Isoflavanones from the allelopathic aqueous root exudate of Desmodium uncinatum

Non-alkaloid components with inhibitory activity against LPS induced NO production in RAW 264.7 cells isolated from the roots of Sophora flavescens

Sophora flavescens Aiton is a plant in the Leguminosae family. As a traditional Chinese medicine, it is used to treat eczema, bloody stool, skin pruritus, and so on. By studying non-alkaloid components in the roots of S. flavescens, we obtained a total of 49 compounds (1-49), including three undescribed flavonoids (13, 15 and 18), five undescribed isopentenyl flavonoids (32, 34, 38, 39 and 48), two known coumarins (1-2), three phenolic acids (3-5), one known isopentenyl flavonoids (19-31, 33, 35-37, 40-47 and 49). On the basis of chemical evidences and spectral data analysis (UV, ECD, Optical rotation data, 1D/2D-NMR and HR-ESI-MS), the structures of undescribed compounds were elucidated. The inhibitory effect of compounds 1-49 on LPS induced NO production in RAW 264.7 cells was detected. Compounds 11, 19, 21-24, and 28-30 showed significant inhibitory effects, and the IC50 values of compounds 11 and 22 even reached 4.58 ± 0.66 and 4.53 ± 0.66 μM. This study suggests that flavonoids may be the main component that exerts anti-inflammatory effects in the non-alkaloid extraction layer of the extract from the roots of S. flavescens.

Evaluating local plant species for effective fall armyworm management strategies in Taiwan

BACKGROUND

The emergence of Spodoptera frugiperda (fall armyworm; FAW) in the world has raised concerns regarding its impact on crop production, particularly on corn and sorghum. While chemical control and Bt crops have been effective in managing FAW damage, the development of pesticide-resistant and Bt-resistant strains necessitates alternative control methods. The push-pull farming system has gained attention, but direct utilization of African plant species in Taiwan faces challenges due to invasive potential and climatic disparities. Therefore, identifying and evaluating suitable local plant species, such as Napier grass (Pennisetum purpureum), Desmodium species, and signal grass (Brachiaria brizantha), is crucial for implementing effective FAW management strategies in Taiwan.

RESULTS

In screening fifty Napier grass germplasms, all demonstrated an antibiotic effect, reducing leaf consumption compared to corn. Notably, thirty-five germplasms exhibited robust antibiotic traits, decreasing FAW consumption and increasing mortality rates. Three Napier grass germplasms also attracted more female moths for oviposition. Further evaluation of selected Napier grass germplasms and signal grass demonstrated efficacy in reducing FAW larval weight and survival duration. Additionally, Desmodium species, particularly D. uncinatum, showed promising toxicity against FAW larvae.

CONCLUSION

Our findings support the effectiveness of selected Napier grass germplasms and signal grass as pull plants, and highlight the potential of D. uncinatum as a push plant in FAW management strategies in Taiwan.

Comparative microbiome diversity in root-nodules of three Desmodium species used in push-pull cropping system

Background

Desmodium species used as intercrops in push-pull cropping systems are known to repel insect-pests, suppress Striga species weeds, and shift soil microbiome. However, the mechanisms through which Desmodium species impact the soil microbiome, either through its root exudates, changes in soil nutrition, or shading microbes from its nodules into the rhizosphere, are less understood. Here, we investigated the diversity of root-nodule microbial communities of three Desmodium species- Desmodium uncinatum (SLD), Desmodium intortum (GLD), and Desmodium incanum (AID) which are currently used in smallholder maize push-pull technology (PPT).

Methods

Desmodium species root-nodule samples were collected from selected smallholder farms in western Kenya, and genomic DNA was extracted from the root-nodules. The amplicons underwent paired-end Illumina sequencing to assess bacterial and fungal populations.

Results

We found no significant differences in composition and relative abundance of bacterial and fungal species within the root-nodules of the three Desmodium species. While a more pronounced shift was observed for fungal community compositions compared to bacteria, no significant differences were observed in the general diversity (evenness and richness) of fungal and bacterial populations among the three Desmodium species. Similarly, beta diversity was not significantly different among the three Desmodium species. The root-nodule microbiome of the three Desmodium species was dominated by Bradyrhizobium and Fusarium species. Nevertheless, there were significant differences in the proportion of marker gene sequences responsible for energy and amino acid biosynthesis among the three Desmodium species, with higher sequence proportions observed in SLD.

Conclusion

There is no significant difference in the microbial community of the three Desmodium species used in PPT. However, root-nodule microbiome of SLD had significantly higher marker gene sequences responsible for energy and amino acid biosynthesis. Therefore, it is likely that the root-nodules of the three Desmodium species host similar microbiomes and influence soil health, consequently impacting plant growth and agroecosystem functioning.

