At high concentrations, free radicals and radical-derived, nonradical reactive species are hazardous for living organisms and damage all major cellular constituents. At moderate concentrations, how...

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Free Radicals in the Physiological Control of Cell Function

Rapid generation of superoxide and accumulation of H2O2 is a characteristic early feature of the hypersensitive response following perception of pathogen avirulence signals. Emerging data indicate that the oxidative burst reflects activation of a membrane-bound NADPH oxidase closely resembling that operating in activated neutrophils. The oxidants are not only direct protective agents, but H2O2 also functions as a substrate for oxidative cross-linking in the cell wall, as a threshold trigger for hypersensitive cell death, and as a diffusible signal for induction of cellular protectant genes in surrounding cells. Activation of the oxidative burst is a central component of a highly amplified and integrated signal system, also involving salicylic acid and perturbations of cytosolic Ca2+, which underlies the expression of disease-resistance mechanisms.

The oxidative burst in plant disease resistance

It is concluded from a review of the literature that plant cell culture itself generates genetic variability (somaclonal variation). Extensive examples are discussed of such variation in culture subclones and in regenerated plants (somaclones). A number of possible mechanisms for the origin of this phenomenon are considered. It is argued that this variation already is proving to be of significance for plant improvement. In particular the phenomenon may be employed to enhance the exchange required in sexual hybrids for the introgression of desirable alien genes into a crop species. It may also be used to generate variants of a commercial cultivar in high frequency without hybridizing to other genotypes.

Somaclonal variation — a novel source of variability from cell cultures for plant improvement

Systemic acquired resistance (SAR) is a mechanism of induced defense that confers long-lasting protection against a broad spectrum of microorganisms. SAR requires the signal molecule salicylic acid (SA) and is associated with accumulation of pathogenesis-related proteins, which are thought to contribute to resistance. Much progress has been made recently in elucidating the mechanism of SAR. Using the model plant Arabidopsis, it was discovered that the isochorismate pathway is the major source of SA during SAR. In response to SA, the positive regulator protein NPR1 moves to the nucleus where it interacts with TGA transcription factors to induce defense gene expression, thus activating SAR. Exciting new data suggest that the mobile signal for SAR might be a lipid molecule. We discuss the molecular and genetic data that have contributed to our understanding of SAR and present a model describing the sequence of events leading from initial infection to the induction of defense genes.

Systemic acquired resistance

https://dspace.library.uu.nl/bitstream/handle/1874/1590/PMPP-van+Loon-1999.pdf?sequence=1

The families of pathogenesis-related proteins, their activities, and comparative analysis of PR-1 type proteins

Acetylsalicylic acid (aspirin) induces resistance to tobacco mosaic virus in tobacco.

Salicylic acid (SA) is absolutely required for establishment of acquired resistance in non-infected tissues following localized challenge of other leaves with a necrotizing pathogen. Although not directly responsive to SA, or induced systemically following pathogen challenge, the expression of defence gene promoter fusions AoPR1—GUS and PAL-3—GUS after wounding or pathogen challenge could be enhanced by pre-treating tobacco plants hydroponically with SA, a phenomenon designated ‘potentiation’. Potentiation of AoPR1—GUS wound-responsiveness was also demonstrated locally, but not systemically, in tobacco tissue exhibiting acquired resistance following infection with either viral or bacterial pathogens. Potentiation of wound-responsive expression by prior wounding could not be demonstrated. In contrast, potentiation of pathogen-responsive AoPR1—GUS expression was exhibited both locally and systemically in non-infected tissue. The spatial and temporal exhibition of defence gene potentiation correlated directly with the acquisition of resistance in non-infected tissue. Pathogen-responsive potentiation was obtained at about 10-fold lower levels of salicylic acid than wounding-responsive potentiation in AoPR1—GUS tobacco plants prefed with salicylate. These results may explain the failure to observe systemic potentiation of the wound-responsive defence gene expression. The data suggest a dual role for SA in terms of gene induction in acquired immunity: a direct one by induction of genes such as pathogenesis-related proteins, and an indirect one by potentiation of expression of other local defence genes (such as PAL and AoPR1) which do not respond directly to SA but become induced on pathogen attack or wounding.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313X.1996.09040559.x

Salicylic acid potentiates defence gene expression in tissue exhibiting acquired resistance to pathogen attack

The effects of aspirin and polyacrylic acid on soluble leaf proteins and resistance to virus infection in five cultivars of tobacco.

Summary
Leaves of tobacco plant cv. Xanthi-nc inoculated or systemically infected with potato virus Y, cucumber mosaic virus, potato virus X, potato aucuba mosaic virus or alfalfa mosaic virus showed varying degrees of resistance to infection with tobacco mosaic virus. The resistance was correlated with the appearance of at least three proteins not present in healthy plants. These were the proteins that appear in leaves injected with polyacrylic acid. Both the resistance to second infection and proteins decreased when the plants were kept for 2 days before inoculation at 32 °C.

A Possible Explanation of the Resistance of Virus-infected Tobacco Plants to Second Infection

SummaryA simple indirect enzyme-linked immunosorbent assay (ELISA) was used to detect and quantify acute paralysis virus (APV) in individual female Varroa jacobsoni collected from naturally infested honeybee (Apis mellifera) colonies in the Netherlands and Germany. The assay was very specific and sensitive, detecting the virus in concentrations as low as 3 ng/ml. Mites collected from both adult honeybees and sealed brood cells contained APV but virus concentrations varied widely between individuals. The possible role of V. jacobsoni as a vector of honeybee virus diseases is discussed.

R. F. White

Papers

Acetylsalicylic acid (aspirin) induces resistance to tobacco mosaic virus in tobacco.

Salicylic acid potentiates defence gene expression in tissue exhibiting acquired resistance to pathogen attack

The effects of aspirin and polyacrylic acid on soluble leaf proteins and resistance to virus infection in five cultivars of tobacco.

A Possible Explanation of the Resistance of Virus-infected Tobacco Plants to Second Infection

The detection of acute paralysis virus in Varroa jacobsoni by the use of a simple indirect ELISA.