Induction of seed germination in <Emphasis Type="Italic">Orobanche</Emphasis> spp. by extracts of traditional Chinese medicinal herbs

The co-evolution of Orobanche spp. and their hosts within the same environment has resulted in a high degree of adaptation and effective parasitism whereby the host releases parasite germination stimulants, which are likely to be unstable in the soil. Our objective was to investigate whether extracts from non-host plants, specifically, Chinese medicinal plants, could stimulate germination of Orobanche spp. Samples of 606 Chinese medicinal herb species were extracted with deionized water and methanol. The extracts were used to induce germination of three Orobanche species; Orobanche minor, Orobanche cumana, and Orobanche aegyptiaca. O. minor exhibited a wide range of germination responses to the various herbal extracts. O. cumana and O. aegyptiaca exhibited an intermediate germination response to the herbal extracts. O. minor, which has a narrow host spectrum, showed higher germination rates in response to different herbal extracts compared with those of O. cumana and O. aegyptiaca, which have a broader host spectrum. Methanolic extracts of many Chinese herbal species effectively stimulated seed germination among the Orobanche spp., even though they were not the typical hosts. The effective herbs represent interesting examples of potential trap crops. Different countries can also screen extracts from indigenous herbaceous plants for their ability to induce germination of Orobanche spp. seeds. The use of such species as trap plants could diminish the global soil seed bank of Orobanche.     

Recognition of root exudates by seeds of broomrape (Orobanche and Phelipanche) species

Background and Aims

The long co-existence of broomrapes and their hosts within the same environment has culminated in a strong adaptation and effective parasitism. As a first step of specialization in the parasitic process, seed receptors of parasitic plant species vary in their ability to recognize compounds released by their hosts. This work aims to investigate potential patterns for the reception requirements needed to activate germination within Orobanche and Phelipanche species.

Methods

Induction of the germination of seeds of nine Orobanche and Pheliphanche species by root exudates of 41 plant species was studied and subjected to biplot multivariate analysis.

Key Results

A high level of specialization in root exudate recognition was found in Orobanche densiflora, O. gracilis and O. hederae, which germinated almost exclusively in contact with root exudates from the plants they infect in nature. At the opposite extreme, Phelipanche aegyptiaca, P. ramosa and O. minor were highly generalist, germinating when in contact with the root exudates of most plant species. Orobanche crenata, O. cumana and O. foetida showed intermediate behaviour.

Conclusions

A universal germination stimulant for all broomrape species has not being identified to date. The synthetic stimulant GR24 is active against most of the weedy broomrape species, but fails with the non-weedy species tested in this study and with the very recent weedy species O. foetida. In addition, germination behaviour of broomrape species depends on the crop plant tested. Weedy broomrapes with a broad host spectrum respond better to the different exudates released by a wide range of crops and wild species than do non-weedy broomrapes, which have a narrow host spectrum and are more restricted to their host range. Root exudates of many plant species were active in stimulating germination of seeds of Orobanche and Phelipanche species for which they are not described as hosts, representing interesting examples of potential trap crops.Key words: Xenognosis, broomrape, root exudate, germination, Orobanche, Phelipanche     

The potential of Ulocladium botrytis for biological control of Orobanche spp.

A strain of Ulocladium botrytis isolated from diseased Orobanche crenata shoots caused disease on the parasitic weed in pathogenicity tests. The potential of the fungus to be developed as a mycoherbicide for Orobanche spp. was further investigated. Although the fungus significantly decreased O. crenata germination in vitro by 80%, it did not generally lead to a decreased number of O. crenata shoots or tubercles in inoculated root chambers or pots. However, the number of diseased or dead tubercles and underground shoots was significantly increased compared to the noninoculated treatments. Postemergence inoculation of O. crenata shoots with a conidial suspension resulted in the death of almost all inoculated plants 14 days after application under greenhouse conditions. In preliminary host-range studies, the pathogen caused disease on Orobanche cumana on sunflower whereas on Orobanche aegyptiaca shoots parasitizing tomato only minimal disease symptoms could be detected after postemergence inoculation. Based on the results of our investigations, we conclude that Ulocladium botrytis has only a limited potential to be used as a biocontrol agent against Orobanche spp.     

