Parasitic Angiosperms Research Articles

The impact of Rhinanthus spp. on sward productivity and composition: Implications for the restoration of species-rich grasslands

The impact of parasitic angiosperms of the genus Rhinanthus on grassland productivity and floristic composition was monitored at five sites in Britain and N. Italy. A combination of non-intervention and perturbation studies were used to detect differences in the yield and community composition of swards in the presence and absence of Rhinanthus spp. Productivity was found to be consistently lower in areas where Rhinanthus was present. The degree of suppression ranged between 8 and 73%, and was found to be positively correlated with site productivity. Community composition differed significantly between parasite infected and uninfected regions of the sward, the most notable differences being a decline in the proportion of grasses, and a concomitant increase in the proportion of dicotyledons, in the presence of the parasite. The results are discussed in relation to the use of Rhinanthus spp. as management tools for the restoration of species-rich grasslands. The practicalities associated with this option are considered.

Actin genes with unusual organization in the parasitic angiosperm Striga asiatica L. (Kuntze)

In order to better understand the regulation of cellular differentiation during haustorial development in parasitic angiosperms, we have begun to examine the structure and expression characteristics of genes encoding various components of the plant cytoskeleton in Striga asiatica L. (Kuntze). We describe here the cloning and characterization of three actin genes from Striga with significant similarity at the nucleotide level and encoding proteins having greater than 98% identity. However, the three genes (designated SAAc-1, SAAc-2 and SAAc-3) differ from each other in their organization and SAAc-2 contains an unusual exon-intron arrangement relative to genes encoding actins described in other higher plants. The significance of these observations concerning the evolutionary origins and potential roles of Striga actin genes is discussed.

Host-Formed Tyloses in Vessels of the Mistletoe Phoradendron (Viscaceae)

The xylem-tapping mistletoe, Phoradendron, forms vessel-to-vessel connections with its dicotyledonous host, Juglans. Contact vessels of the host may become embolized as evidenced by the presence of tyloses. Tyloses may also occur in parasite vessels contiguous with embolized host vessels. Differences between parasite and host in nuclear size, shape and intensity of staining make it possible to determine the origin of the tyloses present in contact vessels. Host-formed tyloses occluded not only emboli zed host vessels, but also those of the parasite with which they were continuous. Some parasite vessels may terminate in vessel members lacking perforations at the juncture with host vessels; these vessels lacked tyloses. In parasitic angiosperms in general tyloses within haustoria may be formed by either host or parasite. Several functional roles are reported for tyloses including; absorption of nutrients, defence against invasion of the parasite, and the occlusion of embolized vessels, as exemplified in the present study.

Screening for resistance to broomrape (Orobanche cernua) in cultivated sunflower

AbstractRacial evolution of sunflower broomrape (Orobanche cernua) has been very rapid in Spain during recent years, in which resistance has been overcome several times and there has been an important increase in areas infested with this parasitic angiosperm. In order to find resistance to a highly virulent population of sunflower broomrape that could be used directly in breeding programmes, three different sets of cultivated plant material composed of 429 entries were tested by artificial inoculation. All evaluated inbred lines from Moden, Canada, were fully susceptible. Out of the 240 P.I. accessions tested, only 10 segregated for resistance to broomrape, the rest being susceptible. From the 160 USDA breeding lines evaluated, 5% were resistant and 19% segregated for resistance to O. cernua. These lines traced back mainly to crosses of RHA 274 and RHA 801 with Russian, Turkish and Romanian hybrids. The origin of P.I. accessions that segregated for resistance were primarily derived from the former USSR and from Romania.

Flange-type parenchyma cells: occurrence and structure in the haustorium of the dwarf mistletoeKorthalsella (Viscaceae)

Flange cells are an unusual type of parenchyma cells with an open reticulate pattern of secondary wall thickenings. The cells superficially resemble tracheary elements but are otherwise fundamentally different. Flange cells were found in haustorial sucker tissue of the dwarf mistletoeKorthalsella. Such cells were previously unknown for a mistletoe, or other parasitic angiosperm. Flange cells are confined to the xylem of the sucker and occur as either diffuse aggregates amongst the ordinary parenchyma tissue lying between the tracts of vessels, or abut the vessels. Typical flange cells are absent at the parasite/host xylem interface. The cells contain a well differentiated protoplast, including chloroplasts with extensive granal stacks. Histochemical staining and fluorescence microscopy indicate lignification of the flange wall. In thin section, the flange wall is often stratified into dark and light staining layers. Flange cells inKorthalsella resemble contact cells, vessel associated cells and certain types of transfer cells reported in the literature. Based on morphological considerations, it is suggested that flange cells inKorthalsella are involved in absorption and transport between host and parasite. As host sap moves through the sucker apoplasm, substance might be selectively absorbed by the flange cell, before the remaining the sap passes into the vessels for long distance transport in the mistletoe.

