Striga Seeds Research Articles

Inheritance of Low Production of Striga Germination Stimulant in Sorghum

Host‐plant resistance to the parasitic weed Striga (Striga spp. Lour.) is a manifestation of one or more potential mechanisms. One of the better understood mechanisms of resistance against Striga by sorghum [Sorghum bicolor (L.) Moench] is low production of compounds by the host root that Striga seeds require as stimulants for germination. A recently developed, efficacious laboratory screening technique, the agar gel assay, distinguishes resistance to Striga among sorghum cultivars based on the capacity of their root exudates to stimulate Striga seeds to germinate in a water agar medium. This study was conducted to determine the inheritance of low stimulant production in sorghum by the use of the agar gel assay. F1, F2, and backcross progenies between resistant sorghum line ‘SRN‐39’ and three susceptible lines, ‘Shanqui Red’, ‘P‐954063’, and ‘IS 4225’, were evaluated for stimulant production. Segregation ratios indicated that low stimulant production is inherited as a single, nuclear, recessive gene which is largely additive in action.

Electron Microscopic Analysis of the Penetration Process ofFusarium nygamai,a Hyperparasite ofStriga hermonthica

Abstract In a previous study we showed that the facultative fungal parasite Fusarium nygamai was able to reduce the germination of seeds of the root-parasitic weed Striga hermonthica and kill more than 70% of the germinated Striga by attacking the parasite's germ tube. As a result, Striga emergence was reduced by about 90%. We are now providing electron microscopic evidence of the infection process. The free, mature conidia of the fungus was able to infect S. hermonthica. The mycelium penetrated the seed coat and destroyed the endosperm. The fungus was also detected in the weed's germ tube. As a soilborne pathogen, F. nygamai could be very effective in controlling the parasitic weed since Striga seeds are attacked underground. A further advantage is that the application of the fungus as a bioherbicide could be done to any crop under crop rotation.

Estimation of the seedbank of Striga spp. (Scrophulariaceae) in Malian fields and the implications for a model of biocontrol of Striga hermonthica

Summary Striga seeds were extracted from soils collecled in Mali and the viahility of these seeds was estimated. Striga seeds were found in 45 of 46 samples taken from 23 fields. Siriga hermonthica (Del.) Benth. growmg on the host crop millet, was present at all 46 sites sampled. The a size uf the Striga seedbaiik measured lo a depth of 15 cm was estimated to be 38 800 m ‐2 of surface area (geometnc mean 11 500). The average number of S. hermonthica plants observed per millei hill was 8.83 (geometric mean 3.89) or 13.98 m‐2 (gcometrie mean 5.69 m‐2). Higher crop hill densities tended to result in higher densities of emerged S. hermonthica per hill. The relationship between seeds m‐2 and S. her‐monhica plants m‐2 was fitied to a reetungular hyperbola and used to reassess an existing model of S. herniouhica control.

Relative Roles of Wind, Crop Seeds, and Cattle in Dispersal ofStrigaspp.

Parasitic flowering plants of the genus Striga cause extensive damage to cereal and legume crops in Africa, but factors affecting seed dispersal have not been well understood. Petrolatum-coated microscope slides placed at regular intervals from Striga hermonthica plants and suspended at 1-, 2-, and 3-m heights from trees within and around S. hermanthica-infested fields indicated that distribution of seeds by wind was not extensive. The maximum horizontal distance that seeds were caught was 12 m and the maximum vertical distance was 2 m. Samples of local market supplies of cowpea, maize, millet, and sorghum from six areas of Nigeria over 2 yr contained an average of 20.9, 32.4, 24.2, and 27.3 Striga seeds, respectively

Enhancement of ethylene biosynthesis and germination by cytokinins and 1‐aminocyclopropane‐l‐carboxylic acid in Striga asiatica seeds

