Management of Infection by Parasitic Weeds: A Review

What we know so far and what we can expect next: A molecular investigation of plant parasitism

The review explores parasitic plants' evolutionary success and adaptability, highlighting their widespread occurrence and emphasizing the role of an invasive organ called haustorium in nutrient acquisition from hosts. It discusses the genetic and physiological adaptations that facilitate parasitism, including horizontal gene transfer, and the impact of environmental factors like climate change on these relationships. It addresses the need for further research into parasitic plants' genomes and interactions with their hosts to better predict environmental changes' impacts.

Heat Stress and Plant-Biotic Interactions: Advances and Perspectives

Climate change presents numerous challenges for agriculture, including frequent events of plant abiotic stresses such as elevated temperatures that lead to heat stress (HS). As the primary driving factor of climate change, HS threatens global food security and biodiversity. In recent years, HS events have negatively impacted plant physiology, reducing plant's ability to maintain disease resistance and resulting in lower crop yields. Plants must adapt their priorities toward defense mechanisms to tolerate stress in challenging environments. Furthermore, selective breeding and long-term domestication for higher yields have made crop varieties vulnerable to multiple stressors, making them more susceptible to frequent HS events. Studies on climate change predict that concurrent HS and biotic stresses will become more frequent and severe in the future, potentially occurring simultaneously or sequentially. While most studies have focused on singular stress effects on plant systems to examine how plants respond to specific stresses, the simultaneous occurrence of HS and biotic stresses pose a growing threat to agricultural productivity. Few studies have explored the interactions between HS and plant-biotic interactions. Here, we aim to shed light on the physiological and molecular effects of HS and biotic factor interactions (bacteria, fungi, oomycetes, nematodes, insect pests, pollinators, weedy species, and parasitic plants), as well as their combined impact on crop growth and yields. We also examine recent advances in designing and developing various strategies to address multi-stress scenarios related to HS and biotic factors.

Tomato Mutants Reveal Root and Shoot Strigolactone Involvement in Branching and Broomrape Resistance

The phytohormones strigolactones (SLs) control root and shoot branching and are exuded from roots into the rhizosphere to stimulate interaction with mycorrhizal fungi. The exuded SLs serve as signaling molecules for the germination of parasitic plants. The broomrape Phelipanche aegyptiaca is a widespread noxious weed in various crop plants, including tomato (Solanum lycopersicum). We have isolated three mutants that impair SL functioning in the tomato variety M82: SHOOT BRANCHING 1 (sb1) and SHOOT BRANCHING 2 (sb2), which abolish SL biosynthesis, and SHOOT BRANCHING 3 (sb3), which impairs SL perception. The over-branching phenotype of the sb mutants resulted in a severe yield loss. The isogenic property of the mutations in a determinate growth variety enabled the quantitative evaluation of the contribution of SL to yield under field conditions. As expected, the mutants sb1 and sb2 were completely resistant to infection by P. aegyptiaca due to the lack of SL in the roots. In contrast, sb3 was more susceptible to P. aegyptiaca than the wild-type M82. The SL concentration in roots of the sb3 was two-fold higher than in the wild type due to the upregulation of the transcription of SL biosynthesis genes. This phenomenon suggests that the steady-state level of root SLs is regulated by a feedback mechanism that involves the SL signaling pathway. Surprisingly, grafting wild-type varieties on sb1 and sb2 rootstocks eliminated the branching phenotype and yield loss, indicating that SL synthesized in the shoots is sufficient to control shoot branching. Moreover, commercial tomato varieties grafted on sb1 were protected from P. aegyptiaca infection without significant yield loss, offering a practical solution to the broomrape crisis.

Genetic potential and inheritance pattern of agronomic traits in faba bean under free and infested Orobanche soil conditions

BACKGROUND

Orobanche is an obligate parasite on faba bean in the Mediterranean region, causes considerable yield losses. Breeding tolerant faba bean genotypes to Orobanche is pivotal to sustain production and ensuring global food security, particularly considering the challenges posed by population growth. In the present study, seven faba bean lines and four testers were used in a line×tester mating design during 2020-2021 and 2021-2022 growing seasons. The eleven parents and their 28 F1 crosses were evaluated under Orobanche free and naturally infested soils.

RESULTS

The results demonstrated considerable variations among the evaluated genotypes, wide diversity among the parental materials, and heterotic effects for all studied agronomic traits under Orobanche-free and infested soils. Orbanche infestation displayed a significant adverse impact on all the studied agronomic traits. The genotypes Line1, Line2, Line3, and Line5 displayed superior performance under Orobanche-infested conditions and recorded the highest values of all studied agronomic traits. Additionally, Line1, Line2, Line3, Line5, and Line7 exhibited desirable significant GCA for most evaluated traits under the two infestation conditions. The obtained crosses displayed significant negative or positive heterosis for studied agronomic characters such as plant height, number of branches per plant, number of pods per plant, number of seeds per plant, and seed weight per plant were observed. Furthermore, specific cross combinations such as Line2×Sakha3, Line3×Nubaria5, Line7 × Nubaria5, Line6×Nubaria1, Line5×Sakha3, Line1×Sakha3, and Line1 × Nubaria5 exhibited superior performance in seed yield and contributing traits under Orobanche-infested conditions. Moreover, these specific crosses showed superior efficacy in reducing dry weight of Orobanche spikes. The results obtained from GGE biplot analysis closely aligned with those from the line×tester procedure, affirming the significance of GGE biplot as a valuable statistical tool for assessing genotype combining ability in line× tester data. Both additive and non-additive gene actions were reported to be predominantly involved in the inheritance of the studied agronomic traits in faba bean.

