) production during the resistant response. Tobacco suspension cells were inoculated with zoospores from compatible or incompatible races of the pathogen. Subsequent HO2·/O2− production was monitored by following the formation of XTT formazan. In the incompatible interaction only, HO2·/O2− was produced in a minor burst between 0 and 2 h and then in a major burst between 8 and 10 h postinoculation. During this second burst, rates of XTT reduction equivalent to a radical flux of 9.9 × 10−15 mol min−1 cell−1 were observed. The HO2·/O2− scavengers O2− dismutase and Mn(III)desferal each inhibited dye reduction. An HR was observed in challenged, resistant cells immediately following the second burst of radical production. Both scavengers inhibited the HR when added prior to the occurrence of either radical burst, indicating that O2− production is a necessary precursor to the HR.

Developmental changes in DNA methylation of the two tobacco pollen nuclei during maturation

Changes in DNA methylation during tobacco pollen development have been studied by confocal fluorescence microscopy using a monoclonal anti-5-methylcytosine (anti-m5C) antibody and a polyclonal anti-histone H1 (anti-histone) antibody as an internal standard. The specificity of the anti-m5C antibody was demonstrated by a titration series against both single-stranded DNA and double-stranded DNA substrates in either the methylated or unmethylated forms. The antibody was found to show similar kinetics against both double- and single-stranded DNA, and the fluorescence was proportional to the amount of DNA used. No signal was observed with unmethylated substrates. The extent of methylation of the two pollen nuclei remained approximately constant after the mitotic division that gave rise to the vegetative and generative nuclei. However, during the subsequent development of the pollen, the staining of the generative nucleus decreased until it reached a normalized value of \documentclass[12pt]{minimal} \usepackage{wasysym} \usepackage{amsmath} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}\frac{1}{5}\end{equation*}\end{document} of that of the vegetative nucleus. The use of a confocal microscope makes these data independent of possible focusing artefacts. The anti-histone antibody was used as a control to show that, while the antibody staining directed against 5-methylcytosine changed dramatically during pollen maturation, the histone signal did not. We observed the existence of structural dimorphism amongst tobacco pollen grains, the majority having three pollen apertures and the rest with four. However, the methylation changes observed occurred to the same extent in both subclasses.

Use of Ubiquitin Fusions to Augment Protein Expression in Transgenic Plants1

A major goal of plant biotechnology is the production of genetically engineered crops that express natural or foreign proteins at high levels. To enhance protein accumulation in transgenic plants, we developed a set of vectors that express proteins and peptides as C-terminal translational fusions with ubiquitin (UBQ). Studies of several proteins in tobacco (Nicotiana tabacum) showed that: (a) proteins can be readily expressed in plants as UBQ fusions; (b) by the action of endogenous UBQ-specific proteases (Ubps), these fusions are rapidly and precisely processed in vivo to release the fused protein moieties in free forms; (c) the synthesis of a protein as a UBQ fusion can significantly augment its accumulation; (d) proper processing and localization of a protein targeted to either the apoplast or the chloroplast is not affected by the N-terminal UBQ sequence; and (e) single amino acid substitutions surrounding the cleavage site can inhibit in vivo processing of the fusion by Ubps. Noncleavable UBQ fusions of β-glucuronidase became extensively modified, with additional UBQs in planta. Because multiubiquitinated proteins are the preferred substrates of the 26S proteasome, noncleavable fusions may be useful for decreasing protein half-life. Based on their ability to augment protein accumulation and the sequence specificity of Ubps, UBQ fusions offer a versatile way to express plant proteins.

Plasma Membrane-Associated Actin in Bright Yellow 2 Tobacco Cells1 : Evidence for Interaction with Microtubules

Plasma membrane ghosts form when plant protoplasts attached to a substrate are lysed to leave a small patch of plasma membrane. We have identified several factors, including the use of a mildly acidic actin stabilization buffer and the inclusion of glutaraldehyde in the fixative, that allow immunofluorescent visualization of extensive cortical actin arrays retained on membrane ghosts made from tobacco (Nicotiana tabacum L.) suspension-cultured cells (line Bright Yellow 2). Normal microtubule arrays were also retained using these conditions. Membrane-associated actin is random; it exhibits only limited coalignment with the microtubules, and microtubule depolymerization in whole cells before wall digestion and ghost formation has little effect on actin retention. Actin and microtubules also exhibit different sensitivities to the pH and K+ and Ca2+ concentrations of the lysis buffer. There is, however, strong evidence for interactions between actin and the microtubules at or near the plasma membrane, because both ghosts and protoplasts prepared from taxol-pretreated cells have microtubules arranged in parallel arrays and an increased amount of actin coaligned with the microtubules. These experiments suggest that the organization of the cortical actin arrays may be dependent on the localization and organization of the microtubules.

Heat-Shock-Induced Changes in Intracellular Ca2+ Level in Tobacco Seedlings in Relation to Thermotolerance1

Exposure of plants to elevated temperatures results in a complex set of changes in gene expression that induce thermotolerance and improve cellular survival to subsequent stress. Pretreatment of young tobacco (Nicotiana plumbaginifolia) seedlings with Ca2+ or ethylene glycol-bis(β-aminoethylether)-N,N,N′,N′-tetraacetic acid enhanced or diminished subsequent thermotolerance, respectively, compared with untreated seedlings, suggesting a possible involvement of cytosolic Ca2+ in heat-shock (HS) signal transduction. Using tobacco seedlings transformed with the Ca2+-sensitive, luminescent protein aequorin, we observed that HS temperatures induced prolonged but transient increases in cytoplasmic but not chloroplastic Ca2+. A single HS initiated a refractory period in which additional HS signals failed to increase cytosolic Ca2+. However, throughout this refractory period, seedlings responded to mechanical stimulation or cold shock with cytosolic Ca2+ increases similar to untreated controls. These observations suggest that there may be specific pools of cytosolic Ca2+ mobilized by heat treatments or that the refractory period results from a temporary block in HS perception or transduction. Use of inhibitors suggests that HS mobilizes cytosolic Ca2+ from both intracellular and extracellular sources.

Use of Arabidopsis thaliana defense-related mutants to dissect the plant response to pathogens.

The plant defense response to microbial pathogens had been studied primarily by using biochemical and physiological techniques. Recently, several laboratories have developed a variety of pathosystems utilizing Arabidopsis thaliana as a model host so that genetic analysis could also be used to study plant defense responses. Utilizing a pathosystem that involves the infection of Arabidopsis with pathogenic pseudomonads, we have cloned the Arabidopsis disease-resistance gene RPS2, which corresponds to the avirulence gene avrRpt2 in a gene-for-gene relationship. RPS2 encodes a 105-kDa protein containing a leucine zipper, a nucleotide binding site, and 14 imperfect leucine-rich repeats. The RPS2 protein is remarkably similar to the product of the tobacco N gene, which confers resistance to tobacco mosaic virus. We have also isolated a series of Arabidopsis mutants that synthesize decreased levels of an Arabidopsis phytoalexin called camalexin. Analysis of these mutants indicated that camalexin does not play a significant role in limiting growth of avirulent Pseudomonas syringae strains during the hypersensitive defense response but that it may play a role in limiting the growth of virulent strains. More generally, we have shown that we can utilize Arabidopsis to systematically dissect the defense response by isolation and characterization of appropriate defense-related mutants.