Maik Kleinwächter

Stress Enhances the Synthesis of Secondary Plant Products: The Impact of Stress-Related Over-Reduction on the Accumulation of Natural Products

Spice and medicinal plants grown under water deficiency conditions reveal much higher concentrations of relevant natural products compared with identical plants of the same species cultivated with an ample water supply. For the first time, experimental data related to this well-known phenomenon have been collected and a putative mechanistic concept considering general plant physiological and biochemical aspects is presented. Water shortage induces drought stress-related metabolic responses and, due to stomatal closure, the uptake of CO2 decreases significantly. As a result, the consumption of reduction equivalents (NADPH + H(+)) for CO2 fixation via the Calvin cycle declines considerably, generating a large oxidative stress and an oversupply of reduction equivalents. As a consequence, metabolic processes are shifted towards biosynthetic activities that consume reduction equivalents. Accordingly, the synthesis of reduced compounds, such as isoprenoids, phenols or alkaloids, is enhanced.

Stress metabolism in green coffee beans (Coffea arabica L.): expression of dehydrins and accumulation of GABA during drying.

In order to produce tradeable standard green coffee, processed beans must be dried. The drying procedure affects the abundance of relevant aroma substances, e.g. carbohydrates. Using molecular tools, the corresponding metabolic basis is analyzed. A decrease in water potential of the still living coffee seeds induces massive drought stress responses. As a marker for these stress reactions, accumulation of a general stress metabolite, GABA (gamma-aminobutyric acid), and associated gene expression of drought stress-associated dehydrins were monitored. The results of this study indicate that metabolism in drying coffee beans is quite complex since several events trigger accumulation of GABA. The first peak of GABA accumulation during drying is correlated with expression of isocitrate lyase and thus with ongoing germination processes in coffee seeds. Two subsequent peaks of GABA accumulation correspond to maxima of dehydrin gene expression and are thought to be induced directly by drought stress in the embryo and endosperm tissue, respectively. Apart from the significance for understanding basic seed physiology, metabolic changes in coffee seeds during processing provide valuable information for understanding the role and effect of the steps of green coffee processing on the quality of the resulting coffee.

New insights explain that drought stress enhances the quality of spice and medicinal plants: potential applications

Drought stress is generally considered as a negative factor in agriculture, being responsible for severe yield losses. However, medicinal plants grown under semi-arid conditions usually produce higher concentrations of active substances than same species grown under moderate climates. Currently, only limited information is available on the physiological background of this well-known phenomenon. To elucidate this issue, we review here the existing findings to unveil the underlying metabolic mechanisms. Indeed, we found that the drought stress-related metabolic changes are responsible for the accumulation of active substances in semi-arid regions. In particular, the biochemical coherences are as follows: due to limited water supply and much higher light intensities, the plants suffer from drought stress. Then, due to water shortage, stomata are closed and the uptake of CO2 decreases markedly. As a result, CO2 fixation via Calvin cycle decreases. This decline of CO2 fixation leads to a massive decrease in the consumption of reduction equivalents (NADPH + H+), generating in turn a massive oversupply of NADPH + H+. As a consequence, metabolic processes are pushed towards the synthesis of highly reduced compounds, such as isoprenoids, phenols, or alkaloids. Our findings open the path to design practical approaches for enhancing the product quality of spice and medicinal plants. Indeed, by deliberately applying moderate drought stress during their cultivation, the quality of spice and medicinal plants can be enhanced significantly.

