Margot Schulz

Role of natural benzoxazinones in the survival strategy of plants.

Benzoxazinoid acetal glucosides are a unique class of natural products abundant in Gramineae, including the major agricultural crops maize, wheat, and rye. These secondary metabolites are also found in several dicotyledonous species. Benzoxazinoids serve as important factors of host plant resistance against microbial diseases and insects and as allelochemicals and endogenous ligands. Interdisciplinary investigations by biologists, biochemists, and chemists are stimulated by the intention to make agricultural use of the benzoxazinones as natural pesticides. These natural products are not only constituents of a plant defense system but also part of an active allelochemical system used in the competition with other plants. This review covers biological and chemical aspects of benzoxazinone research over the last decade with special emphasis on recent advances in the elucidation of the biosynthetic pathway.

Effects of salicylic acid on growth and stomatal movements ofVicia faba L.: Evidence for salicylic acid metabolization

The influence of salicylic acid on the growth and stomatal movements ofVicia faba L. was investigated. Whereas shoot length, fresh weight, and transpiration rates, which are directly correlated with stomatal pore widths, were only affected at salicylic acid concentrations higher than 3.5 mM after long-term treatments, guard cells in epidermal peels exhibited a high sensitivity at concentrations as low as 0.001 mM, resulting in stomatal closing. HPLC analysis of methanolic extracts from roots and leaves revealed the presence of free salicylic acid and a metabolite, whose amount increased with time in plants previously incubated with a medium containing salicylic acid. The possible ability ofVicia faba to detoxify the phenolic acid may be one explanation of the discrepancy between the stomatal reaction in epidermal peels directly treated with the phenolic acid and after application through the transpiration stream. The results indicate that, under natural conditions, salicylic acid will not act as an allelopathic compound whose toxic properties severely affect the growth ofVicia faba.

Rapid response reactions of roots to boron deprivation

Upon B removal from the nutrient solution, several response reactions of root cells can be measured within minutes. These include: reduction of cell wall elasticity modulus e, increase of hydraulic conductivity, reduced activity of plasmalemma-bound inducible (NADH) reductase, (smaller) changes of the membrane potential, and liberation of Ca2+ (apoplastic and membrane-bound). The B most demanding (root) tissues are epidermal and outer cortical cells of the extension zone, xylem vessels, and root hair tips. Deprivation of B leads to morphological changes which can be noticed within hours to days, including browning of tissues, growth inhibition, death of apical meristems, and lack of root hairs. How the primary response reaction(s) lead to the expression of visible symptoms, however, is not yet clear. The present review summarizes rapid responses to B deprivation and shows several possibilities how primary might be linked to secondary reactions, including cytoskeleton-mediated responses. Schnelle Reaktionen von Wurzeln auf Bor-Mangel Nach Umsetzen auf ein B-Mangelmedium konnen verschiedene Reaktionen von Wurzelzellen innerhalb von Minuten beobachtet werden, wie z.B. Verringerung des Zellwandelastizitatsmoduls e, Zunahme der hydraulischen Leitfahigkeit, verringerte Plasmalemma-gebundene induzierbare (NADH) Reduktaseaktivitat, (kleinere) Anderungen des Membranpotenzials und eine Freisetzung von apoplastischem und membrangebundenem Ca2+. Die Wurzelgewebe mit dem hochsten B-Bedarf scheinen die epidermalen und auseren Rindenzellen der Streckungszone, Xylem und Wurzelhaarspitzen zu sein. B-Entzug fuhrt zu morphologischen Anderungen, die innerhalb von Stunden und Tagen sichtbar werden, wie Gewebeverbraunung, Wachstumshemmung und Absterben apikaler Meristeme, sowie Fehlen von Wurzelhaaren. Wie die raschen Reaktionen zur (sekundaren) Auspragung von Mangelsymptomen fuhren, ist jedoch noch nicht klar. In dieser Ubersicht werden rasche Reaktionen auf B-Entzug zusammengefasst und Moglichkeiten zur Verknupfung primarer und sekundarer Reaktionen diskutiert.

