Reactive oxygen species (ROS) are formed by fungi in the course of metabolic activity. ROS production increases in fungi due to various stress agents such as starvation, light, mechanical damage, and interactions with some other living organisms. Regulation of ROS level appears to be very important during development of the fungal organism. ROS sources in fungal cells, their sensors, and ROS signal transduction pathways are discussed in this review. Antioxidant defense systems in different classes of fungi are characterized in detail. Particular emphasis is placed on ROS functions in interactions of phytopathogenic fungi with plant cells.
This study focuses on the transient and complex nature of phenolics that accumulate in the extracellular environment of plant suspension cells during the first few hours of the interaction between these plant cells and bacterial pathogens. Using suspension cells of Nicotiana tabacum we identified four acetophenones and four hydroxycinnamic acid amides that accumulate in this extracellular environment. Treatment of the suspension cells with isolates of the plant pathogen Pseudomonas syringae or heat-killed bacteria increased elicitation of extracellular phenolics and changed the composition of the compounds that accumulated. These phenolics were sensitive to oxidative stress; when suspension cells were treated with bacterial strains or elicitors that triggered an oxidative burst, these phenolics were oxidized and depleted for the duration of the burst. The qualitative and quantitative makeup of phenolics produced by N. tabacum suspensions was also affected by plant cell age and density. To our knowledge, this is the first study that closely follows the kinetics of individual extracellular phenolic compounds and the concurrent oxidative stress during the first few hours of a plant-bacterial interaction.
In this study, acetosyringone was identified as one of the major extracellular phenolics in tobacco suspension cells and was shown to have bioactive properties that influence early events in plant-bacterial pathogenesis. In our model system, tobacco cell suspensions treated with bacterial isolate Pseudomonas syringae WT (HR+) undergo a resistant interaction characterized by a burst in oxygen uptake several hours after inoculation. When the extracellular concentration of acetosyringone in tobacco cell suspensions was supplemented with exogenous acetosyringone, the burst in oxygen uptake occurred as much as 1.5h earlier. The exogenous acetosyringone had no effect on tobacco suspensions undergoing susceptible interactions with Pseudomonas tabaci or a non-resistant interaction with a near-isogenic mutant derivative of isolate P. syringae WT (HR+). Resistant interactions with isolate P. syringae WT (HR+) also produce an oxidative burst which oxidizes the extracellular acetosyringone. This study demonstrates that acetosyringone, and likely other extracellular phenolics, may have bioactive characteristics that can influence plant-bacterial pathogenesis.
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