Biologically active secondary metabolites of barley. II. Phytotoxicity of barley allelochemicals

Metabolic Activation and Cytotoxicity of Gramine Mediated by CYP3A in Rats

Gramine (GRM), which occurs in Gramineae plants, has been developed to be a biological insecticide. Exposure to GRM was reported to induce elevations of serum ALT and AST in rats, but the mechanisms of the observed hepatotoxicity have not been elucidated. The present study aimed to identify reactive metabolites that potentially participate in the toxicity. In rat liver microsomal incubations fortified with glutathione or N-acetylcysteine, one oxidative metabolite (M1), one glutathione conjugate (M2), and one N-acetylcysteine conjugate (M3) were detected after exposure to GRM. The corresponding conjugates were detected in the bile and urine of rats after GRM administration. CYP3A was the main enzyme mediating the metabolic activation of GRM. The detected GSH and NAC conjugates suggest that GRM was metabolized to a quinone imine intermediate. Both GRM and M1 showed significant toxicity to rat primary hepatocytes.

Product safety aspects of plant molecular farming

Plant molecular farming (PMF) has been promoted since the 1990s as a rapid, cost-effective and (most of all) safe alternative to the cultivation of bacteria or animal cells for the production of biopharmaceutical proteins. Numerous plant species have been investigated for the production of a broad range of protein-based drug candidates. The inherent safety of these products is frequently highlighted as an advantage of PMF because plant viruses do not replicate in humans and vice versa. However, a more nuanced analysis of this principle is required when considering other pathogens because toxic compounds pose a risk even in the absence of replication. Similarly, it is necessary to assess the risks associated with the host system (e.g., the presence of toxic secondary metabolites) and the production approach (e.g., transient expression based on bacterial infiltration substantially increases the endotoxin load). This review considers the most relevant host systems in terms of their toxicity profile, including the presence of secondary metabolites, and the risks arising from the persistence of these substances after downstream processing and product purification. Similarly, we discuss a range of plant pathogens and disease vectors that can influence product safety, for example, due to the release of toxins. The ability of downstream unit operations to remove contaminants and process-related toxic impurities such as endotoxins is also addressed. This overview of plant-based production, focusing on product safety aspects, provides recommendations that will allow stakeholders to choose the most appropriate strategies for process development.

Effects of Different Beer Compounds on Biometrically Assessed Emotional Responses in Consumers

The study of emotional responses from consumers toward beer products is an important digital tool to obtain novel information about the acceptability of beers and their optimal physicochemical composition. This research proposed the use of biometrics to assess emotional responses from Mexican beer consumers while tasting top- and bottom-fermented samples. Furthermore, a novel emotional validation assessment using proven evoking images for neutral, negative, and positive emotions was proposed. The results showed that emotional responses obtained from self-reported emoticons and biometrics are correlated to the specific emotions evoked by the visual, aroma, and taste aspects of beers. Consumers preferred bottom-fermentation beers and disliked the wheat-based and higher-bitterness samples. Chemical compounds and concentrations were in accordance to previously reported research for similar beer styles. However, the levels of hordenine were not high enough to evoke positive emotions in the biometric assessment, which opens additional research opportunities to assess higher concentrations of this alkaloid to increase the happiness perception of low or non-alcoholic beers.

Phytotoxicity of organic extracts of five medicinal plants of the Neotropical savanna

Medicinal plants produce a high diversity of secondary metabolites with different biological activities, which are commonly evaluated when prospecting for bioherbicides. We analyzed the phytotoxic activity of organic extracts from the leaves of five medicinal species, Byrsonima intermedia, Moquiniastrum polymorphum, Luehea candicans, Miconia chamissois, and Qualea cordata. Phytotoxicity was evaluated on the initial growth of cucumber seedlings through tests with different concentrations of hexane, ethyl acetate, and methanol extracts. The results showed that all organic extracts and all concentrations affected cucumber development, with methanol extracts generally showing the greatest negative effect on the initial growth of the target species. The only exception was for M. chamissois extracts, in which the hexane extract had the greatest phytotoxicity. Furthermore, the organic extracts were subjected to preliminary phytochemical analysis, revealing the widespread presence of alkaloids along with other chemical classes. All the study species are thus potential candidates for use as natural herbicides.