Intercropping of Narrow-Leafed Lupin (Lupinus angustifolius L.) and Barley (Hordeum vulgare L.) Affects the Flavonoid Composition of Both Crops

Barley (Hordeum vulgare L.) is a common cereal crop in agricultural production and is often included in legume-cereal intercropping. Flavonoids, a major class of secondary metabolites found in barley, are involved in plant defense and protection. However, the effect of intercropping on barley flavonoids remains unknown. Herein, an intercropping system involving barley and lupin (Lupinus angustifolius L.) was studied. Intercropping increased the level of luteolin in lupin roots. Lupin-barley intercropping considerably increased genistein, rutin, and apigenin in barley shoots. Genistein and apigenin were also detected in intercropped barley roots and rhizosphere soil. The three flavonoids have been reported as defense compounds, suggesting that lupin triggers a defense response in barley to strengthen its survival ability.

The dynamics and transmission of antibiotic resistance associated with plant microbiomes

Antibiotic resistance genes (ARGs) have been widely found and studied in soil and water environments. However, the propagation of ARGs in plant microbiomes has attracted insufficient attention. Plant microbiomes, especially the rhizosphere microorganisms, are closely connected with water, soil, and air, which allows ARGs to spread widely in ecosystems and pose a threat to human health after entering the human body with bacteria. Therefore, it is necessary to deeply understand and explore the dynamics and the transmission of ARGs in rhizosphere microorganisms and endophytes of plants. In this review, the transmission and influencing factors of ARGs in the microorganisms associated with plants, especially the influence of root exudates on plant microbiomes, are analyzed. Notably, the role of intrinsic genes of plants in determining root exudates and their potential effects on ARGs are proposed and analyzed. The important role of phyllosphere microorganisms and endophytes in the transmission of ARGs and co-resistance of antibiotics and other substances are also emphasized. The proliferation and transmission of ARGs associated with plant microbiomes addressed in this review is conducive to revealing the fate of ARGs in plant microorganisms and alleviating ARG pollution.

Characterization of Conyza bonariensis Allelochemicals against Broomrape Weeds

The study of allelopathic activity of plants and the isolation and characterization of the responsible allelochemicals can lead to the development of environment friendly alternative approaches to weed control. Conyza species are invasive weeds that use allelopathic activity as part of a successful strategy to outcompete neighboring plants. Broomrape weeds are parasitic plants that use host-induced germination and the formation of a haustorium as strategies to infect host plants. The control of broomrape infection in most affected crops is limited or non-existing. In the current study, we investigated the allelopathic activity of Conyza bonariensis organic extracts in suicidal germination and radicle growth of four broomrape species (Orobanche crenata, Orobanche cumana, Orobanche minor and Phelipanche ramosa). A bioactivity-driven fractionation of Conyza bonariensis extracts led to the identification of two germination-inducing molecules and two growth-inhibitory compounds. The germination-inducing metabolites had species-specific activity being hispidulin active on seeds of O. cumana and methyl 4-hydroxybenzoate active in P. ramosa. The growth-inhibitory metabolites (4Z)-lachnophyllum lactone and (4Z,8Z)-matricaria lactone strongly inhibited the radicle growth of all parasitic weed species studied. Some structure-activity relationships were found as result of the study herein presented.

Isoflavone levels, nodulation and gene expression profiles of a CRISPR/Cas9 deletion mutant in the isoflavone synthase gene of red clover

Isoflavones are not involved in rhizobial signaling in red clover, but likely play a role in defense in the rhizosphere. Red clover (Trifolium pratense) is a high-quality forage legume, well suited for grazing and hay production in the temperate regions of the world. Like many legumes, red clover produces a number of phenylpropanoid compounds including anthocyanidins, flavan-3-ols, flavanols, flavanones, flavones, and isoflavones. The study of isoflavone biosynthesis and accumulation in legumes has come into the forefront of biomedical and agricultural research due to potential for medicinal, antimicrobial, and environmental implications. CRISPR/Cas9 was used to knock out the function of a key enzyme in the biosynthesis of isoflavones, isoflavone synthase (IFS1). A hemizygous plant carrying a 9-bp deletion in the IFS1 gene was recovered and was intercrossed to obtain homozygous mutant plants. Levels of the isoflavones formononetin, biochanin A and genistein were significantly reduced in the mutant plants. Wild-type and mutant plants were inoculated with rhizobia to test the effect of the mutation on nodulation, but no significant differences were observed, suggesting that these isoflavones do not play important roles in nodulation. Gene expression profiling revealed an increase in expression of the upstream genes producing the precursors for IFS1, namely, phenylalanine ammonium lyase and chalcone synthase, but there were no significant differences in IFS1 gene expression or in the downstream genes in the production of specific isoflavones. Higher expression in genes involved in ethylene response was observed in the mutant plants. This response is normally associated with biotic stress, suggesting that the plants may have been responding to cues in the surrounding rhizosphere due to lower levels of isoflavones.

