Cumana Seeds Research Articles

First Report of Karelinia caspia (Pall.) Less as a new host of Orobanche cumana Wallr. in Xinjiang, China.

Sunflower broomrape (Orobanche cumana Wallr.) is a holoparasitic plant species which mainly parasitizes a few species of the Asteraceae in the wild and is exclusively found growing on sunflower in agricultural fields (Fernández-Martínez et al. 2015). O. cumana is a serious threat to sunflower production in Xinjiang and Inner Mongolia (Shi et al. 2015). Karelinia caspia (Pall.) Less. (Asteraceae) is an ecologically important plant species occurring across the desert ecosystems of Russia, Central Asia, and northwest China. It plays an important role in reducing wind erosion and desertification (Xu et al. 2018). During the 2018 and 2019 growing seasons, sunflower broomrape was observed parasitizing K. caspia in non-cultivated areas adjacent to sunflower fields near Beitun city (87°51'E, 47°15'N) in Xinjiang, China. Sunflower broomrape plants were identified morphologically as O. cumana according to Pujadas-Salvà and Velasco (2000). The host plants were identified morphologically as K. caspia according to Lin et al (1979). The ribosomal DNA internal transcribed spacer (ITS) and the trnL-F region of the parasite were amplified by PCR using primer pairs ITS1/ITS4 and trnL-FF/trnL-FR, respectively (Taberlet et al. 1991; Anderson et al. 2004). The ITS sequence of the parasite (Accession No. MT795725.1) showed 100% identity (675bp out of 689bp) to that of O. cernua var. cumana (KC811228.1). The trnl-F sequence of the parasite (Accession No. ON843707) showed 98% identity (675 of 689 bp) to O.cernua var. cumana (KT387722.1). Multi-locus phylogenetic analysis of the two sequences showed clustering with sunflower broomrape. The ITS region of the parasite and host was were amplified by PCR using the primer pair ITS1F/ITS4R (Taberlet et al.1991), and the ITS sequences of the host (Accession No. MT791995.1) showed 99.86% identity (728bp of 802bp) to that of K. caspia (LN607483.1). Rhizotron and pot experiments were carried out to assess the parasitic relationship between O. cumana and K. caspia. In the rhizotron experiment, 2-week-old seedlings of K. caspia were inoculated with sterilized 400 O. cumana seeds in a 15-cm petri dish filled with a sponge overlaid with glass fiber filter paper. The parasitic state of O. cumana was observed 9 days after inoculation. In another trial, seeds of K. caspia were sowed in 2-L and 4-L pots containing sand-vermiculite-compost (1:1:1 v:v:v). These pots were artificially inoculated with 50 mg of O. cumana seeds per 1 kg of substrate. After 20 and 70 days, corresponding to the early parasitic and flowering stages, respectively, of O. cumana, K. caspia plants were uprooted from the media and washed carefully. The parasitic relationship was confirmed by the attachment position of the broomrape to the K. caspia root. To our knowledge, this is the first report of O. cumana parasitizing K. caspia in Xinjiang, China. This phenomenon means that sunflower broomrape can raise up seed on a newly recognized host. Weed eradication in and near sunflower fields is a key measure to control sunflower broomrape.

Maize Rotation Combined with Streptomyces rochei D74 to Eliminate Orobanche cumana Seed Bank in the Farmland

Orobanche cumama wallr. is the sunflower root parasitic weed with special life stage in which seed germination and parasitism take place in the soil. In practice, applying microbial agents and trapping crop rotation are utilized separately, or just one of them is selected to control O. cumana. The development of the sunflower industry is severely constrained on the farmland, where there is high density of O. cumana’s seed banks. In this study, two biological control methods were combined to solve the problem of O. cumana parasitism. The bioassay experiment showed that the high concentration fermentation filtrates of Streptomyces rochei D74 could effectively inhibit the germination and growth of the germ tube of O. cumana seeds. As the concentration was increased to 3.1 mg/mL, O. cumana was almost unable to sprout. A two-year pot experiment revealed that the use of D74 agents and sunflower–maize–sunflower rotation together promoted sunflower growth, as shown by the biomass accumulation, plant height, and denser root systems. The combined method resulted in a significant decrease in the number of O. cumana parasitism, compared to one method alone. Additionally, it affected the bacterial community composition of sunflower rhizosphere, mostly leading to an increase in Streptomyces and Brevibacterium and a decrease in Arthrobacter. This experiment, combined with multiple biological control, means significantly reducing the parasitism of O. cumana, which provides an effective foundation for practical application.