Root exudates contribute to belowground ecosystem hotspots: A review

Root exudates are an essential carrier for material cycling, energy exchange, and information transfer between the belowground parts of plants and the soil. We synthesize current properties and regulators of root exudates and their role in the belowground ecosystem as substances cycle and signal regulation. We discussed the composition and amount of root exudates and their production mechanism, indicating that plant species, growth stage, environmental factors, and microorganisms are primary influence factors. The specific mechanisms by which root secretions mobilize the soil nutrients were summarized. First, plants improve the nutrient status of the soil by releasing organic acids for acidification and chelation. Then, root exudates accelerated the SOC turnover due to their dual impacts, forming and destabilizing aggregates and MASOC. Eventually, root exudates mediate the plant-plant interaction and plant-microbe interaction. Additionally, a summary of the current collection methods of root exudates is presented.

The Allelopathic Activity of Extracts and Isolated from Spirulina platensis

We determined the allelopathic effects of crude organic (hexane, ethyl acetate, and methanol) extracts of the cyanobacterial Spirulina platensis on barnyardgrass (Echinochloa crus-galli (L.) Beauv.) and Chinese amaranth (Amaranthus tricolor L.). The crude ethyl acetate extract showed the highest inhibitory activity and was subsequently fractionated by column chromatography into 23 fractions based on thin-layer chromatography band pattern similarities. Four concentrations (2000, 1000, 500, and 250 ppm) of each fraction were tested for their allelopathic activity. Fractions E6 and E13 exhibited the most significant inhibitory effects against Chinese amaranth. The constituents of the highly active E6F3-E6F5 fractions determined by GC-MS, chromatography, and spectroscopy included the fatty acids, γ-linolenic acid 15, oleic acid 12, and predominantly palmitic acid 7; minor constituents included 2-ethyl-3-methylmaleimide 9 and C₁₁ norisoprenoids (dihydroactinidiolide 10 and 4-oxo-β-ionone 13). Isolation of E13 fraction by column chromatography revealed four C₁₃ norisoprenoids: 3-hydroxy-β-ionone 17, 3-hydroxy-5α,6α-epoxy-β-ionone 18, 3-hydroxy-5β,6β-epoxy-β-ionone 19, and loliolide 20. Their structures were elucidated by NMR spectroscopy. All six isolated norisoprenoids inhibited seed germination and seedling growth of Chinese amaranth at concentrations of 250–1000 ppm. Allelochemicals from S. platensis could be utilized in the development of novel bioactive herbicides.

Plant Secondary Metabolites: An Opportunity for Circular Economy

Moving toward a more sustainable development, a pivotal role is played by circular economy and a smarter waste management. Industrial wastes from plants offer a wide spectrum of possibilities for their valorization, still being enriched in high added-value molecules, such as secondary metabolites (SMs). The current review provides an overview of the most common SM classes (chemical structures, classification, biological activities) present in different plant waste/by-products and their potential use in various fields. A bibliographic survey was carried out, taking into account 99 research articles (from 2006 to 2020), summarizing all the information about waste type, its plant source, industrial sector of provenience, contained SMs, reported bioactivities, and proposals for its valorization. This survey highlighted that a great deal of the current publications are focused on the exploitation of plant wastes in human healthcare and food (including cosmetic, pharmaceutical, nutraceutical and food additives). However, as summarized in this review, plant SMs also possess an enormous potential for further uses. Accordingly, an increasing number of investigations on neglected plant matrices and their use in areas such as veterinary science or agriculture are expected, considering also the need to implement "greener" practices in the latter sector.

