Search

Background and Aims: Obligate root holoparasites of the genus Orobanche attack dicotyledonous crops and cause severe losses in many parts of the world. Chemical induction of plant defence systems such as systemic acquired resistance was proposed to be an available strategy to control the root parasite, but the detailed mechanisms involved have not been clarified. The aim of this study was to elucidate the effects of salicylic acid (SA), jasmonic acid (JA) and their analogues on resistance of red clover to Orobanche parasitism., Methods: Roots of red clover grown in plastic chambers were applied with SA, S-methyl benzo[1,2,3]thiadiazole-7-carbothioate (BTH), methyl jasmonate (MeJA) and n-propyl dihydrojasmonate (PDJ), and then were inoculated with O. minor seeds. Attachments of the parasite were observed after 5 weeks., Key Results: SA and BTH, inducers of SA-mediated defences, significantly reduced the number of established parasites by more than 75 %. By contrast, MeJA and PDJ, inducers of JA-mediated defences, did not affect parasitism. The reduction in the number of established parasites by SA and BTH was due to the inhibited elongation of O. minor radicles and the activation of defence responses in the host root including lignification of the endodermis., Conclusions: These results suggest that SA-induced resistance, but not JA-induced resistance, is effective in inhibiting Orobanche parasitism and that the resistance is expressed by the host root both externally and internally.

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.

Niche theory predicts that species which share resources should evolve strategies to minimise competition for those resources, or the less competitive species would be extirpated. Some plant species are constrained to co-occur, for example parasitic plants and their hosts, and may overlap in their pollination niche if they flower at the same time and attract the same pollinators. Using field observations and experiments between 1996 and 2006, we tested a series of hypotheses regarding pollination niche overlap between a specialist parasitic plant Orobanche elatior (Orobanchaceae) and its host Centaurea scabiosa (Asteraceae). These species flower more or less at the same time, with some year-to-year variation. The host is pollinated by a diverse range of insects, which vary in their effectiveness, whilst the parasite is pollinated by a single species of bumblebee, Bombus pascuorum, which is also an effective pollinator of the host plant. The two species therefore have partially overlapping pollination niches. These niches are not finely subdivided by differential pollen placement, or by diurnal segregation of the niches. We therefore found no evidence of character displacement within the pollination niches of these species, possibly because pollinators are not a limiting resource for these plants. Direct observation of pollinator movements, coupled with experimental manipulations of host plant inflorescence density, showed that Bombus pascuorum only rarely moves between inflorescences of the host and the parasite and therefore the presence of one plant is unlikely to be facilitating pollination in the other. This is the first detailed examination of pollination niche overlap in a plant parasite system and we suggest avenues for future research in relation to pollination and other shared interactions between parasitic plants and their hosts.

Plant derived sesquiterpene strigolactones, which have previously been characterized as germination stimulants for root parasitic plants, have recently been identified as the branching factors which induce hyphal branching morphogenesis, a critical step in host recognition by arbuscular mycorrhizal (AM) fungi. We show here that, in red clover plants (Trifolium pratense L.), which is known as a host for both AM fungi and the root holoparasitic plant Orobanche minor Sm., reduced supply of phosphorus (P) but not of other elements examined (N, K, Mg, Ca) in the culture medium significantly promotes the release of a strigolactone, orobanchol, by the roots of this plant. In red clover plants, the level of orobanchol exudation appeared to be regulated by P availability and was in good agreement with germination stimulation activity of the root exudates. This implies that under P deficiency, plant roots attract not only symbiotic fungi but also root parasitic plants through the release of strigolactones. This is the first report demonstrating that nutrient availability influences both symbiotic and parasitic interactions in the rhizosphere.

We have developed a simple genetic engineering strategy for conferring resistance against parasitic weeds on host plants. Transgenic tomato plants expressing the sarcotoxin IA gene were grown either in polyethylene bags (PE) or in pots inoculated with Orobanche aegyptiaca seeds. The results indicate that transgenic plants exhibited strong inhibition of parasite growth and significantly increased yield as compared with non-transgenic ones. In both PE and pot systems most of the parasite tubercles attached to the transgenic root plants turned necrotic and developed abnormally. Integration and expression of the gene were confirmed by Southern blot, RT-PCR and Western blot analysis. Our results indicate that the insect gene produced in the plant cells was selectively toxic to the parasite and non-toxic to the host plant.