Effects of 2,4-dihydroxy-1,4-benzoxazin-3-ones on the activity of plasma membrane H+-ATPase

Abstract The action of 2,4-dihydroxy-1,4-benzoxazin-3-ones and their corresponding benzoxazolinone (BOA) derivatives on the activity of plasma membrane H + -ATPase from roots of Avena sativa and Vicia faba was investigated. Significant inhibitory effects were found with 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one in concentrations of 0.25 mM or higher. Benzoxazolinone compounds caused a weaker inhibition of the enzyme. At low effector concentrations stimulatory effects were observed. The H + -ATPase activity of A. sativa and V. faba was reduced by DIBOA to the same extent. The allelopathic effects of DIBOA and BOA on radicle elongation of A. sativa are correlated with the H + -ATPase activity.

Variation in metabolism of BOA among species in various field communities – biochemical evidence for co-evolutionary processes in plant communities?

Summary. The capacity of benzoxazolinone metabolization of dicotyledonous species characteristic for the former vegetation classes Secalietea (grain field weed communities) and Chenopodietea (hoed vegetable communities) was estimated by the production of BOA-6-OH, BOA-6-β-O-glucoside, and BOA-N-glucoside. Except for Urtica urens, Galinsoga ciliata, and Polygonum aviculare (Chenopodietea), all species tested were able to synthesize BOA-6-OH, its glucoside, and BOA-N-glucoside, but effectiveness of BOA metabolism differed highly depending on species and plant organ. There was no correlation between bacterial phenoxazinone production and appearance of metabolites in the plants. Bioassays demonstrated that N-glucosylation is more efficient in BOA detoxification than O-glucosylation. The intermediate BOA-6-OH, however, is more harmful than BOA itself. It is therefore assumed that the ability to synthesize BOA-N-glucoside reduces the sensitivity to BOA strikingly. Since the detoxification capacity did not correlate with the taxonomic position, the affiliation of the species tested with the corresponding plant communities was taken into consideration. Evidently, the ecobiochemical potential of species to detoxify benzoxazolinone, regarded as an essential secondary compound in rye and wheat fields, reflects their occurrence in those plant associations. The ability to cope with the compound could be the result of co-evolutionary processes and presents a hidden aspect of allelopathic interaction.

Phytotoxins from shoot extracts and root exudates of Agropyron repens seedlings

Abstract Allelopathic constituents of ethylacetate extracts from shoots and root exudates of 10-day old Agropyron repens seedlings were investigated. The allelochemicals were identified by GC-mass spectrometry and comparison of retention times and mass spectra to data of respective reference compounds. In shoot extracts the cyclic hydroxamic acids 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one (DIMBOA) and 2,4-dihydroxy-2H-1,4-benzoxazin-3-one (DIBOA), as well as the corresponding lactam derivative 2-hydroxy-1,4-benzoxazin-3-one (HBOA), were found. The concentration of major component DIBOA was 0.5 mg g−1 fr. wt, the concentration of DIMBOA was 0.02 mg g−1 fr. wt. Futhermore maleic, t-aconitic and citric acid were found. In order to estimate the allelopathic potential of living plants an investigation of root exudates was performed. The cyclic hydroxamic acids were identified as important constituents. Their concentrations were 0.4 μmoll−1 DIMBOA and 0.2 μmoll−1 DIBOA. Additionally 2,4-dihydroxy-7,8-dimethoxy-2H-1,4-benzoxazin-3-one (DIM2BOA) was detected. Vanillic, ferulic and β-hydroxybutyric acid are also phytotoxins released by intact, living quackgrass seedlings.

Benzoxazinones in plants: Occurrence, synthetic access, and biological activity

Abstract Acetal glycosides of the 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one skeleton are naturally occurring in Acanthaceae, Poaceae, Ranunculaceae, and Scrophulariaceae, i.e. in plants of different taxonomic positions. They act as plant own resistance factors towards pests, like microbial diseases, insects and fungi. Structurally, they are unique because only in their case a nitrogen atom is part of the aglyconic cyclohemiacetal unit. In case of a pest attack, the glycosides undergo a two step degradation, consisting in an enzymatic deglycosylation followed by a chemical ring contraction of the 1,4-benzoxazinone aglycone to form a benzoxazolinon-2(3H)-one derivative. The latter process is taking place, when plants release such aglycones into the environment by root exudation. Both series of degradation products are bioactive towards pests and can also act as allelochemicals The driving forcefor all investigations is the possibility to make agricultural use of the results in the growing of main cereals, like maize, rye, and wheat (Poaceae). A future goal consists in the gene transfer for the benzoxazinone biosynthesis into other plants of agricultural interest. A detailed overview on natural occurring benzoxazinone acetal glucosides, benzoxazinone aglycones, and benzoxazolinones is presented. Three subjects of the benzoxazinone research are especially emphasised: synthetic access to aglucones and glucosides, medical effects of structures derived from natural product leads and molecular allelopathy, i.e. detoxification strategies of plants coexisting with benzoxazinone forming species in comparison with those belonging to other plant associations. Rapidaccess to further fields of research, like biosynthesis, plant-pest interaction, and molecular mode of action is given by citation of leading references.