Importance of Direct and Indirect Effects in the Interaction between a Parasitic Angiosperm (Cuscuta epithymum) and Its Host Plant (Hormathophylla spinosa)

In this paper, I analyse the interaction between a parasitic angiosperm (Cuscuta epithymum, Cuscutaceae) and its host plant (Hormathophylla spinosa, Cruciferae), taking into account the other species which directly affect the fitness of the host plant. I studied the spatial pattern of Cuscuta infestation in H. spinosa in the Sierra Nevada, Spain, for 6 years (1988-1993). In addition, I assessed the composition and abundance of the pollinator assemblage of Cuscuta during 1991. I determined the effect of dodder on H. spinosa reproductive success by comparing the fruit and seed production of both parasitized and non-parasitized host plants, and determining the mortality factors affecting reproductive potential. Cuscuta epithymum was found to parasitize the labelled host plants only in three of the six years of study, and only in two of the three host plant populations. The maximum percentage of infestation ocurred in 1990, in which 20% of the labelled host plants were parasitized by dodders in one population and 15% in the other population. No labelled host plant had more than 35% of its canopy surface covered by dodders. Parasitized plants were visited by fewer pollinators than were non-parasitized plants. In addition, the diversity of the pollinator assemblage also differed between these two groups of plants. The dodder did not significatively affect the fruit or seed production of its host plant, either during the study years or the years following the strong infestation. The dodder causes little harm mainly because it is scarce. However, even the relationship between an individual dodder and an individual host plant is neutral, due to the array of direct and indirect, positive and negative effects appearing between Cuscuta and H. spinosa during the sucessives phases of the reproductive cycle of the host plant.

The attachment organ of the parasitic angiosperms Orobanche cumana and O. aegyptiaca and its development

Some species of Orobanche are parasitic weeds attacking the roots of various crops and causing extensive damage. The development of the young parasite was investigated using light and electron microscopy. Germinating O. aegyptiaca and O. cumana form a short root, with no root cap, which is covered by a thin cuticle. As soon as the root reaches a host, elongation stops, the apex expands, and peripheral cells become papillate. The extended apex becomes the young attachment organ. The outer papillar surface of the extended apex bears wall protuberances that are encircled with a thick cuticular belt and covered with a thin cuticle. These protuberances secrete a carbohydrate that accumulates in a subcuticular space and is released to the surface, forming a thin adhesive layer that binds the parasite to its host. Bacteria are commonly found in the secretion pool, on the papillae, and in the outer cell walls. The event of attachment of the young parasite to host surface signals a shift from the independent to parasitic phase. This change also includes a shift from the accumulation of lipids to the accumulation of starch. Key words: Orobanche, broomrape, parasitic plants, haustorium, attachment organ, root.

Effect ofCuscuta infection on chloroplast lipid composition ofBrassica leaves

Brassica campestris was infected with the angiosperm parasite,Cuscuta reflexa, on 24 DAS (days after sowing). A significant reduction (43%) in carbon dioxide assimilation in leaves was found compared to normal plant, when both were assayed on 38 DAS. At this stage, the chloroplast lipid in infected leaves had decreased by 34%, with a reduction of 13, 38 and 55%, respectively, in neutral, glyco- and phospholipids. Among the glycolipids, the decrease in the content of MGDG, DGDG and SQDG was 21, 67 and 44%, respectively, with 2.4-fold increase in the MGDG/DGDG ratio. The chlorophyll content was reduced by 24%, while the carotenoid level increased by 44%. The free fatty acid content was enhanced by 47% which may be attributed to an increase in lipase activity. The contents of saturated fatty acids (capric, lauric, myristic, palmitic and stearic) increased while the contents of unsaturated fatty acids (palmitoleic, oleic, linoleic and linolenic) decreased. The ratio of unsaturated to saturated fatty acids decreased over 50%. These findings may suggest chloroplasts as reactive sites of the host-parasite interaction in case of the angiosperm parasite.