Resistance to broomrape (Orobanche crenata) in faba bean (Vicia faba): cell wall changes associated with prehaustorial defensive mechanisms

Broomrapes (Orobanche spp.) are parasitic angiosperms, which attach to the roots of the hosts to take water and nutrients from them. No complete control measures are available to date, but breeding for resistance remains as one of the most feasible and environmentally friendly methods. However, the mechanisms governing the interaction between these parasites and the host are not yet well understood. We studied the cellular changes associated with the resistance to Orobanche crenata in faba bean as mechanisms involved or responsible for resistance. Two cultivars of faba bean, resistant and susceptible to O. crenata infection, were used. The evolution of the infection and the changes in the cell and tissue organisation and wall components of the host cells were followed and evaluated in both genotypes. Samples of compatible and incompatible interactions were fixed and sectioned, and specific cytochemical methods for different cell components were applied, results being analysed under light and epifluorescence microscopy. A higher proportion of O. crenata seedlings unable to penetrate the root was found on the resistant genotype. Reinforcement of cell walls by callose deposition hampers parasite penetration through the cortex. Lignification of endodermal cells prevents further penetration of the parasite into the central cylinder.     

A quantitative model for loss of primary dormancy and induction of secondary dormancy in imbibed seeds of Orobanche spp

Seeds of three species of Orobanche were conditioned (e.e. stored fully imbibed in darkness) for periods up to 210 d in order to model relief of primary dormancy and induction of secondary dormancy. The data were consistent with the hypothesis that periods for loss and induction of formancy and loss of viability are normally distributed in populations of imbibed sees and that these three processes are independent. There was a positive, linear relationship between the rate of loss of primary dormancy and temperature from 10-30C in O. aegyptiaca and O. cernua and 10-25C in O. crenata. In all three species, the rate of induction of secondary dormancy was highest at 10°C and decreased with increase of temperature up to about 20°C, above which there was little further change in the rate with temperature. The resulting model explained over 90% of the variation in germination after conditioning in both O. aegyptiaca and O. cernua. In O. crenata, however, this model was only satisfactory at 10 and 15°C. At higher temperatures, dormancy was relatively stable for periods of conditioning from 70 to about 154 d. Possible explanations for this are discussed. Applications of these models for estimating the time required to reduce Orobanche infestations in the field are also briefly discussed.Key words: Orobanche spp., broomrape, seed dormancy, temperature, dormancy model      

A Model of the Effects of a Wide Range of Constant and Alternating Temperatures on Seed Germination of FourOrobanche Species

Seeds of the obligate parasitic plants, Orobanche spp., wereconditioned in water or GA₃for 2 or 12 weeks and then stimulatedto germinate by the synthetic stimulant GR24. Temperature treatmentsduring the germination tests comprised 169 different constantand alternating temperature regimes on a two-dimensional gradientplate. Optimum temperatures for germination of seeds of O. aegyptiacaand O. crenata were 18–21 °C and 18 °C, respectively.However, longer conditioning periods slightly lowered the optimain both species, and the maximum germination percentage wasalso reduced due to an induction of secondary dormancy. At agiven mean temperature, more seeds germinated at constant thanat alternating temperatures. Results were analysed in termsof characteristics of alternating temperatures that appearedto control germination, i.e. mean temperature, maximum temperature,amplitude (difference between daily maximum and minimum temperatures)and thermoperiod (the time spent at the maximum temperatureeach day). Final germination was modelled on the basis of therebeing two prerequisites for germination: a minimum mean temperaturewhich must be exceeded and a maximum temperature above whichthe seed will not germinate. These two requirements were assumedto be independent and to be normally distributed in the seedpopulation so that final germination could be described by amultiplicative probability model. Because of the response tomaximum temperature, inhibitory effects were more evident atalternating temperatures. Amplitude and thermoperiod influencedthis effect of maximum temperature. The implications of thedetrimental effect of alternating temperatures for germinationofOrobanche spp. in the field are discussed. Copyright 1999Annals of Botany Company Orobanche aegyptiaca, O. crenata, O. cernua, O. minor, broomrape, seed germination, temperature, germination model, secondary dormancy.     

