Summary: High acidity, low temperature and extremely low concentration of nutrients form Sphagnum bogs into extreme habitats for organisms. Little is known about the bacteria associated with living Sphagnum plantlets, especially about their function for the host. Therefore, we analysed the endo‐ and ectophytic bacterial populations associated with two widely distributed Sphagnum species, Sphagnum magellanicum and Sphagnum fallax , by a multiphasic approach. The screening of 1222 isolates for antagonistic activity resulted in 326 active isolates. The bacterial communities harboured a high proportion of antifungal (26%) but a low proportion of antibacterial isolates (0.4%). Members of the genus Burkholderia (38%) were found to be the most dominant group of antagonistic bacteria. The finding that a large proportion (89%) of the antagonistic bacteria produced antifungal compounds may provide an explanation for the well‐known antimicrobial activity of certain Sphagnum species. The secondary metabolites of the Sphagnum species themselves were analysed by HPLC‐PDA. The different spectra of detected compounds may not only explain the antifungal activity but also the species specificity of the microbial communities. The latter was analysed using cultivation‐independent single‐stranded conformation polymorphism (SSCP) analysis. Using Burkholderia ‐specific primers we found a high diversity of Burkholderia isolates in the endophytic and ectophytic habitats of Sphagnum . Furthermore, a high diversity of nitrogen–fixing bacteria was detected by using nifH‐specific primers, especially inside Sphagnum mosses. In conclusion, this study provides evidence that both Sphagnum species were colonized by characteristic bacterial populations, which appear to be important for pathogen defence and nitrogen fixation.

Flavonoids are ubiquitous phenolic plant metabolites. Many of them are well known for their pro- and antioxidant properties. Myricetin has been reported to be either a potent antioxidant or a pro-oxidant depending on the conditions. The reaction conditions for the pro- and antioxidant activities were therefore investigated using variations of the deoxyribose degradation assay systems. The deoxyribose degradation assay systems were conducted as follows; H(2)O(2)/Fe(III)/ascorbic acid, H(2)O(2)/Fe(III), Fe(III)/ascorbic acid, and Fe(III). Each system was carried out in two variants, FeCl(3) (iron ions added as FeCl(3)) and FeEDTA (iron added in complex with ethylenediaminetetraacetic acid). When ascorbic acid was present, myricetin showed antioxidant properties, especially when it occurred in complex with iron. In ascorbic acid-free systems, pro-oxidant activities prevailed, which where enhanced if iron was in complex with EDTA. Myricetin's antioxidant activity depends on both the reactive oxygen species (ROS) scavenging and iron ions chelation properties. The pro-oxidative properties are caused by reduction of molecular oxygen to ROS and iron(III) to iron(II). Myricetin is able to substitute for ascorbic acid albeit less efficiently.

Sphagnum mosses represent a main vegetation component in ombrotrophic wetlands. They harbor a specific and diverse microbial community with essential functions for the host. To understand the extend of host specificity and impact of environment, Sphagnum fallax and Sphagnum angustifolium, two phylogenetically closely related species, which show distinct habitat preference with respect to the nutrient level, were analyzed by a multifaceted approach. Microbial fingerprints obtained by PCR-single-strand conformation polymorphism of 16S rRNA and nitrogenase-encoding (nifH) genes were highly similar for both Sphagnum species. Similarity was confirmed for colonization patterns obtained by fluorescence in situ hybridization (FISH) coupled with confocal laser scanning microscopy (CLSM): Alphaproteobacteria were the main colonizers inside the hyaline cells of Sphagnum leaves. A deeper survey of Alphaproteobacteria by 16S rRNA gene amplicon sequencing reveals a high diversity with Acidocella, Acidisphaera, Rhodopila, and Phenylobacterium as major genera for both mosses. Nitrogen fixation is an important function of Sphagnum-associated bacteria, which is fulfilled by microbial communities of Sphagna in a similar way. NifH libraries of Sphagnum-associated microbial communities were characterized by high diversity and abundance of Alphaproteobacteria but contained also diverse amplicons of other taxa, e.g., Cyanobacteria and Deltaproteobacteria. Statistically significant differences between the microbial communities of both Sphagnum species could not be discovered in any of the experimental approach. Our results show that the same close relationship, which exists between the physical, morphological, and chemical characteristics of Sphagnum mosses and the ecology and function of bog ecosystems, also connects moss plantlets with their associated bacterial communities.