New insights in the allelopathic traits of different barley genotypes: Middle Eastern and Tibetan wild-relative accessions vs. cultivated modern barley

The two alkaloids gramine and hordenine have been known for playing a role in the allelopathic ability in barley (Hordeum vulgare L.). These allelochemicals can be both found in leaves and roots in some barley cultivars whereas in others one seems to exclude the other. In this study eighteen accessions of barley from the Middle-East area, one accession from Tibet and the modern spring cultivar Barke, already used as parental donor in a nested associated mapping (NAM) population, were screened for their gramine, hordenine and N-methyltyramine (the direct precursor of hordenine) content in leaves, roots and exudates. Moreover, the toxicity of the three allelochemicals on root growth inhibition on lettuce (Lactuca sativa L.) was evaluated. Results of this study showed the preferential production of gramine and hordenine in leaves and roots, respectively, in the nineteen barley accessions. On the other hand, in the modern barley cultivar Barke, the highest content of hordenine in roots and the general lack of gramine suggests a favored biosynthesis of the former. Gramine was not detected in the root exudates. In additions, different metabolomic profiles were observed in wild relatives compared to modern barley genotypes. The results also showed the phytotoxic effects of the three compounds on root growth of lettuce seedlings, with a reduction in root length and an increase of root surface area and diameter. In conclusion, this study highlighted the impact of the domestication effects on the production and distribution of the two allelopathic alkaloids gramine and hordenine in barley.

Allelopathic Plants: Models for Studying Plant-Interkingdom Interactions

Allelopathy is a biochemical interaction between plants in which a donor plant releases secondary metabolites, allelochemicals, that are detrimental to the growth of its neighbours. Traditionally considered as bilateral interactions between two plants, allelopathy has recently emerged as a cross-kingdom process that can influence and be modulated by the other organisms in the plant's environment. Here, we review the current knowledge on plant-interkingdom interactions, with a particular focus on benzoxazinoids. We highlight how allelochemical-producing plants influence not only their plant neighbours but also insects, fungi, and bacteria that live on or around them. We discuss challenges that need to be overcome to study chemical plant-interkingdom interactions, and we propose experimental approaches to address how biotic and chemical processes impact plant health.

Identification of Barley (Hordeum vulgare L. subsp. vulgare) Root Exudates Allelochemicals, Their Autoallelopathic Activity and Against Bromus diandrus Roth. Germination

Crops with weed suppressive root exudates or the direct use of bioherbicidal allelochemicals is a new approach in integrated weed management systems. In this context, the allelopathic activity and chemical composition of root exudates from six genotypes (modern varieties and landraces) of barley were characterized. The phenolic acids appeared to be particularly implicated in the inhibitory action of barley root exudates against Bromus diandrus. The amount of these compounds was higher in sandy substrate than in sandy-clay-loam substrate. Ten phenolic acids and one phenylpropanoid derivative were present, in addition to saponarin, a newly identified flavonoid in barley root exudates. Seven compounds explaining variability in the inhibitory activity of barley roots (stepwise analysis) and one compound detected only in highly allelopathic genotypes were toxic against receiver plants. Most compounds had a greater inhibitory effect on the growth of great brome than the barley genotypes. The synergistic and/or additive effect of the eight compounds appeared to be the source of the toxicity. Benzoic acid, the mixture of compounds, saponarin and salicylic acid were the most efficient compounds against the great brome and the less aggressive against barley. Overall, the results revealed the allelopathic potential of the water-soluble compounds exuded by the roots of living barley plants. These compounds included saponarin, a flavonoid not yet recognized as a barley root allelochemical.

Interaction between the barley allelochemical compounds gramine and hordenine and artificial lipid bilayers mimicking the plant plasma membrane

Some plants affect the development of neighbouring plants by releasing secondary metabolites into their environment. This phenomenon is known as allelopathy and is a potential tool for weed management within the framework of sustainable agriculture. While many studies have investigated the mode of action of various allelochemicals (molecules emitted by allelopathic plants), little attention has been paid to their initial contact with the plant plasma membrane (PPM). In this paper, this key step is explored for two alkaloids, gramine and hordenine, that are allelochemicals from barley. Using in vitro bioassays, we first showed that gramine has a greater toxicity than hordenine towards a weed commonly found in northern countries (Matricaria recutita L.). Then, isothermal titration calorimetry was used to show that these alkaloids spontaneously interact with lipid bilayers that mimic the PPM. The greater impact of gramine on the thermotropic behaviour of lipids compared to hordenine was established by means of infrared spectroscopy. Finally, the molecular mechanisms of these interactions were explored with molecular dynamics simulations. The good correlation between phytotoxicity and the ability to disturb lipid bilayers is discussed. In this study, biophysical tools were used for the first time to investigate the interactions of allelochemicals with artificial PPM.