Use of random amplified polymorphic DNA (RAPD) markers in the study of the parasitic weed Orobanche

Despite the tremendous economic impact of broomrapes (Orobanche spp.) on agriculture in many countries little is known of the pattern of genetic variation within this group of parasitic weeds. The present paper describes the use of RAPD markers for the study of five Orobanche species in agricultural fields in Israel. Pronounced genetic differentiation was found between the species, and RAPD markers were raised for the identification of each of them. Southern-hybridization patterns of RAPD products of the various species were used to confirm the interpretation. The same markers were valid both for broomrapes collected in agricultural fields and for those collected in natural habitats. The validity of the markers found for O. cumana and O. crenata was confirmed on plants of the same species that were collected in Spain. Parsimony analysis of 86 RAPD characters produced a tree that clearly distinguishes between the five studied Orobanche species, separates the two Orobanche species belonging to sect. Trionychon from those belonging to sect. Osproleon, and supports the separation of O. cumana from O. cernua and of O. aegyptiaca from O. ramosa.     

Characterization of the LTR retrotransposon repertoire of a plant clade of six diploid and one tetraploid species

Comparisons of closely related species are needed to understand the fine‐scale dynamics of retrotransposon evolution in flowering plants. Towards this goal, we classified the long terminal repeat (LTR) retrotransposons from six diploid and one tetraploid species of Orobanchaceae. The study species are the autotrophic, non‐parasitic Lindenbergia philippensis (as an out‐group) and six closely related holoparasitic species of Orobanche [O. crenata, O. cumana, O. gracilis (tetraploid) and O. pancicii] and Phelipanche (P. lavandulacea and P. ramosa). All major plant LTR retrotransposon clades could be identified, and appear to be inherited from a common ancestor. Species of Orobanche, but not Phelipanche, are enriched in Ty3/Gypsy retrotransposons due to a diversification of elements, especially chromoviruses. This is particularly striking in O. gracilis, where tetraploidization seems to have contributed to the Ty3/Gypsy enrichment and led to the emergence of seven large species‐specific families of chromoviruses. The preferential insertion of chromoviruses in heterochromatin via their chromodomains might have favored their diversification and enrichment. Our phylogenetic analyses of LTR retrotransposons from Orobanchaceae also revealed that the Bianca clade of Ty1/Copia and the SMART‐related elements are much more widely distributed among angiosperms than previously known.     

Recent advances in the biocontrol of Orobanche (broomrape) species

Parasitic broomrapes (Orobanche spp.) are majoruncontrolled weeds in the Mediterranean regions of Europe and the NearEast causing major losses to vegetable, grain legume, and sunflowercrops. Selective herbicides alone cannot provide persistent, season-longcontrol of these parasites, and much methyl bromide is used for theircontrol, where affordable. Thus they are excellent targets forbiocontrol. The recent progress by the COST 816 Orobancheworking group in this area is reviewed herein. Natural infestation bythe fly Phytomyza orobanchia of seed capsules of Orobanchecrenata parasitising faba bean halved Orobanche seedproduction while inundative releases of adults reduced it to 5%of viable seeds. The fungi Fusarium arthrosporioides E4a andF. oxysporum E1d, as well as strains of bacteria were isolatedfrom diseased, juvenile, Orobanche flower stalks. They arepathogenic to O. aegyptiaca, O. crenata and O. ramosaon most vegetable crops. A F. oxysporum f. sp.orthoceras was specifically pathogenic to O. cumana onsunflowers. All were used in various experiments with a modicum ofsuccess. Methods were developed to formulate isolated mycelia, whichcould eventually allow the use of transgenic hypervirulent pathogens inasporogenic (deletion) mutants (as a failsafe against spread).Mycotoxins were also isolated from different Fusarium and otherfungal species that kill Orobanche, and are being consideredfor direct use, or to augment other strategies. All threeFusarium spp. used have been transformed with gusand/or gfp genes allowing tracing their movement in theenvironment, and opening the way to future transformations tohypervirulence.     

