Root Parasitic Weeds Research Articles

A new major parasitic plant in rice in Togo: Rhamphicarpa fistulosa

Rhamphicarpa fistulosa (Hochst.) Benth. (Scrophulariaceae) is an annual facultative root-parasitic weed found in the lowlands and floodplains of tropical Africa. It has been reported to be the major biotic constraint to rice cultivation in the Savannah region of Togo. A survey was conducted in 2012 to determine its geographical spread and to collect information on farmers' knowledge of its control. An investigation based on semi-open questions was carried out among the people visited. Rhamphicarpa fistulosa was found in 80% of the 33 lowlands visited. The period of abundance is in August-September with the highest infestation (90%) being found on sandy and clay soils. When the parasite was present, 94 other weeds belonging to 24 botanical families were recorded. As a method for parasite control, farmers use hand weeding and apply chemical fertilizers but always at lower input rates than national standards. Heavy infestation levels lead to the abandonment of plots while other reported types of damage provoke growth reduction of rice plants and plant yellowing.

Heliolactone, a non-sesquiterpene lactone germination stimulant for root parasitic weeds from sunflower

Root exudates of sunflower (Helianthus annuus L.) line 2607A induced germination of seeds of root parasitic weeds Striga hermonthica, Orobanche cumana, Orobanche minor, Orobanche crenata, and Phelipanche aegyptiaca. Bioassay-guided purification led to the isolation of a germination stimulant designated as heliolactone. FT-MS analysis indicated a molecular formula of C20H24O6. Detailed NMR spectroscopic studies established a methylfuranone group, a common structural component of strigolactones connected to a methyl ester of a C14 carboxylic acid via an enol ether bridge. The cyclohexenone ring is identical to that of 3-oxo-α-ionol and the other part of the molecule corresponds to an oxidized carlactone at C-19. It is a carlactone-type molecule and functions as a germination stimulant for seeds of root parasitic weeds. Heliolactone induced seed germination of the above mentioned root parasitic weeds, while dehydrocostus lactone and costunolide, sesquiterpene lactones isolated from sunflower root exudates, were effective only on O. cumana and O. minor. Heliolactone production in aquacultures increased when sunflower seedlings were grown hydroponically in tap water and decreased on supplementation of the culture with either phosphorus or nitrogen. Costunolide, on the other hand, was detected at a higher concentration in well-nourished medium as opposed to nutrient-deficient media, thus suggesting a contrasting contribution of heliolactone and the sesquiterpene lactone to the germination of O. cumana under different soil fertility levels.

The genuine structure of alectrol: end of a long controversy

Alectrol was first isolated from root exudates of cowpea (Vigna unguiculata), a genuine host of a root parasitic weed Striga gesnerioides, as a germination stimulant for seeds of the parasite. The proposed structure, an isomer of strigol, was disproven by chemical synthesis. Recently, another structure, namely orobanchyl acetate, was proposed. Surprisingly, however, the synthetic compound having this proposed structure for alectrol was not active in inducing germination of S. gesnerioides seeds although it was active toward seeds of other root parasitic weeds such as S. hermonthica and Orobanche minor. Detailed studies on 1H NMR, mass and CD spectra of naturally occurring alectrol, re-isolated from cowpea root exudates, revealed that the genuine structure of the germination stimulant is not orobanchyl acetate but its stereoisomer ent-2′-epi-orobanchyl acetate. Accordingly, the structure of natural orobanchol was revised to ent-2′-epi-orobanchol 12 years after a tentative structure of orobanchol was proposed. Strict stereochemical requirements of strigolactones for germination induction of S. gesnerioides seeds, authentic samples of synthetic strigolactones and advanced analytical instruments made the structural assignment possible, thus ending a 20 years controversy concerning the true structure of alectrol.

Legume breeding for broomrape resistance

Legume cultivation is hampered in Mediterranean regions by the occurrence of the root parasitic weeds Orobanche crenata (crenate broomrape) and Orobanche foetida (foetida broomrape). Strategies of control have been developed but only marginal successes have been achieved. Most control methods are unfeasible, uneconomical, and hard to achieve or result in incomplete protection. Breeding for resistance is possible, but is hampered by the lack of sufficient levels of resistance, the complexity of its inheritance and the unreliability of available screening methods. Recent achievements in the identification of resistance levels and their deployment in breeding programmes will be presented and critically discussed.

