Phytoalexin

Abstract: Some of the more important evidence is presented which supports the view that accumulation of phytoalexins at the site of attempted infection in plants. The accumulation of phytoalexins represents only one of a number of disease-resistance mechanisms in plants. Experiments are described demonstrating that the enzymes that catalyze the synthesis of phytoalexins are themselves synthesized de novo when plant cells are exposed to microbes of other effective stimuli. Evidence is also presented showing that phytoalexins, once accumulated, are catabolized or detoxified by many microorganisms as well as by the plants themselves. Elicitors are the major concern of this review. The literature is summarized in which elicitors are shown to be not only constituents of some microbes but also to be present in the cell walls of plants. A hypothesis is presented that suggests that all of the abiotic and some of the biotic elicitors stimulate phytoalexin accumulation by causing the release of an endogenous elicitor from the cell walls of plants. 108 references, 9 figures.

Abstract: In Arabidopsis, systemic acquired resistance against pathogens has been associated with the accumulation of salicylic acid (SA) and the expression of the pathogenesis-related proteins PR-1, PR-2, and PR-5. We report here the isolation of two nonallelic mutants impaired in the pathway leading to SA biosynthesis. These SA induction-deficient (sid) mutants do not accumulate SA after pathogen inoculation and are more susceptible to both virulent and avirulent forms of Pseudomonas syringae and Peronospora parasitica. However, sid mutants are not as susceptible to these pathogens as are transgenic plants expressing the nahG gene encoding an SA hydroxylase that degrades SA to catechol. In contrast to NahG plants, only the expression of PR-1 is strongly reduced in sid mutants, whereas PR-2 and PR-5 are still expressed after pathogen attack. Furthermore, the accumulation of the phytoalexin camalexin is normal. These results indicate that SA-independent compensation pathways that do not operate in NahG plants are active in sid mutants. One of the mutants is allelic to eds5 (for enhanced disease susceptibility), whereas the other mutant has not been described previously.

Abstract: Plant cells produce active oxygen during interactions with potential patho­ gens. Active oxygen species, including superoxide, hydrogen peroxide, and the hydroxyl radical, could potentially affect many cellular processes in­ volved in plant/pathogen interactions. Active oxygen can be difficult to monitor in plant cells because many of the species are short-lived and are subject to cellular antioxidant mechanisms such as superoxide dismutases, peroxidases, the ascorbate/glutathione cycle, and catalase. Modifications of the luminol-dependent chemiluminescence assay have facilitated studies on both the production and scavenging of active oxygen that occurs during incompatible plantlbacteria interactions. Many potential sources for active oxygen production have been identified such as NADPH oxidases and per­ oxidases, but it is still unclear which mechanisms predominate during plant! pathogen interactions. The active oxygen produced in response to pathogens and elicitors has been hypothesized to have direct antimicrobial effects and to play a role in other defense mechanisms including lignin production, lipid peroxidation, phytoalexin production, and the hypersensitive response. tThe US Govenunent has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper.

Abstract: Potato tuber tissue discs, which were aged after wounding in order to acquire hypersensitive reactivity, reduced extracellular cytochrome c and nitroblue tetrazolium (NBT) following inoculation with an incompatible, but not compatible, race of Phytophthora infestans . The cytochrome c -reducing activity rapidly increased from l to 4 h after inoculation along with an increase in the percentage of hypersensitively dead cells, and then decreased from the time when most of the penetrated cells had died. A localized activation of NBT reduction around invading hyphae of the incompatible, but not those of the compatible, race was observed at early stages of penetration before cell death. The reductive activity of the discs was also elicited by treatment with a hypersensitivity-eliciting substance, hyphal wall components of the fungus. Superoxide dismutase (SOD), an enzyme catalysing the conversion of the superoxide anion (O 2 − ) to H 2 O 2 and O 2 inhibited the enhanced reducing activity of the discs when added to the assay solution, indicating that cytochrome c and NBT may be reduced by O 2 − generated from the discs. Pre-infectional, vacuum infiltration of the discs with a solution containing SOD significantly delayed the occurrence of hypersensitive cell death caused by infection with the incompatible race as well as the accumulation of phytoalexin. Application of SH-binding reagents and NADP + , but not respiratory inhibitors, inhibited the elicitation of the reducing activity caused by infection with the incompatible race. These results indicate that an O 2 − -generating system may be activated in potato tissues during the incompatible interaction induced by invading fungi or fungal wall components, and also that the generation of O 2 − may be involved during hypersensitive cell death as a trigger of the sequence of resistance reactions.

Abstract: Stimulation of cultured plant cells with elicitors of the defense response leads to the rapid destruction of a variety of water-soluble compounds including indoleacetic acid and certain fluorescent dyes. This destructive activity, which is often vigorously manifested within 5 minutes of elicitor addition, is shown to derive from the rapid production of H2O2 and its use by extracellular peroxidases. Because of its speed of appearance, this oxidative burst may qualify as the first induced line of defense against invading pathogens. Since H2O2 has been implicated as a second messenger of hormone-stimulated metabolic changes in some animal cells, its possible role in transduction of the defense signal in plants was also examined. Not only did exogenous H2O2 alone stimulate phytoalexin production in the plant cell suspension, but inhibition of elicitor-stimulated phytoalexin production was observed upon addition of catalase and other inhibitors of the oxidative burst. Furthermore, for inhibition to occur, the presence of catalase was required during elicitor addition, since if introduction of the enzyme was delayed until 1 hour after addition of the elicitor, no inhibition resulted. These results suggest that H2O2 also plays an important role in inducing subsequent defense responses such as phytoalexin production.