Parasitic plants are widely distributed in various ecological environments, with different growth habits and host recognition mechanisms. This paper discussed the distinctive seed germination characteristics of root parasitic plants such as Orobanche and Striga, summarized the signals for parasitic seed germination discovered up to now, and reviewed the effects of various germination signals, plant hormones and several fungal metabolites on the host recognition of parasitic plants, as well as the respiration characteristics during the conditioning, and the activating mechanism of the signals for parasitic seed germination. The induction of various differentiated calli in different Orobanche species, and the establishment of novel in vitro aseptic infection system and its application in the host recognition of parasitic plants were also discussed, with the present problems in researching the recognition mechanisms between parasitic plants and hosts put forward, and the further work prospected.

Root holoparasitic angiosperms, like Orobanche spp, completely lack chlorophyll and totally depend on their host for their supply of nutrients. O. crenata is a severe constraint to the cultivation of legumes and breeding for resistance remains the most economical, feasible, and environmentally friendly method of control. Due to the lack of resistance in commercial pea cultivars, the use of wild relatives for breeding is necessary, and an understanding of the mechanisms underlying host resistance is needed in order to improve screening for resistance in breeding programmes. Compatible and incompatible interactions between O. crenata and pea have been studied using cytochemical procedures. The parasite was stopped in the host cortex before reaching the central cylinder, and accumulation of H2O2, peroxidases, and callose were detected in neighbouring cells. Protein cross-linking in the host cell walls appears as the mechanism of defence, halting penetration of the parasite. In situ hybridization studies have also shown that a peroxidase and a beta-glucanase are differently expressed in cells of the resistant host (Pf651) near the penetration point. The role of these proteins in the resistance to O. crenata is discussed.

Sunflower broomrape (Orobanche cumana Wallr.) is a root holoparasitic angiosperm considered as one of the major constraints for sunflower production in Mediterranean areas. Breeding for resistance is regarded as the most effective, feasible, and environmentally friendly solution to control this parasite. However, the existing sources of genetic resistance are defeated by the continuous emergence of new more virulent races of the parasite. In this work, the interaction between sunflower and O. cumana has been analysed in order to gain insights into the mechanisms involved in resistance. Two sunflower genotypes were selected showing different behaviour against the new race F of O. cumana, HE-39998 (susceptible) and HE-39999 (resistant), and both compatible and incompatible interactions were compared. Pot and Petri dish bioassays revealed that only HE-39998 plants were severely affected, supporting a high number of successfully established broomrapes to mature flowering, whereas in HE-39999 root tubercles were never observed, resistance being associated with browning symptoms of both parasite and host tissues. Histological aspects of the resistance were further investigated. Suberization and protein cross-linking at the cell wall were seen in the resistant sunflower cells in contact with the parasite, preventing parasite penetration and connection to the host vascular system. In addition, fluorescence and confocal laser microscopy (CLM) observations revealed accumulation of phenolic compounds during the incompatible reaction, which is in agreement with these metabolites playing a defensive role during H. annuus-O. cumana interaction.

Orobanche spp. (broomrapes) are holoparasites lacking in chlorophyll and totally dependent on their host for their supply of nutrients. O. crenata is a severe constraint to legumes cultivation and breeding for resistance remains as one of the best available methods of control. However, little is known about the basis of host resistance to broomrapes. It is a multicomponent event, and resistance based on hampering development and necrosis of broomrape tubercles has been reported. In the present work, the formation of mucilage and occlusion of host xylem vessels associated with the death of O. crenata tubercles were studied histologically. Samples of necrotic O. crenata tubercles established on resistant and susceptible vetch genotypes were collected. The samples were fixed, sectioned and stained using different procedures. The sections were observed at the light microscopy level, either under bright field, epi-fluorescence or confocal laser scanning microscopy. A higher proportion of necrotic tubercles was found on the resistant genotype and this was associated with a higher percentage of occluded vessels. Mucilage is composed mainly by carbohydrates (non-esterified pectins) and the presence of polyphenols was also detected. The mucilage and other substances composed by parasite secretions and host-degraded products was found to block host vessels and obstruct the parasite supply channel, being a quantitative defensive response against O. crenata in vetch, and probably also in other legumes and plants. The presence of foreign substances (i.e. parasite secretions) and host-degraded products (i.e. carbohydrates from cell walls) inside host vessels seems to activate this response and leads to xylem occlusion and further death of established Orobanche tubercles.

