Effect of intercropping maize and soybeans on Striga hermonthica parasitism and yield of maize

Striga hermonthica a major biotic constraint to cereal production can be controlled by trap crops. Soybean cultivars vary in ability to stimulate suicidal germination of the weed. An experiment was conducted to select soybean (Glycine max) varieties with the ability to stimulate germination of S. hermonthica seeds. Experiments were conducted with strigol Nijmegen 1® (GR 24), a synthetic stimulant, as a check. In the pot and field experiments, maize (variety WH507) was intercropped with soybeans. Variation occurred among soybean varieties in inducing germination of S. hermonthica. The relative germination induction by soybean varieties ranged from 8% to 66% compared to 70% for synthetic stimulant check. Varieties TGX1448-2E, Tgm 1576, TGX1876-4E and Tgm 1039 had the highest relative germination. Soybean varieties TGX 1831-32E, Tgm944, Tgm 1419 and Namsoy4m had high stimulation but low attachment. Intercropping maize with soybeans in the field led to a low S. hermonthica count and high maize yield.     

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.     

Variation of Seed-coat Ornamentation in Striga hermonthica (Scrophulariaceae)

The surface features of the seed of the parasitic floweringplant Striga hermonthica were examined with the scanning electronmicroscope. The details of ornamentation were constant on seedsfrom one plant but varied within and between populations andare probably due to out-breeding. The variation was not relatedto geographical origin or to host-preference. Testa patternssimilar to those reported for other Striga species were foundin the samples. Striga hermonthica, Scrophulariaceae: Rhinanthoideae, hemi-parasite, testa, seed-coat, scanning electron microscope, genetic variation     

Germination strategy of Striga hermonthica involves regulation of ethylene biosynthesis

Ethylene involvement in germination of Striga hermonthica (Del.) Benth., an important root parasitic weed on poaceous crops, was investigated at the physiological and molecular levels. Seeds, conditioned at 30°C for 14 days, were treated with ethylene, ethephon or 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene consistently induced low germination. Ethephon and ACC effectively stimulated germination at concentrations of 0.01 and 1 m M , respectively. In contrast to ethylene, both ethephon and ACC acted in a concentration-dependent manner. Germination induced by the synthetic strigolactone GR24 was inhibited by aminoethoxyvinylglycine (AVG) and 1-methylcyclopropene. ACC reversed the inhibition caused by AVG. When seeds were treated with GR24 in sealed vials, ethylene concentration in headspace gas increased prior to the onset of germination. Total RNA extracted from germinating seeds 12 h after GR24 treatment was used for PCR-based amplification of cDNA fragments encoding the ACC synthase- and oxidase-active site domains. Two distinct cDNA fragments encoding ACC synthase ( SHACS1 and SHACS2 ) and one encoding ACC oxidase ( SHACO1 ) were cloned and sequenced. Southern analysis suggested that each of the cloned genes was present as a single copy in the genome of S. hermonthica . Northern analyses showed that SHACS1 exhibited a temporal change in expression peaking at 10 h after GR24 treatment, which coincided with a steady increase in ethylene concentration. SHACS2 was expressed at a low level with a similar trend. SHACO1 exhibited a temporal change in expression peaking at 15 days during conditioning, when seed response to GR24 was maximal. In summary, expression of ACC synthase and ACC oxidase genes was found to be responsive to a germination stimulant and to conditioning, respectively. The implications of these findings with respect to germination of S. hermonthica under field conditions are discussed.     

