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PubMed2006

Strigol: biogenesis and physiological activity.

The role played by molecules of the strigolactone family in stimulating the germination of seeds of parasitic weeds of the genera Striga, Orobanche and Alectra has never been clearly elucidated. The biogenesis of these unusual terpenoid lactones, originally identified in minute quantities in the root exudates of a small number of host plants and two or three "false hosts", also remains obscure. These lactones, as the chemical signals which initiate the life cycle of Striga, are consequently at the forefront of the Striga research effort. This paper reviews recent key discoveries relating to the biosynthesis and mode of action of strigolactones, and summarises the evidence suggesting that these molecules may be far more widely distributed and have a greater physiological significance than has hitherto been appreciated.

Germination↗

PubMed1998

On becoming a parasite: evaluating the role of wall oxidases in parasitic plant development.

BACKGROUND: The temporal and spatial control of the transition from vegetative to parasitic growth is critical to any parasite, but is essential to the sessile parasitic plants. It has been proposed that this transition in Striga spp. is controlled simply by an exuded oxidase that converts host cell-surface phenols into benzoquinones which act as developmental signals that mediate the transition. An understanding of this mechanism may identify the critical molecular events that made possible the evolution of parasitism in plants. RESULTS: PoxA and PoxB are identified as the only apoplastic phenol oxidases in Striga asiatica seedlings, and the genes encoding them have been cloned and sequenced. These peroxidase enzymes are capable of oxidizing the 60 known inducing phenols into a small set of benzoquinones, and it is these quinones that induce parasitic development. Analysis of the reaction requirements and comparisons to host enzymes, however, lead us to argue that PoxA and PoxB are not necessary for host recognition. CONCLUSIONS: A new model is proposed where constitutive production of an activated oxygen species (in the case of Striga, H2O2) mediates host recognition. This strategy would allow a parasite to exploit abundant host enzymes to produce the diffusible recognition signals by converting a standard host defense into a parasitic offense.

Amino Acid Sequence↗

PubMed2003

Synthesis and bioactivity of labelled germination stimulants for the isolation and identification of the strigolactone receptor.

Strigolactones are highly potent germination stimulants for seeds of the parasitic weeds Striga and Orobanche spp. The induction of seed germination is thought to proceed via a receptor-mediated mechanism. Isolation and purification of the strigolactone receptor by affinity chromatography using immobilized avidin or streptavidin requires a biotin labelled strigolactone analogue. For this purpose biotin has been attached, directly as well as indirectly, via a hydrophilic linker to the amino function of optically active amino-GR24. Using the same amino substituted synthetic stimulant GR24, labelled stimulants have been prepared which may be suitable for the identification of the receptor by means of fluorescence correlation spectroscopy, scanning force microscopy or photoaffinity techniques. Bioassays of the labelled stimulants reveal that the germination activity on seeds of Striga hermonthica is retained. Crystal data for the diastereoisomer (+)-8 are reported.

Avidin↗

PubMed2000

Signaling Organogenesis in Parasitic Angiosperms: Xenognosin Generation, Perception, and Response.

Parasitic strategies within the angiosperms generally succeed by tightly coupling developmental transitions with host recognition signals in a process referred to as xenognosis. Within the Scrophulariaceae, Striga asiatica is among the most studied and best understood parasitic member with respect to the processes of host recognition. Specific xenognosins regulate seed germination, the development of the host attachment organ, the haustorium, and several later stages of host-parasite integration. Here we discuss the signals regulating the development of the haustorium, the critical vegetative/parasitic transition in the life cycle of this obligate parasite. We provide evidence for the localized production of H(2)O(2) at the Striga root tip and suggest how this oxidant is used to exploit host peroxidases and cell wall pectins to generate a simple benzoquinone signal. This benzoquinone xenognosin proves to be both necessary and sufficient for haustorial induction in cultured seedlings. Furthermore, evidence is provided that benzoquinone binding to a redox active site completes a "redox circuit" to mediate signal perception. This redox reaction regulates the time-dependent expression of specific marker genes critical for the development of the mature host attachment organ. These studies extend the emerging series of events necessary for the molecular regulation of organogenesis within the parasitic plants and suggest novel signaling features and molecular mechanisms that may be common across higher plants.

Journal Article↗

PubMed1987

Carbon isotope ratios demonstrate carbon flux from c(4) host to c(3) parasite.

