The dynamics of apoplast phenolics in tobacco leaves following inoculation with bacteria.

This study demonstrates that the accumulation of apoplastic phenolics is stimulated in planta in response to bacterial inoculation. Past studies have shown that levels of extracellular phenolics are elicited in plant cell suspensions in response to bacteria, and that tomato plants infected with viroids showed changes in apoplastic phenolics. The method described here monitored changes in apoplastic phenolics in tobacco leaves following bacterial inoculation of the same tissue. Inoculation with a saprophyte, Pseudomonas fluorescens, which does not cause visible symptoms or physical damage, was used to elicit phenolics and examine the effects of variable parameters on phenolic composition. Location of the inoculation on the leaf, position, or developmental age of the leaf on the plant, and inoculum concentration were standardized for further experiments. The patterns of phenolic change in the apoplast were compared for tobacco inoculated with P. syringae pathovars, pv. syringae, which causes a resistant HR reaction within 15 h, and pv. tabaci, which causes a susceptible reaction with delayed visible symptoms. Both pathogens elicited lower increased levels of acetosyringone compared to the saprophyte, P. fluorescens but had greatly increased levels of the chlorogenic acid derivatives. The latter metabolites appear to have come from the intracellular stores, which could indicate a weakening of the apoplast/symplast barrier. This unexpected aspect will require further study of intracellular phenolics.

Pre-illumination of rice blast conidia induces tolerance to subsequent oxidative stress.

Many environmental factors, alone or combined, affect organisms by changing a pro-/antioxidant balance. Here we tested rice blast fungus (Magnaporthe oryzae) for possible cross-adaptations caused by relatively intense light and protecting from artificially formed reactive oxygen species (ROS) and ROS-dependent fungitoxic response of the host plant. Spore germination was found to be suppressed under 4-h and, to larger extent, 5-h illumination. The effect was diminished by antioxidants and, therefore, suggests involvement of ROS. One-hour of light did not affect spore germination, but stimulated their chemically assayed superoxide production. The illuminated spores were more tolerant (than non-illuminated ones) to artificially generated H(2)O(2), O(2)(-), or OH or to toxic diffusate of rice leaf. They also caused more severe disease symptoms if applied to leaves of the susceptible rice cultivar at low concentration. Spore diffusates decomposed hydrogen peroxide. They detoxified exogenous H(2)O(2) and superoxide radical as well as leaf diffusates. Spore illumination increased some of these protective effects. It is suggested that short-term light led to mild oxidative stress, which induced spore antioxidant capacity, enhancing spore tolerance to subsequent stronger oxidative stress and its aggressiveness in planta. Such tolerance depends partly on the antidotal action of spore extracellular compounds, which may also be light-stimulated. Therefore, a certain ROS-related environmental factor may adapt a fungus to other factors and so modulate its pathogenic properties.

Involvement of acetosyringone in plant-pathogen recognition.

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.