Glutamine Synthetase Isoenzymes of Striga hermonthica and other Angiosperm Root Parasites

The occurrence of multiple forms of glutamine synthetase inStriga hermonthica and other angiosperm root parasites was investigated.The facultative chlorophyllous parasite Melampyrum arvense exhibitedtwo isoenzymes in leaf tissue, the cytosolic component (GS₁)comprised less than 30% of total glutamine synthetase. In contrastGS₁ was the major component (<70%) in photosynthetic tissueof Striga hermonthica and S. gesnerioides. Only a single isoenzyme(GS₁) was detectable in the achlorophyllous root parasites Orobancheand Lathraea and in non-photosynthetic tissue of S. gesnerioides.The kinetic and physical properties of GS₁ and GS₂ of theseangiosperm parasites were similar to those of the isoenzymesin other non-parasitic angiosperms. Key words: Glutamine synthetase, Angiosperms, Root parasites     

A study on the susceptibility of rice cultivars to Striga hermonthica and mapping of Striga tolerance quantitative trait loci in rice

Striga is a parasitic weed attacking mainly maize, sorghum, millet and cowpea. Studying the interaction between rice and Striga is valuable since rice is a model monocot. In this paper, the susceptibility of different rice cultivars to S. hermonthica was tested and quantitative trait loci (QTL) for Striga tolerance mapped on the Bala x Azucena F(6) population. Seven rice cultivars were grown with and without S. hermonthica for 14 wk. For the mapping experiment, 115 recombinant inbred lines (RILs), along with Azucena and Bala, were grown with and without Striga for 11 wk. Rice cultivars tested had different susceptibilities to Striga, ranging from highly susceptible to completely resistant. Azucena and Bala differed in the speed of Striga emergence and the impact on host growth. A genomic region between positions 139 and 166 cM on chromosome 1 was identified containing strong QTL (LOD = 4.9-15.7) for all traits measured. This indicates that genes for Striga tolerance exist in rice germplasm and the mapped QTL can be further studied to promote understanding of the nature of resistance/tolerance and breeding for Striga-resistant crop plants.     

A novel form of resistance in rice to the angiosperm parasite Striga hermonthica

The root hemiparasitic weed Striga hermonthica is a serious constraint to grain production of economically important cereals in sub-Saharan Africa. Breeding for parasite resistance in cereals is widely recognized as the most sustainable form of long-term control; however, advances have been limited owing to a lack of cereal germplasm demonstrating postattachment resistance to Striga. Here, we identify a cultivar of rice (Nipponbare) that exhibits strong postattachment resistance to S. hermonthica; the parasite penetrates the host root cortex but does not form parasite-host xylem-xylem connections. In order to identify the genomic regions contributing to this resistance, a mapping population of backcross inbred lines between the resistant (Nipponbare) and susceptible (Kasalath) parents were evaluated for resistance to S. hermonthica. Composite interval mapping located seven putative quantitative trait loci (QTL) explaining 31% of the overall phenotypic variance; a second, independent, screen confirmed four of these QTL. Relative to the parental lines, allelic substitutions at these QTL altered the phenotype by at least 0.5 of a phenotypic standard deviation. Thus, they should be regarded as major genes and are likely to be useful in breeding programmes to enhance host resistance.     

Effect of cultural methods on Striga (Striga hermonthica (Del.)) Benth management and yield of cereals in the Savanna Zone of Nigeria: a review

Data were obtained from the research done in the Guinea Savanna (Zaria 11°11′N; 070 38′E) and Sudan Savanna (Maiduguri ?11°51′N; 13°15′E) regions of Nigeria, respectively on different cultural methods of Striga control and management. In the Guinea Savanna, trials on the effects of nitrogen on the response of resistant and susceptible upland rice varieties to Striga hermonthica infestation and the effect of resistant and susceptible varieties of maize and crop rotation on Striga infestation was carried out, while the effect of inter-cropping trap crop (Bambaranut) with resistant sorghum varieties on S. hermonthica was studied in the Sudan Savanna Zone of Nigeria. In the Guinea Savanna, it was observed that a combination of upland rice variety, Faro 40 and an application of 90 kg N/ha in the wet season and WAB 56-50 upland rice variety and 120 kg N/ha in the dry season, respectively reduced Striga infestation and produced maximum grain yield. Also, the growing of resistant variety of maize (Across 97ITZ comp. I-W) after 1 or 2 years' rotation with cowpea or soybean was observed not only to be effective in Striga control, but resulted in higher grain yield of maize. In the Sudan Savanna, the use of resistant varieties of sorghum, 1CSV1002 and 1CSV1007 intercropped with bambaranut significantly reduced Striga infestation, but the grain yield of the resistant varieties was low. From these studies, Faro 40 with 90 kg N/ha application rate and WAB56–50 with 120 kg N/ha were suitable for the management of Striga and for higher grain yield of upland rice in both wet and dry seasons, respectively, while Across 97ITZ comp. I-W, resistant maize variety and 1 or 2 years rotation with cowpea or soybean were also the best for the management of S. hermonthica and for higher maize yield in the Guinea Savanna zone. Further research needs to be carried out in the Sudan Savanna to select a high yielding resistant variety of sorghum which when intercropped with bambaranut will not only control Striga infestation but will also give high grain yield.     