CONCLUSIONS

The detected genetic diversity within the evaluated faba bean genotypes and their developed crosses exhibits substantial potential for improving faba bean productivity under Orobanche-infested conditions. The parental genotypes, Line1, Line2, Line3, Line5, and Line7, were identified as effective and promising combiners. Moreover, the developed crosses Line2×Sakha3, Line3×Nubaria5, Line7×Nubaria5, Line6×Nubaria1, Line5×Sakha3, Line1×Sakha3, and Line1×Nubaria5 could be considered valuable candidates for developing high-yielding and tolerant faba bean genotypes to Orobanche.

(4Z)-Lachnophyllum Lactone, a Metabolite with Phytotoxic and Antifungal Activity against Pests Affecting Mediterranean Agriculture: A New Versatile and Easy Scalable Parallel Synthesis

A methodology for the total and modulable synthesis of (4Z)-lachnophyllum lactone (1), on a gram scale, is reported for the first time. The present work started with the design of a retrosynthetic pathway for the target compound, with the key step identified in Pd-Cu bimetallic cascade cross-coupling cyclization. (4Z)-Lachnophyllum lactone (1) is an acetylenic furanone previously isolated, in a low amount, from the organic extract of the autotrophic weedConyza bonariensis. Tested against the stem parasitic weed Cuscuta campestris in a seedling growth bioassay, (4Z)-lachnophyllum lactone (1) showed almost 85% of inhibitory activity up to 0.3 mM in comparison with the control. At the same concentration, the compound displayed radicle growth inhibitory activity of the root parasitic weeds Orobanche minor and Phelipanche ramosa higher than 70 and 40%, respectively. Surprisingly, the compound showed a high percentage of inhibition, up to 0.1 mM, on C. bonariensis seed germination too. This versatile synthetic strategy was also used to obtain two further natural analogues, namely, (4E)-lachnophyllum lactone (8) and (4Z,8Z)-matricaria lactone (9), that showed, in most cases, the same inhibitory trend with slight differences, highlighting the importance of the stereochemistry and unsaturation of the side chain. Furthermore, all of the compounds showed antifungal activity at 1 mM reducing the mycelial growth of the olive pathogen Verticillium dahliae. The design and implementation of scalable and modulable total synthesis on a gram scale of acetylenic furanones allow the production of a large amount of these natural products, overcoming the limit imposed by isolation from natural sources. The results of the present study pave the way for the development of ecofriendly bioinspired pesticides with potential application in agrochemical practices as alternative to synthetic pesticides.

Sesquiterpene Lactones Isolated from Centaurea cineraria L. subsp. cineraria Inhibit the Radicle Growth of Broomrape Weeds

The plant Centaurea cineraria L. subsp. cineraria has been investigated as a potential source of inhibitors of broomrape radicle growth. The latter are weeds that pose a threat to agriculture and for which there are few methods available for the control of infestations. Four sesquiterpene lactones have been isolated from C. cineraria L. subsp. cineraria aerial parts and identified as isocnicin, cnicin, salonitenolide, and 11β,13-dihydrosalonitenolide using spectroscopic, spectrometric, and optical methods. Salonitenolide and 11β,13-dihydrosalonitenolide have been isolated for the first time from this plant. Tested at 1.0-0.1 mM against the broomrape species Phelipanche ramosa, Orobanche minor, Orobanche crenata, and Orobanche cumana, isocnicin, cnicin, and salonitenolide demonstrated remarkable inhibitory activity (over 80% in most of the cases) at the highest concentrations. Structure-activity relationship conclusions indicated the significance of the α,β-unsaturated lactone ring. In addition, the synthetic acetylated derivative of salonitenolide showed the strongest activity among all compounds tested, with inhibitions close to 100% at different concentrations, which has been related to a different lipophilicity and the absence of H-bond donor atoms in its structure. Neither the extracts nor the compounds exhibited the stimulating activity of broomrape germination (induction of suicidal germination). These findings highlight the potential of C. cineraria to produce bioactive compounds for managing parasitic weeds and prompt further studies on its sesquiterpene lactones as tools in developing natural product-based herbicides.

Genome editing for healthy crops: traits, tools and impacts

Crop cultivars in commercial use have often been selected because they show high levels of resistance to pathogens. However, widespread cultivation of these crops for many years in the environments favorable to a pathogen requires durable forms of resistance to maintain "healthy crops". Breeding of new varieties tolerant/resistant to biotic stresses by incorporating genetic components related to durable resistance, developing new breeding methods and new active molecules, and improving the Integrated Pest Management strategies have been of great value, but their effectiveness is being challenged by the newly emerging diseases and the rapid change of pathogens due to climatic changes. Genome editing has provided new tools and methods to characterize defense-related genes in crops and improve crop resilience to disease pathogens providing improved food security and future sustainable agricultural systems. In this review, we discuss the principal traits, tools and impacts of utilizing genome editing techniques for achieving of durable resilience and a "healthy plants" concept.