Resistance of cowpea [Vigna unguiculata (L.) Walp.] to Striga gesnerioides (Willd.) Vatke, a parasitic angiosperm.

An in vitro growth system was used to investigate the expression of resistance of cowpea [Vigna unguiculata (L.) Walp.] varieties B301 and 58-57 to the angiosperm parasite Striga gesnerioides (Willd.) Vatke. Variety Blackeye was included in experiments as a susceptible control. Seeds of S. gesnerioides germinated on the roots of all varieties. Two different mechanisms of resistance were expressed in B301 and 58-57 after penetration of host roots by parasite radicles. The first mechanism was expressed in both varieties; host tissue around invading Striga radicles became necrotic in association with the early death of the parasite and lack of tubercle formation. On variety 58-57, the necrosis response was expressed only when the roots were inoculated with Striga from Burkina Faso but Striga from Mali successfully parasitized the roots of variety 58-57. The second mechanism was observed only on variety B301. Striga radicles infected cowpea roots, tubercles were formed but remained at 1-2 mm in diameter with very limited stem growth. No further development of the parasite occurred in vitro or when infected cowpeas were grown in soil or vermiculite. Similarities between the expression of resistance of cowpea to Striga and ot plants to plant pathogenic fungi are discussed.

Exploitation of the xylem stream by parasitic organisms

A taxonomically diverse group of angiosperms and certain homopteran insects derive water, inorganic and organic solutes from angiosperm xylem sap. Parasitic angiosperms are connected to their host(s) by a specialized organ (the haustorium) and form close cellular contacts with host xylem tissue, while insects tap xylem vessels by means of stylets. Adaptations to phytophagy are discussed with respect to gaining access to xylem tissue and the nutrition of sap feeders. Parasitic angiosperm-host interactions are examined in relation to recent advances in our understanding of root-to-shoot communication via the xylem (the influence of host-sourced signals on the parasite) and the functional significance of high rates of transpiration in parasitic angiosperms.

Role of Nitrogen in Resistance to Striga Parasitism of Maize

Striga [Striga hermonthica (Del.) Benth.] is a parasitic angiosperm that infects tropical cereals causing severe yield losses. This study was conducted to determine if Striga damage in maize (Zea mays L.) can be mediated by the amount, form, and timing of N availability; and if the efficacy of N is contingent upon its regulation of assimilate partitioning. Two experiments were conducted in Kibos, western Kenya, in 1989 and 1990 on fields that had uniform Striga infestation. One experiment evaluated N rates of 0, 30, 60, and 90 kg N ha−1 supplied as either urea, calcium ammonium nitrate, ammonium sulfate, or ammonium sulfate plus the nitrification inhibitor dicyandiamide, while a second experiment evaluated similar N rates applied at 14, 21, 28, and 35 d after planting. Although Striga infection generally declined with increasing N availability, the impact was partially dependent on the severity of infestation as all N rates decreased infection in 1990, while only 90 kg N ha−1 reduced infection in 1989. Under high parasite densities in 1989, only urea reduced Striga (26%), while in 1990, infection was significantly decreased (an average of 30%) by all sources of N. In both years, N application at 28 d after planting resulted in the least Striga infection. Although assimilate partitioning during vegetative growth was unresponsive to N treatments, N availability during reproductive growth altered dry matter partitioning in favor of the ear over the vegetation. Averaged across N rates, this alteration resulted in increases in grain yield (64%) and harvest index (27%), and a decrease in source‐sink partitioning (41%) and in the concentration of nonstructural carbohydrates in the stalk (16%). Based on these data, N fertility can mediate the impact of Striga infestation on maize by altering assimilate partitioning in favor of the ear.

Screening techniques and sources of resistance to parasitic angiosperms

Screening techniques and sources of resistance to parasitic angiosperms

Virulence characteristics of a new race of the parasitic angiosperm, Striga gesnerioides, from southern Benin on cowpea (Vigna unguiculata)

An in vitro growth system was used to determine the virulence of two samples of Striga gesnerioides from Zakpota in southern Benin. Cowpea variety B301, previously considered resistant to all races of S. gesnerioides, was susceptible to both samples of the parasite. Two other cowpea varieties, 58–57 and IT81D-994, were totally resistant. Resistance in 58–57 was associated with a hypersensitive necrosis of infected roots, whilst IT81D-994 supported production of small S. gesnerioides tubercles with stems which failed to develop. Striga gesnerioides from southern Benin is the fourth race of the parasite to be identified, and the first with virulence on variety B301. The implications for breeding cowpeas with resistance to S. gesnerioides are discussed.