Striga hermonthica decreases photosynthesis in Zea mays through effects on leaf cell structure

Maize seedlings were grown in pots either with or without preconditionedseeds of the parasitic weed, Striga hermonthica. After between4 and 8 weeks, net photosynthesis in the leaves of maize plantsinfected with Striga decreased compared to leaves of uninfectedcontrol plants. The activities of four enzymes of photosyntheticmetabolism were, however, little affected by infection. A pulse-chaseexperiment using ¹⁴CO₂ showed that C₄ acids were the main earlyproducts of assimilation even when the rate of photosynthesiswas much decreased by infection, but more radio-activity appearedin glycine and serine than in leaves of healthy maize plants.Leaves of infected maize required longer to reach a steady rateof photosynthesis upon enclosure in a leaf chamber than leavesof uninfected plants after similar treatment. Electron microscopy of transverse sections of the leaves ofinfected maize indicated that the cell walls in the bundle sheathand vascular tissue were less robust than in leaves of healthyplants. The results suggest that infection with Striga causesan increase in the permeability of cell walls in the bundlesheath, leakage of CO₂ from the bundle sheath cells and decreasedeffectiveness of C₄ photosynthesis in host leaves. Key words: Zea mays, Striga hermonthica, photosynthesis, photorespiration, enzyme activity     

Microorganisms of Striga hermonthica in Northern Ghana with Potential as Biocontrol Agents

A survey of microorganisms of Striga hermonthica Del. Benth., a root parasite of graminaceous crops, was conducted in northern Ghana in 1992. Thirteen fungal species were isolated from infected S. hermonthica plants. Fusarium spp. were the most prevalent, and were isolated from more than 90% of the S. hermonthica samples collected. Other fungi isolated were Alternaria alternata, Bipolaris specifera, Cladosporium oxysporum, Curvularia falax, Macrophomina phaseolina, Nodulisporium gregarium, Phoma sorghina and Sclerotium rolfsii.     

Biochemical composition and photosynthetic activity of chloroplasts from Striga hermonthica and Striga aspera, root parasites of field-grown cereals

Striga hermonthica (Del.) Benth. and Striga aspera (Willd.) Benth. are root parasites causing dramatic losses in field-grown cereals in semi-and tropics. Being achlorophyllous and obligate parasites during their underground development, upon emergence from the soil, they become green leafy plants; but, despite the presence of chlorophyll, they exhibit only low rates of photosynthesis. To investigate if deficiency in the photosynthetic apparatus could account for the low rates of photosynthesis, chloroplasts were isolated from S. hermonthica parasitizing sorghum [Sorghum bicolor (L.) Moench cv. Tiemarifing] and from S. aspera parasitizing maize ( Zea mays L. cv. Tiémantié ) grown under greenhouse conditions or in their natural surroundings. Isolated chloroplasts exhibited the characteristics of chloroplasts from C₃ plants but displayed low levels of chlorophyll and polar lipids, while the protein content was less reduced. Main changes occurred in polar lipid composition, with decreases in monogalactosyldiacylglycerol and digalactosyldiacylglycerol. All polar lipids showed a decrease in the degree of unsaturation of fatly acids. All these changes were particularly pronounced in chloroplasts from plants that experienced heavy drought in Africa. On a chlorophyll basis, chloroplasts did not display a dramatic decrease in photosynthetic activities. These results are discussed in relation to parasitism and drought adaptation.     