Tyrosine biosynthesis, metabolism, and catabolism in plants

L-Tyrosine (Tyr) is an aromatic amino acid (AAA) required for protein synthesis in all organisms, but synthesized de novo only in plants and microorganisms. In plants, Tyr also serves as a precursor of numerous specialized metabolites that have diverse physiological roles as electron carriers, antioxidants, attractants, and defense compounds. Some of these Tyr-derived plant natural products are also used in human medicine and nutrition (e.g. morphine and vitamin E). While the Tyr biosynthesis and catabolic pathways have been extensively studied in microbes and animals, respectively, those of plants have received much less attention until recently. Accumulating evidence suggest that the Tyr biosynthetic pathways differ between microbes and plants and even within the plant kingdom, likely to support the production of lineage-specific plant specialized metabolites derived from Tyr. The interspecies variations of plant Tyr pathway enzymes can now be used to enhance the production of Tyr and Tyr-derived compounds in plants and other synthetic biology platforms.

Research Progress on the use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy

Allelopathy is a common biological phenomenon by which one organism produces biochemicals that influence the growth, survival, development, and reproduction of other organisms. These biochemicals are known as allelochemicals and have beneficial or detrimental effects on target organisms. Plant allelopathy is one of the modes of interaction between receptor and donor plants and may exert either positive effects (e.g., for agricultural management, such as weed control, crop protection, or crop re-establishment) or negative effects (e.g., autotoxicity, soil sickness, or biological invasion). To ensure sustainable agricultural development, it is important to exploit cultivation systems that take advantage of the stimulatory/inhibitory influence of allelopathic plants to regulate plant growth and development and to avoid allelopathic autotoxicity. Allelochemicals can potentially be used as growth regulators, herbicides, insecticides, and antimicrobial crop protection products. Here, we reviewed the plant allelopathy management practices applied in agriculture and the underlying allelopathic mechanisms described in the literature. The major points addressed are as follows: (1) Description of management practices related to allelopathy and allelochemicals in agriculture. (2) Discussion of the progress regarding the mode of action of allelochemicals and the physiological mechanisms of allelopathy, consisting of the influence on cell micro- and ultra-structure, cell division and elongation, membrane permeability, oxidative and antioxidant systems, growth regulation systems, respiration, enzyme synthesis and metabolism, photosynthesis, mineral ion uptake, protein and nucleic acid synthesis. (3) Evaluation of the effect of ecological mechanisms exerted by allelopathy on microorganisms and the ecological environment. (4) Discussion of existing problems and proposal for future research directions in this field to provide a useful reference for future studies on plant allelopathy.

Gramine-induced growth inhibition, oxidative damage and antioxidant responses in freshwater cyanobacterium Microcystis aeruginosa