Germination of the seeds of parasitic angiosperms

AbstractTotalling more than 3000 species, parasitic angiosperms are a most diverse group of organisms. Classified as either shoot or root parasites according to site of attachment, plant parasites are further classified, according to the degree of parasitism, into one of two divisions: holoparasitic or hemiparasitic. Within these divisions, angiosperm parasites are either obligate or facultative.Obligate parasites cannot survive to maturity without attaching to a host plant, thus the need to germinate in close proximity to a host first characterizes the specialized germination and seed dispersal strategies of these plants. However, seeds of all species of the rootholoparasitic Orobanchaceae and four genera within the Scrophulariaceae have evolved very specialized germination strategies. Once the seeds have undergone an after-ripening period, they require a prolonged imbibition period at temperatures of approx. 30°C before they acquire the potential to germinate. Minute seeds with very little reserves, they must germinate within millimetres of the host root for successful host attachment and continued survival. To this end, germination only occurs in responseto specific chemical germination triggers released from the host root. InStriga hermonthica(Del.) Benth., the perception of the host-derived trigger results in stimulation of endogenous ethylene production and it is this ethylene that initiates germination. It is proposed that the chemical triggers released from the host be consideredas ‘elicitors’ which function primarily as allelochemicals and that the parasite seeds have become adapted in such a way as to have an absolute requirement for external germination triggers.

Étude de quelques aspects du métabolisme carboné et azoté chez l'Arceuthobium oxycedri, gui nain du genévrier

In juniper dwarf mistletoe, Arceuthobium oxycedri (DC) M. Bieb., a hemiparasitic and epiphytic Angiosperm characterized by very reduced and slow development, the chlorophyll content of the shoots is relatively high for a parasitic plant. However, gas exchanges reveal a low net gain in carbon, and the real photosynthetic intensity is only about 1.5 to 2 times higher than the respiratory intensity in these organs. In the endophytic system, which is also chlorophyllous, a small amount of photosynthetic activity also occurs. By studying 14CO2 assimilation and carboxylase activities in both organs, it was possible to determine the ability of the parasite to satisfy part of its carbon requirements. Following the administration of 14CO2, radioactivity was mainly recovered in carbohydrates and to a lesser extent in organic acids and amino acids. This demonstrates that A. oxycedri has some autonomy for the fixation and distribution of carbon into various compounds. On the other hand, the parasite, since it is epiphytic, is completely dependent upon its host for nitrogen. The study of the activities of the key enzymes of nitrogen metabolism ([Formula: see text] and [Formula: see text] reductases, glutamine synthetase, glutamate dehydrogenase, glutamate-oxalacetate transaminase) shows that nitrogen supply from the host certainly occurs mainly in the form of reduced nitrogenous compounds (ammonia and (or) aminated organic compounds). The numerous labelled amino acids recovered after administration of 14CO2 show that A. oxycedri is able to redistribute the imported nitrogen in various ways. Key words: Arceuthobium oxycedri, parasitic Angiosperms, carbon nutrition, nitrogen nutrition.

Natural selection and the plastid genome of parasitic angiosperms

Natural selection and the plastid genome of parasitic angiosperms

New Methods for Control of Leafy Mistletoe (Phoradendron spp.) on Landscape Trees