Maternal tissue is involved in stimulant reception by seeds of the parasitic plant Orobanche

Background and Aims

A fundamental element in the evolution of obligate root-parasitic angiosperms is their ability to germinate only in response to chemical stimulation by roots, to ensure contact with a nearby nourishing host. The aim of this study was to explore inheritance of the unique germination control in this group of plants.

Methods

Analysis was made of the segregation of spontaneous (non-induced) germination that appeared in hybrid progenies derived from crosses between Orobanche cernua and O. cumana, which, like all other Orobanche species, are totally dependent on chemical stimulation for the onset of germination, and show negligible spontaneous germination in their natural seed populations.

Key Results and Conclusions

F₁ and F₂ seeds did not germinate in the absence of chemical stimulation, but significant spontaneous germination was found in some F₃ seed families. This indicates that the prevention of non-induced germination in Orobanche seeds, i.e. dependence on an external chemical stimulation for seed germination, is genetically controlled, that this genetic control is expressed in a seed tissue with maternal origin (presumably the perisperm that originates from the nucellus) and that genetic variation for this trait exists in Orobanche species. Similar segregation results were obtained in reciprocal crosses, suggesting that stimulated germination is controlled by nuclear genes.     

新疆列当的种类、分布及其防治技术研究进展

列当是一种根寄生种子植物,是我国进境植物检疫对象,同时也是我国的内检对象。新疆是我国列当发生面积最大、危害最重和种类最多的地区,对新疆的主要经济作物,如加工番茄、向日葵、甜瓜等已造成严重危害。在新疆文献记载的列当种类有18种,常见的有埃及列当、大麻列当、向日葵列当和弯管列当,并已广泛分布于新疆各县、市、团场。列当的防治技术主要包括检疫、田间管理、轮作等,但目前的防治效果均不理想。本文通过对新疆列当的相关文献搜集、整理分析,总结了目前新疆列当的种类、分布情况,以及综合防治技术,为今后列当的研究和防治提供参考。     

The carbohydrate and water balance of beans (Vicia faba) attacked by broomrape (Orobanche crenata)

Orobanche crenata parasitizing beans maintained a slightly higher osmotic pressure than the bean roots, largely because of the higher concentration of sugars in the broomrape tissues. The sugar was withdrawn from the bean mainly as sucrose, which was hydrolysed to glucose and fructose by the Orobanche. These sugars were then rapidly translocated to the developing flower spike. As well as maintaining a high osmotic pressure this hydrolysis ensured a sucrose concentration gradient between host and parasite. In the field, bean plants showed wilt symptoms at about the time that the Orobanche flower spikes emerged. It was found that the higher the level of infection the lower was the water content of the host. This fall in water content was not due to increased water loss by the bean shoots and it seemed unlikely that it was due to water removal by the parasite. It was concluded that the death of the bean was due to desiccation brought about largely by the reduced ability of the carbohydrate-starved roots to extract water from the soil.     