Effects of oregano on performance and immunmodulating factors in weaned piglets

Many health effects can be attributed to the Mediterranean herb oregano (Origanum vulgare L.) and several studies demonstrated the improving effect on performance, changes in blood count, antibacterial, antifungal and immunmodulating abilities. The majority of these investigations were carried out with processed essential oil, while whole plant material was only used in a few studies. Thus, the aim of the present experiment was to test the effect of increasing proportions of dried oregano in piglet feed on health and performance, with a special focus on immune modulation. A total of 80 male castrated weaned piglets (body weight [BW] 7.9 kg ±1.0 kg) were used in a feeding experiment lasting 5 weeks. They were assigned to 4 experimental groups: a control diet, and three diets with an oregano supplementation at 2 g, 4 g and 8 g per kg feed, respectively, corresponding to 23.5 mg, 46.9 mg and 93.9 mg carvacrol/kg DM. After 3 weeks, half of each group was challenged with 5 µg lipopolysaccharides (LPS) per kg BW. Blood samples were collected 2 h after LPS stimulation and analysed for T-cell phenotypes, granulocyte activity, clinical-chemistry as well as white and red blood count. The results indicate no effects of oregano on performance. In contrast, oregano altered the lymphocyte proportion and the ratio of CD4+ and CD8+ T-cells as well as the triglyceride concentration in the serum of non-stimulated and in LPS-stimulated piglets. In conclusion, whole plant supplementation of oregano to piglet feed altered immune-related parameters, but did not modulate the acute inflammatory response induced by LPS stimulation.

The glucosinolate-myrosinase system in nasturtium (Tropaeolum majus L.): variability of biochemical parameters and screening for clones feasible for pharmaceutical utilization.

Leaves of Tropaeolum majus L. contain high amounts of the glucosinolate glucotropaeolin. They are used in traditional medicine to treat infections of the urinary tract. When Tropaeolum leaves are consumed, glucotropaeolin is hydrolyzed to yield mustard oils, which are absorbed in the intestine and excreted in the urine, exhibiting their antimicrobial activity. For a corresponding phytopharmacon, a sufficiently high glucotropaeolin concentration is required and any degradation of glucosinolates while drying must be minimized, i.e. the post mortal cleavage by myrosinases, which are activated by ascorbic acid. In extensive screenings, the dominant parameters determining the glucotropaeolin content in the dried leaves were quantified. It turned out that the glucotropaeolin concentration in the dried leaves represented the most suitable screening parameter. The screening of several hundred Tropaeolum plants resulted in the selection of eight high-yield varieties, from which in vitro plants had been generated and propagated as a source for large field trials.

Influencing the Contents of Secondary Metabolites in Spice and Medicinal Plants by Deliberately Applying Drought Stress during Their Cultivation

Medicinal plants grown under semi-arid conditions generally reveal significantly higher concentrations of relevant natural products than identical plants of the same species, which are cultivated in moderate climates. So far, there is very limited information on this well-known phenomenon. In the present study, corresponding data are compiled and relevant aspects are discussed. It turns out that metabolic reactions triggered by drought stress are responsible for the higher natural product accumulation in plants grown in semi-arid regions. The related plant physiological and biochemical background can be summarized as follows: In plants suffering drought stress, the water shortage caused by limited water supply triggers stomata closure. As a result, the uptake of CO2 is markedly decreased and the consumption of reduction equivalents (NADPH+H + ) consumed in the course of CO2-fixation via Calvin cycle declines considerably, generating a massive oversupply of NADPH+H + . Consequently, all metabolic processes are pushed towards the synthesis of highly reduced compounds, such as isoprenoids, phenols or alkaloids. Based on these considerations, impulses for novel practical approaches for enhancing the product quality by deliberately applying drought stress during the cultivation of medicinal plants are outlined. However, as drought stress concomitantly also leads to massive reductions in biomass production, special emphasis must be put on the interference of these stress related effects.

Nicotine uptake by peppermint plants as a possible source of nicotine in plant-derived products

Recently, nicotine has been detected in a large number of food crops and plant-derived products such as spices and herbal teas, but the origin of this nicotine is unknown. This study aimed to elucidate the putative sources of nicotine. We investigate the uptake of nicotine from nicotine-contaminated soils and tobacco smoke using peppermint plants, Mentha × piperita, as a model system in mulching and fumigation experiments. Results show that all the peppermint plants contain minor amounts of nicotine before treatment, but the experiments revealed that the plants also incorporate nicotine considerably from the soil as well as from tobacco smoke. These findings demonstrate for the first time that the reported occurrence of nicotine indeed may originate from tobacco. The incorporated nicotine was subsequently metabolised by the plants. Apart from the nutritional aspects, the results on nicotine uptake may also affect basic plant biology, because they demonstrate that alkaloids can be transferred from one plant, after its death, to another plant species.