Variation for host range within and among populations of the parasitic plant Striga hermonthica

Striga hermonthica is an angiosperm parasite that causes substantial damage to a wide variety of cereal crop species, and to the livelihoods of subsistence farmers in sub-Saharan Africa. The broad host range of this parasite makes it a fascinating model for the study of host-parasite interactions, and suggests that effective long-term control strategies for the parasite will require an understanding of the potential for host range adaptation in parasite populations. We used a controlled experiment to test the extent to which the success or failure of S. hermonthica parasites to develop on a particular host cultivar (host resistance/compatibility) depends upon the identity of interacting host genotypes and parasite populations. We also tested the hypothesis that there is a genetic component to host range within individual S. hermonthica populations, using three rice cultivars with known, contrasting abilities to resist infection. The developmental success of S. hermonthica parasites growing on different rice-host cultivars (genotypes) depended significantly on a parasite population by host-genotype interaction. Genetic analysis using amplified fragment length polymorphism (AFLP) markers revealed that a small subset of AFLP markers showed 'outlier' genetic differentiation among sub-populations of S. hermonthica attached to different host cultivars. We suggest that, this indicates a genetic component to host range within populations of S. hermonthica, and that a detailed understanding of the genomic loci involved will be crucial in understanding host-parasite specificity and in breeding crop cultivars with broad spectrum resistance to S. hermonthica.     

Photosynthesis and stomatal characteristics of Striga hermonthica in relation to its parasitic habit

Photosynthesis and stomatal characteristics of the angiosperm parasite Striga her-monthica (Del.) Benth. have been compared with those of Antirrhinum majus L. cv. Suttons Yellow Rust Resistant, a related, non-parasitic species. The concentration of photosynthetic pigments in S. hermonthica leaves was less than 40% of those of A. majus leaves. Light saturated CO₂ assimilation rate of S. hermonthica was less than 40% that of A. majus on a chlorophyll basis and under 20% than of A. majus on a leaf area basis.
Stomata of Striga showed only partial closure in darkness, remained open in water stressed leaves and showed little response to exogenously applied abscisic acid. Stomatal conductance and transpiration were considerably higher in Striga compared with Antirrhinum . The high transpiration and low photosynthetic rates of S. hermonthica resulted in a low water use efficiency. The water relations of Striga leaves, while seemingly inappropriate for growth in drought prone environments, do appear to maximise nutrient and water acquisition from the host and as such may be an adaptation to the parasitic habit.     

Effect of Jasmonates and Related Compounds on Seed Germination of Orobanche minor Smith and Striga hermonthica (Del.) Benth

Jasmonates and related compounds were found to elicit the seed germination of the important root parasites, clover broomrape (Orobanche minor Smith) and witchweed [Striga hermonthica (Del.) Benth]. The stimulation of seed germination by the esters was more effective than by the corresponding free acids, and methyl jasmonate (MJA) was the most active stimulant among the compounds tested.     

Infection time and density influence the response of sorghum to the parasitic angiosperm Striga hermonthica

Two cultivars of sorghum (CSH-1 and Ochuti) were grown in the presence and absence of the root hemiparasite Striga hermonthica in uniform conditions in the field in Kenya, Africa. S. hermonthica had a marked influence on growth and photosynthesis of 'CSH-1'; however, 'Ochuti' showed a less severe response to infection and tolerance of the parasite. The variation in genotype response might be partly explained by later attachment of the parasite and a lower level of infection. Laboratory studies were used to determine the importance of both variables in determining host response to infection. Early infection by S. hermonthica had a more negative effect on the host than late infection. The level of parasite biomass supported by the host also influenced host productivity but the relationship was nonlinear. Low degrees of parasite infection had a proportionately much greater effect on host grain weight than at greater parasite loading. Early infection of 'Ochuti' in laboratory conditions resulted in lower stem dry weight than in uninfected plants but not in smaller total plant biomass or lower rates of photosynthesis. In conclusion, the time of parasite attachment affected host performance and might explain much of the variation in host sensitivity both within and between studies. The level of parasite infection affected host performance to a lesser extent. In addition, late attachment and low levels of infection might have implications for control management strategies.     