In recent years, the exploration and development of the effective methods of treatment and prevention to algal blooms, especially Microcystis aeruginosa blooms has been an important issue in the field of water environment protection. Allelochemicals (natural plant toxins) are considered promising sources of algicides to control algal blooms. The objective of this study is to determine the inhibitory effects and potential mechanisms of a well-known allelochemical gramine (N,N-dimethyl-3-amino-methylindole) on bloom-forming cyanobacterium M. aeruginosa. The results showed that this indole alkaloid effectively inhibited the growth of M. aeruginosa. The effective concentration causing a 50% inhibition at 3 d (EC(50, 3 d)) increased with the initial algal density (IAD) increasing. When IAD increased from 5x10(4) to 5x10(5)cellsmL(-1), the values of EC(50, 3 d) increased from 0.5 to 2.1mgL(-1). In the cells of M. aeruginosa, gramine caused an obvious increase in the level of reactive oxygen species (ROS). The lipid-peroxidation product malondialdehyde (MDA) increased significantly in gramine-treated cells. The effects of gramine on enzymatic and non-enzymatic antioxidants were in different manners. The activity of superoxide dismutase (SOD) was decreased after gramine exposure. The catalase (CAT) activity was increased after 4h but decreased from 60h. Both the contents and the regeneration rates of ascorbic acid (AsA) and reduced glutathione (GSH) were increased after 4h of exposure to gramine. However, only GSH content was still increased after 40h of exposure. These results suggested that the activation of antioxidants in M. aeruginosa played an important role to resist the stress from gramine at initial time, the inactivation of SOD is crucial to the growth inhibition of M. aeruginosa by gramine, and the phytotoxicity of gramine on M. aeruginosa may be due to oxidative damage via oxidation of ROS.

Soybean (Glycine max) root lignification induced by ferulic acid. The possible mode of action

Ferulic acid, in the form of feruloyl CoA, occupies a central position as an intermediate in the phenylpropanoid pathway. Due to the allelopathic function, its effects were tested on root growth, H(2)O(2) and lignin contents, and activities of cinnamyl alcohol dehydrogenase (CAD, EC 1.1.1.195) and peroxidase (POD, EC 1.11.1.7) from soybean (Glycine max (L.) Merr.) root seedlings. Three-day-old seedlings were cultivated in half-strength Hoagland's solution (pH 6.0), with or without 1.0 mM ferulic acid in a growth chamber (25 degrees C, 12/12 hr light/dark photoperiod, irradiance of 280 micromol m(-2) s(-1)) for 24 or 48 hr. Exogenously supplied ferulic acid induced premature cessation of root growth, with disintegration of the root cap, compression of cells in the quiescent center, increase of the vascular cylinder diameter, and earlier lignification of the metaxylem. Moreover, the allelochemical decreased CAD activity and H(2)O(2) level and increased the anionic isoform PODa5 activity and lignin content. The lignin monomer composition of ferulic acid-exposed roots revealed a significant increase of guaiacyl (G) units. When applied jointly with piperonylic acid (an inhibitor of the cinnamate 4-hydroxylase, C4H), ferulic acid increased lignin content. By contrast, the application of 3,4-(methylenedioxy) cinnamic acid (an inhibitor of the 4-coumarate:CoA ligase, 4CL) with ferulic acid did not. Taken together, these results suggest that ferulic acid may be channeled into the phenylpropanoid pathway (by the 4CL reaction) and, further, may increase the lignin monomer amount solidifying the cell wall and restricting the root growth.

The natural compound benzoxazolin-2(3H)-one selectively retards cell cycle in lettuce root meristems

Benzoxazolin-2(3H)-one (BOA) is a natural plant product that is phytotoxic to target plant species, inhibiting germination and growth and causing oxidative damage. We investigated its effects on the root meristems of seedlings of lettuce (Lactuca sativa) by means of light and transmission electron microscopy, flow cytometry, and conventional determination of mitotic index. Flow cytometry analyses and mitotic index showed a retard of cell cycle in BOA-treated meristems with selective activity at G2/M checkpoint.

Caffeic acid affects early growth, and morphogenetic response of hypocotyl cuttings of mung bean (Phaseolus aureus)