Germination of witchweed (Striga asiatica [L.] Kuntze), an important parasitic weed on several poaceous crops, is stimulated by several synthetic and natural compounds. We investigated the role of ethylene biosynthesis and action in cytokinin‐induced germination. Conditioned Striga seeds treated with distilled water, 1‐aminocyclopro‐pane‐1‐carboxylic acid (ACC) or the cytokinins thidiazuron (TDZ), trans zeatin (TZ), benzyladenine (BA) and kinetin (KIN) produced little ethylene. Treatments with cytokinin‐ACC combinations enhanced ethylene production. The relative order of activity of the cytokinins in elicitation of the phytohormone was TDZ > TZ > BA > KIN. Germination in response to distilled water and ACC treatments was negligible. Induction of germination by cytokinins varied from low (0%) to moderate (52%). Seeds treated with cytokinin‐ACC combinations displayed high rates of germination. The observed germination was positively correlated (γ= 0. 8 and 0. 9) with ethylene production. Germination was reduced by silver thiosulphate (STS) and CoCl2, inhibitors of ethylene action and ACC oxidase, respectively. Aminoethoxyvi‐nylglycine (AVG), an ACC‐synthase inhibitor, reduced TDZ‐induced Striga germination. However, the inhibitory effect of AVG was overcome by addition of ACC. The results are consistent with a model in which Striga germination and embryo growth are limited by low capacity of the seeds to oxidize ACC. The cytokinins promote ACC conversion into ethylene and consequent Striga germination by enhancing ACC oxidase activity and/or synthesis.

Fusarium nygamai, a potential bioherbicide for Striga hermonthica control in sorghum

Abstract Glasshouse trials were performed to investigate the control of the parasitic weed Striga hermonthica by Fusarium nygamai and the performance of the host plant sorghum (Sorghum bicolor) using different inoculum substrates and inoculum amounts of the fungus. Optimal constant and alternating temperatures for the growth of the fungus were 25°C and 30 20° C , respectively. Striga incidence was decreased up to 100% when the fungus was incorporated into the soil preplanting. Emerged Striga plants at different stages of growth up to the flowering stage were killed by the fungus when the fungus was applied postemergent. In root-chamber trials none of the Striga seeds germinated when 10 ml inoculum suspension of 8 × 106 spores/ml of F. nygamai was applied on seeds of the parasitic weed sprinkled on the surface of filter paper. F. nygamai has potential as a bioherbicide for Striga control. Further studies regarding its performance under field conditions and its safety to the environment and humans should be assessed.

Tentative molecular mechanism for germination stimulation of Striga and Orobanche seeds by strigol and its synthetic analogs

Analysis of the bioactivity of strigol analogues revealed that the CD part of these molecules is primarily responsible for germination stimulation of seeds of parasitic weeds. Preparation and bioassays of a reduced analogue of GR24, in which the enol ether carbon double bond has been replaced by a carbon single bond, showed that this double bond is essential for stimulatory activity. A molecular mechanism for germination stimulation is proposed which can account for the bioactivity of all strigol analogues. This mechanism involves addition of a nucleophilic species in a Michael fashion, followed by elimination of ring D []

Synthesis and biological evaluation of A‐ring analogs of the natural germination stimulant strigol

AbstractAn A‐ring‐derived analog of the natural germination stimulant, Strigol (1), has been prepared from citral in an unambiguous manner. This analog, 2, a γ‐hydroxy aldehyde for which a high stimulant activity was claimed in the literature, has been re‐evaluated as a germination stimulant for seeds of parasitic weeds and was found to be inactive. It was also shown that analog 2 is rather stable in aqueous solution, in contrast to other reports.In addition, some related A‐ and AB‐ring‐derived analogs have been prepared and biologically evaluated. The γ‐hydroxy acid 12 was obtained in one step from a mixture of α‐ and β‐cyclocitral in 58% yield. The dihydroxy analog 14 was prepared by reduction of the γ‐hydroxy ester 13. The AB‐ring analog 15 has been synthesized in five steps from ester 13 following published procedures. All analogs have been evaluated using Striga and Orobanche seeds; none of the compounds induced germination. It was concluded that the actiphore of strigol does not reside in the AB part of the molecule.