Secondary metabolites that could contribute to the monodominance of Erythrina fusca in the Brazilian Pantanal

Erythrina fusca is a dominant species in the Brazilian Pantanal. We hypothesized that E. fusca possess allelopathic potential and we evaluated effects of extracts on germination and development of Lactuca sativa, a bioindicator species. We tested the effect of leaves, bark, roots, and seeds extracts of E. fusca on germination and speed index, using high, moderate and low concentration (0.2, 1 and 5 mg mL-1). To evaluate effects on development, we subjected seedlings of L. sativa to the same treatments and measured root and aerial part length. High concentration of extracts reduced L. sativa germination; leaves extract caused the maximum reduction on germination of L. sativa, similar to 2,4-Dichlorophenoxyacetic acid (2,4-D); this extract has flavonoids and saponins as main compounds, classes that also occur in the bark and roots extracts in lower concentrations; bark and roots (5 mg mL-1), leaves and roots (1 mg mL-1) decreased these traits as well, but in lower magnitude. A significant reduction in root length was induced by highest concentration of all extracts (5 mg mL-1); the results suggest that erythrinic alkaloids should interfere in the root length once the seeds accumulate almost exclusively this class of compounds. Our results showed that all parts of E. fusca had adverse effects on germination or development of L. sativa, showing that different class of compounds secondary metabolites is involved in this activity. Possibly, this phytotoxicity influences monodominance of E. fusca in Pantanal, but studies are essential to evaluate effects of it on other native species.

Drought-tolerant Desmodium species effectively suppress parasitic striga weed and improve cereal grain yields in western Kenya

The parasitic weed Striga hermonthica Benth. (Orobanchaceae), commonly known as striga, is an increasingly important constraint to cereal production in sub-Saharan Africa (SSA), often resulting in total yield losses in maize (Zea mays L.) and substantial losses in sorghum (Sorghum bicolor (L.) Moench). This is further aggravated by soil degradation and drought conditions that are gradually becoming widespread in SSA. Forage legumes in the genus Desmodium (Fabaceae), mainly D. uncinatum and D. intortum, effectively control striga and improve crop productivity in SSA. However, negative effects of climate change such as drought stress is affecting the functioning of these systems. There is thus a need to identify and characterize new plants possessing the required ecological chemistry to protect crops against the biotic stress of striga under such environmental conditions. 17 accessions comprising 10 species of Desmodium were screened for their drought stress tolerance and ability to suppress striga. Desmodium incanum and D. ramosissimum were selected as the most promising species as they retained their leaves and maintained leaf function for longer periods during their exposure to drought stress conditions. They also had desirable phenotypes with more above ground biomass. The two species suppressed striga infestation, both under controlled and field conditions, and resulted in significant grain yield increases, demonstrating the incremental capability of Desmodium species in striga suppression. These results demonstrate beneficial effects of Desmodium species in enhancing cereal productivity in dry areas.

The biosynthesis of allelopathic di-C-glycosylflavones from the roots of Desmodium incanum (G. Mey.) DC

The allelopathic root exudate of the drought-tolerant subsistence cereal intercrop D. incanum, protecting against the parasitic weed Striga hermonthica, comprises a number of di-C-glycosylflavones specifically containing C-glucosyl, C-galactosyl and C-arabinosyl moieties. Here we demonstrate that the biosynthesis of all compounds containing a C-glucose involves C-glucosylation of 2-hydroxynaringenin with subsequent C-galactosylation, C-glucosylation or C-arabinosylation. In addition, the crude soluble enzyme extract converts two fluorinated 2-hydroxyflavanone analogues to corresponding mono- and di-C-glycosylflavones demonstrating that some differences in C-ring substitution can be tolerated by the plant enzymes. Elucidating the biosynthesis of these C-glycosylflavones (CGFs) has the potential to open up opportunities for transferring the enzymic and genetic basis for the S. hermonthica inhibiting allelopathic trait to food crop plants.