Structure-Activity Relationship (SAR) Study of trans-Cinnamic Acid and Derivatives on the Parasitic Weed Cuscuta campestris

Cuscuta campestris Yunck. is a parasitic weed responsible for severe yield losses in crops worldwide. The selective control of this weed is scarce due to the difficult application of methods that kill the parasite without negatively affecting the infected crop. trans-Cinnamic acid is secreted by plant roots naturally into the rhizosphere, playing allelopathic roles in plant-plant communities, although its activity in C. campestris has never been investigated. In the search for natural molecules with phytotoxic activity against parasitic weeds, this work hypothesized that trans-cinnamic acid could be active in inhibiting C. campestris growth and that a study of a series of analogs could reveal key structural features for its growth inhibition activity. In the present structure-activity relationship (SAR) study, we determined in vitro the inhibitory activity of trans-cinnamic acid and 24 analogs. The results showed that trans-cinnamic acid's growth inhibition of C. campestris seedlings is enhanced in eight of its derivatives, namely hydrocinnamic acid, 3-phenylpropionaldehyde, trans-cinnamaldehyde, trans-4-(trifluoromethyl)cinnamic acid, trans-3-chlorocinnamic acid, trans-4-chlorocinnamic acid, trans-4-bromocinnamic acid, and methyl trans-cinnamate. Among the derivatives studied, the methyl ester derivative of trans-cinnamic acid was the most active compound. The findings of this SAR study provide knowledge for the design of herbicidal treatments with enhanced activity against parasitic weeds.

Genomic and Epigenomic Mechanisms of the Interaction between Parasitic and Host Plants

Parasitic plants extract nutrients from the other plants to finish their life cycle and reproduce. The control of parasitic weeds is notoriously difficult due to their tight physical association and their close biological relationship to their hosts. Parasitic plants differ in their susceptible host ranges, and the host species differ in their susceptibility to parasitic plants. Current data show that adaptations of parasitic plants to various hosts are largely genetically determined. However, multiple cases of rapid adaptation in genetically homogenous parasitic weed populations to new hosts strongly suggest the involvement of epigenetic mechanisms. Recent progress in genome-wide analyses of gene expression and epigenetic features revealed many new molecular details of the parasitic plants' interactions with their host plants. The experimental data obtained in the last several years show that multiple common features have independently evolved in different lines of the parasitic plants. In this review we discuss the most interesting new details in the interaction between parasitic and host plants.

Crop diversification and parasitic weed abundance: a global meta-analysis

Parasitic weeds cause huge annual losses to food production globally. A small number of species from the genera Cuscuta, Orobanche, Phelipanche and Striga have proliferated across many agroecological zones. Their control is compromised due to the lack of efficacy of conventional herbicides and their rapid adaptation to new resistant crop cultivars. A broad range of studies suggest consistent reductions in parasitic weed densities owing to increased spatial (intercropping) and temporal diversity (crop rotation). However, to date, no synthesis of this body of research has been published. Here we report the results of a meta-analysis using 1525 paired observations from 67 studies across 24 countries, comparing parasitic weed density and crop yields from monocrop and more diverse cropping systems. We found both spatial and temporal crop diversification had a significant effect on parasitic weed density reduction. Furthermore, our results show effects of spatial diversification are stronger in suppressing parasitic weeds than temporal effects. Furthermore, the analysis indicates intercrops which alter both microclimate and soil chemistry (e.g. Crotalaria, Stylosanthes, Berseem clover and Desmodium) are most effective in parasitic weed management. This analysis serves to underline the viability of crop diversification as a tool to enhance food security globally.

Mitigating the Mistletoe Menace: Biotechnological and Smart Management Approaches

Mistletoes have been considered a keystone resource for biodiversity, as well as a remarkable source of medicinal attributes that attract pharmacologists. Due to their hemiparasitic nature, mistletoes leach water and nutrients, including primary and secondary metabolites, through the vascular systems of their plant hosts, primarily trees. As a result of intense mistletoe infection, the hosts suffer various growth and physiological detriments, which often lead to tree mortality. Because of their easy dispersal and widespread tropism, mistletoes have become serious pests for commercial fruit and timber plantations. A variety of physical and chemical treatment methods, along with silvicultural practices, have shaped conventional mistletoe management. Others, however, have either failed to circumvent the growing range and tropism of these parasitic plants or present significant environmental and public health risks. A biocontrol approach that could sidestep these issues has never achieved full proof of concept in real-field applications. Our review discusses the downsides of conventional mistletoe control techniques and explores the possibilities of biotechnological approaches using biocontrol agents and transgenic technologies. It is possible that smart management options will pave the way for technologically advanced solutions to mitigate mistletoes that are yet to be exploited.

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.