Aims: To develop an in-situ, non-destructive method for observation and monitoring of the underground developmental stages of the root parasite Orobanche cumana., Scope: The parasitic weed Orobanche causes severe damage to vegetables and field crops. Most of the damage caused to the crops occurs during the underground, unobservable parasitism stage. Sunflower (Helianthus annuus 'Adi') plants were planted in soil that was artificially inoculated with O. cumana seeds. Clear Plexiglas mini-rhizotron plastic observation tubes were inserted into the soil. Seed germination, early stage of penetration, and formation of tubercles and spikes were observed non-destructively and were monitored throughout the growing season by mean of a mini-rhizotron camera. Use of this technology enabled the complete individual parasite life cycle from the very early development (including germination) to Orobanche shoot to be monitored. In addition, the effect of the systemic herbicide Cadre (imazapic) on the development of O. cumana was inspected and quantified., Conclusions: This novel methodology facilitates the in-situ study of major aspects of the host-parasite interaction and of parasite suppression, such as parasitism dynamics, parasite growth rate, and the effect of chemical treatments on the parasite.

Bulked segregant analysis and amplification of RTS loci were used for searching of DNA-markers to Or 3 gene that controls resistance of sunflower to broomrape. Polymorphic amplifyed fragments have been discovered which differentiate resistant and non-resistant parental lines and F2 plants. Linkage between codominant marker and locus Or 3 is defined.

Parasitic plants present some of the most intractable weed problems for agriculture in much of the world. Species of root parasites such as Orobanche can cause enormous yield losses, yet few control measures are effective and affordable. An ideal solution to this problem is the development of parasite-resistant crops, but this goal has been elusive for most susceptible crops. Here we report a mechanism for resistance to the parasitic angiosperm Orobanche based on expression of sarcotoxin IA in transgenic tobacco. Sarcotoxin IA is a 40-residue peptide with antibiotic activity, originally isolated from the fly, Sarcophaga peregrina. The sarcotoxin IA gene was fused to an Orobanche-inducible promoter, HMG2, which is induced locally in the host root at the point of contact with the parasite, and used to transform tobacco. The resulting transgenic plants accumulated more biomass than non-transformed plants in the presence of parasites. Furthermore, plants expressing sarcotoxin IA showed enhanced resistance to O. aegyptiaca as evidenced by abnormal parasite development and higher parasite mortality after attachment as compared to non-transformed plants. The transgenic plants were similar in appearance to non-transformed plants suggesting that sarcotoxin IA is not detrimental to the host.

Background and Aims: Orobanche species represent major constraints to crop production in many parts of the world as they reduce yield and alter root/shoot allometry. Although much is known about the histology and effect of Orobanche spp. on susceptible hosts, less is known about the basis of host resistance to these parasites. In this work, histological aspects related to the resistance of some legumes to Orobanche crenata have been investigated in order to determine which types of resistance responses are involved in the unsuccessful penetration of O. crenata., Methods: Samples of resistance reactions against O. crenata on different genotypes of resistant legumes were collected. The samples were fixed, sectioned and stained using different procedures. Sections were observed using a transmission light microscope and by epi-fluorescence., Key Results: Lignification of endodermal and pericycle host cells seems to prevent parasite intrusion into the root vascular cylinder at early infection stages. But in other cases, established tubercles became necrotic and died. Contrary to some previous studies, it was found that darkening at the infection site in these latter cases does not correspond to death of host tissues, but to the secretion of substances that fill the apoplast in the host-parasite interface and in much of the infected host tissues. The secretions block neighbouring host vessels. This may interfere with the nutrient flux between host and parasite, and may lead to necrosis and death of the developing parasite., Conclusions: The unsuccessful penetration of O. crenata seedlings into legume roots cannot be attributed to cell death in the host. It seems to be associated with lignification of host endodermis and pericycle cells at the penetration site. The accumulation of secretions at the infection site, may lead to the activation of xylem occlusion, another defence mechanism, which may cause further necrosis of established tubercles.

In this study, the isolation and characterization of a phytochrome A (PHYA) homologous cDNA (OmPHYA) in the non-photosynthetic holoparasitic plant Orobanche minor are described. The present findings provide the first report of the presence of a PHYA homolog in the holoparasite. This study found that OmPHYA is of similar size to the other PHYAs of green plants and shows 72, 77, and 77% amino acid sequence identity with PHYA in Arabidopsis, potato, and tobacco respectively. The OmPHYA contains a conserved chromophore attachment cysteine at position 323. Although OmPHYA shows high sequence identity with other PHYAs in green plants, 13 amino acid substitutions located in both the N and C-terminal domains are observed (a total of 26 amino acids). OmPHYA is encoded by a single gene within the O. minor genome. The abundance of the OmPHYA transcript as well as nuclear translocation of OmphyA occurs in a light-dependent manner.