First Report of Cirsium arvense (Canada Thistle) as a New Host of Orobanche cumana in Xinjiang, China.

Cirsium arvense (Canada thistle) is a perennial herb native to Eurasia that has been introduced to temperate regions of the world where it is considered one of the serious weeds for arable and pastoral agriculture (Schröder et al. 1993). C. arvense reproduces both clonally and sexually. The weed is highly competitive, causes yield reductions in crops such as wheat, alfalfa, sugarbeet, and can reduce forage availability and production (Wilson 1981). Canada thistle is also a harbour for plant pathogens such as plant-parasitic nematodes (Tenuta et al. 2014). Sunflower broomrape (Orobanche cumana Wallr.) is a holoparasitic plant species with a restricted range of hosts both in the wild, where it mainly parasitizes a few species of the Asteraceae, and in agricultural fields, where it is exclusively found growing on sunflower (Fernández-Martínez et al. 2015). O. cumana infection can cause up to 80% of the yield loss in sunflower, which is a serious threat for sunflower production in Xinjiang and Inner Mongolia, China (Parker 2009). In July 2019, broomrape was observed parasitizing C. arvense in the greenhouse used for sunflower resistance identification (Shihezi, 86° 3' 36" E, 44° 18' 36" N, 500 m elevation) in Xinjiang, China. Fifty percent of the plants were parasitized by broomrape in the greenhouse and the host had an average of 1-2 broomrape shoots per plant. For molecular analysis, total genomic DNA was extracted from the flowers of broomrape and the rps2, rbcL, trnL-F genes, and ribosomal DNA internal transcribed spacer (ITS) region were amplified by PCR using the primer pairs rps2F/rps2R, rbcLF/rbcLR, C/F, ITS1/ITS4, respectively (Park et al. 2007; Manen et al. 2004; Taberlet et al. 1991; Anderson et al. 2004). The ITS (659bp), rps2 (451 bp), trnL-F (914 bp), and rbcL (961 bp) gene sequences of the broomrape were deposited in GenBank, the accession numbers are MT856745, MW809407, MW809408, and MW809409. The results of BLAST analysis showed that ITS sequence shared 100% similarity with O. cumana (659/659 nucleotide identity, MK567978), the rps2 sequence shared 99% similarity with O. cumana (449/451 nucleotide identity, KT387722), trnL-F sequence shared 99% similarity with O. cumana (907/911 nucleotide identity, MT027325), rbcL sequence shared 99% similarity with O. cumana (956/964 nucleotide identity, MK577840). The morphological characteristics such as stem, inflorescence, corolla, bracts, calyx, stamens, gynoecium are consistent with O. cumana described by Pujadas-Salvá and Velasco (2000). Morphological and molecular identification strongly support that the broomrape parasitic on C. arvense belonged to the O. cumana. Greenhouse pot experiments were carried out to assess the parasitic relationship between sunflower broomrape and C. arvense (Fernández-Martínez et al. 2000). In January 2020, C. arvense roots were harvested from an extant field of C. arvense in the greenhouse at Shihezi University (Supplementary Figure S1A). The soil was dug to 30-40 cm depth and C. arvense roots were removed and carefully washed in water. The healthy and living C. arvense roots were selected and cut into 10-11 cm pieces. Four C. arvense root pieces were grown (buried at a depth of 10-12 cm) in 8-L pots containing a mixture of sand-vermiculite-compost (1:1:1 v:v:v) and O. cumana seeds (50 mg of O. cumana seeds per 1 kg of the substrate) with 5 replicates. Three non-infected plants were grown and evaluated in parallel. Approximately 80 days after planting, at the flowering stage of the O. cumana, C. arvense plants were uprooted from the soil. Compared to non-infected plants, the hosts' symptoms were slow growth, leaf wilting, and chlorosis, and similiar to the broomrape-infected C. arvense plants observed in the greenhouse field. The roots of C. arvense and broomrape were carefully washed in water and observed the parasitism of O. cumana. The infection was confirmed by observation of the attachment of the O. cumana to the C. arvense roots (Supplementary Figure S1D). To the best of our knowledge, this is the first report of O. cumana parasitizing C. arvense in Xinjiang, China. C. arvense as a new host of O. cumana indicates that sunflower broomrape can also propagate and survive in a host such as Canada thistle grown in sunflower fields. This finding suggests that it may be more difficult to control sunflower broomrape by rotation. In the next study, the contaminated area and the degree of parasitism of broomrape on C. arvense in the field will be investigated, and better-integrated control methods for controlling O. cumana will be designed.