Identification of Structural Features of Hydrocinnamic Acid Related to Its Allelopathic Activity against the Parasitic Weed Cuscuta campestris

Cuscuta campestris is a parasitic weed species that inflicts worldwide noxious effects in many broadleaf crops due to its capacity to withdraw nutrients and water directly from the crop vascular system using haustorial connections. Cuscuta campestris control in the majority of crops affected is non-existent, and thus, research for the development of control methods is needed. Hydrocinnamic acid occurs naturally in the rhizosphere, playing regulatory roles in plant-plant and plant-microbe communities. The toxicity of hydrocinnamic acid against C. campestris was recently identified. In the present work, a structure-activity relationship study of 21 hydrocinnamic acid analogues was performed to identify key structural features needed for its allelopathic action against the seedling growth of this parasitic plant. The findings of this study provide the first step for the design of herbicides with enhanced activity for the control of C. campestris infection.

Wild Helianthus species: A reservoir of resistance genes for sustainable pyramidal resistance to broomrape in sunflower

Orobanche cumana Wall., sunflower broomrape, is one of the major pests for the sunflower crop. Breeding for resistant varieties in sunflower has been the most efficient method to control this parasitic weed. However, more virulent broomrape populations continuously emerge by overcoming genetic resistance. It is thus essential to identify new broomrape resistances acting at various stages of the interaction and combine them to improve resistance durability. In this study, 71 wild sunflowers and wild relatives accessions from 16 Helianthus species were screened in pots for their resistance to broomrape at the late emergence stage. From this initial screen, 18 accessions from 9 species showing resistance, were phenotyped at early stages of the interaction: the induction of broomrape seed germination by sunflower root exudates, the attachment to the host root and the development of tubercles in rhizotron assays. We showed that wild Helianthus accessions are an important source of resistance to the most virulent broomrape races, affecting various stages of the interaction: the inability to induce broomrape seed germination, the development of incompatible attachments or necrotic tubercles, and the arrest of emerged structure growth. Cytological studies of incompatible attachments showed that several cellular mechanisms were shared among resistant Helianthus species.

Iridoid Glycosides Isolated from Bellardia trixago Identified as Inhibitors of Orobanche cumana Radicle Growth

Orobanche cumana is an obligate holoparasitic plant with noxious effects in sunflower crops. Bellardia trixago is a facultative hemiparasitic plant that infects ruderal plants without noxious significance in agriculture and is known to produce a wide spectrum of bioactive metabolites. The objective of this study was to evaluate the allelopathic effects of B. trixago on the growth of O. cumana seedlings. Three different extracts using solvents of increasing polarity (n-hexane, dichloromethane and ethyl acetate) were prepared from the flowers, aerial green organs and roots of two populations, a white-flowered and a yellow-flowered population of B. trixago, both collected in southern Spain. Each extract was studied using allelopathic screenings on O. cumana which resulted in the identification of allelopathic activity of the ethyl acetate extracts against Orobanche radicles. Five iridoid glycosides were isolated together with benzoic acid from the ethyl acetate extract of aerial green organs by bio-guided purification. These compounds were identified as bartsioside, melampyroside, mussaenoside, gardoside methyl ester and aucubin. Among them, melampyroside was found to be the most abundant constituent in the extract (44.3% w/w), as well as the most phytotoxic iridoid on O. cumana radicle, showing a 72.6% inhibition of radicle growth. This activity of melampyroside was significantly high when compared with the inhibitory activity of benzoic acid (25.9%), a phenolic acid with known allelopathic activity against weeds. The ecotoxicological profile of melampyroside was evaluated using organisms representing different trophic levels of the aquatic and terrestrial ecosystems, namely producers (green freshwater algae Raphidocelis subcapitata and macrophyte Lepidium sativum), consumers (water flea Daphnia magna and nematode Caenorhabditis elegans) and decomposers (bacterium Aliivibrio fischeri). The ecotoxicity of melampyroside differed significantly depending on the test organism showing the highest toxicity to daphnia, nematodes and bacteria, and a lower toxicity to algae and macrophytes. The findings of the present study may provide useful information for the generation of green alternatives to synthetic herbicides for the control of O. cumana.

Specialized Metabolites from the Allelopathic Plant Retama raetam as Potential Biopesticides

To cope with the rising food demand, modern agriculture practices are based on the indiscriminate use of agrochemicals. Although this strategy leads to a temporary solution, it also severely damages the environment, representing a risk to human health. A sustainable alternative to agrochemicals is the use of plant metabolites and plant-based pesticides, known to have minimal environmental impact compared to synthetic pesticides. Retama raetam is a shrub growing in Algeria’s desert areas, where it is commonly used in traditional medicine because of its antiseptic and antipyretic properties. Furthermore, its allelopathic features can be exploited to effectively control phytopathogens in the agricultural field. In this study, six compounds belonging to isoflavones and flavones subgroups have been isolated from the R. raetam dichloromethane extract and identified using spectroscopic and optical methods as alpinumisoflavone, hydroxyalpinumisoflavone, laburnetin, licoflavone C, retamasin B, and ephedroidin. Their antifungal activity was evaluated against the fungal phytopathogen Stemphylium vesicarium using a growth inhibition bioassay on PDA plates. Interestingly, the flavonoid laburnetin, the most active metabolite, displayed an inhibitory activity comparable to that exerted by the synthetic fungicide pentachloronitrobenzene, in a ten-fold lower concentration. The allelopathic activity of R. raetam metabolites against parasitic weeds was also investigated using two independent parasitic weed bioassays to discover potential activities on either suicidal stimulation or radicle growth inhibition of broomrapes. In this latter bioassay, ephedroidin strongly inhibited the growth of Orobanche cumana radicles and, therefore, can be proposed as a natural herbicide.