The germination stimulants for root parasitic plants Striga and Orobanche produced by cotton (Gossypium hirsutum L.) were examined in detail. Seeds of cotton were germinated and grown on glass wool wetted with sterile distilled water in sterile filter units. The root exudate was collected daily and extracted with ethyl acetate. Each of these ethyl acetate extracts was analyzed directly by high-performance liquid chromatography linked with tandem mass spectrometry (LC/MS/MS). The results demonstrate that cotton roots exuded strigol and strigyl acetate, but no other known strigolactones such as orobanchol and alectrol. The production of strigol was detected even in the root exudate collected during the first 24 h of incubation and reached a maximum 5-7 days later. The average exudation of strigol and strigyl acetate during the incubation period was ca. 15 and 2 pg/plant/day, respectively, indicating that strigol mainly contributed to germination stimulation by the cotton root exudate.

Background and Aims: Previous research has made clear the intrinsic taxonomic difficulties in identifying species in the genus Orobanche. The aim of this study, therefore, was to investigate the systematic utility of seed characteristics., Methods: Light and scanning electron microscopy was used to examine the seeds of 33 taxa of Orobanche from the Iberian Peninsula and the Balearic Islands., Key Results: Characters such as size, shape and ornamentation of the seeds were not found to be very useful in differentiation of taxa; however, other characters of the epidermal seed coat cells proved to be very helpful in this respect. Ornamentation of the periclinal walls could be used to discriminate four morphological types. Other features related to the anticlinal walls of the cells, such as thickness, presence/absence of a narrow trough, or relative depth, all contributed to the characterization of a large number of species., Conclusions: The usefulness of micromorphological studies on seeds of Orobanche in relation to differentiating taxa is demonstrated, and a key is provided to distinguish species or groups of species.

Broomrape (Orobanche cumana Wallr.) is a root parasite of sunflower that is regarded as one of the most important constraints of sunflower production in the Mediterranean region. Breeding for resistance is the most effective method of control. P-96 is a sunflower line which shows dominant resistance to broomrape race E and recessive resistance to the very new race F. The objective of this study was to map and characterize quantitative trait loci (QTL) for resistance to race E and to race F of broomrape in P-96. A population from a cross between P-96 and the susceptible line P-21 was phenotyped for broomrape resistance in four experiments, two for race E and two for race F, by measuring different resistance parameters (resistance or susceptibility, number of broomrape per plant, and proportion of resistant plants per F(3) family). This population was also genotyped with microsatellite and RFLP markers. A linkage map comprising 103 marker loci distributed on 17 linkage groups was developed, and composite interval mapping analyses were performed. In total, five QTL ( or1.1, or3.1, or7.1 or13.1 and or13.2) for resistance to race E and six QTL ( or1.1, or4.1, or5.1, or13.1, or13.2 and or16.1) for resistance to race F of broomrape were detected on 7 of the 17 linkage groups. Phenotypic variance for race E resistance was mainly explained by the major QTL or3.1 associated to the resistance or susceptibility character ( R(2)=59%), while race F resistance was explained by QTL with a small to moderate effect ( R(2) from 15.0% to 38.7%), mainly associated with the number of broomrape per plant. Or3.1 was race E-specific, while or1.1, or13.1 and or13.2 of were non-race specific. Or13.1, and or13.2 were stable across the four experiments. Or3.1, and or7.1 were stable over the two race E experiments and or1.1 and or5.1 over the two race F experiments. The results from this study suggest that resistance to broomrape in sunflower is controlled by a combination of qualitative, race-specific resistance affecting the presence or absence of broomrape and a quantitative non-race specific resistance affecting their number.