Isolation and identification of Desmodium root exudates from drought tolerant species used as intercrops against Striga hermonthica

Plants from the genus Desmodium, in particular D. uncinatum, are used on sub-Saharan small-holder farms as intercrops to inhibit parasitism of cereal crops by Striga hermonthica and Striga asiatica via an allelopathic mechanism. The search for Desmodium species which are adapted to more arid conditions, and which show resilience to increased drought stress, previously identified D. intortum, D. incanum and D. ramosissimum as potential drought tolerant intercrops. Their potential as intercrops was assessed for resource poor areas of rain-fed cereal production where drought conditions can persist through normal meteorological activity, or where drought may have increasing impact through climate change. The chemical composition of the root exudates were characterised and the whole exudate biological activity was shown to be active in pot experiments for inhibition of Striga parasitism on maize. Furthermore, rain fed plot experiments showed the drought tolerant Desmodium intercrops to be effective for Striga inhibition. This work demonstrates the allelopathic nature of the new drought tolerant intercrops through activity of root exudates and the major compounds seen in the exudates are characterised as being C-glycosylflavonoid. In young plants, the exudates show large qualitative differences but as the plants mature, there is a high degree of convergence of the C-glycosylflavonoid exudate chemical profile amongst active Desmodium intercrops that confers biological activity. This defines the material for examining the mechanism for Striga inhibition.

Antiosteoporotic activity and constituents of Podocarpium podocarpum

Our study aimed to investigate the antiosteoporotic properties of the ethanol extract of Podocarpium podocarpum (DC.) Yang et Huang (PE) in ovariectomized (OVX) rats and to characterize the active constituents. As a result, PE significantly inhibited the increased urinary Ca excretion and activity of bone resorption markers including tartrate-resistant acid phosphatase (TRAP), deoxypyridinoline crosslinks and cathepsin K in OVX rats, whereas exhibited little effects on the body, uterus and vagina weight. Detailed micro-CT analysis showed that PE notably enhanced bone quality, with increased bone mineral content (BMC), bone volume fraction (BVF), connectivity density (CD), tissue mineral content (TMC), tissue mineral density (TMD) and trabecular number (Tb. N), and decreased trabecular separation (Tb. Sp), in OVX animal. Those findings implied that PE had notable antiosteoporotic effect, especially effective in preventing bone resorption, with little side-effects on reproductive tissue. Further chemical investigation led to the isolation of 17 flavonoids, most of which showed significantly stimulatory effect on osteoblastic proliferation, ALP activity and mineralized nodes formation as well as inhibitory effect on osteoclastic TRAP activity in osteoblastic and osteoclastic cells. Our results indicated that PE, with abundant flavonoids, had remarkable antiosteoporotic activity and therefore can be a promising candidate for the treatment of postmenopausal osteoporosis induced by estrogen deficiency through herbal remedy.

Effect of sunn hemp (Crotalaria juncea L.) cutting date and planting density on weed suppression in Georgia, USA

A field study was conducted in 2008 and 2009 at the USDA, ARS, Plant Genetic Resources Conservation Unit in Griffin, GA, to investigate weed suppression by sunn hemp (Crotalaria juncea L). The objectives were to (1) evaluate the effects of apical meristem removal (AMR) at three dates [5, 6, and 7 wks after planting (WAP) on May 14, 2008 and May 21, 2009] and (2) assess the impact of seeding rates (11, 28, and 45 kg ha(-1)) on weed biomass reduction. Weed species were identified at 4, 8, and 12 wks after sunn hemp planting. Sunn hemp cutting date had no significant effect on weed suppression in 2008 but significant differences for grass weeds at 4, 8, and 12 WAP and for yellow nutsedge at 8 and 12 WAP did occur when compared to the control in 2009. In comparison to the sunn hemp-free control plot in 2009, all three seeding rates had reduced grass weed dry weights at 4, 8, and 12 WAP. The total mass of yellow nutsedge when grown with sunn hemp was reduced compared to the total mass of yellow nutsedge grown in the weedy check for all seeding rates at 8 and 12 WAP. Lower grass weed biomass was observed by 12 WAP for cutting dates and seeding rates during 2008 and 2009. Sunn hemp cutting date and seeding rate reduced branch numbers in both years. The reduction in sunn hemp seeding rates revealed a decrease in weed populations.

Engineering the plant rhizosphere

Plant natural products are low molecular weight compounds playing important roles in plant survival under biotic and abiotic stresses. In the rhizosphere, several groups of plant natural products function as semiochemicals that mediate the interactions of plants with other plants, animals and microorganisms. The knowledge on the biosynthesis and transport of these signaling molecules is increasing fast. This enables us to consider to optimize plant performance by changing the production of these signaling molecules or their exudation into the rhizosphere. Here we discuss recent advances in the understanding and metabolic engineering of these rhizosphere semiochemicals.