In plants, accumulation in specific compartments and huge structural diversity of secondary metabolites is one trait that is not understood yet. By exploring the diverse abiotic and biotic interactions of plants above- and belowground, we provide examples that are characterized by nonlinear effects of the secondary metabolites. We propose that redox chemistry, specifically the reduction of reactive oxygen species (ROS) and, in their absence, reduction of molecular oxygen by the identical secondary metabolite, is an important component of the hormetic effects caused by these compounds. This is illustrated for selected phenols, terpenoids, and alkaloids. The redox reactions are modulated by the variable availability of transition metals that serve as donors of electrons in a Fenton reaction mode. Low levels of ROS stimulate growth, cell differentiation, and stress resistance; high levels induce programmed cell death. We propose that provision of molecules that can participate in this redox chemistry is the raison d'ětre for secondary metabolites. In this context, the presence or absence of functional groups in the molecule is more essential than the whole structure. Accordingly, there exist no constraints that limit structural diversity. Redox chemistry is ubiquitous, from the atmosphere to the soil.

(±)-Catechin is a flavan-3-ol that occurs in the organs of many plant species, especially fruits. Health-beneficial effects have been studied extensively, and notable toxic effects have not been found. In contrast, (±)-catechin has been implicated as a ‘chemical weapon’ that is exuded by the roots of Centaurea stoebe, an invasive knapweed of northern America. Recently, this hypothesis has been rejected based on (±)-catechin’s low phytotoxicity, instability at pH levels higher than 5, and poor recovery from soil. In the current study, (±)-catechin did not inhibit the development of white and black mustard to an extent that was comparable to the highly phytotoxic juglone, a naphthoquinone that is allegedly responsible for the allelopathy of the walnut tree. At high stress levels, caused by sub-lethal methanol concentrations in the medium, and a 12 h photoperiod, (±)-catechin even attenuated growth retardation. A similar effect was observed when (±)-catechin was assayed for brine shrimp mortality. Higher concentrations reduced the mortality caused by toxic concentrations of methanol. Further, when (±)-catechin was tested in variants of the deoxyribose degradation assay, it was an efficient scavenger of reactive oxygen species (ROS) when they were present in higher concentrations. This antioxidant effect was enhanced when iron was chelated directly by (±)-catechin. Conversely, if iron was chelated to EDTA, pro-oxidative effects were demonstrated at higher concentrations; in this case (±)-catechin reduced molecular oxygen and iron to reagents required by the Fenton reaction to produce hydroxyl radicals. A comparison of cyclic voltammograms of (±)-catechin with the phytotoxic naphthoquinone juglone indicated similar redox-cycling properties for both compounds although juglone required lower electrochemical potentials to enter redox reactions. In buffer solutions, (±)-catechin remained stable at pH 3.6 (vacuole) and decomposed at pH 7.4 (cytoplasm) after 24 h. The results support the recent rejection of the hypothesis that (±)-catechin may serve as a ‘chemical weapon’ for invasive plants. Instead, accumulation and exudation of (±)-catechin may help plants survive periods of stress.

Juglone, 5-hydroxy-1,4-naphthoquinone, is known for its wide range of biological activities. It has been suggested that juglone’s excellent redox cycling properties contribute to this reputation. Many biological activities are nonlinear with low concentrations exerting stimulating effects, whereas only higher concentrations cause inhibition. Here, we corroborate studies on the nematode Caenorhabditis elegans that point out hormetic effects by showing that juglone may cause a nonlinear effect on postgerminative shoot and root growth of Sinapis alba. This effect was only significantly visible, however, when seedlings were stressed with methanol. Classic and modified versions of the deoxyribose assay were applied successfully to characterize antioxidative (purposeful generation of hydroxyl radicals) and prooxidative (no purposeful generation of hydroxyl radicals) activities. Variants of the assay with and without the addition of the iron chelator EDTA showed that the antioxidant activity is independent on chelation of iron ions by juglone; by contrast, the strength of the prooxidative activity depended on the chelation of iron ions by juglone. The hormetic effects of lower concentrations on germination of Sinapis alba, thus, may be caused by the antioxidant activities of this compound, which are especially effective when the test organism is subjected to higher oxidative challenge. The present study suggests that pronounced prooxidative activities, which are considerably accelerated by chelation of iron ions, may contribute to the toxic effects of juglone at higher concentrations.