Characterisation of Simple In Vitro Cultures of Striga hermonthica Suitable for Metabolic Studies

Abstract: The aim of this study is to develop simplified models for standardised screenings of xenobiotics, especially targeted against mannitol production, to control the harmful parasite, S. hermonthica. Chlorophyllous protoplasts and calli were produced from the young leaves of the parasite. Best yield from protoplast isolation was obtained when leaf segments were incubated at 30 °C, in the light, under shaking in an enzyme cocktail containing 2 % cellulase Onozuka R10, 0.1 % Pectolyase and sorbitol 1 M as the osmoticum. Oxygen exchange measurements, as well as labelling experiments with ¹⁴C-bicarbonate, emphasised a significant decrease in photosynthetic capacity of protoplasts, mannitol remaining, however, a major primary product of photosynthesis. Initiation of cell cultures was unsuccessful and instability of protoplasts prevents their standardised utilisation for herbicide screening. In contrast, globular calli produced first on MS medium containing 0.5 mg L^-1 NAA, 2.5 mg L^-1 BAP and 2 % sucrose were stable for two years, after monthly subculturing on fresh medium. Sucrose substitution by mannose in the medium did not change kinetic growth and stability. Potential autotrophy was lost for calli by increasing exogenous sugar level. Biochemical analyses and labelling experiments with ¹⁴C-bicarbonate or ¹⁴C-sucrose or -mannose showed that carbon partitioning is modified in calli, in comparison with young leaves or protoplasts, sucrose or mannose accumulation being favoured in sucrose- or mannose-fed calli, respectively. However, carbon flow towards mannitol was more marked in calli growing on high mannose medium. Stability and preservation of an active mannitol biosynthetic pathway allows planning of xenobiotic assays with calli as a simplified model for Striga hermonthica.     

Growth and carbon allocation in Pennisetum typhoides infected with the parasitic angiosperm Striga hermonthica

Abstract Growth and gas exchange measurements are used in conjunction with a carbon balance model to describe the millet (Pennisetum typhoides)–witchweed (Striga hermonthica) host—parasite association. Striga hermonthica reduces the growth of millet by 28% and radically alters the architecture of infected plants. Whilst grain yield and stem dry weight are reduced (by 80 and 53%, respectively), leaf and root growth are stimulated (by 41 and 86%, respectively). The difference in production between infected and uninfected millet plants can be accounted for by two processes: first, export of carbon to the parasite (accounting for 16% of the dry weight not gained); and second, parasite-induced reductions in host photosynthesis (accounting for 84% of the dry weight not gained). Striga hermonthica is dependent on carbon exported from the host, since the plant has low rates of photosynthesis coupled with high rates of respiration. The carbon balance model suggests that in mature S. hermonthica plants parasitic on millet, 85% of the carbon is host-derived. Carbon fluxes are also estimated using δ¹³C measurements, since S. hermonthica is a C₃ plant parasitizing a C₄ host. In conjunction with gas exchange measurements, these suggest that in root, stem and leaf of S. hermonthica, 87, 70 and 49% of carbon is hostderived, respectively.     

Effect of Growth Medium and Method of Application of Fusarium oxysporum on Infestation of Sorghum by Striga hermonthica in Burkina Faso

A 2-year (1997–1998) study was conducted at Kouaré, Burkina Faso, to investigate effect of growth medium and application method of Fusarium oxysporum isolate 4-3-B to control Striga hermonthica. In 1997, growth medium and isolate 4-3-B delayed striga emergence by 9 days. Isolate 4-3-B reduced emerged striga number by 33% compared to treatments without Fusarium. In 1998, striga emergence was delayed by 13 days by growth medium and Fusarium. The fungus reduced the number of emerged striga by 27% and, as a result, sorghum straw and grain yields were significantly improved by 10 and 38%, respectively.