Role of Ethylene in the Germination of the Hemiparasite Striga hermonthica

Seed germination of the hemiparasitic angiosperm Striga hermonthica (Del.) Benth is elicited by compounds present in the root exudates of the host plant. Although a variety of compounds can substitute for the host-derived signal, the mechanism through which these act is unknown. In the present study, an inhibitor of ethylene biosynthesis, aminoethoxyvinyl glycine, was found to inhibit germination. Addition of an intermediate in ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid, was found to override this inhibition and to act as a substitute for the host-derived signal. 2,5-Norbornadiene, an inhibitor of ethylene action, was also found to inhibit germination. Ethylene is rapidly produced by Striga seeds after treatment with host root exudates. These results are consistent with a model for Striga seed germination in which host-derived signals and other compounds act by eliciting the synthesis of ethylene and in which ethylene itself initiates the biochemical changes leading to germination.

An in‐vitro growth system for studying the parasitism of cowpea (Vigna unguiculata) by Striga gesnerioides

Summary:An in‐vitro growth system for investigating the behaviour of S. gesnerioides on cowpea has been developed. The roots of young cowpea plants were spread over glass fibre filter paper in a shallow plastic tray, and a known number of germinated Striga seeds were placed on the cowpea root surface. This allowed the infection process of the parasite to be quantified. Good access to the host‐parasite association enabled the entire infection process to be easily monitored and tissue removed for cytological studies. Observations revealed for the first time that contact of S. gesnerioides radicles with host roots stimulated the development of radicular hairs and swelling of the tip of these radicles. Furthermore, penetration of cowpea roots by Striga was shown to be localized and to cause very little damage to the host roots. Swollen parasite tubercles developed on the surface of the roots only after the host stele had been penetrated by invading Striga cells. This growth system is suitable for in‐vestigating the mechanisms of host resistance. and also for rapidly screening cowpea germplasm for new sources of resistance to the Parasite. Resistant cowpea cultivars offer the best potential solution for control of this Parasite.

A time-course study of early establishment stages of parasitic angiosperm Striga asiatica on susceptible sorghum roots

Summary A time-course study of the early establishment stages of Striga asiatica was carried out on a susceptible sorghum hybrid, CSH 1, using polyethylene bags and whole-root clearing and staining techniques. Preconditioned Striga seeds were applied to different aged segments of primary root but the results did not differ for these different aged segments. Most of the Strzga seeds (63%) germinated within 24 h of inoculation on the host roots. The attachment of Striga radicles to host root was rapid and it occurred between 36 and 48 h after inoculation. Only 9% of the germinated Striga seeds attached to the host root but 659; of these attachments successfully penetrated through the epidermis and entered the host cortex within 72 h. Penetration through the cortical cells was difficult; only 17% of attachments were able to reach the endodermis. Penetration took from 12 to 43 h after the first appearance of haustorial cells in the cortex; a total of 84 to 120 h after inoculation on the host root. Penetration through the endodermis and establishment on the host stele was relatively easier, as most of the haustoria reaching the endodermis were able to establish on the host stele. But this is a slow proces. taking a minimum of 24 h, and a maximum of 60 h after first contact of haustorial cells with the endodermis. The minimum time taken from inoculation of ungerminated Striga seed on the host root to establishment is about 108 h. The results are discussed in relation to published reports on other parasitic species such as Agalinis purpurea.

The effect of soil temperature, moisture and nitrogen on Striga asiatica (L.) Kuntze seed germination, viability and emergence on sorghum (Sorghum bicolor L. Moench) roots under field conditions

