The Janus face of reactive oxygen species in resistance and susceptibility of plants to necrotrophic and biotrophic pathogens

Plant pathogens can be divided into biotrophs and necrotrophs according to their different life styles; biotrophs prefer living, while necrotrophs prefer dead cells for nutritional purposes. Therefore tissue necrosis caused by reactive oxygen species (ROS) during pathogen infection increases host susceptibility to necrotrophic, but resistance to biotrophic pathogen. Consequently, elevation of antioxidant capacity of plants enhances their tolerance to development of necroses caused by necrotrophic pathogens. Plant hormones can strongly influence induction of ROS and antioxidants, thereby influencing susceptibility or resistance of plants to pathogens. Pathogen-induced ROS themselves are considered as signaling molecules. Generally, salicylic acid (SA) signaling induces defense against biotrophic pathogens, whereas jasmonic acid (JA) against necrotrophic pathogens. Furthermore pathogens can modify plant's defense signaling network for their own benefit by changing phytohormone homeostasis. On the other hand, ROS are harmful also to the pathogens, consequently they try to defend themselves by elevating antioxidant activity and secreting ROS scavengers in the infected tissue. The Janus face nature of ROS and plant cell death on biotrophic and on necrotrophic pathogens is also supported by the experiments with BAX inhibitor-1 and the mlo mutation of Mlo gene in barley. It was found that ROS and elevated plant antioxidant activity play an important role in systemic acquired resistance (SAR) and induced systemic resistance (ISR), as well as in mycorrhiza induced abiotic and biotic stress tolerance of plants.

Suppression of tobacco mosaic virus-induced hypersensitive-type necrotization in tobacco at high temperature is associated with downregulation of NADPH oxidase and superoxide and stimulation of dehydroascorbate reductase

Tissue necroses and resistance during the hypersensitive response (HR) of tobacco to tobacco mosaic virus (TMV) are overcome at temperatures above 28 degrees C and the virus multiplies to high levels in the originally resistant N-gene expressing plants. We have demonstrated that chemical compounds that generate reactive oxygen species (ROS) or directly applied hydrogen peroxide (H(2)O(2)) are able to induce HR-type necroses in TMV-inoculated Xanthi-nc tobacco even at high temperatures (e.g. 30 degrees C). The amount of superoxide (O(2)(*-)) decreased, while H(2)O(2) slightly increased in TMV- and mock-inoculated leaves at 30 degrees C, as compared with 20 degrees C. Activity of NADPH oxidase and mRNA levels of genes that encode NADPH oxidase and an alternative oxidase, respectively, were significantly lower, while activity of dehydroascorbate reductase was significantly higher at 30 degrees C, as compared with 20 degrees C. It was possible to reverse or suppress the chemically induced HR-type necrotization at 30 degrees C by the application of antioxidants, such as superoxide dismutase and catalase, demonstrating that the development of HR-type necroses indeed depends on a certain level of superoxide and other ROS. Importantly, high TMV levels at 30 degrees C were similar in infected plants, whether the HR-type necrotization developed or not. Suppression of virus multiplication in resistant, HR-producing tobacco at lower temperatures seems to be independent of the appearance of necroses but is associated with temperatures below 28 degrees C.

In vitro microspore selection in maize anther culture with oxidative-stress stimulators

In order to produce doubled-haploid maize plants tolerant of oxidative stress, in vitro microspore selection was carried out in anther culture with reactive oxygen species (ROS) progenitors such as paraquat, menadione, tert-butylhydroperoxide (t-BHP), and methionine combined with riboflavin. All the ROS progenitors reduced the anther induction, the formation of microspore-derived structures, and their regeneration potential. Abnormal cell divisions and progeny cell degradation could be observed during the development of microspores treated with ROS progenitors. Menadione and t-BHP influenced the microspore developmental pathway, as menadione induced the formation of embryoids, while t-BHP increased the proportion of calli in the microspore-derived structures. As the result of in vitro selection, 15, 10, 10, and 3 fertile doubled-haploid plants were obtained in cultures treated with paraquat, t-BHP, methionine combined with riboflavin, and menadione, respectively.

In vivo detection of tobacco mosaic virus-induced local and systemic oxidative burst by electron paramagnetic resonance spectroscopy

This is the first demonstration that tobacco mosaic virus-induced oxidative stress in a necrotic host plant is signalled by an elevated level of monodehydroascorbate (MDA) radicals detected by electron paramagnetic resonance spectroscopy. Furthermore, systemic acquired resistance induced in remote leaves of Xanthi-nc tobacco is also associated with stimulated MDA signals indicative of a microoxidative burst.

Plants ectopically expressing the iron-binding protein, ferritin, are tolerant to oxidative damage and pathogens

Transgenic tobacco plants that synthesize alfalfa ferritin in vegetative tissues--either in its processed form in chloroplasts or in the cytoplasmic nonprocessed form--retained photosynthetic function upon free radical toxicity generated by iron excess or paraquat treatment. Progeny of transgenic plants accumulating ferritin in their leaves exhibited tolerance to necrotic damage caused by viral (tobacco necrosis virus) and fungal (Alternaria alternata, Botrytis cinerea) infections. These transformants exhibited normal photosynthetic function and chlorophyll content under greenhouse conditions. We propose that by sequestering intracellular iron involved in generation of the very reactive hydroxyl radicals through a Fenton reaction, ferritin protects plant cells from oxidative damage induced by a wide range of stresses.