Assessment of weed root extracts for allelopathic activity against Orobanche and Phelipanche species

Broomrapes (Orobanche and Phelipanche species) are holoparasitic weeds that infect roots of crop hosts from Asteraceae, Brassicaceae, Apiaceae, Fabaceae, and Solanaceae. The parasitic weeds are difficult to control selectively without crop damage once attached to their roots. Identification of natural compounds with herbicidal activity against pre-attached broomrape stages can provide control alternatives. With the aim to identify plant species with efficacy for broomrape control, organic and residual aqueous phase extracts from roots of seventeen weed species common in south Spanish broomrape-infested fields were assessed as potential inducers of suicidal broomrape germination and as inhibitors of broomrape radicle growth. Assessments were carried out in vitro using seeds and seedlings of four noxious broomrape species, Orobanche crenata Forsk., Orobanche cumana Wallr., Orobanche minor Sm. and Phelipanche ramosa (L.) Pomel. While root extracts from all the weed species did not induce suicidal germination on O. crenata seeds, most of the extracts induced germination of P. ramosa except for those obtained from Amaranthus albus L., Amaranthus retroflexus L. and Convolvulus arvensis L. Moderate levels of germination activity were induced in O. cumana and O. minor seeds by some of the root extracts tested, with strongest induction obtained from Heliotropium europaeum L. on O. cumana seeds, and from Silybum marianum (L.) Gaertn. on O. minor seeds. For root extract inhibition of broomrape radicles, the extract from C. arvensis roots strongly inhibited radicles of all the broomrape species. While extracts from the other weed species induced low or negligible inhibition of O. cumana and O. crenata radicle growth, many inhibited P. ramosa and O. minor radicles. Exceptions were root extracts from Datura stramonium L., Heliotropium europaeum L., Malva sylvestris L., Solanum nigrum L. and Urtica dioica L., which did not inhibit P. ramosa radicles, and those from A. retroflexus, Datura stramonium L., Malva sylvestris L., Portulaca oleracea L. and S. nigrum, which did not inhibit O. minor radicles. Among the active organic extracts assessed, those showing promising chemical profiles were selected for future studies to characterize natural compounds with potential herbicidal activity on early stages of broomrape growth.

Costunolide Influences Germ Tube Orientation in Sunflower Broomrape - A First Step Toward Understanding Chemotropism.

Orobanche cumana WALLR. is a host-specific root parasite of cultivated sunflowers with increasing economic importance in Europe, North Africa, and parts of Asia. While sesquiterpene lactones (STLs) released from sunflower roots were identified as natural germination stimulants of O. cumana seeds in the soil, the chemical nature of the signals guiding the emerging germ tube toward the host root has remained unknown hitherto. Thus, we designed a bioassay that allowed the observation of broomrape germination and subsequent germ tube development in the presence of substances with putative chemotropic activity. Root exudates and sunflower oil extracts, both containing STLs in micromolar concentrations, caused the positive chemotropic orientation of germ tubes. A similar positive chemotropic effect was achieved with costunolide, one of the four STLs of sunflower present in the exudate and oil extracts. In contrast, GR24, a synthetic strigolactone (SL) with germination-inducing activity on O. cumana seeds, showed no effect on the germ tube orientation. The effect of costunolide was concentration-dependent and within the range of its natural micromolar occurrence in roots. We assume that an STL gradient is responsible for the stronger inhibition of elongation growth on the host-facing flank of the germ tube compared with the far side flank. This would confer a double role of STLs from sunflower root exudates in the sunflower–broomrape interaction, namely, as germination stimulants and as chemotropic signals.

Prohexadione calcium is herbicidal to the sunflower root parasite Orobanche cumana.