Carbon isotope ratios of mature leaves from the C(3) angiosperm root hemiparasites Striga hermonthica (Del.) Benth (-26.7 per thousand) and S. asiatica (L.) Kuntze (-25.6 per thousand) were more negative than their C(4) host, sorghum (Sorghum bicolor [L.] Moench cv CSH1), (-13.5 per thousand). However, in young photosynthetically incompetent plants of S. hermonthica this difference was reduced to less than 1 per thousand. Differences between the carbon isotope ratios of two C(3)-C(3) associations, S. gesnerioides (Willd.) Vatke-Vigna unguiculata (L.) Walp. and Oryza sativa L.-Rhamphicarpa fistulosa (Hochst.) Benth differed by less than 1 per thousand. Theoretical carbon isotope ratios for mature leaves of S. hermonthica and S. asiatica, calculated from foliar gas exchange measurements, were -31.8 and -32.0 per thousand, respectively. This difference between the measured and theoretical delta(13)C-values of 5 to 6 per thousand suggests that even in mature, photosynthetically active plants, there is substantial input of carbon from the C(4) host. We estimate this to be approximately 28% of the total carbon in S. hermonthica and 35% in S. asiatica. This level of carbon transfer contributes to the host's growth reductions observed in Striga-infected sorghum.

Journal Article↗

PubMed2003

Isoflavanones from the allelopathic aqueous root exudate of Desmodium uncinatum.

Three isoflavanones, 5,7,2',4'-tetrahydroxy-6-(3-methylbut-2-enyl)isoflavanone (1), 4",5"-dihydro-5,2',4'-trihydroxy-5"-isopropenylfurano-(2",3";7,6)-isoflavanone (2) and 4",5"-dihydro-2'-methoxy-5,4'-dihydroxy-5"-isopropenylfurano-(2",3";7,6)-isoflavanone (3) and a previously known isoflavone 5,7,4'-trihydroxyisoflavone [genistein (4)] were isolated and characterised spectroscopically from the root exudate of the legume Desmodium uncinatum (Jacq.) DC. We propose the names uncinanone A, B, and C for compounds 1, 2 and 3, respectively. Isolated fractions containing uncinanone B (2) induced germination of seeds from the parasitic weed Striga hermonthica (Del.) Benth. and fractions containing uncinanone C (3) moderately inhibited radical growth, the first example of a newly identified potential allelopathic mechanism to prevent S. hermonthica parasitism.

Biological Assay↗

PubMed2004

Chemical biology of multi-host/pathogen interactions: chemical perception and metabolic complementation.

The xenognostic mechanisms of two multi-host pathogens, the causative agent of crown gall tumors Agrobacterium tumefaciens and the parasitic plant Striga asiatica, are compared. Both organisms are general plant pathogens and require similar information prior to host commitment. Two mechanistic strategies, chemical perception and metabolic complementation, are used to ensure successful host commitment. The critical reactions at host-parasite contact are proton and electron transfer events. Such strategies may be common among multi-host pathogens.

Agrobacterium tumefaciens↗

PubMed2006

[Seed germination characteristics of parasitic plant and its host recognition mechanisms].

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.

Germination↗

PubMed2005

Impact of phenolic compounds and related enzymes in sorghum varieties for resistance and susceptibility to biotic and abiotic stresses.

Contents of phenolic compounds and related enzymes before and after sorghum grain germination were compared between varieties either resistant or susceptible to biotic (sooty stripe, sorghum midge, leaf anthracnose, striga, and grain molds) and abiotic (lodging, drought resistance, and photoperiod sensitivity) stresses. Independent of grain germination, sorghum varieties resistant to biotic and abiotic stresses had on average higher contents of proanthocyanidins (PAs), 3-deoxyanthocyanidins (3-DAs), and flavan-4-ols than susceptible varieties. Results show that content of 3-DAs is a good marker for sorghum resistance to both biotic and abiotic stresses because it correlates with resistance to all stresses except for photoperiod sensitivity. The second good marker for stress resistance is content of PAs. Total phenolic compounds and the activities of related enzymes are not good markers for stress resistance in sorghum grains.

Adaptation, Physiological↗

PubMed1998

Host-root exudates increase gene expression of asparagine synthetase in the roots of a hemiparasitic plant Triphysaria versicolor (Scrophulariaceae).

Triphysaria is a facultative root parasite in the Scrophulariaceae family. Similar to other related parasites, the development of the parasitic life cycle is initiated by molecular signals released from appropriate host roots. Using a differential display, we isolated cDNAs preferentially abundant in T. versicolor roots exposed to Trifolium repens (white clover) root exudates in vitro. Sequence analysis indicated that one of the differentially expressed cDNAs had significant homology to the nitrogen-assimilating enzyme, asparagine synthetase (AS). T. versicolor AS cDNA clones were isolated and placed into three distinct classes on the basis of nucleotide sequence variations. All three classes encoded identical AS proteins. AS was expressed in both roots and shoots of in-vitro-cultured T. versicolor. Steady-state levels of AS mRNA increased in T. versicolor roots several-fold when seedlings were exposed to exudate obtained from hydroponically grown Arabidopsis thaliana roots. Therefore, AS transcript levels increased in response to exudates from two different hosts (Trifolium and Arabidopsis). The T. versicolor AS message levels increased to a similar magnitude when seedlings were incubated in the dark. Interestingly, AS levels were unaffected by treatment with the Striga haustoria inducer 2,6-dimethoxybenzoquinone. The potential role of AS in root parasitism is discussed.