67. Heliolactone, a non-sesquiterpene lactone germination stimulant for root parasitic weeds from sunflower

67. Heliolactone, a non-sesquiterpene lactone germination stimulant for root parasitic weeds from sunflower

7α- and 7β-Hydroxyorobanchyl acetate as germination stimulants for root parasitic weeds produced by cucumber

Cucumber (Cucumis sativus) plants were found to exude at least 12 germination stimulants for root parasitic weeds including 5 known strigolactones, 7-oxoorobanchol, 7-oxoorobanchyl acetate, orobanchol, orobanchyl acetate, and 4-deoxyorobanchol. Two novel germination stimulants were purified from cucumber root exudates and their structures were determined to be 7αand 7β-hydroxyorobanchyl acetate by 1D and 2D NMR spectroscopy, and ESIand EI-MS spectrometry. The stereochemistry was determined by NOE measurement and by comparing the CD spectra with those of the synthetic standards of four stereoisomers of orobanchol. 7αand 7β-Hydroxyorobanchol were detected by LC-MS/MS, and GC-MS analysis of purified sample confirmed their structures. The germination stimulation activities of 7αand 7β-hydroxyorobanchyl acetate on Orobanche minor were comparable to those of orobanchyl acetate and 7-oxoorobanchyl acetate. By contrast, 7β-hydroxyorobanchyl acetate was a highly potent germination stimulant for Phelipanche ramosa inducing more than 50% germination at 10 pM. © Pesticide Science Society of Japan

Selective Mimics of Strigolactone Actions and Their Potential Use for Controlling Damage Caused by Root Parasitic Weeds

Strigolactones (SLs) are a novel class of plant hormones and rhizosphere communication signals, although the molecular mechanisms underlying their activities have not yet been fully determined. Nor is their application in agriculture well developed. The importance of plant hormone agonists has been demonstrated in both basic and applied research, and chemicals that mimic strigolactone functions should greatly facilitate strigolactone research. Here, we report our discovery of a new phenoxyfuranone compound, 4-Br debranone (4BD), that shows similar activity to that of the major strigolactone (SL) analog GR24 in many aspects of a biological assay on plants. 4BD strongly inhibited tiller bud outgrowth in the SL-deficient rice mutant d10 at the same concentration as GR24, with no adverse effects, even during prolonged cultivation. This result was also observed in the Arabidopsis thaliana SL-deficient mutants max1, max3, and max4. However, the application of 4BD to the Arabidopsis SL-insensitive mutant max2 induced no morphological changes in it. The expression of SL biosynthetic genes was also reduced by 4BD treatment, probably via negative feedback regulation. However, in a seed germination assay on Striga hermonthica, a root parasitic plant, 4BD showed far less activity than GR24. These results suggest that 4BD is the first plant-specific strigolactone mimic.

Potential of Some Hybrid Maize Lines to Induce Germination of Sunflower Broomrape

Orobanche cumana Wallr. (sunflower broomrape) is a devastating root parasitic weed, causing enormous crop losses throughout the world. Maize (Zea mays L.) is able to induce germination of at least two Orobanche species. Our question was whether or not maize has potential to be used as a “trap crop” for O. cumana. To answer this question, we screened four maize hybrids and their parental lines for their ability to induce O. cumana germination. The results indicated that rhizosphere soil, rhizosphere soil extracts, root extracts, and shoot extracts from three of the four maize hybrids and their parental lines induced significant O. cumana germination. Root extracts generally induced higher germination rates than shoot extracts. Ten‐fold dilutions of the extracts generally induced higher germination rates than either undiluted extracts or 100‐fold dilutions. The ability to induce germination varied significantly among maize hybrid and inbred lines, with the hybrid 3255 × 335 and its parental lines generally inducing the highest O. cumana germination rates. The genetic mechanism for the production of the chemical stimulant that induces O. cumana germination needs to be analyzed further; however, we propose that there is potential for a breeding program to be developed to produce maize lines with greater ability to induce O. cumana germination. These lines could be used as trap crops for the control of this devastating root parasitic weed.

88. Study on distinctive sugar metabolism in germinating Orobanche minor seeds for development of a method for selective control of root parasitic weeds

88. Study on distinctive sugar metabolism in germinating Orobanche minor seeds for development of a method for selective control of root parasitic weeds

45. Characterization of germination stimulants for root parasitic weed seeds exuded by sunflower

45. Characterization of germination stimulants for root parasitic weed seeds exuded by sunflower

The Potential of Cell-free Cultures of Rhizobium leguminosarum, Azotobacter chroococcum and Compost Tea as Biocontrol Agents for Faba Bean Broomrape (Orobanche crenata Forsk.)