Production of Strigolactones by Arabidopsis thaliana responsible for Orobanche aegyptiaca seed germination

The germination stimulants produced by Arabidopsis thaliana, a host of root parasitic plants Orobanche spp. but not of arbuscular mycorrhizal (AM) fungi were examined. Root exudates from the hydroponically grown A. thaliana plants were subjected to reverse phase high performance liquid chromatography (HPLC) and retention times of germination stimulants inducing O. aegyptiaca seed germination were compared with those of strigolactone standards. In addition, the root exudates were analyzed by using HPLC linked with tandem mass spectrometry (LC/MS/MS). A. thaliana was found to exude at least three different germination stimulants of which one was identified as orobanchol. This is the first report of strigolactone production by a non-mycotrophic plant. These results together with recent knowledge imply that strigolactones have other unrevealed functions in plant growth and development.     

Die Chromosomenzahlen der in Mitteleuropa vorkommenden Arten vonOrobanche sect.Orobanche

The chromosome numbers of five species ofOrobanche sect.Orobanche (O. alsatica, O. laserpitii-sileris, O. loricata, O. salviae, O. teucrii) are reported for the first time and previous counts could be verified in ten other species. Now the chromosome numbers of all species of sect.Orobanche occurring in Central Europe are known: they are diploid (2n = 38) with the exception ofO. gracilis (tetra- and hexaploid, aneusomatic).

     

Characterization of Strigolactones, Germination Stimulants for the Root Parasitic Plants Striga and Orobanche, Produced by Maize, Millet and Sorghum

The germination stimulants for root parasitic plants Striga and Orobanche produced by sorghum (Sorghum bicolor (L.) Moench), maize (Zea mays L.), and pearl millet (Pennisetum typhoideum Rich.) were examined. Characterization of strigolactones in the root exudates from the plants grown hydroponically was conducted by comparing retention times of germination stimulants on reverse phase high performance liquid chromatography (HPLC) with those of synthetic standards, and by using HPLC linked with tandem mass spectrometry (LC/MS/MS). All the plants tested, except for a sorghum cultivar Swarna, were found to exude two major stimulants, 5-deoxy-strigol, which is known as a branching factor for arbuscular mycorrhizal (AM) fungi, and an isomer of strigol, tentatively named sorghumol. Swarna was found to exude 5-deoxy-strigol and strigol. These results imply that 5-deoxy-strigol is one of major germination stimulants of gramineous plants and that major stimulants may differ even among cultivars within the same species.     

Gap dynamics in climaxFagus crenata forests

Gap characteristics and gap regeneration were studied in several climaxFagus crenata forests in Japan. 278 gaps were observed. Gaps covered 12% of the total land area of 20.05 ha. Gap density was 13.9 gaps per ha and, mean gap size was 92.0 m². Smaller gaps were much more frequent than larger ones. Gaps larger than 400 m² were rare. Most gaps were created by the death of single trees. Canopy trees died more often standing or with broken trunks than by uprooting, although uprooted trees were relatively abundant in the site with poor soil drainage and in the site on upper slope. Differences of gap regeneration behaviour were recognized among tree species.F. crenata regenerates in gaps from saplings recruited before gap creation and can replace not only its own gaps but also gaps of other species. Most species other thanF. crenata andMagnolia obovata could not regenerate in their own gaps. More successful regeneration ofF. crenata may occur in gaps smaller than 200 m², althought it regenerated in a wide range of gap size. However, increased relative density ofF. crenata in the canopy layer seems to prevent its successful regeneration. Gap regeneration of other species did not clearly depend on a species-specific gap size.     

Germination of resting spores in Synchytrium species parasitic on cucurbitaceae

Germination of resting spores is described in 2 Synchytrium species viz. Synchytrium lagenariae Mhatre and Mundkur and S. trichosanthidis Mhatre and Mundkur, parasitic on cucurbitaceous hosts. The resting spores of both species behave as prosorus in germination giving rise to an attached superficial sorus of sporangia. Several anomalies observed in germination are briefly described. Efficacy of methods inducing germination is discussed relative to the field conditions.     