Amino Acid Sequence↗

PubMed2003

Secondary metabolite signalling in host-parasitic plant interactions.

The parasitic weeds Orobanche and Striga spp. are a serious threat to agriculture in large parts of the world. The lifecycle of the parasitic weeds is closely regulated by the presence of their hosts, and secondary metabolites that are produced by host plants play an important role in this interaction. Model plants, such as Arabidopsis and maize mutant collections, have been increasingly used to study these chemical signals, especially those host-produced stimulants that induce the germination of parasite seeds.

Germination↗

PubMed1999

Degradation and transformation of a potential natural herbicide in three soils.

The methyl ester of fusaric acid (ME) is one of four toxins produced by the fungus Fusarium nygamai, which could be used as a natural herbicide against Striga hermonthica, a parasitic weed of sorghum and corn in a vast zone of West and Central Africa. A laboratory study was performed to measure the degradation of ME in three soil types and under different temperature and soil moisture conditions, so as to ascertain whether a single ME treatment would protect the crops against this weed during the critical phases of growth. The results show that the persistence in all soils and under all incubation conditions is long enough to protect the crops for the first week of growth, excluding the trial at 30 degrees C in the humic soil, where the half-life of 6 days would require more than one treatment. A degradation product of ME (butylpyridine, BP) was identified by gas chromatography/mass spectrometry and its degradation measured. The sum of ME and BP residues for the first 7 days was almost 100% of the applied compound in all soils and incubation conditions, thus indicating that BP may be the only transformation product of ME at this stage.

Africa↗

PubMed1990

Loss of photosynthetic and chlororespiratory genes from the plastid genome of a parasitic flowering plant.

Photosynthesis is the hallmark of plant life and is the only plastid metabolic process known to be controlled by plastid genes. The complete loss of photosynthetic ability, however, has occurred on several independent occasions in parasitic flowering plants. Some of these plants are known to lack chlorophyll and certain photosynthetic enzymes, but it is not known to what extent changes have occurred in the genes encoding the photosynthetic apparatus or whether the plants even maintain a plastid genome. Here we report that the nonphotosynthetic root parasite Epifagus virginiana has a plastid chromosome only 71 kilobases in size, far smaller than any previously characterized land plant plastid genome. The Epifagus plastid genome has lost most, if not all, of the 30 or more chloroplast genes for photosynthesis and most of a large family of plastid genes, the ndh genes, whose products may be involved in a plastid respiratory chain. The extensive changes in Epifagus plastid gene content must have occurred in a relatively short time (5-50 x 10(6) yr), because Striga asiatica, a related photosynthetic parasite, has a typical complement of chloroplast genes for photosynthesis and chlororespiration. The plastid genome of Epifagus has retained transcribed ribosomal RNA and ribosomal protein genes, suggesting that it expresses one or more gene products for plastid functions not related to photosynthesis.

Blotting, Northern↗

PubMed2005

Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi.

Arbuscular mycorrhizal (AM) fungi form mutualistic, symbiotic associations with the roots of more than 80% of land plants. The fungi are incapable of completing their life cycle in the absence of a host root. Their spores can germinate and grow in the absence of a host, but their hyphal growth is very limited. Little is known about the molecular mechanisms that govern signalling and recognition between AM fungi and their host plants. In one of the first stages of host recognition, the hyphae of AM fungi show extensive branching in the vicinity of host roots before formation of the appressorium, the structure used to penetrate the plant root. Host roots are known to release signalling molecules that trigger hyphal branching, but these branching factors have not been isolated. Here we have isolated a branching factor from the root exudates of Lotus japonicus and used spectroscopic analysis and chemical synthesis to identify it as a strigolactone, 5-deoxy-strigol. Strigolactones are a group of sesquiterpene lactones, previously isolated as seed-germination stimulants for the parasitic weeds Striga and Orobanche. The natural strigolactones 5-deoxy-strigol, sorgolactone and strigol, and a synthetic analogue, GR24, induced extensive hyphal branching in germinating spores of the AM fungus Gigaspora margarita at very low concentrations.

Chromatography, High Pressure Liquid↗

PubMed1996

A mechanism for inducing plant development: the genesis of a specific inhibitor.