Experiments were conducted in a Striga-sick field to study the effect of soil temperature, moisture and nitrogen on Striga parasitism on sorghum. Striga seeds contained in nylon bags and buried at 2 cm in the soil, were exposed to different temperature and moisture treatments. Clear polythene, hay mulch and bare soil treatments were used to vary soil temperature. These treatments gave mean maximum temperatures of 60°, 48° and 37°C, respectively at 2-cm soil depth. Irrigation levels of 0, 30 and 60 mm were applied to change soil moisture. Striga seed germination, viability and emergence were studied. After 34 days of preconditioning, the exhumed Striga seeds from polythene-covered plots (solarized plots) did not germinate or retain viability when these seeds were exposed to sorghum root exudate. However, seeds similarly buried under hay mulch or bare soil, with mean maximum soil temperatures of 48° and 37°C, respectively, had similar germination and viability percentages. Of these 75% germinated and 85% of them were viable, regardless of the temperature treatment. Although seeds stored at high temperature and humidity (solarization) were killed, more Striga plants emerged under the polythene treatment compared to hay mulch and bare soil treatments. The observed Striga plants in the polythene mulch treatment were, therefore, assumed to have come from deeper layers where solarization was not effective. Irrigation treatments did not have significant effects on Striga seed germination and viability, but a slightly higher number of plants emerged at 60-mm irrigation level than at 30-mm and 0-mm. Striga emergence, on the other hand, was directly related to the rate of N application. Nitrogen rates of 0, 25, 50 and 100 kg ha−1 resulted in the emergence of 11, 34, 38 and 40 Striga plants per plot, respectively. Despite the high infestation at high N levels, sorghum plants did not show a loss of vigor. Nitrogen application, therefore, does not reduce Striga incidence, but seems to neutralize the harmful effects of Striga without reducing the extent of parasitism.

Control of Germination in Striga asiatica: Chemistry of Spatial Definition

Striga asiatica (Scrophulariaceae), a member of a heterogeneous group known as the parasitic plants, is totally dependent on host root attachment for survival. In agar, Striga seeds germinated in high percentages within 5 millimeters of a sorghum (Sorghum bicolor (L.) Moench) host root surface, and no germination was observed at distances greater than 1 centimeter. This spatially restricted germination may be explained by the chemistry of a single compound, 2-hydroxy-5-methoxy-3-[8'Z, 11'Z)-8', 11', 14' -pentadecatriene]-p-hydroquinone, structure 1, which is exuded by sorghum roots. The presence of the compound was chemically imaged with pigments such as methylene blue. The use of methylene blue suggested that structure 1 was exuded along the entire surface of the root for long periods. This exudation and the inherent instability of structure 1 together establish an apparent steady state concentration gradient of the germination stimulant around the sorghum root. The Striga seed must be exposed to micromolar concentrations of 1 for >/=5 hours before high germination percentages were observed. Such a requirement for a long term exposure to a steady state concentration of an inherently labile, exuded compound would provide an extra degree of resolution to signal detection and host commitment in Striga parasitism.

Sorghum root length density and the potential for avoiding striga parasitism

Striga hermonthica is a serious root parasite of sorghum in the semiarid tropics. Successful parasitism is dependent on interactions of Striga seeds and host roots. Several sorghum cultivars have been found which resist parasitism. The basis of resistance is not well known. One possible method for reducing the chances of parasitism is by restricted host root development. This research was conducted to evaluate this hypothesis in sorghum known to possess resistance to parasitism by Striga. Root length density of 21-day-old pot-grown resistant cultivars, Framida, N-13, IS-9830, Tetron and P-967083, were compared to that of the susceptible check, Dabar, using the line intercept method of measuring root length. There was no significant difference between resistant cultivars and the susceptible cultivar Dabar. The RLD of resistant P-967083 however was significantly less than Framida, another resistant cultivar. The RLD of Dabar was compared to that of Framida and P-967083 in USA and Niger field trials. Root length density was determined on soil cores taken at flowering with a Giddings Soil Sampler. Each core was divided into 10-cm fractions for estimating RLD by the line intercept method. In the USA Dabar had significantly greater RLD than the two resistant cultivars in the upper 10-cm portion of the soil profile, but only significantly greater than P-967083 in the 10–20-cm portion. Significant differences in RLD between susceptible and resistant cultivars were not found at depths between 20–60 cm. In field trials in Niger, RLD of Dabar was significantly greater than either resistant cultivar in the (0 to 30 cm) portion of the soil core. These results suggest that part of the Striga resistance of P-967083 and perhaps Framida may be a result of avoiding interactions between parasitic seeds and host roots.