Recolonizing native wildlife facilitates exotic plant invasion into Singapore's rain forests

Halting biological invasions and rewilding extirpated native fauna are conservation interventions to bolster biodiversity, species interactions, and ecosystems. These actions are often considered separately and the potential for reintroduced wildlife to facilitate invasive plants has been largely overlooked. Here, we investigated the role of Singapore's recolonizing native wild pigs (Sus scrofa) in facilitating an invasive weed Miconia crenata into tropical rainforests, which are normally highly resistant to invasion. We conducted line-transect surveys in 11 Singaporean rain forests and used generalized linear mixed models to consider the contribution of pigs' soil disturbances, human forest paths, and other environmental covariates, on the density of M. crenata. We found that M. crenata was more abundant at forest edges and invasion into forest interior was facilitated by pigs, paths, and canopy gaps, but that these effects were all additive, not synergistic (i.e., not multiplicative). These results highlight how modern invasions are driven by multiple disturbances as well as propagule pressure (e.g., urban birds dispersing seeds at forest edges where they establish in pig soil disturbances). Singapore's extensive native forest restoration efforts may have provided plentiful edge and secondary forests that are well suited to pigs and M. crenata, which in turn undermine the aims of fostering later-successional native plant communities. To prevent negative externalities, we suggest that plant restoration and rewilding projects consider the potential role of wildlife in facilitating non-native plants, and couple these actions with preliminary screening of unintended consequences and continued monitoring, as well as limiting human-mediated weed invasion to minimize propagule sources.     

Habitat requirements and host selectivity of Thesium species (Santalaceae)

The aim of this study was to identify the habitat requirements and host specificity of three root parasites in the genus Thesium (T. linophyllon, T. bavarum and T. ebracteatum). We conducted a vegetation survey and tested whether some plant species occur significantly more often in plots with Thesium spp. than expected, thereby representing potential hosts of the parasites. To test if the potential hosts are the species to which Thesium is physically attached, we also conducted an excavating and sowing experiment. We found a positive association with surrounding species for T. linophyllon, but not for the other two species. The number of haustoria of T. linophyllon attached to roots of host species was significantly affected by the number of roots of the surrounding species and by their identity (species, family and plant group). Thesium was, however, attached to 94% of all species occurring in the plots. We suggest that Thesium spp. do not specifically select hosts, but rather occur in microhabitats with specific conditions. This conclusion is also supported by the fact that there were differences in the microhabitats with and without Thesium and that germination of Thesium in the sowing experiment was higher in pots without any host. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 164 , 394–408.     

Biological control of Orobanche spp. with Phytomyza orobanchia, a review

This review summarises theavailable information on the biocontrol agentPhytomyza orobanchia Kalt. (Diptera, Agromyzidae). It gives an overview of a rareexample of weed biocontrol using insects in aninundative approach. A high diversity ofphytophagous insects has been collected onparasitic weeds of the genus Orobanche(Orobanchaceae). For biological control of Orobanche spp., only insects like P. orobanchia whose host range is restricted toOrobanche spp. are of interest. Of the140 Orobanche spp. described in total,P. orobanchia is reported from 21 species. The larvae of P. orobanchia minein Orobanche shoots and capsules. As aconsequence, a natural reduction of Orobancheseed production by 30 to almost 80%has been reported from different countries. Theefficacy of P. orobanchia under naturalconditions is limited by low temperatures, cultural practices and natural enemies. Tostrengthen the natural population and itsimpact, inundative releases of P. orobanchia adults at the beginning of Orobanche emergence have to be undertaken. Different methods for the application of P. orobanchia in biocontrol of Orobanche spp. have been developed in theformer Soviet Union. Releases of 500 to 1000adults/ha resulted in a reduction of up to 96%of the Orobanche seed production.However, due to the 10 to 15 year longevity ofOrobanche seeds, further infestationswill occur in the following cropping seasons.Consequently, releases of P. orobanchiahave to be repeated continuously over severalyears to reduce the infestation to a tolerablelevel.