Involvement of α-galactosidase OmAGAL2 in planteose hydrolysis during seed germination of Orobanche minor

Root parasitic weeds of the Orobanchaceae, such as witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.), cause serious losses in agriculture worldwide, and efforts have been made to control these parasitic weeds. Understanding the characteristic physiological processes in the life cycle of root parasitic weeds is particularly important to identify specific targets for growth modulators. In our previous study, planteose metabolism was revealed to be activated soon after the perception of strigolactones in germinating seeds of O. minor. Nojirimycin inhibited planteose metabolism and impeded seed germination of O. minor, indicating a possible target for root parasitic weed control. In the present study, we investigated the distribution of planteose in dry seeds of O. minor by matrix-assisted laser desorption/ionization-mass spectrometry imaging. Planteose was detected in tissues surrounding-but not within-the embryo, supporting its suggested role as a storage carbohydrate. Biochemical assays and molecular characterization of an α-galactosidase family member, OmAGAL2, indicated that the enzyme is involved in planteose hydrolysis in the apoplast around the embryo after the perception of strigolactones, to provide the embryo with essential hexoses for germination. These results indicate that OmAGAL2 is a potential molecular target for root parasitic weed control.

Fungal Naphthalenones; Promising Metabolites for Drug Discovery: Structures, Biosynthesis, Sources, and Pharmacological Potential

Fungi are well-known for their abundant supply of metabolites with unrivaled structure and promising bioactivities. Naphthalenones are among these fungal metabolites, that are biosynthesized through the 1,8-dihydroxy-naphthalene polyketide pathway. They revealed a wide spectrum of bioactivities, including phytotoxic, neuro-protective, cytotoxic, antiviral, nematocidal, antimycobacterial, antimalarial, antimicrobial, and anti-inflammatory. The current review emphasizes the reported naphthalenone derivatives produced by various fungal species, including their sources, structures, biosynthesis, and bioactivities in the period from 1972 to 2021. Overall, more than 167 references with 159 metabolites are listed.

A novel strigolactone receptor antagonist provides insights into the structural inhibition, conditioning, and germination of the crop parasite Striga

Crop parasites of the Striga genera are a major biological deterrent to food security in Africa and are one of the largest obstacles to poverty alleviation on the continent. Striga seeds germinate by sensing small-molecule hormones, strigolactones (SLs), that emanate from host roots. Although SL receptors (Striga hermonthica HYPOSENSITIVE TO LIGHT [ShHTL]) have been identified, discerning their function has been difficult because these parasites cannot be easily grown under laboratory conditions. Moreover, many Striga species are obligate outcrossers that are not transformable, hence not amenable to genetic analysis. By combining phenotypic screening with ShHTL structural information and hybrid drug discovery methods, we discovered a potent SL perception inhibitor for Striga, dormirazine (DOZ). Structural analysis of this piperazine-based antagonist reveals a novel binding mechanism, distinct from that of known SLs, blocking access of the hormone to its receptor. Furthermore, DOZ reduces the flexibility of protein–protein interaction domains important for receptor signaling to downstream partners. In planta, we show, via temporal additions of DOZ, that SL receptors are required at a specific time during seed conditioning. This conditioning is essential to prime seed germination at the right time; thus, this SL-sensitive stage appears to be critical for adequate receptor signaling. Aside from uncovering a function for ShHTL during seed conditioning, these results suggest that future Ag-Biotech Solutions to Striga infestations will need to carefully time the application of antagonists to exploit receptor availability and outcompete natural SLs, critical elements for successful parasitic plant invasions.

Genetic and physiological characterization of sunflower resistance provided by the wild-derived OrDeb2 gene against highly virulent races of Orobanche cumana Wallr

Or_Deb2 confers post-attachment resistance to Orobanche cumana and is located in a 1.38 Mbp genomic interval containing a cluster of receptor-like kinase and receptor-like protein genes with nine high-confidence candidates. Sunflower broomrape is a holoparasitic angiosperm that parasitizes on sunflower roots, severely constraining crop yield. Breeding for resistance is the most effective method of control. Or_Deb2 is a dominant resistance gene introgressed into cultivated sunflower from a wild-related species that confers resistance to highly virulent broomrape races. The objectives of this study were as follows: (i) locate Or_Deb2 into the sunflower genome and determine putative candidate genes and (ii) characterize its underlying resistance mechanism. A segregating population from a cross between the sunflower resistant line DEB2, carrying Or_Deb2, and a susceptible line was phenotyped for broomrape resistance in four experiments, including different environments and two broomrape races (F_GV and G_TK). This population was also densely genotyped with microsatellite and SNP markers, which allowed locating Or_Deb2 within a 0.9 cM interval in the upper half of Chromosome 4. This interval corresponded to a 1.38 Mbp genomic region of the sunflower reference genome that contained a cluster of genes encoding LRR (leucine-rich repeat) receptor-like proteins lacking a cytoplasmic kinase domain and receptor-like kinases with one or two kinase domains and lacking an extracellular LRR region, which were valuable candidates for Or_Deb2. Rhizotron and histological studies showed that Or_Deb2 determines a post-attachment resistance response that blocks O. cumana development mainly at the cortex before the establishment of host-parasite vascular connections. This study will contribute to understand the interaction between crops and parasitic weeds, to establish durable breeding strategies based on genetic resistance and provide useful tools for marker-assisted selection and Or_Deb2 map-based cloning.