The obligatory sunflower root parasite Orobanche cumana Wallr. deprives its host of essential nutrients, resulting in a dramatic reduction in yield and biomass. A post-emergence application with an imidazolinone herbicide on an imidazolinone-tolerant sunflower is highly effective against O. cumana. The herbicide inhibits the enzyme acetohydroxy acid synthase and consequently, growth of the parasite is inhibited, although the sunflower survives the treatment through mutations in the target enzyme. Interestingly, field studies have shown that a combined application of an imidazolinone herbicide with prohexadione resulted in reduced emergence of O. cumana compared with the sole application of the herbicide. The aim of this study was to investigate whether prohexadione is herbicidal to O. cumana. Prohexadione was rapidly distributed within the sunflower, reaching the roots, the site of O. cumana attack, as early as 6 h after application (HAA) on sunflower leaves. A direct impact of prohexadione on O. cumana germination was investigated and a half-maximal inhibitory concentration (IC50 ) of 84 μm prohexadione was found. In addition, the inhibition of germination by prohexadione was terminal, meaning that O. cumana seeds died after prohexadione contact as soon as they were primed for germination. Additionally, excretion studies showed that a small proportion of the applied prohexadione was excreted by sunflower roots. We show that prohexadione is an inhibitor of O. cumana germination and that the growth regulator is found in sunflower roots shortly after application. We hypothesize that prohexadione is excreted in sufficient amounts from the sunflower roots, therefore having a direct impact on O. cumana germination. © 2020 Society of Chemical Industry.

Phytochemical Study of Safflower Roots (Carthamus tinctorius) on the Induction of Parasitic Plant Germination and Weed Control.

Weeds have been a major threat in agriculture for several generations as they lead to decreases in productivity and cause significant economic losses. Parasitic plants are a specific type of weed causing losses in crops of great relevance. A new strategy has emerged in the fight against parasitic plants, which is called 'suicidal germination' or the 'honey-pot strategy'. Regarding the problem of weed control from an ecological point of view, it is interesting to investigate new natural compounds with allelopathic activity with the aim of developing new natural herbicides that can inhibit the growth of weeds without damaging the environment. Safflower crops have been affected by parasitic plants and weeds and, as a consequence, the secondary metabolites exuded by safflower roots have been studied. The sesquiterpene lactone dehydrocostuslactone was isolated and characterised, and the structurally related costunolide was identified by UHPLC-MS/MS in safflower root exudates. These sesquiterpene lactones have been shown to stimulate germination of Phelipanche ramosa and Orobanche cumana seeds. In addition, these compounds were phytotoxic on three important weeds in agriculture, namely Lolium perenne, Lolium rigidum and Echinochloa crus-galli. The exudation of the strigolactones solanacol and fabacyl acetate have also been confirmed by UHPLC-MS/MS. The study reported here contributes to our knowledge of the ecological role played by some secondary metabolites. Moreover, this knowledge could help identify new models for the development of future agrochemicals based on natural products.

A new race of sunflower broomrape (Orobanche cumana) with a wider host range due to changes in seed response to strigolactones

AbstractBroomrapes (Orobanche and Phelipanche species, Orobanchaceae) are obligate root parasites of dicotyledonous plants. This taxonomic group includes seven weedy parasites of agricultural crops that damage vegetables, sunflower (Helianthus annuus L.), and legumes. Processing-tomato (Solanum lycopersicum L.) fields in Israel have been recently found infested with a new broomrape, first identified as nodding broomrape (Orobanche cernua Loefl.) based on its host. However, its morphology resembled the closely related sunflower broomrape (Orobanche cumana Wallr.), an obligate parasite of sunflower. The new race (CUCE) parasitized sunflower, tomato, and tobacco (Nicotiana tabacum L.) in vitro, in a polyethylene bag system and in pots. Its seeds germinated in response to strigolactones (orobanchol, 5-deoxystrigol, 2′-epiorobanchol, and GR24) and dehydrocostus lactone (DCL), whereas O. cumana seeds responded only to DCL and GR24, and O. cernua only to strigolactones. Based on morphological similarities with O. cumana, shared molecular markers with O. cumana, ability to parasitize sunflower and respond to sunflower-germination stimulants, it was concluded that CUCE is a new race of O. cumana, with a host range expanding to Solanaceae crops. While being an important noxious weed of sunflower, this new O. cumana race is currently spreading and posing a threat to processing tomato in Israel. This finding is an alarming indication that broomrapes can shift host range and that similar new races of O. cumana could potentially appear in other countries.