13,14-dihydrocoptisine--the genuine alkaloid from Chelidonium majus.

The genuine major benzylisoquinoline alkaloid occurring in the traditional medicinal plant greater celandine (Chelidonium majus L.) is 13,14-dihydrocoptisine and not - as described previously - coptisine. Structure of 13,14-dihydrocoptisine was elucidated. The discrepancy between the alkaloid pattern of the living plants and that of detached and dried leaves is due to the rapid and prompt conversion of 13,14-dihydrocoptisine to coptisine in the course of tissue injuries. Indeed, apart from the major alkaloid, some minor alkaloids might also be converted; this however is not in the centre of focus of this paper. This conversion is initiated by the change of pH. In vivo 13,14-dihydrocoptisine is localized in the acidic vacuoles, where it is stable. In contrast, in the neutral milieu, which results when vacuoles are destroyed in the course of tissue injuries, the genuine alkaloid is oxidized to yield coptisine. Accordingly, when alkaloids from C.majus should be analyzed, any postmortal conversion of 13,14-dihydrocoptisine has to be prevented.

Influencing the Product Quality by Applying Drought Stress During the Cultivation of Medicinal Plants

When medicinal plants are grown under semi-arid conditions, they generally reveal significantly higher concentrations of relevant natural products than identical plants of the same species, which however are cultivated in moderate climates. Up to now, only limited information on this well-known phenomenon is available. In this treatise, corresponding data are compiled and relevant aspects are discussed. It becomes obvious that drought stress-related metabolic changes are responsible for the higher natural product accumulation in plants grown in semi-arid regions. The corresponding plant physiological and biochemical background is outlined as follows: Due to limited water supply and much higher light intensities, the plants suffer drought stress. The related water shortage leads to stomata closure and as a result the uptake of CO2 is markedly decreased. Accordingly, the consumption of reduction equivalents (NADPH+H+) for the CO2-fixation via Calvin cycle declines considerably, generating a massive oversupply of NADPH+H+. As a consequence, metabolic processes are pushed towards the synthesis of highly reduced compounds like isoprenoids, phenols or alkaloids. Based on these coherences, impulses for novel practical approaches for enhancing the product quality by deliberately applying drought stress during the cultivation of medicinal plants are given. However, as drought stress concomitantly leads to massive reductions in biomass production, special emphasis is put on the interference of these stress-related effects.

Sulfate determines the glucosinolate concentration of horseradish in vitro plants (Armoracia rusticana Gaertn., Mey. & Scherb.).

BACKGROUND Horseradish plants (Armoracia rusticana) contain high concentrations of glucosinolates. Former studies have revealed that Armoracia plants cultivated in vitro have markedly lower glucosinolate concentrations than those grown in soils. Yet, these studies neglected that the sulfate concentration in the growth medium may have had a strong impact on glucosinolate metabolism. Accordingly, in this study horseradish in vitro plants were cultivated with differing sulfate concentrations and the glucosinolate concentrations were quantified by ion pair HPLC. RESULTS Cultivation in 1.7 mmol L(-1) sulfate (as used in the prior studies) resulted in the accumulation of 16.2 µmol g(-1) DW glucosinolates, while the glucosinolate concentration increased to more than 23 µmol g(-1) DW when 23.5 mmol L(-1) sulfate was used in the medium. Correspondingly, the glucosinolate concentration decreased to 1.6 µmol g(-1) DW when sulfate concentration was lowered to 0.2 mmol L(-1). CONCLUSION Since the glucosinolate accumulation in relation to the sulfate concentration follows a typical saturation curve, we deduce that the availability of sulfate determines the glucosinolate concentration in horseradish in vitro plants.