Phytotoxic compounds from endophytic fungi

Weeds represent one of the most challenging biotic factors for the agricultural sector, responsible for causing significant losses in important agricultural crops. Traditional herbicides have managed to keep weeds at bay, but overuse has resulted in negative environmental and toxicological impacts, including the increase of herbicide-resistant species. Within this context, the use of biologically derived (bio-)herbicides represents a promising solution because they are able to provide the desired phytotoxic effects while causing less toxic environmental damage. In recent years, bioactive secondary metabolites, in particular those bio-synthesized by endophytic fungi, have been shown to be promising sources of novel compounds that can be exploited in agriculture, including their use in weed control. Endophytic fungi have the ability to produce volatile and nonvolatile compounds with broad phytotoxic activity. In addition, as a result of the beneficial relationships they establish with their host plants, they are part of the colonization mechanism and can provide protection for their hosts. As such, endophytic fungi can be exploited as bioherbicides and as research tools. In this review, we cover 100 nonvolatile secondary metabolites with phytotoxic activity and more than 20 volatile organic compounds in a mixture, produced by 28 isolates of endophytic fungi from 21 host plant families, collected in 8 countries. This information can form the basis for the application of endophytic fungal compounds in weed control. KEY POINTS: • Endophytic fungi produce a wide variety of secondary metabolites with unique and complex structures. • Fungal endophytes produce volatile and nonvolatile compounds with promising phytotoxic activity. • Endophytic fungi are a promising source of useful bioherbicides.

Imazapic Herbigation for Egyptian Broomrape (Phelipanche aegyptiaca) Control in Processing Tomatoes—Laboratory and Greenhouse Studies

Parasitic plants belonging to the Orobanchaceae family include species that cause heavy damage to crops in Mediterranean climate regions. Phelipanche aegyptiaca is the most common of the Orobanchaceae species in Israel inflicting heavy damage to a wide range of broadleaf crops, including processing tomatoes. P. aegyptiaca is extremely difficult to control due to its minute and vast number of seeds and its underground association with host plant roots. The highly efficient attachment of the parasite haustoria into the host phloem and xylem enables the diversion of water, assimilates and minerals from the host into the parasite. Drip irrigation is the most common method of irrigation in processing tomatoes in Israel, but the delivery of herbicides via drip irrigation systems (herbigation) has not been thoroughly studied. The aim of these studies was to test, under laboratory and greenhouse conditions, the factors involved in the behavior of soil-herbigated imazapic, and the consequential influence of imazapic on P. aegyptiaca and tomato plants. Dose-response Petri dish studies showed that imazapic does not impede P. aegyptiaca seed germination and non-attached seedlings, even at the high rate of 5000 ppb. Imazapic applied to tomato roots inoculated with P. aegyptiaca seeds in a PE bag system revealed that the parasite is killed only after its attachment to the tomato roots, at concentrations as low as 2.5 ppb. Imazapic sorption curves and calculated Kd and Koc values indicated that the herbicide Kd is similar in all soils excluding a two-fold higher coefficient in the Gadash farm soil, while the Koc was similar in all soils except the Eden farm soil, in which it was more than twofold lower. In greenhouse studies, control of P. aegyptiaca was achieved at >2.5 ppb imazapic, but adequate control requires repeated applications due to the 7-day half-life (t_1/2) of the herbicide in the soil. Tracking of imazapic in soil and tomato roots revealed that the herbicide accumulates in the tomato host plant roots, but its movement to newly formed roots is limited. The data obtained in the laboratory and greenhouse studies provide invaluable knowledge for devising field imazapic application strategies via drip irrigation systems for efficient and selective broomrape control.

Germination stimulatory activity of bacterial butenolide hormones from Streptomyces albus J1074 on seeds of the root parasitic weed Orobanche minor

Damage caused by Orobanchaceae root parasitic weeds is a substantial agricultural problem for global food security. Many studies have been conducted to establish practical methods of control, but efforts are still required for successful management. Seed germination of root parasitic weeds requires host-derived germination stimulants including strigolactones (SLs). Studies on SLs have revealed that a butenolide ring is the essential moiety for SL activity as a germination stimulant. Interestingly, recent studies have revealed that butenolide hormones regulate the biosynthesis of secondary metabolites and mediate communication in actinomycete bacteria. Because of the structural similarity between SLs and the bacterial butenolides, we evaluated the germination stimulatory activity of butenolides isolated from Streptomyces albus J1074 on root parasitic weeds. These butenolides were found to specifically induce seed germination of Orobanche minor. Our findings contribute to understanding the molecular mechanisms of germination stimulant perception and to the development of a method for their biological control.