Biocontrol potential of the antagonistic microorganism Streptomyces enissocaesilis against Orobanche cumana

The root parasitic weed Orobanche cumana (sunflower broomrape) constrains sunflower (Helianthus annuus L.) production in many areas of the world. A seed germination experiment, a co-culture experiment, and a pot experiment were conducted to study the effect of some soilborne antagonistic microorganisms on O. cumana. Results showed that culture filtrate of Streptomyces enissocaesilis significantly reduced the germination rate of O. cumana seeds both in the seed germination experiment and the co-culture experiment, with more than 50 and 40 % (after cultivation for eight days) reduction respectively over the control. In the pot experiment, applying S. enissocaesilis reduced the epigaeous number of O. cumana tubercles by 47.5 % after 130 days. Furthermore, S. enissocaesilis also increased polyphenol oxidase activity in sunflower roots and improved the beneficial microflora in the rhizosphere soil of sunflower. Selecting biocontrol agents from soilborne antagonistic microorganisms might be a promising strategy for controlling O. cumana.

Stimulating effect of the root exudates of sorghum, millet, and Sudan grass on the seed germination of broomrape (Orobanche cumana Wallr.) infesting sunflowers in Russia

Broomrape (Orobanche cumana Wallr.) is a harmful obligate parasite of sunflower. We found that the negative effect of broomrape on the cultivation of this crop can be eliminated by planting the crops that stimulate the germination of broomrape seeds but do not serve as its hosts. Under laboratory conditions, the root exudates of eight samples of grain sorghum, three samples of sugar sorghum, two samples of Sudan grass, and five cultivars of common millet stimulated the germination of broomrape seeds in different ways. It was observed that Gureeva millet, Sudan grasses Anastasia and Aleksandrina, grain sorghum Khasine 28, and sugar sorghum Listvenit are the most suitable provocative crops for the elimination of O. cumana seeds.

Switchgrass (Panicum virgatum L.) has ability to induce germination of Orobanche cumana

Orobanche cumana Wallr. (sunflower broomrape) is a devastating root parasitic weed, causing enormous crop losses worldwide. The question was whether or not switchgrass has the potential to be a ‘trap crop’ for O. cumana control. To answer this question, the field experiments, pot experiments, and laboratory experiments were conducted in this study. The ability of nine switchgrass (Panicum virgatum L.) cultivars to induce O. cumana seeds germination was tested. Results indicated that root extracts shoot extracts, rhizosphere soil, and root exudates from switchgrass induced O. cumana germination. Ability to induce germination varied significantly among growing stages, with the earlier part of growing season (grown for 2 weeks) generally inducing the highest O. cumana germination rates. The methanol was a more suitable solvent than distilled water for extracting germination stimulants from switchgrass plants. Ten-fold dilutions of the extracts generally induced higher germination rates than either undiluted or 100-fold dilutions. The germination rates of O. cumana seeds in shoot extracts treatments were positively correlated with those in the root extracts (R2 = 0.6397; p < 0.01) and negatively correlated with those in the rhizosphere soil treatments ( R2 = 0.4433; p < 0.05). In conclusion, it is believed that switchgrass is a potential trap crop for the control of root parasitic weed O. cumana.

N–P–K ratio affects exudation of germination stimulants and resistance of tobacco seedlings to broomrapes

Nutrient deficiency is known to aggravate the infestation of broomrape in a number of plant species. This effect is hypothesised to result from the promotion of the release of germination stimulants from the roots of the host plant. In the present study, three experiments (hydroponic culture, Petri dish culture and soil culture) were conducted to investigate the effects of macro-nutrient ratios on the production and release of germination stimulants and the resistance of host tobacco cultivars to broomrapes. The hydroponic culture study showed that nitrogen/phosphate deficiency and potassium excess increased the germination-inducing activity of root exudates on Phelipanche aegyptiaca and Orobanche cumana seeds, with two tobacco cultivars showing different responses to nutrient ratios. The production of germination stimulants was significantly increased under nitrogen- and phosphate-deficient soil conditions. In the Petri dish study, increased vulnerability to P. aegyptiaca infection was observed in RG tobacco seedlings under nitrogen- and potassium-deficient conditions and in WF tobacco seedlings under nitrogen- and phosphate-deficient conditions. The results showed that macro-nutrient ratios can affect both the exudation of germination stimulants and the resistance of tobacco seedlings. Finally, we suggest that the classification of the resistance or tolerance of host species should fully consider the nutritional conditions of cultivation and the strategy of fertilisation in terms of the potential control of parasitic weeds prior to the identification of the key mechanism of parasite resistance.