Alternative weed control using the allelopathic effect of natural benzoxazinoids from rye mulch

In conventional agriculture, weed control by herbicides is an expensive practice and can also have a negative effect on the environment. Allelopathy permits sustainable weed management while reducing the impact of agriculture on the environment. We studied the content of 2,4-dihydroxy-1,4 (2H)-benzoxazin-3-one (DIBOA) and benzoxazolin-2(3H)-one (BOA), indicated as benzoxazinoids and considered effective for weed control, in 8 cultivars of rye and 1 of triticale grown in a greenhouse. We also tested the ability of mulches to inhibit the germination of four warm-season weeds. Our results show that all rye cultivars produced DIBOA, while BOA was found only in four of them. Benzoxazinoids were absent in triticale. Total benzoxazinoid content ranged from 177 to 545 μg g−1 and was statistically different among cultivars. Rye mulches were not able to suppress velvetleaf and common lambsquarters seedlings, while redroot pigweed and common purslane were significantly affected. Weed suppression ranged from 40% to 52% for redroot pigweed and from 40% to 74% for common purslane. The inhibitory activity of triticale mulch was observed only for common purslane, with a suppression percentage of 33%. No correlation was found between total benzoxazinoid content and the number of weed seedlings suppressed, with R2 of 0.076 for redroot pigweed and R2 of 0.003 for common purslane, indicating that benzoxazinoids are not the only source of phytotoxicity.

Arylsufatase activity in the rhizosphere and roots of different crop species

Abstract It is assumed that arylsulfatase contributes to the mineralization of organic S to SO42− for plant uptake. However, the impacts of agronomic measures (long-term organic manure) or crop species on enzyme activity are not well understood. Therefore, in the present investigations arylsulfatase activity was quantified within the rhizosphere of Sinapis album, Lolium perenne, Triticum aestivum and Brassica napus, grown on three different soils from a long-term field experiment. Highest activities were found within 0.25 mm from the root surface. Highest over all activities were found with B. napus and T. aestivum, lowest with S. album and L. perenne at a distance of 1.75 mm. As compared to soil supplemented with mineral fertilizer, compost and farmyard manure, respectively, arylsulfatase activity was higher in the soil with long-term compost application. In the protein extracts of sterile-grown roots a low arylsulfatase activity could be detected. The enzyme is induced under sulfate deficiency conditions. Preliminary biochemical data obtained from studies with T. aestivum arylsulfatase indicate conformity with biochemical properties of arylsulfatases originated from micro-organisms, vertebrates and invertebrata. From our results we assume that arylfulfatase activity determined in roots of sterile-grown plants is derived from endophytic bacteria and not by higher plants.

Subcellular localization of ubiquitin in plant protoplasts and the function of ubiquitin in selective degradation of outer-wall plasmodesmata in regenerating protoplasts

Immunocytochemical localizations in Vicia faba L. protoplasts and cultures of regenerating Solanum nigrum L. protoplasts support former observations that in plant cells ubiquitin occurs within the cytoplasm, the nucleus, the chloroplasts and at the plasmalemma, but not within the vacuole or the cell wall. Immunoresponses were also observed within mitochondria and associated with the endoplasmic reticulum, which is in accordance with previous findings on animal cells. Moreover, the tonoplast membrane system was found to be labelled. For regenerating S. nigrum protoplasts, evidence is given that ubiquitin plays a role in selective degradation even of whole subcellular structures. Most of the discontinuous plasmodesmata formed in the newly deposited outer cell walls during the early stages of culture disappear later on, except for those near the periphery of division walls or of non-division walls, which are probably used for the formation of continuous cell connections during further culture. Outer-wall plasmodesmata which are destined to disappear show high immunoreactivity to ubiquitin antibody, but no conspicuous immunolabelling was observed with the remaining plasmodesmata. Thus, the selective disintegration of whole plasmodesmatal structures is obviously regulated by ubiquitination of plasmodesmatal proteins. A model for the mechanism of degradation of outer-wall plasmodesmata during extension growth of the cell wall is presented.