Effects of Benzoquinones on Radicles of Orobanche and Phelipanche Species

The holoparasitic broomrape weeds (Orobanche and Phelipanche species) cause severe yield losses throughout North Africa, the Middle East, and Southern and Eastern Europe. These parasitic weeds form an haustorium at the tip of their radicles to infect the crop upon detection of the host-derived haustorium-inducing factors. Until now, the haustorial induction in the broomrapes remains less studied than in other parasitic plant species. Known haustorium-inducing factors active in hemiparasites, such as Striga and Triphysaria species, were reported to be inefficient for the induction of haustoria in broomrape radicles. In this work, the haustorium-inducing activity of p-benzoquinone and 2,6-dimethoxy-p-benzoquinone (BQ and DMBQ) on radicles of three different broomrapes, namely Orobanche cumana, Orobanche minor and Phelipanche ramosa, is reported. Additional allelopathic effects of benzoquinones on radicle growth and radicle necrosis were studied. The results of this work suggest that benzoquinones play a role in the induction of haustorium in broomrapes. Although dependent on the broomrape species assayed and the concentration of quinones used in the test, the activity of BQ appeared to be stronger than that of DMBQ. The redox property represented by p-benzoquinone, which operates in several physiological processes of plants, insects and animals, is invoked to explain this different activity. This work confirms the usefulness of benzoquinones as haustorium-inducing factors for holoparasitic plant research. The findings of this work could facilitate future studies in the infection process, such as host-plant recognition and haustorial formation.

Search for Resistant Genotypes to Cuscuta campestris Infection in Two Legume Species, Vicia sativa and Vicia ervilia

The dodders (Cuscuta spp.) are parasitic plants that feed on the stems of their host plants. Cuscuta campestris is one of the most damaging parasitic plants for the worldwide agricultural production of broad-leaved crops. Its control is limited or non-existent, therefore resistance breeding is the best alternative both economically and environmentally. Common vetch (Vicia sativa) and bitter vetch (Vicia ervilia) are highly susceptible to C. campestris, but no resistant genotypes have been identified. Thus, the aim of this study was to identify in V. sativa and V.ervilia germplasm collections genotypes resistant to C. campestris infection for use in combating this parasitic plant. Three greenhouse screening were conducted to: (1) identify resistant responses in a collection of 154 accessions of bitter vetch and a collection of 135 accessions of common vetch genotypes against infection of C. campestris; (2) confirm the resistant response identified in common vetch accessions; and (3) characterize the effect of C. campestris infection on biomass of V. sativa resistant and susceptible accessions. Most common vetch and bitter vetch genotypes tested were susceptible to C. campestris. However, the V. sativa genotype Vs.1 exhibited high resistance. The resistant phenotype was characterized by a delay in the development of C. campestris posthaustorial growth and a darkening resembling a hypersensitive-like response at the penetration site. The resistant mechanism was effective in limiting the growth of C. campestris as the ratio of parasite/host shoot dry biomass was more significantly reduced than the rest of the accessions. To the best or our knowledge, this is the first identification of Cuscuta resistance in V. sativa genotypes.

Allelopathic Effect of Quercetin, a Flavonoid from Fagopyrum esculentum Roots in the Radicle Growth of Phelipanche ramosa: Quercetin Natural and Semisynthetic Analogues Were Used for a Structure-Activity Relationship Investigation

Allelopathic potential of buckwheat roots on the radicle growth of the broomrape weed species Orobanche cumana and Phelipanche ramosa was studied. Buckwheat root exudates induced a significant growth inhibition in P. ramosa radicles but radicles of O. cumana were not affected. Among the metabolites present in the root organic extract we identified the flavonol quercetin and the stilbene p-coumaric acid methyl ester with only quercetin showing inhibitory effect on P. ramosa. The activity of quercetin was compared with other two similar flavanoids, the flavone apigenin and the dihydroflavanol 3-O-acetylpadmatin extracted respectively from Lavandula stoechas and Dittrichia viscosa plants. In this comparative assay only 3-O-acetylpadmatin besides quercetin, showed inhibition activity of radicle growth while apigenin was inactive. These results indicated that the presence of two ortho-free hydroxy groups of C ring, like catechol, could be an important feature to impart activity while the carbon skeleton of B ring and substituents of both A and B rings are not essential. Besides reduction of radicle growth, haustorium induction was observed at the tip of P. ramosa radicles treated with quercetin which swelled and a layer of papillae was formed. Activity of quercetin on haustorium induction in P. ramosa was assayed in comparison with the known haustorium-inducing factor 2,6-dimethoxy-p-benzoquinone (DMBQ) and a three partial methyl ether derivatives semisynthetized from quercetin. Results indicated that P. ramosa haustorium was induced by DMBQ at concentrations of 1–0.5 mM and quercetin and its derivatives at concentration range 0.1–0.05 mM.