Parasitic strategies are widely distributed in the plant kingdom and frequently involve coupling parasite organogenesis with cues from the host. In Striga asiatica, for example, the cues that initiate the development of the host attachment organ, the haustorium, originate in the host and trigger the transition from vegetative to parasitic mode in the root meristem. This system therefore offers a unique opportunity to study the signals and mechanisms that control plant cell morphogenesis. Here we establish that the biological activity of structural analogs of the natural inducer displays a marked dependence on redox potential and suggest the existence of a semiquinone intermediate. Building on chemistry that exploits the energetics of such an intermediate, cyclopropyl-p-benzoquinone (CPBQ) is shown to be a specific inhibitor of haustorial development. These data are consistent with a model where haustorial development is initiated by the completion of a redox circuit.

Journal Article↗

PubMed2006

Strigolactones: chemical signals for fungal symbionts and parasitic weeds in plant roots.

AIMS: Arbuscular mycorrhizae are formed between >80 % of land plants and arbuscular mycorrhizal (AM) fungi. This Botanical Briefing highlights the chemical identification of strigolactones as a host-recognition signal for AM fungi, and their role in the establishment of arbuscular mycorrhizae as well as in the seed germination of parasitic weeds. SCOPE: Hyphal branching has long been described as the first morphological event in host recognition by AM fungi during the pre-infection stages. Host roots release signalling molecules called 'branching factors' that induce extensive hyphal branching in AM fungi. Strigolactones exuded from host roots have recently been identified as an inducer of hyphal branching in AM fungi. Strigolactones are a group of sesquiterpenes, previously isolated as seed germination stimulants for the parasitic weeds Striga and Orobanche. Parasitic weeds might find their potential hosts by detecting strigolactones, which are released from plant roots upon phosphate deficiency in communication with AM fungi. In addition to acting as a signalling molecule, strigolactones might stimulate the production of fungal symbiotic signals called 'Myc factors' in AM fungi. CONCLUSIONS: Isolation and identification of plant symbiotic signals open up new ways for studying the molecular basis of plant-AM-fungus interactions. This discovery provides a clear answer to a long-standing question in parasitic plant biology: what is the natural role for germination stimulants? It could also provide a new strategy for the management and control of beneficial fungal symbionts and of devastating parasitic weeds in agriculture and natural ecosystems.

Germination↗

PubMed2002

Research needs to improve agricultural productivity and food quality, with emphasis on biotechnology.

Research into agricultural productivity, especially for crops in the developing world, should include resistance to plant viruses, fungi and the parasitic weed Striga. It must also include research into the development of resistance to Bacillus thuringiensis (Bt) toxin-expressing crops. Drought- and heat-tolerant crops, and those that can combat the problems of soil deficiencies, are required, and vaccine production in plants should be a high priority. Research into food quality should include the equivalent of "golden rice" in maize, the enhancement of the production of phytosterols and improved qualities of vegetable oils.

Agriculture↗

PubMed1998

The effect of relaxed functional constraints on the photosynthetic gene rbcL in photosynthetic and nonphotosynthetic parasitic plants.

The photosynthetic gene rbcL has been lost or dramatically altered in some lineages of nonphotosynthetic parasitic plants, but the dynamics of these events following loss of photosynthesis and whether rbcL has sustained functionally significant changes in photosynthetic parasitic plants are unknown. To assess the changes to rbcL associated with the loss of functional constraints for photosynthesis, nucleotide sequences from nonparasitic and parasitic plants of Scrophulariales were used for phylogeny reconstruction and character analysis. Plants in this group display a broad range of parasitic abilities, from photosynthetic ("hemiparasites") to nonphotosynthetic ("holoparasites"). With the exception of Conopholis (Orobanchaceae), the rbcL locus is present in all parasitic plants of Scrophulariales examined. Several holoparasitic genera included in this study, including Boschniakia, Epifagus, Orobanche, and Hyobanche, have rbcL pseudogenes. However, the holoparasites Alectra orobanchoides, Harveya capensis, Harveya purpurea, Lathraea clandestina, Orobanche corymbosa, O. fasciculata, and Striga gesnerioides have intact open reading frames (ORFs) for the rbcL gene. Phylogenetic hypotheses based on rbcL are largely in agreement with those based on sequences of the nonphotosynthetic genes rps2 and matK and show a single origin of parasitism, and loss of photosynthesis and pseudogene formation have been independently derived several times in Scrophulariales. The mutations in rbcL in nonparasitic and hemiparasitic plants would result in largely conservative amino acid substitutions, supporting the hypothesis that functional proteins can experience only a limited range of changes, even in minimally photosynthetic plants. In contrast, ORFs in some holoparasites had many previously unobserved missense substitutions at functionally important amino acid residues, suggesting that rbcL genes in these plants have evolved under relaxed or altered functional constraints.

Amino Acid Substitution↗