In the present study, cell-free cultures of four isolates of Rhizobium leguminosarum, an isolate of Azotobacter chroococcum and compost tea were investigated for their biocontrol potential against the root parasitic weed Orobanche crenata. Individual cell-free cultures of Azotobacter chroococcum or Rhizobium sp., dual and mixture of cell-free cultures of Rhizobium spp. or compost tea were applied to infested pots in greenhouse conditions. The treatments showed variable effects on many developmental parameters of both faba bean and broomrape. Significant decrease in the number of broomrape attachments, dry weight of the attached tubercles on faba bean roots and the reduction in percentage of broomrape seed germination were recorded. Compost tea, individual and mixture of R. leguminosarum isolates were more reducing on broomrape germination and growth than A. chroococcum alone did; being the former treatment is the best. The reduction in broomrape incidence by compost tea was due to certain phenotypic mechanisms, which acted alone or in combination. These mechanisms included negative effect of natural stimulant broomrape on seed germination, prevention of radical penetration inside the host roots, parasite yield reduction, and thus increasing the growth and vitality of faba bean. In vitro experiment indicated that seed germination percentage of broomrape was also negatively affected by the combination of root-exudates and compost tea. Radical apexes of the germinated seeds were distorted. These distortions may prevent the radicals to follow up the infestation. In conclusion, the study presents the potential of R. leguminosarum isolates and compost tea in biocontrol of broomrape. More investigations should be carried out with viable bacterial cells on the parasite plant before use in sustainable agricultural systems.

Application of qRT-PCR and RNA-Seq analysis for the identification of housekeeping genes useful for normalization of gene expression values during Striga hermonthica development

Striga is a root parasitic weed that attacks many of the staple crops in Africa, India and Southeast Asia, inflicting tremendous losses in yield and for which there are few effective control measures. Studies of parasitic plant virulence and host resistance will be greatly facilitated by the recent emergence of genomic resources that include extensive transcriptome sequence datasets spanning all life stages of S. hermonthica. Functional characterization of Striga genes will require detailed analyses of gene expression patterns. Quantitative real-time PCR is a powerful tool for quantifying gene expression, but correct normalization of expression levels requires identification of control genes that have stable expression across tissues and life stages. Since no S. hermonthica housekeeping genes have been established for this purpose, we evaluated the suitability of six candidate housekeeping genes across key life stages of S. hermonthica from seed conditioning to flower initiation using qRT-PCR and high-throughput cDNA sequencing. Based on gene expression analysis by qRT-PCR and RNA-Seq across heterogeneous Striga life stages, we determined that using the combination of three genes, UBQ1, PP2A and TUB1 provides the best normalization for gene expression throughout the parasitic life cycle. The housekeeping genes characterized here provide robust standards that will facilitate powerful descriptions of parasite gene expression patterns.

A thermal time model for predicting parasitism of Orobanche cumana in irrigated sunflower—Field validation

A major part of its life cycle, the root parasitic weed broomrapes (Orobanche and Phelipanche species) grow underground. Predicting their developmental stages underground is a challenge that can be met by using a modeling approach, as reported for Phelipanche aegyptiaca and Orobanche cumana, in sunflower grown under temperature-controlled conditions. In those studies the relation between parasitism dynamics and thermal time was described by Sigmoid functions. To confirm these relations and calibrate the parameters under field conditions, a 5-year field study was conducted in nine fields in Israel, in various locations under different climatic conditions. As a result, a robust thermal time model that allows precisely estimating the parasitism dynamics was developed. Development of O. cumana in the subsurface was detected, monitored and recorded non-destructively in situ by the use of a minirhizotron. The data from field experiments, conducted in 2006, 2007, 2008 and 2009, were used for model calibration. For model validation, five field experiments were independently performed in 2009 and 2010. To relate the number of attachments and shoot emergence to thermal time, the following equations were tested: Sigmoid, Gompertz (both three-parameter) and Weibull (four parameters with lag phase). In the calibration studies, the number of attachments was best fitted to thermal time using the Weibull equation, which further resulted in very good fit in the validation studies for number of attachments (RMSE=0.066; R2=0.99) and O. cumana shoot emergence (RMSE=0.0096; R2=0.97). The Weibull equation thus adds a biological dimension (lag phase) to the model that the other equations lack, as the lag phase allows estimating the precise timing of parasite attachment to host roots and shoot emergence. This information is crucial in any attempt to develop control strategies for these parasitic weeds.