Ryecyanatines A and B and ryecarbonitrilines A and B, substituted cyanatophenol, cyanatobenzo[1,3]dioxole, and benzo[1,3]dioxolecarbonitriles from rye (Secale cereale L.) root exudates: Novel metabolites with allelopathic activity on Orobanche seed germination and radicle growth

Orobanche and Phelipanche species (the broomrapes) are root parasitic plants, some of which represent serious weed problems causing heavy yield losses on important crops. Current control relies on the use of certain agronomic practices, resistant crop varieties, and herbicides, albeit success has been marginal. Agronomic practices such as the use of allelopathic species in intercropping or cover crops, or the use of direct seedling over residues of allelopathic species incorporate the principle of allelopathy exerted by molecules exuded from roots or released by crop residues to control broomrapes. In addition, the isolation of natural substances from root exudates of plants with potential to inhibit broomrape development opens the door to the design of new herbicides based on natural and benign sources. Ryecyanatines A and B and ryecarbonitrilines A and B, the first new substituted cyanatophenol, substituted cyanatobenzo[1,3]dioxole, and the latter two new substituted benzo[1,3]dioxolecarbonitriles were isolated from rye (Secale cereale L.) root exudates. They were characterized as 4-cyanato-2-methoxyphenol, 2-cyanato-benzo[1,3]dioxole, 2-methoxybenzo[1,3]dioxole-5-carbonitrile and benzo[1,3]dioxole-2-carbonitrile by spectroscopic (essentially NMR and HRESI MS spectra) methods. These compounds were investigated for allelopathic activity on Orobanche germination and development. Ryecarbonitriline A induced germination of Orobanche cumana seeds, and this germination can be considered as suicidal because O. cumana does not parasite rye roots and cannot survive without host resources beyond germination stage. In addition, ryecyanatine A promotes a rapid cessation of O. cumana, Orobanche crenata and Orobanche minor radicle growth with the promotion of a layer of papillae at the radicle tip in O. cumana and O. crenata hampering the contact of the parasite to the host. Ryecarbonitriline B also displayed the same activity although being less active than ryecyanatine A and mainly restricted to O. cumana.

ChemInform Abstract: Synthesis of Strigolactones Analogues (I) by Intramolecular [2 + 2] Cycloaddition of Ketene‐Iminium Salts to Olefins and Their Activity on Orobanche cumana Seeds.

ChemInform Abstract: Synthesis of Strigolactones Analogues (I) by Intramolecular [2 + 2] Cycloaddition of Ketene‐Iminium Salts to Olefins and Their Activity on Orobanche cumana Seeds.

Synthesis of strigolactones analogues by intramolecular [2+2] cycloaddition of ketene-iminium salts to olefins and their activity on Orobanche cumana seeds

Strigolactones have been the latest identified phytohormones. Among the strigolactones analogues described recently, GR-24 remains the most studied derivative which is used as standard in this field. In order to improve several properties of GR-24 for potential agronomical applications, we investigated the effect of substituents on the B and C-rings on the activity for seed germination induction. We report here the synthesis of 9 GR-24 analogues via a [2+2] intramolecular cycloaddition of ketene-iminium salts and a summary of their activity for the germination of Orobanche cumana (broomrape) seeds.

New Sesquiterpene Lactones from Sunflower Root Exudate as Germination Stimulants for Orobanche cumana

Orobanche cumana is a serious threat for cultivation of sunflower in Europe and Asia. Germination of the parasite is induced by metabolites released from the host root system. The first germination stimulant from sunflower root exudate was recently identified as dehydrocostus lactone, a sesquiterpene lactone. Bioassay-guided fractionation of root exudates now showed the release of additional sesquiterpene lactones. Besides dehydrocostus lactone, costunolide, tomentosin, and 8-epixanthatin were purified and identified spectroscopically. All four compounds induced germination of O. cumana at nano- to micromolar concentrations. Costunolide and dehydrocostus lactone concentrations above 1 μM reduced the activity, and application of 100 μM inhibited germination irreversibly. Seeds of Phelipanche ramosa could not be induced with costunolide. O. cumana seeds also germinated with GR24, a synthetic strigolactone. No bioactive fraction of sunflower contained compounds of this type. This supports previous findings that sesquiterpene lactones instead of strigolactones trigger the sunflower/O. cumana interaction.