Desmodianone H and uncinanone B, potential tyrosinase inhibitors obtained from Lespedeza maximowiczii by using bioactivity-guided isolation

A new bioactive compound, namely desmodianone H(1), and another known compound uncinanone B(2) were first isolated using bioactivity-guided isolation from the leaves of Lespedeza maximowiczii and structures were elucidated by comprehensive analysis of their nuclear magnetic resonance and mass spectrometry data. Compounds 1 and 2 exhibited strong inhibitory effects on mushroom tyrosinase activity.

Achieving food security for one million sub-Saharan African poor through push-pull innovation by 2020

Food insecurity is a chronic problem in Africa and is likely to worsen with climate change and population growth. It is largely due to poor yields of the cereal crops caused by factors including stemborer pests, striga weeds and degraded soils. A platform technology, 'push-pull', based on locally available companion plants, effectively addresses these constraints resulting in substantial grain yield increases. It involves intercropping cereal crops with a forage legume, desmodium, and planting Napier grass as a border crop. Desmodium repels stemborer moths (push), and attracts their natural enemies, while Napier grass attracts them (pull). Desmodium is very effective in suppressing striga weed while improving soil fertility through nitrogen fixation and improved organic matter content. Both companion plants provide high-value animal fodder, facilitating milk production and diversifying farmers' income sources. To extend these benefits to drier areas and ensure long-term sustainability of the technology in view of climate change, drought-tolerant trap and intercrop plants are being identified. Studies show that the locally commercial brachiaria cv mulato (trap crop) and greenleaf desmodium (intercrop) can tolerate long droughts. New on-farm field trials show that using these two companion crops in adapted push-pull technology provides effective control of stemborers and striga weeds, resulting in significant grain yield increases. Effective multi-level partnerships have been established with national agricultural research and extension systems, non-governmental organizations and other stakeholders to enhance dissemination of the technology with a goal of reaching one million farm households in the region by 2020. These will be supported by an efficient desmodium seed production and distribution system in eastern Africa, relevant policies and stakeholder training and capacity development.

Inhibition of Orobanche crenata seed germination and radicle growth by allelochemicals identified in cereals

Orobanche crenata is a parasitic weed that causes severe yield losses in important grain and forage legume crops. Cereals have been reported to inhibit O. crenata parasitism when grown intercropped with susceptible legumes, but the responsible metabolites have not been identified. A number of metabolites have been reported in cereals that have allelopathic properties against weeds, pests, and pathogens. We tested the effect of several allelochemicals identified in cereals on O. crenata seed germination and radicle development. We found that 2-benzoxazolinone, its derivative 6-chloroacetyl-2-benzoxazolinone, and scopoletin significantly inhibited O. crenata seed germination. Benzoxazolinones, l-tryptophan, and coumalic acid caused the stronger inhibition of radicle growth. Also, other metabolites reduced radicle length, this inhibition being dose-dependent. Only scopoletin caused cell necrotic-like darkening in the young radicles. Prospects for their application to parasitic weed management are discussed.

Elucidation of the biosynthesis of the di-C-glycosylflavone isoschaftoside, an allelopathic component from Desmodium spp. that inhibits Striga spp. development

Isoschaftoside, an allelopathic di-C-glycosylflavone from Desmodium spp. root exudates, is biosynthesised through sequential glucosylation and arabinosylation of 2-hydroxynaringenin with UDP-glucose and UDP-arabinose. Complete conversion to the flavone requires chemical dehydration implying a dehydratase enzyme has a role in vivo to complete the biosynthesis. The C-glucosyltransferase has been partially characterised and its activity demonstrated in highly purified fractions.

Rapid isolation and identification of minor natural products by LC-MS, LC-SPE-NMR and ECD: isoflavanones, biflavanones and bisdihydrocoumarins from Ormocarpum kirkii

The combination of the hyphenated techniques LC-MS and LC-SPE-NMR constitutes a powerful platform for the rapid isolation and identification of minor components from natural sources. Electronic circular dichroism (ECD) is a useful tool to determine the absolute configuration of small quantities of chiral molecules. In order to search for minor constituents present in an Ormocarpum kirkii extract, these techniques were applied for the separation and structure elucidation of a series of isoflavanones, biflavanones and biscoumarins. After optimization of chromatographic conditions and subsequent isolation, MS and 1D and 2D NMR data were collected. Experimental and calculated ECD spectra were used in conjunction with NMR data to confirm the absolute configuration of these compounds. Eight compounds were identified for the first time and six have been previously reported. The present approach offers a strategy for accelerating research on natural products.

