No abstract

Lipopolysaccharides (LPS) are cell-surface components of Gramnegative bacteria and are microbe-͞pathogen-associated molecular patterns in animal pathosystems. As for plants, the molecular mechanisms of signal transduction in response to LPS are not known. Here, we show that Arabidopsis thaliana reacts to LPS with a rapid burst of NO, a hallmark of innate immunity in animals. Fifteen LPS preparations (among them Burkholderia cepacia, Pseudomonas aeruginosa, and Erwinia carotovora) as well as lipoteichoic acid from Gram-positive Staphylococcus aureus were found to trigger NO production in suspension-cultured Arabidopsis cells as well as in leaves. NO was detected by confocal laserscanning microscopy in conjunction with the fluorophore 4-amino-5-methylamino-2,7-difluorofluorescein diacetate, by electron paramagnetic resonance, and by a NO synthase (NOS) assay. The source of NO was addressed by using T-DNA insertion lines. Interestingly, LPS did not activate the pathogen-inducible varP NOS, but AtNOS1, a distinct NOS previously associated with hormonal signaling in plants. A prominent feature of LPS treatment was activation of defense genes, which proved to be mediated by NO. Northern analyses and transcription profiling by using DNA microarrays revealed induction of defense-associated genes both locally and systemically. Finally, AtNOS1 mutants showed dramatic susceptibility to the pathogen Pseudomonas syringae pv. tomato DC3000. In sum, perception of LPS and induction of NOS contribute toward the activation of plant defense responses.

Aroxyl radicals of fifteen structurally distinct flavonoids were generated by attack of azide radicals (N3.) on the parent compounds dissolved in aqueous solution at pH 11.5. Generation rate constants were all found to be very high (2.4-8.8 x 10(9) dm3mol-1 s-1), whereas the decay rates differed considerably, ranging from 10(5) to 10(8) dm3mol-1 s-1. In most cases the spectral characteristics of the transient aroxyl radicals relate to structural features of the parent compounds and according to spectral similarities they can be classed in three distinct groups (with only two exceptions). Although the data do not conclusively prove that the biological function of flavonoids might be the scavenging of radicals, the very high rate constants of formation and the relative stability of some of the aroxyl radicals, are in support of such a hypothesis.

Most plant-derived polyphenols exhibit strong antioxidant potentials, established by various assay procedures. With pulse radiolysis experiments, absolute scavenging rate constants can be obtained with a variety of oxidizing radicals which allow further structure-activity correlations and, combined with EPR spectroscopy, detailed insight into the mechanisms governing these antioxidant reactions. The most striking difference occurs between regular flavonoids and both condensed and hydrolyzable tannins. The tannins are considered superior antioxidants as their eventual oxidation may lead to oligomerization via phenolic coupling and enlargement of the number of reactive sites, a reaction which has never been observed with the flavonoids themselves.