Resistance against Orobanche crenata in Bitter Vetch (Vicia ervilia) Germplasm Based on Reduced Induction of Orobanche Germination

Bitter vetch (Vicia ervilia (L.) Willd.) is a legume well adapted to cultivation in marginal areas, being an important source of protein for animal feed in low input cropping systems. Surprisingly, it is an underutilized crop as it could be a good alternative to increase the sustainability of extensive rainfed cropping systems. In Mediterranean rainfed cropping systems, the productivity of bitter vetch is severely reduced by the parasitic weed species Orobanche crenata (Forsk). To date, few resistant bitter vetch genotypes have been identified. O. crenata infection process initiates with the recognition of germination factors exuded by roots of susceptible hosts. In this work, the interaction of a collection of bitter vetch accessions and O. crenata has been analyzed in order to discover accessions with low germination induction activity. Through a combination of field and rhizotron experiments, two bitter vetch accessions were selected showing low germination-induction activity, which resulted in less infection. In addition, in vitro germination assays revealed that the low germination activity was due to low exudation of germination factors and not due to the exudation of germination inhibitors. The selected low germination-inducers genotypes could be the basis for a new breeding program generating locally adapted alternatives with resistance to O. crenata.

Cytinus under the Microscope: Disclosing the Secrets of a Parasitic Plant

Well over 1% of all flowering plants are parasites, obtaining all or part of the nutrients they need from other plants. Among this extremely heterogeneous assemblage, the Cytinaceae form a small group of holoparasites, with Cytinus as the main representative genus. Despite the small number of known species and the fact that it doesn't attack crops or plants of economic importance, Cytinus is paradigmatic among parasitic plants. Recent research has indeed disclosed many aspects of host-parasite interactions and reproductive biology, the latter displaying a vast array of adaptive traits to lure a range of animal pollinators. Furthermore, analysis of biological activities of extracts of the most common species of Cytinus has provided evidence that this plant could be a valuable source of compounds with high potential in key applicative areas, namely food production (nutraceuticals) and the development of antimicrobial therapeutics. This article offers a complete overview of our current knowledge of Cytinus.

Identification of Vicia ervilia Germplasm Resistant to Orobanche crenata

Bitter vetch (Vicia ervilia L.) is an ancient grain legume used as animal feed in the Mediterranean basin. This legume has a large economical potential because of its high yield under low inputs and good protein content, as well as resistance to cold and drought. Nevertheless, its growth and production area are affected in the presence of the broomrape weed species Orobanche crenata. Due to the small bitter vetch size, infection by as few as two or three O. crenata per vetch plant can be devastating. There are no efficient methods of selectively controlling O. crenata in this crop, for which reason the development of varieties resistant and tolerant to O. crenata infection is needed. Phytogenetic resources are valuable reserves for species survival. They represent important genetic variability and allow the possibility of finding characters of interest, such as new resistance sources. A large-scale field screening of a collection of 102 bitter vetch accessions indicated that most bitter vetch accessions were susceptible but allowed us to select 16 accessions with low levels of O. crenata infection. Next, we used a combination of field and rhizotron experiments to investigate the resistant response of selected bitter vetch genotypes in detail by studying the performance and resistance mechanisms. These experiments led to the identification of three different mechanisms that block O. crenata parasitism. A pre-attachment mechanism of low induction of O.crenata germination was identified in two bitter vetch accession Ve.055 and Ve.155. In addition, a post-attachment mechanism of resistance to O. crenata penetration was identified inthe accession Ve.125. In addition, the field-resistant accession Ve.123 showed susceptible response in rhizotron, indicating that a late mechanism acting after vascular connection, most probably related with bitter vetch of escape due to fructification precocity was acting against O. crenata development.

Infestation of Field Dodder (Cuscuta campestris Yunck.) Promotes Changes in Host Dry Weight and Essential Oil Production in Two Aromatic Plants, Peppermint and Chamomile

Peppermint (Mentha piperita L.) and chamomile (Chamomilla recutita (L.) Rausch.) are aromatic plants with considerable economic value. These plants and their essential oils are used in medicine, cosmetics, and the food industry. One of the main limiting factors in peppermint and chamomile commercial cultivation is weed competition since weeds are able to decrease both oil amount and biomass yield. The purpose of the present study was to determine the effect of parasitism by field dodder (Cuscuta campestris Yunck.) on peppermint and chamomile dry weight and their essential oil yield and composition. Essential oils from both noninfested and infested peppermint and chamomile plants were obtained by hydrodistillation and characterized chemically by gas chromatography (GC) coupled with mass spectrometry (MS). The amount of dry matter accumulated by peppermint and chamomile plants infested by field dodder was lower (25% and 63%, respectively) compared to noninfested plants. Essential oil yield increased for peppermint (3.87% (v/w) and 3.63% (v/w)), but decreased for chamomile (0.2% (v/w) and 0.5% (v/w)) both from infested and noninfested plants, respectively. The oil composition profile significantly differed in terms of content. In peppermint plants, field dodder infestation increased menthone content by 23%, and decreased the content of both menthol by 11% and pulegone by 67%. Furthermore, δ-cadinene was detected only in oil extracted from infested peppermint plants. Compared to peppermint, chamomile plants were significantly more affected by field dodder in terms of essential oil yield, as well as oil composition and plant dry weight. In chamomile plants, (E)-dendrolasin was detected in the oil of noninfested plants, and 1,4-dimethyl-7-(1-methylethyl)-azulen-2-ol was detected only in the oil of infested plants.