In vitro phytotoxicity and antioxidant activity of selected flavonoids

The knowledge of flavonoids involved in plant-plant interactions and their mechanisms of action are poor and, moreover, the structural characteristics required for these biological activities are scarcely known. The objective of this work was to study the possible in vitro phytotoxic effects of 27 flavonoids on the germination and early radical growth of Raphanus sativus L. and Lepidium sativum L., with the aim to evaluate the possible structure/activity relationship. Moreover, the antioxidant activity of the same compounds was also evaluated. Generally, in response to various tested flavonoids, germination was only slightly affected, whereas significant differences were observed in the activity of the various tested flavonoids against radical elongation. DPPH test confirms the antioxidant activity of luteolin, quercetin, catechol, morin, and catechin. The biological activity recorded is discussed in relation to the structure of compounds and their capability to interact with cell structures and physiology. No correlation was found between phytotoxic and antioxidant activities.

The genus Desmodium (Fabaceae)-traditional uses in Chinese medicine, phytochemistry and pharmacology

ETHNOPHARMACOLOGICAL RELEVANCE

Plants of the genus Desmodium (Fabaceae), such as Desmodium styracifolium (Osbeck) Merr. and Desmodium gyrans (L. f.) DC., have a long history of medical use in Traditional Chinese Medicine to treat various ailments including rheumatism, pyrexia, dysentery, wounds, cough, malaria, hepatitis, hemoptysis, etc. In the theory of Traditional Chinese Medicine, most species have the effect of relieving internal heat or fever, neutralizing toxins, inhibiting pain, invigorating blood circulation, suppressing cough and alleviating dyspnea.

MATERIALS AND METHODS

A bibliographic investigation was accomplished by analyzing secondary sources including Chinese Herbal Classics, and worldwide accepted scientific databases (Pubmed, Scopus and Web of Science, SciFinder) were scrutinized for the available information on the ethnopharmacological uses in Chinese medicine, phytochemistry, pharmacology and toxicology of Desmodium species.

RESULTS

The genus Desmodium is a large member of the Papilionaceae (Fabaceae) family. It contains about 350 plant species used for both feeding stuffs and herbal medicines, of which only about 30 species have been phytochemically or pharmacologically investigated. Desmodium plant extracts, as well as the active principles, have been experimentally studied for their anti-inflammatory, cytotoxic, antidiabetic, antinephrolithic, antibacterial, and nootropic activities in vitro or in vivo. And so far, a total of 212 compounds have been isolated from 15 Desmodium species and characterized mainly as flavonoids and alkaloids, followed by terpenoids, steroids, phenols, phenylpropanoids, glycosides and a number of volatile oils. The remaining unrevealed species are recorded chiefly in Asia and Africa being used in empirical treatment for various diseases.

CONCLUSIONS

Desmodium species have long been used in TCM to treat various ailments. Available scientific references revealed that the traditional medical uses of some important Desmodium species in TCM have been evaluated by modern pharmacological studies. As literature demonstrated, flavonoids and alkaloids are perhaps responsible for most of the activities shown by the plants of this genus. Further studies are still required to reveal the structure-activity relationship of these active constituents.

Isoschaftoside, a C-glycosylflavonoid from Desmodium uncinatum root exudate, is an allelochemical against the development of Striga

In East African small-holder farming of maize, the cattle forage legume, Desmodium uncinatum is used as an intercrop due to its allelopathic inhibition of parasitism by Striga hermonthica, an obligate parasitic weed that can devastate the maize crop. Bioassay-guided fractionation of the root extract of D. uncinatum revealed isoschaftoside to be the main compound in the most potent fraction inhibiting growth of germinated S. hermonthica radicles. Bioassays repeated with isoschaftoside isolated from a different plant source, Passiflora incarnata, proved it to be a biologically active component. Analysis of the root exudates produced by hydroponically grown D. uncinatum showed isoschaftoside to be present in the hydroponic media at biologically active concentrations of 10-100 nM.

Arbuscular mycorrhizal fungi protect a native plant from allelopathic effects of an invader

The allelopathic potential of the Eurasian invasive plant Alliaria petiolata has been well documented, with the bulk of the effects believed to be mediated by arbuscular mycorrhizal fungi (AMF). We exposed the herbaceous annual Impatiens pallida, which is native to North America, to fractionated A. petiolata extracts at four developmental stages (germination, presymbiosis growth, symbiosis formation, and symbiosis growth) by using exposure levels expected to be similar to field levels. Surprisingly, we found strong direct effects on I. pallida germination and growth, but no indirect effects on I. pallida growth mediated by AMF. We also observed strong synergistic effects with a complete A. petiolata extract that inhibited I. pallida germination and presymbiosis root growth more than either a glucosinolate or flavonoid enriched fraction alone. In fact, the flavonoid enriched fraction tended to stimulate germination and presymbiosis root growth. In contrast to these strong direct effects, I. pallida plant growth during both the symbiosis formation and symbiosis growth phases was unaffected by A. petiolata extracts. We also found no inhibition of AMF colonization of roots or soils by A. petiolata extracts. We show that AMF can actually ameliorate allelopathic effects of an invasive plant, and suggest that previously observed allelopathic effects of A. petiolata may be due to direct inhibition of plant and fungal growth before symbiosis formation.