Leafy mistletoes in the genus Phoradendron parasitize many trees common in amenity and natural landscapes in the United States from coast to coast, where winter temperatures are moderate (Fig. 1). These parasitic angiosperms reduce the vigor of host trees such as Fraxinus, Juglans, Quercus, Gleditsia, Alnus, and others (5), by tapping the host tree xylem for water and mineral nutrients. Branches infected with mistletoe may die (7) and if the whole tree is exposed to other stresses, such as drought, the mistletoe may contribute to its death (8). Current recommendations for control of this pest include not planting susceptible species; removing mistletoe-infected branches at least 12 inches below the site of infection; or pruning the mistletoe cluster flush to the host branch and wrapping the infected area with black plastic, tar paper or aluminum foil (1). Mistletoe can also be pruned to a stub and treated with a formulation of (2, 4-dichlorophenoxy) acetic acid (2, 4-D) applied in a foam (1, 2, 8). While these control methods are effective, they are time consuming, expensive and potentially damaging to plants and humans. The City of Sacramento spent $1.4 million for mistletoe control in 1978-79, including some remedial treatment (5), and now Sacramento spends approximately $350,000 annually. Ethephon ([2-chloroethyl] phosphonic acid) is readily absorbed by plant tissues and then releases ethylene, a plant growth regulator affecting many physiological plant processes (6). When sprayed on intact clusters of leafy mistletoe, ethephon at 2% a.i. can cause complete abscission of the aerial parts of the parasite (3). Stress on the host is thus reduced, and mistletoe seed production is eliminated or delayed. We have observed, however, that regrowth of the mistletoe from buds or from the endophytic system can occur in the same season following such a treatment. In the present study, therefore, we tested for control of mistletoe regrowth by applying ethephon to cut stubs of Phoradendron, rather than to the whole aerial cluster. In our experiments, we tested the effects of ethephon on two species of Phoradendron parasitizing three host tree species. In the same experiments, we compared ethephon treatment to other materials, including the currentlyrecommended control, black plastic wrap. Black plastic works because it blocks light from penetrating to the host branch, so that the mistletoe cannot carry out the photosynthesis necessary to survive. We tested an aerosol pruning paint because it blocked out 97% of incident light, and might have an effect similar to black plastic. In addition, we tested glyphosate (N-[phosphonomethyl] glycine). Mistletoes reportedly do not translocate substances back to the host branches through the phloem (4), so we decided to test whether this systemic herbicide could kill the parasite without harming the host.

Germination of pollen of the dwarf mistletoe Arceuthobium americanum

Pollen of Arceuthobium americanum Nuttall ex Engelmann, a parasitic dwarf mistletoe of Pinus banksiana Lamb, in Manitoba, was germinated on media containing a range of concentrations of sucrose and salts. Highest germination levels occurred on 20% sucrose agar; addition of salts significantly depressed germination. Pollen germinability was below 5% at first but increased to near 50% as the season advanced. Mean germination differed significantly between flowers from 10 staminate plants but peaked at approximately the same times in association with increasing temperatures. Freezing night temperatures were followed by reduced germinability. The optimum temperature for in vitro pollen germination is close to 30 °C, whereas 40 °C impairs germination. Key words: parasitic angiosperms, microclimate, jack pine, reproductive biology.

Phylogenetic relationships of the Santalales and relatives

Determining relationships among parasitic angiosperms has often been difficult owing to frequent morphological reductions in floral and vegetative features. We report 18S (small-subunit) rRNA sequences for representative genera of three families within the Santalales (Olacaceae, Santalaceae, and Viscaceae) and six outgroup dicot families (Celastraceae, Cornaceae, Nyssaceae, Buxaceae, Apiaceae, and Araliaceae). Using Wagner parsimony analysis, one most parsimonious tree resulted that shows the Santalales to be a holophyletic taxon most closely related to Euonymus (Celastraceae). The santalalean taxa showed approximately 13% more transitional mutations than the group of seven other dicot species. This suggests a higher fixation rate for mutations in these organisms, possibly owing to a relaxation of selection pressures at the molecular level in parasitic vs nonparasitic plants. Outgroup relationships are generally in accord with current taxonomic classifications, such as the grouping of Nyssaceae and Cornaceae together (Cornales) and the grouping of Araliaceae with Apiaceae (Apiales). These data provide the first nucleotide sequences for any parasitic flowering plant and support the contention that rRNA sequence analysis can result in robust phylogenetic comparisons at the family level and above.

Anatomy and ultrastructure of haustoria in selected West Australian parasitic angiosperms

Our understanding of nutrient transfer between host and flowering parasitic plants is usually based mainly on physiological concepts, with little information on haustorial structure related to function. The aim of this paper is to study the haustorial interface and possible pathways of water and solute transfer between a number of host and parasites.Haustorial tissues were fixed in glutaraldehyde and embedded in glycol methacrylate (LM), or fixed in glutaraldehyde then OsO4 and embedded in Spurr’s resin (TEM).Our study shows that lumen to lumen continuity occurs between tracheary elements of a host and four S.W. Australian species of aerial mistletoes (Fig. 1), and some root hemiparasites (Exocarpos spp. and Anthobolus foveolatus) (Fig. 2). On the other hand, haustorial interfaces of the root hemiparasites Olax phyllanthi and Santalum (2 species) are comprised mainly of parenchyma, as opposed to terminating tracheads or vessels, implying that direct solution transfer between partners via vessels or tracheary elements may be limited (Fig. 3).