Caffeic acid (CA) is one of the most common cinnamic acids ubiquitously present in plants and implicated in a variety of interactions including allelopathy among plants and microbes. This study investigated the possible interference of CA with root growth and the process of rhizogenesis in hypocotyl cuttings of mung bean (Phaseolus aureus=Vigna radiata). Results indicated that CA (0-1000 microM) significantly suppressed root growth of mung bean, and impaired adventitious root formation and root length in the mung bean hypocotyl cuttings. Further investigations into the role of CA in hampering root formation indicated its interference with the biochemical processes involved in rooting process at the three stages - root initiation (third day; RI), root expression (fifth day; RE), and post-expression (seventh day; PE) - of rhizogenesis. CA caused significant changes in the activities of proteases, peroxidases (PODs), and polyphenol oxidases (PPOs) during root development and decreased the content of total endogenous phenolics (TP) in the hypocotyl cuttings. The enhanced activity of PODs and PPOs, though, relates to lignification and/or phenolic metabolism during rhizogenesis; yet their protective role to CA-induced stress, especially during the PE phase, is not ruled out. At 1000 microM CA, where rooting was significantly affected, TP content was very high during the RI phase, thus indicating its non-utilization. The study concludes that CA interferes with the rooting potential of mung bean hypocotyl cuttings by altering the activities of PODs and PPOs and the endogenous TP content that play a key role in rhizogenesis.

Allelopathy in crop/weed interactions--an update

Since varietal differences in allelopathy of crops against weeds were discovered in the 1970s, much research has documented the potential that allelopathic crops offer for integrated weed management with substantially reduced herbicide rates. Research groups worldwide have identified several crop species possessing potent allelopathic interference mediated by root exudation of allelochemicals. Rice, wheat, barley and sorghum have attracted most attention. Past research focused on germplasm screening for elite allelopathic cultivars and the identification of the allelochemicals involved. Based on this, traditional breeding efforts were initiated in rice and wheat to breed agronomically acceptable, weed-suppressive cultivars with improved allelopathic interference. Promising suppressive crosses are under investigation. Molecular approaches have elucidated the genetics of allelopathy by QTL mapping which associated the trait in rice and wheat with several chromosomes and suggested the involvement of several allelochemicals. Potentially important compounds that are constitutively secreted from roots have been identified in all crop species under investigation. Biosynthesis and exudation of these metabolites follow a distinct temporal pattern and can be induced by biotic and abiotic factors. The current state of knowledge suggests that allelopathy involves fluctuating mixtures of allelochemicals and their metabolites as regulated by genotype and developmental stage of the producing plant, environment, cultivation and signalling effects, as well as the chemical or microbial turnover of compounds in the rhizosphere. Functional genomics is being applied to identify genes involved in biosynthesis of several identified allelochemicals, providing the potential to improve allelopathy by molecular breeding. The dynamics of crop allelopathy, inducible processes and plant signalling is gaining growing attention; however, future research should also consider allelochemical release mechanisms, persistence, selectivity and modes of action, as well as consequences of improved crop allelopathy on plant physiology, the environment and management strategies. Creation of weed-suppressive cultivars with improved allelopathic interference is still a challenge, but traditional breeding or biotechnology should pave the way.

Implementation of card: curve-fitting allelochemical response data

Bioassay techniques are essential methods used to study the effects of allelochemicals on plant processes. It is often observed that the biological processes are stimulated at low allelochemical concentrations and inhibited as the concentrations increase. Liu et al., (2003) developed a simple model to fit this type of allelochemical response data. Based on the model, CARD (curve-fitting allelochemical response data) was developed as a Windows based program that can be used to fit a stimulation-inhibition response data. An example of using CARD is given.

Cellular evidence of allelopathic interference of benzoic acid to mustard (Brassica juncea L.) seedling growth

Cellular changes in the roots of mustard (Brassica juncea L.) grown in soil treated with 1.09, 1.46 and 1.83 mg benzoic acid per g soil, a known allelochemical, were analyzed after 7 days. The recoverable concentration of 1.09, 1.46 and 1.8 mg benzoic acid per g soil (measured by high performance liquid chromatography) was 68, 150 and 250 microg benzoic acid per g soil, respectively. The benzoic acid treatments suppressed root growth by 30.5%, 58.8% and 81.1% with increasing concentrations. Transmission electron microscopy studies of roots showed irregular shaped cells arranged in disorganized manner and disruption of cell organelles at cellular level. Root cells showed dissolution of middle lamella (at 68 and 150 microg benzoic acid per g soil) but intact middle lamella with increased wall deposits was observed with 250 microg benzoic acid per g soil. Damage to the mustard root at cellular level was evidenced by changes in cell morphology and internal organization.