Companion cropping to manage parasitic plants

Parasitic plants, through a range of infestation strategies, can attack crop plants and thereby require management. Because such problems often occur in resource-poor farming systems, companion cropping to manage parasitic plants is an appropriate approach. Many examples of companion cropping for this purpose have been reported, but the use of cattle forage legumes in the genus Desmodium as intercrops has been shown to be particularly successful in controlling the parasitic witchweeds (Striga spp.) that afflict approximately one quarter of sub-Saharan African cereal production. Through the use of this example, the development of effective companion crops is described, together with developments toward widespread adoption and understanding the underlying mechanisms, both for sustainability and ensuring food security, and also for exploitation beyond the cropping systems described here.

Control--the Striga conundrum

There is a wide range of existing and potential control options for Striga. This paper describes and discusses many of the control options, with a focus on technology limitations, adoption limitations (real or potential) and, in the case of novel technologies, development limitations. The paper addresses the question as to why, after many years of research, control method testing, piloting and technology dissemination, the wide-scale effective control of Striga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze is so elusive. Limitations, including variable technology reliability, poor access to control technology, costs (monetary, labour, skills) associated with control technology, limited practicality of methods and poor information, all hamper the adoption and impact of existing control methods. Some of the same issues may impact upon novel control technologies, and this needs careful consideration. Additional issues surround other potential technologies, especially so in the case of transgenic approaches. Suggestions are made as to how the impasse of effective Striga control can be overcome. More effective use of integrated control approaches, improved crop germplasm phenotyping, enhanced understanding of the host/non-host--parasite interaction and better integration and communication among the parasitic plant research, development and extension community are among the suggestions made.

New genetic opportunities from legume intercrops for controlling Striga spp. parasitic weeds

In smallholder farming in East Africa, intercropping of maize with the cattle forage legume, Desmodium uncinatum Jacq., prevents parasitism by Striga hermonthica (Del.) Benth. (witchweed) through an allelopathic mechanism. Isoschaftoside, a di-C-glycosylflavone, isolated from the root extract and root exudate of Desmodium, interferes with in vitro radicle development of germinated Striga. The biosynthetic pathway of this class of compound is already mostly present in edible legumes and in cereals, so characterisation of the enzyme and genes that control C-glycosylflavone biosynthesis has the potential to create this protection mechanism in other agriculturally important plants.

Recognition of root exudates by seeds of broomrape (Orobanche and Phelipanche) species

BACKGROUND AND AIMS

The long co-existence of broomrapes and their hosts within the same environment has culminated in a strong adaptation and effective parasitism. As a first step of specialization in the parasitic process, seed receptors of parasitic plant species vary in their ability to recognize compounds released by their hosts. This work aims to investigate potential patterns for the reception requirements needed to activate germination within Orobanche and Phelipanche species.

METHODS

Induction of the germination of seeds of nine Orobanche and Pheliphanche species by root exudates of 41 plant species was studied and subjected to biplot multivariate analysis.

KEY RESULTS

A high level of specialization in root exudate recognition was found in Orobanche densiflora, O. gracilis and O. hederae, which germinated almost exclusively in contact with root exudates from the plants they infect in nature. At the opposite extreme, Phelipanche aegyptiaca, P. ramosa and O. minor were highly generalist, germinating when in contact with the root exudates of most plant species. Orobanche crenata, O. cumana and O. foetida showed intermediate behaviour.

CONCLUSIONS

A universal germination stimulant for all broomrape species has not being identified to date. The synthetic stimulant GR24 is active against most of the weedy broomrape species, but fails with the non-weedy species tested in this study and with the very recent weedy species O. foetida. In addition, germination behaviour of broomrape species depends on the crop plant tested. Weedy broomrapes with a broad host spectrum respond better to the different exudates released by a wide range of crops and wild species than do non-weedy broomrapes, which have a narrow host spectrum and are more restricted to their host range. Root exudates of many plant species were active in stimulating germination of seeds of Orobanche and Phelipanche species for which they are not described as hosts, representing interesting examples of potential trap crops.

Melanin synthesis inhibitors from Lespedeza cyrtobotrya

From the roots of Lespedeza cyrtobotrya, 45 flavonoids were isolated along with 20 new and 25 known compounds. Lipophilic flavonoids 2, 3, 7, 9, 11, 28, 30, and 39 exhibited strong inhibitory activities on melanin synthesis in normal human epidermal melanocytes.