Comparison of the seed germination effects of synthetic analogs of strigol, gibberellic acid, cytokinins, and other plant growth regulators

Four synthetic multiring analogs of strigol, a naturally occurring sesquiterpene lactone that promotes germination of dormant seeds ofStriga (witchweed), were found to stimulate germination of dormantLactuca (lettuce) seeds. The effects on light-sensitive and light-insensitive lettuce seeds were concentration-dependent and exceeded those produced by equimolar (0.1 mM) solutions of gibberellic acid. Strigol and epistrigol promoted lettuce seed germination to a lesser degree than did the synthetic analogs. The strigol group compounds had minimal effect on the germination of monocot seeds. The results indicate that the synthetic strigol analogs have plant growth regulatory activity in dormant seeds of genera beyondStriga in which germination stimulation by strigol and the synthetic analogs was first demonstrated.

Surface features of Striga seeds (Scrophulariaceæ)

Surface features of Striga seeds (Scrophulariaceæ)

Differential and comparative screening of cowpea varieties to Striga gesnerioides (Willd.) Vatke for race specific identification in Burkina Faso

Significant efforts have been made to develop cowpea (Vigna unguiculata (L.) Walp.) varieties resistant to Striga gesnerioides in Burkina Faso. Despite these efforts, the resistant genotypes developed still express differential responses to Striga gesnerioides in different zones of the country. This suggests existence of intraspecific variability within the parasite. The objective of this study was to assess the intraspecific variability of cowpea genotypes to Striga gesnerioides infection in Burkina Faso. Ten cowpea varieties were screened over two consecutive years, under artificial infestation with 30 ecotypes of Striga seeds at Kamboinsé research station, in a screenhouse in Burkina Faso. Cowpea varieties used included Moussa local, Komsaré and KVx404-8-1, which are susceptible to all Striga ecotypes; and varieties B301, IT93K-693-2 and IT82D-849, which are free from all Striga ecotypes infestation. Cowpea varieties Tiligré, 524B, local Gorom and Niizwè had specific reactions depending on the ecotypes. The study highlights the existence of varietal specificities according to the geographical origin of Striga gesnerioides seeds. The structuring of the intraspecific diversity showed five biotypes, of which three were clearly identified as SG1, SG5 and SG Kp races; and two biotypes could not be identified. Although this study did not allow for a clear determination of the racial affiliation of the two new biotypes, it offers the possibility of developing new strategies to control Striga by focusing on the selection of resistant varieties based on regional specificities of Striga races in each agricultural zone.

A Germination Inhibitor in Striga Seeds

A Germination Inhibitor in Striga Seeds

The effects of D-xyloketose and certain root exudates in extension growth.

This investigation has been designed, to examine the possibility that the substance which is exuded from the roots of Sorghum seedlings, and which stimulates the germination of the seeds of Striga hermonthica , may promote extension growth in the roots of a variety of species. The technique used involves observations on fragments 1·5 mm. long excised from the extend­ing zones of roots of peas and maize. The effects on the growth of these segments of certain solutions of the natural exudate and of D-xyloketose, which also stimulates the germination of Striga seeds in very low concentrations, have been examined. It has been found that the exudates and the sugar promote extension growth in the root by stimulating longitudinal and lateral extension. Moreover, it has been shown that as with stimulation of the germination of Striga seeds, the activity of exudate and sugar solutions is destroyed by heat and it decreases during storage of the solutions. The conclusion is drawn that the substance which is released from the roots of Sorghum seedlings is a generalized root extension stimulator which promotes the germination of Striga seeds. The substance may be a carbohydrate or a simple deriva­tive and may bear a close relationship to D-xyloketose.

Effect of the Striga Germination Stimulant on the Respiration of Striga Seeds

THE demonstration by Vallance1 of the effect of the stimulant solution on the respiration of Striga seeds is a matter of some importance. Vallance, however, suggests that this effect is independent of that described by Brown, Robinson and Johnson2, who showed that the Striga stimulant may promote extension growth.