Crenate broomrape (Orobanche crenata) is a parasitic plant that threatens legume production in Mediterranean areas. Pea (Pisum sativum) is severely affected, and only moderate levels of genetic resistance have so far been identified. In the present work we selected the most resistant accession available (Ps 624) and compared it with a susceptible (Messire) cultivar. Experiments were performed by using pot and Petri dish bioassays, showing little differences in the percentage of broomrape seed germination induced by both genotypes, but a significant hamper in the number of successfully installed tubercles and their developmental stage in the Ps 624 compared to Messire. The protein profile of healthy and infected P. sativum root tissue were analysed by two-dimensional electrophoresis. Approximately 500 individual protein spots could be detected on silver stained gels. At least 22 different protein spots differentiated control, non-infected, Messire and Ps 624 accessions. Some of them were identified by MALDI-TOF mass spectrometry and database searching as cysteine proteinase, beta-1,3-glucanase, endochitinase, profucosidase, and ABA-responsive protein. Both qualitative and quantitative differences have been found among infected and non-infected root extracts. Thus, in the infected susceptible Messire genotype 34 spots were decreased, one increased and three newly detected, while in Ps 624, 15 spots were increased, three decreased and one newly detected. In response to the inoculation, proteins that correspond to enzymes of the carbohydrate metabolism (fructokinase, fructose-bisphosphate aldolase), nitrogen metabolism (ferredoxin-NADP reductase) and mitochondrial electronic chain transport (alternative oxidase 2) decreased in the susceptible check, while proteins that correspond to enzymes of the nitrogen assimilation pathway (glutamine synthetase) or typical pathogen defence, PR proteins, including beta-1,3-glucanase and peroxidases, increased in Ps 624. Results are discussed in terms of changes in the carbohydrate and nitrogen metabolism an induction of defence proteins in response to broomrape parasitism.

Various types of broomrape (Orobanche cermua), such as parasitic weeds, live on the roots of many important crops. Different methods may be advised to eradicate this parasite. But the seed size is so tiny with high stability in the soil and having strong connection with the host, it is very difficult to eradicate this parasite and there is no useful and economic method has not been suggested in this regard yet. The seeds of all types of broomrape germinate in the soil only when they are exposed to the secretions of the host's roots, therefore, providing special combinations that cause the parasite's seeds germinate in the absence of the host, can be an effective way in controlling this parasite. The effect of the tobacco root's extract and the synthetic combinations as well as their mixtures with different concentrations on the germination of the seeds of Orobanche cermua Loefi were tested to introduce an effective combination that can be used in controlling this parasite. This experiment includes 28 treatments in three replications. The tobacco root's extract was derived from the tobacco roots planted in pots containing Perlit in different stages and were prepared in different concentrations. Synthetic combinations (including combinations 1, 2 and 3) were also prepared with concentration of 1, 2 and 3 parts per million (ppm), then these combinations were tested as treatments on Broomrape's seeds. At the end of experiment, it was concluded that all the combinations used in different concentrations in this experiment significantly differ from the control treatment in their effects in causing the Broomrape's seeds germinate. Among all the combinations examined, the following were the most effective ones in the process of the germination of the Broomrape's seeds: 1. combination 2 with the concentration of one part per million, 2. combination 2 with the concentration of two parts per million, and 3 tobacco root's extract in the third stage.

The appearance of the activity of the cyanide insensitive, alternative oxidase (AOX), pathway of oxygen uptake was followed in seeds of Orobanche aegyptiaca during conditioning. The pathway becomes operative during conditioning, up to day three as determined by inhibition of oxygen uptake of the seeds by propyl gallate. At the same time an increasing percentage of oxygen uptake is insensitive to cyanide and an increased oxygen uptake, responsive to propyl gallate, is induced by brief salicylic acid treatment of seeds. By day six of conditioning, these responses decrease and the AOX pathway could not be detected in germinating seeds, after treatment with a germination stimulant. These results were confirmed by following the reaction of extracts of fractions enriched with mitochondria from the conditioned seeds, using a specific antibody against AOX. Treatment of the seeds with inhibitors of AOX during conditioning significantly inhibited their subsequent germination. Addition of hydrogen peroxide after 4 and 7 days of conditioning resulted in reduced germination. In addition treatment of seed with propyl or octyl gallate during conditioning reduced the infection of tomato plants by Orobanche seeds and the development of tubercles of the parasite on the host roots. These results together indicate that the operation of AOX during conditioning has a significant function on the subsequent germination behaviour and pathogenicity of the root parasite. Some potential practical applications of these findings are discussed.

In the Poitou-Charentes district, among the 82 species of winter rape weeds identified, 22 displayed a strong affinity for this crop (Brassica napus L.). In fields, 50% of these weeds were parasitized by Orobanche ramosa, playing the role of host plants. Greenhouse co-cultures (weed/Orobanche ramosa) showed that weeds non-parasitized in fields could be attacked by broomrape, developing a more or less complete cycle. In vitro co-cultures (weed/Orobanche ramosa) revealed that root exudates of non-parasitized weeds, in fields or in greenhouse co-cultures, could induce Orobanche ramosa seed germination, but not attachment. These weeds could play the role of false hosts.