Isoflavanones from Uraria picta and their antimicrobial activity

Two isoflavanones, 5,7-dihydroxy-2'-methoxy-3',4'-methylenedioxyisoflavanone (2) and 4',5-dihydroxy-2',3'-dimethoxy-7-(5- hydroxyoxychromen-7yl)-isoflavanone (4) along with six known compounds including isoflavanones, triterpenes and steroids were isolated from the roots of Uraria picta. The structures of these compounds were established unambiguously by UV, IR, MS and a series of 1D and 2D NMR analyses. The minimum inhibitory concentrations (MIC) for these compounds were found to be in the range of 12.5-200 microg/ml against bacteria (both Gram positive and Gram negative) and fungi.

Isoflavanones and their O-glycosides from Desmodium styracifolium

Two isoflavanones (5,7-dihydroxy-2',3',4'-trimethoxy-isoflavanone and 5,7-dihydroxy-2'-methoxy-3',4'-methylenedioxy-isoflavanone), four isoflavanone O-glycosides (5,7-dihydroxy-2',3',4'-trimethoxy-isoflavanone 7-O-beta-glucopyranoside, 5,7-dihydroxy-2'-methoxy-3',4'-methylenedioxy-isoflavanone 7-O-beta-glucopyranoside, 5,7-dihydroxy-2',4'-dimethoxy-isoflavanone 7-O-beta-glucopyranoside, and 5,7,4'-trihydroxy-2',3'-dimethoxy-isoflavanone 7-O-beta-glucopyranoside), and a coumaronochromone (3,5,7,4'-tetrahydroxy-coumaronochromone), along with 25 known compounds, were isolated from the aerial parts of Desmodium styracifolium. This is for the first time isoflavanone O-glycosides were isolated from a natural source.

Rhizosphere communication of plants, parasitic plants and AM fungi

Plants use an array of secondary metabolites to defend themselves against harmful organisms and to attract others that are beneficial. However, the attraction of beneficial organisms could also lead to abuse by malevolent organisms. An exciting example of such abuse is the relationship between plants, beneficial mutualistic arbuscular mycorrhizal fungi and harmful parasitic plants. Signalling molecules called strigolactones, which are secreted by plant roots in low concentrations, induce the growth of both obligate biotrophs. Here, we review the importance of strigolactones for these two interactions and discuss possible developments that should further clarify the role of these signalling molecules in rhizosphere processes.

Isoflavonoids of the leguminosae

Isoflavonoids are found predominantly in subfamily Papilionoideae of the Leguminosae. This review describes more than 420 new examples of Leguminosae isoflavonoids, giving details of their source, identification, biological activity, synthesis, and ecological or chemosystematic significance. Other topics addressed include the application of hyphenated analytical techniques to the characterisation of legume-derived isoflavonoids, and advances made in biosynthetic studies. A checklist of new compounds by species is given, and 404 references are cited.

C-methylated and C-prenylated isoflavonoids from root extract of Desmodium uncinatum

A pterocarpan, 1,9-dihydroxy-3-methoxy-2-methylpterocarpan (named uncinacarpan) and two isoflavanones, 5,7-dihydroxy-2',3',4'-trimethoxy-6-(3-methylbut-2-enyl)isoflavanone (named uncinanone D) and 5,4'-dihydroxy-7,2'-dimethoxy-6-methylisoflavanone (named uncinanone E), were isolated from the CH(2)Cl(2) root extract of Desmodium uncinatum (Jacq.) DC and characterised by spectroscopic methods. In addition, a rare pterocarpan edudiol and two known abietane diterpenes, 7-oxo-15-hydroxydehydroabietic acid and 7-hydroxycallitrisic acid were identified. The fraction of the root extract that was analysed induced germination of Striga hermonthica seeds, but none of the isolated compounds showed this activity.

The use of push-pull strategies in integrated pest management

Push-pull strategies involve the behavioral manipulation of insect pests and their natural enemies via the integration of stimuli that act to make the protected resource unattractive or unsuitable to the pests (push) while luring them toward an attractive source (pull) from where the pests are subsequently removed. The push and pull components are generally nontoxic. Therefore, the strategies are usually integrated with methods for population reduction, preferably biological control. Push-pull strategies maximize efficacy of behavior-manipulating stimuli through the additive and synergistic effects of integrating their use. By orchestrating a predictable distribution of pests, efficiency of population-reducing components can also be increased. The strategy is a useful tool for integrated pest management programs reducing pesticide input. We describe the principles of the strategy, list the potential components, and present case studies reviewing work on the development and use of push-pull strategies in each of the major areas of pest control.