Global changes in gene expression during compatible and incompatible interactions of cowpea (Vigna unguiculata L.) with the root parasitic angiosperm Striga gesnerioides.

BackgroundCowpea, Vigna unguiculata L. Walp., is one of the most important food and forage legumes in the semi-arid tropics. While most domesticated forms of cowpea are susceptible to the root parasitic weed Striga gesnerioides, several cultivars have been identified that show race-specific resistance. Cowpea cultivar B301 contains the RSG3-301 gene for resistance to S. gesnerioides race SG3, but is susceptible to race SG4z. When challenged by SG3, roots of cultivar B301 develop a strong resistance response characterized by a hypersensitive reaction and cell death at the site of parasite attachment. In contrast, no visible response occurs in B301 roots parasitized by SG4z.ResultsGene expression in the roots of the cowpea cultivar B301 during compatible (susceptible) and incompatible (resistant) interactions with S. gesnerioides races SG4z and SG3, respectively, were investigated at the early (6 days post-inoculation (dpi)) and late (13 dpi) stages of the resistance response using a Nimblegen custom design cowpea microarray. A total of 111 genes were differentially expressed in B301 roots at 6 dpi; this number increased to 2102 genes at 13 dpi. At 13 dpi, a total of 1944 genes were differentially expressed during compatible (susceptible) interactions of B301 with SG4z. Genes and pathways involved in signal transduction, programmed cell death and apoptosis, and defense response to biotic and abiotic stress were differentially expressed in the early resistance response; at the later time point, enrichment was primarily for defense-related gene expression, and genes encoding components of lignifications and secondary wall formation. In compatible interactions (B301 – SG4z), multiple defense pathways were repressed, including those involved in lignin biosynthesis and secondary cell wall modifications, while cellular transport processes for nitrogen and sulfur were increased.ConclusionDistinct changes in global gene expression profiles occur in host roots following successful and unsuccessful attempted parasitism by Striga. Induction of specific defense related genes and pathways defines components of a unique resistance mechanism. Some genes and pathways up-regulated in the host resistance response to SG3 are repressed in the susceptible interactions, suggesting that the parasite is targeting specific components of the host’s defense. These results add to our understanding of plant-parasite interactions and the evolution of resistance to parasitic weeds.

Use of ex vitro composite plants to study the interaction of cowpea (Vigna unguiculata L.) with the root parasitic angiosperm Striga gesnerioides.

BackgroundCowpea (Vigna unguiculata L.) is an important grain and forage legume grown throughout sub-Saharan Africa primarily by subsistence farmers on poor, drought prone soils. Genetic improvement of the crop is being actively pursued and numerous functional genomics studies are underway aimed at characterizing gene controlling key agronomic characteristics for disease and pest resistances. Unfortunately, similar to other legumes, efficient plant transformation technology is a rate-limiting step in analysis of gene function in cowpea.ResultsHere we describe an optimized protocol for the rapid generation of transformed hairy roots on ex vitro composite plants of cowpea using Agrobacterium rhizogenes. We further demonstrate the applicability of cowpea composite plants to study gene expression involved in the resistance response of the plant roots to attack by the root parasitic weed, Striga gesnerioides. The utility of the new system and critical parameters of the method are described and discussed herein.ConclusionsCowpea composite plants offer a rapid alternative to methods requiring stable transformation and whole plant regeneration for studying gene expression in resistance or susceptibility responses to parasitic weeds. Their use can likely be readily adapted to look at the effects of both ectopic gene overexpression as well as gene knockdown of root associated defense responses and to the study of a broader range of root associated physiological and aphysiological processes including root growth and differentiation as well as interactions with other root pests, parasites, and symbionts.