Reduced germination of Orobanche cumana seeds in the presence of Arbuscular Mycorrhizal fungi or their exudates.

Broomrapes (Orobanche and Phelipanche spp) are parasitic plants responsible for important crop losses, and efficient procedures to control these pests are scarce. Biological control is one of the possible strategies to tackle these pests. Arbuscular Mycorrhizal (AM) fungi are widespread soil microorganisms that live symbiotically with the roots of most plant species, and they have already been tested on sorghum for their ability to reduce infestation by witchweeds, another kind of parasitic plants. In this work AM fungi were evaluated as potential biocontrol agents against Orobanche cumana, a broomrape species that specifically attacks sunflower. When inoculated simultaneously with O. cumana seeds, AM fungi could offer a moderate level of protection against the broomrape. Interestingly, this protection did not only rely on a reduced production of parasitic seed germination stimulants, as was proposed in previous studies. Rather, mycorrhizal root exudates had a negative impact on the germination of O. cumana induced by germination stimulants. A similar effect could be obtained with AM spore exudates, establishing the fungal origin of at least part of the active compounds. Together, our results demonstrate that AM fungi themselves can lead to a reduced rate of parasitic seed germination, in addition to possible effects mediated by the mycorrhizal plant. Combined with the other benefits of AM symbiosis, these effects make AM fungi an attractive option for biological control of O. cumana.

Dehydrocostus lactone is exuded from sunflower roots and stimulates germination of the root parasite Orobanche cumana

The germination of the obligate root parasites of the Orobanchaceae depends on the perception of chemical stimuli from host roots. Several compounds, collectively termed strigolactones, stimulate the germination of the various Orobanche species, but do not significantly elicit germination of Orobanche cumana, a specific parasite of sunflower. Phosphate starvation markedly decreased the stimulatory activity of sunflower root exudates toward O. cumana, and fluridone – an inhibitor of the carotenoid biosynthesis pathway – did not inhibit the production of the germination stimulant in both shoots and roots of young sunflower plants, indicating that the stimulant is not a strigolactone. We identified the natural germination stimulant from sunflower root exudates by bioassay-driven purification. Its chemical structure was elucidated as the guaianolide sesquiterpene lactone dehydrocostus lactone (DCL). Low DCL concentrations effectively stimulate the germination of O. cumana seeds but not of Phelipanche aegyptiaca (syn. Orobanche aegyptiaca). DCL and other sesquiterpene lactones were found in various plant organs, but were previously not known to be exuded to the rhizosphere where they can interact with other organisms.

Quantification of the dynamics of Orobanche cumana and Phelipanche aegyptiaca parasitism in confectionery sunflower

Ephrath JE &amp; Eizenberg H (2010). Quantification of the dynamics of Orobanche cumana and Phelipanche aegyptiaca parasitism in confectionery sunflower. Elymus repens. Weed Research50, 140–152.SummaryThe dynamics of the host–parasite relationship between sunflower and two broomrape species, Orobanche cumana and Phelipanche aegyptiaca, are temperature‐related. This relationship can be quantified by thermal time as measured by growing day degrees (d °C; GDD). To obtain a reliable prediction from thermal time values, studies based on a wide range of temperatures are essential. O. cumana was found to be more suited to germination under high temperatures than P. aegyptiaca. The maximal parasite germination percentage, 79% and 49% in O. cumana and P. aegyptiaca, respectively, occurred in the two species in a similar thermal time of about 500 GDD. Although, there was significantly higher germination of O. cumana seeds compared with P. aegyptiaca under the various temperature regimes, there was no effect of temperature on the number of O. cumana attachments, when compared with P. aegyptiaca. The relationship between the progress of O. cumana and P. aegyptiaca attachments and emergence and thermal time followed a sigmoid‐shaped curve. O. cumana and P. aegyptiaca biomass was found to be strongly related to thermal time.

Ecophysiological consideration of Orobanche cumana germination

SUMMARY It is widely known that Orobanche cumana seeds need a conditioning phase of several days under suitable temperatures and wet conditions before being able to germinate in response to germination stimulants. In a series of experiments we showed that O. cumana seeds respond to the synthetic germination stimulants Nijmegen-1 and GR24 even without prior conditioning. These results are consistent with our hypothesis that, under certain field circumstances, non-conditioned Orobanche seeds may also germinate when a host root comes close to them.