A novel laboratory screening bioassay for crop seedling allelopathy

Crops that control weeds by root exudation of allelochemicals are receiving increased attention, and there are efforts to breed allelopathic cultivars in several crops. The genetic improvement of allelopathic traits is based upon parental germ plasm with high allelopathic activity. Identification of allelopathic germplasm is done in laboratory screening bioassays, but experimental protocols are limited. We developed a fast and reliable laboratory screening bioassay for grain crops that includes dose-response considerations as an integral part of the experimental design. The bioassay was conducted in hydroponic culture, and a range of experiments with 2-(3H)-benzoxazolinone (BOA), an allelochemical of several grain crops, was carried out to define the basic protocol. Because of its sensitivity to BOA, Sinapis alba L. was selected as the receiver species. BOA affected growth (fresh weight and length of shoot and root), enzyme activities (ascorbate peroxidase, catalase, glutathione S-transferase, peroxidase, phenylalanine ammonia-lyase), and chlorophyll fluorescence, whereby root length was the most reliable response parameter. BOA sensitivity was dependent on nutrients for all parameters measured, and, thus, no nutrients were added. A set of experiments with Secale cereale L. and Triticum aestivum L. as donor species was carried out to optimize the protocol. Light and pH were eliminated as primary causes for the observed inhibition. The proposed bioassay has several methodological advantages over current bioassays.

Growth inhibition and root ultrastructure of cucumber seedlings exposed to allelochemicals from rye (Secale cereale)

Inhibition of "Calypso" cucumber seedling growth by rye allelochemicals, 2(3H)-benzoxazolinone BOA and 2,4-dihydroxy-1,4(2H)-benzoxazin-3-one DIBOA, was studied by analyzing the growth of seedling tissues and organs. Light and electron microscopy of seedling root cells were also carried out to investigate the mechanism(s) of root growth inhibition and mode of action of these compounds. BOA inhibited root elongation and reduced the number of cucumber lateral roots by 77 and 100% at 0.1 and 0.43 mg BOA/ml deionized (DI) water, respectively. DIBOA also inhibited root growth, but did not affect the number of lateral roots. BOA increased size of cucumber cortical root cells fivefold, but DIBOA had no effect. Both compounds reduced the regeneration of root cap cells and increased the width of cortical cells resulting in increased root diameter. BOA and DIBOA caused increased cytoplasmic vacuolation, reduced ribosome density and dictyosomes, reduced number of mitochondria, and reduced lipid catabolism. Starch granules in amyloplasts of seedling roots treated with BOA and DIBOA were also greatly reduced compared to the control. Changes in cellular ultrastructure indicated that BOA and DIBOA reduced root growth by disrupting lipid metabolism, reducing protein synthesis, and reducing transport or secretory capabilities.

Change in acceptability of barley plants to aphids after exposure to allelochemicals from couch-grass (Elytrigia repens)

The response of the bird cherry-oat aphid, Rhopalosiphum padi, to barley plants was investigated following exposure of the plants to root allelochemicals from the aggressive weed couch-grass, Elytrigia (Agropyron) repens. Plants were treated either with root exudates from living couch-grass plants or with previously identified couch-grass root compounds 15-hydroxyindole-3-acetic acid, DL-5-hydroxytryptophan, L-5-hydroxytryptophan hydrate, and 6-hydroxy-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (carboline)] either separately or in mixtures. In choice and no-choice settling tests, aphid acceptance of barley plants was significantly reduced following treatment with root exudates, and the carboline when tested alone or in combination with the other compounds. In contrast, the other compounds without the carboline were less active in reducing aphid acceptance. In a probing bioassay, individual substances were either neutral or stimulatory to aphids, indicating that the reduced settling was probably not due to direct effects on aphids, but rather due to effects on the plant. This was confirmed in olfactometer assays, in which aphids were repelled by odors from barley plants following treatment with a mixture containing all four chemicals.