A petunia ABC protein controls strigolactone-dependent symbiotic signalling and branching

Strigolactones were originally identified as stimulators of the germination of root-parasitic weeds that pose a serious threat to resource-limited agriculture. They are mostly exuded from roots and function as signalling compounds in the initiation of arbuscular mycorrhizae, which are plant-fungus symbionts with a global effect on carbon and phosphate cycling. Recently, strigolactones were established to be phytohormones that regulate plant shoot architecture by inhibiting the outgrowth of axillary buds. Despite their importance, it is not known how strigolactones are transported. ATP-binding cassette (ABC) transporters, however, are known to have functions in phytohormone translocation. Here we show that the Petunia hybrida ABC transporter PDR1 has a key role in regulating the development of arbuscular mycorrhizae and axillary branches, by functioning as a cellular strigolactone exporter. P. hybrida pdr1 mutants are defective in strigolactone exudation from their roots, resulting in reduced symbiotic interactions. Above ground, pdr1 mutants have an enhanced branching phenotype, which is indicative of impaired strigolactone allocation. Overexpression of Petunia axillaris PDR1 in Arabidopsis thaliana results in increased tolerance to high concentrations of a synthetic strigolactone, consistent with increased export of strigolactones from the roots. PDR1 is the first known component in strigolactone transport, providing new opportunities for investigating and manipulating strigolactone-dependent processes.

Role of the Sucrose Synthase Encoding PrSus1 Gene in the Development of the Parasitic Plant Phelipanche ramosa L. (Pomel)

Phelipanche ramosa L. (Pomel) is a major root-parasitic weed attacking many important crops. Success in controlling this parasite is rare and a better understanding of its unique biology is needed to develop new specific control strategies. In the present study, quantitative polymerase chain reaction experiments showed that sucrose synthase encoding PrSus1 transcripts accumulate at their highest level once the parasite is connected to the host (tomato) vascular system, mainly in the parasite tubercles, which bear numerous adventitious roots. In situ hybridization experiments revealed strong PrSus1 expression in both shoot and root apices, especially in shoot apical meristems and in the vascular tissues of scale leaves and stems, and in the apical meristems and developing xylem in roots. In addition, immunolocalization experiments showed that a sucrose synthase protein co-localized with cell-wall thickening in xylem elements. These findings highlight the role of PrSus1 in the utilization of host-derived sucrose in meristematic areas and in cellulose biosynthesis in differentiating vascular elements. We also demonstrate that PrSus1 is downregulated in response to 2,3,5-triiodobenzoic acid-induced inhibition of polar auxin transport in the host stem, suggesting that PrSus1 activity in xylem maturation is controlled by host-derived auxin.

Strigolactones—intriguing biologically active compounds: perspectives for deciphering their biological role and for proposing practical application

Strigolactones are a class of bioactive natural metabolites produced by plant roots and released into the rhizosphere. They were discovered as signals indispensable for the induction of germination of seeds of root parasitic weeds, but since then, interestingly, many other biological, physiological and ecological roles have been described. This has suddenly provoked huge scientific interest in these compounds within different research fields. In this short perspective the attention is focused mainly on the ongoing and future research aimed at deciphering the biological roles of strigolactones that could positively affect, more or less directly, the management of parasitic weeds.

Agronomic, breeding, and biotechnological approaches to parasitic plant management through manipulation of germination stimulant levels in agricultural soils

A number of plant species have adapted to parasitize other plants, and some parasitic species pose severe constraints to major crops. The role of strigolactones and other metabolites present in host root exudates as germination stimulants for weedy root parasitic weed seeds has been known for the last 40 years. Recently, the ecological and developmental roles of strigolactones have been clarified by the discovery that they are a new class of plant hormone that controls shoot branching and serve as host recognition signals for mycorrhizal fungi. Parasitic plants also recognize these chemicals and use them to coordinate their life cycle with that of their host. Here we review agronomic practices that use parasitic germination stimulant production as a target for manipulation to control parasitic weeds.

Ent-2′-epi-Orobanchol and Its Acetate, As Germination Stimulants for Striga gesnerioides Seeds Isolated from Cowpea and Red Clover

Striga gesnerioides is a root parasitic weed of economic significance to cowpea (Vigna unguiculata) crops in Western Africa. Seeds of the parasite germinate in response to cowpea root exudates. Germination stimulants for the seeds were isolated from the hydroponic culture filtrate of cowpea, and their structures were unambiguously determined as (-)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchol and (+)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchyl acetate, on the basis of mass, CD, and (1)H NMR spectra; optical rotatory power; and chromatographic behavior on HPLC. The alcohol was first isolated and identified from the cowpea root exudates, and the acetate may be the same compound that had been previously isolated from the exudates and designated as alectrol. Identity of the stimulants produced by cowpea to those produced by red clover (Trifolium pratense) was confirmed.