Parasitic Angiosperms Research Articles

Hemiparasitic plant impacts animal and plant communities across four trophic levels.

Understanding the impact of species on community structure is a fundamental question in ecology. There is a growing body of evidence that suggests that both subdominant species and parasites can have disproportionately large effects on other organisms. Here we report those impacts for a species that is both subdominant and parasitic, the hemiparasite Rhinanthus minor. While the impact of parasitic angiosperms on their hosts and, to a lesser degree, coexisting plant species, has been well characterized, much less is known about their effects on higher trophic levels: We experimentally manipulated field densities of the hemiparasite Rhinanthus minor in a species-rich grassland, comparing the plant and invertebrate communities in plots where it was removed, present at natural densities, or present at enhanced densities. Plots with natural and enhanced densities of R. minor had lower plant biomass than plots without the hemiparasite, but enhanced densities almost doubled the abundance of invertebrates within the plots across all trophic levels, with effects evident in herbivores, predators, and detritivores. The hemiparasite R. minor, despite being a subdominant and transient component within plant communities that it inhabits, has profound effects on four different trophic levels. These effects persist beyond the life of the hemiparasite, emphasizing its role as a keystone species in grassland communities.

Apoplastic interactions between plants and plant root intruders.

Numerous pathogenic or parasitic organisms attack plant roots to obtain nutrients, and the apoplast including the plant cell wall is where the plant cell meets such organisms. Root parasitic angiosperms and nematodes are two distinct types of plant root parasites but share some common features in their strategies for breaking into plant roots. Striga and Orobanche are obligate root parasitic angiosperms that cause devastating agricultural problems worldwide. Parasitic plants form an invasion organ called a haustorium, where plant cell wall degrading enzymes (PCWDEs) are highly expressed. Plant-parasitic nematodes are another type of agriculturally important plant root parasite. These nematodes breach the plant cell walls by protruding a sclerotized stylet from which PCWDEs are secreted. Responding to such parasitic invasion, host plants activate their own defense responses against parasites. Endoparasitic nematodes secrete apoplastic effectors to modulate host immune responses and to facilitate the formation of a feeding site. Apoplastic communication between hosts and parasitic plants also contributes to their interaction. Parasitic plant germination stimulants, strigolactones, are recently identified apoplastic signals that are transmitted over long distances from biosynthetic sites to functioning sites. Here, we discuss recent advances in understanding the importance of apoplastic signals and cell walls for plant–parasite interactions.

Anther and pollen development in the lodgepole pine dwarf mistletoe (Arceuthobium americanum) staminate flower

The lodgepole pine dwarf mistletoe, Arceuthobium americanum Nutt. ex Engelm., is a parasitic angiosperm that infects conifers in western Canadian forests. While production of viable pollen in anthers is critical to dwarf mistletoe reproduction, the few existing reports that examine staminate development in Arceuthobium are often incomplete or conflicting. The objective of this work was to investigate the developmental anatomy of anther and pollen of A. americanum using modern microscopy. We found that the microsporangium was toroidal from the outset and gave rise to a central peg-shaped sterile “columella” early in anther development. The endothecium was absent, the epidermis persisted as an “exothecium” fulfilling the role of the endothecium, and a primary parietal layer generated a secretory tapetum and middle layer. Thus, we suggest that a new category of anther wall development, the Arceuthobium type, be created. Microsporogenesis produced tetrahedral microspores via simultaneous cytokinesis and involved callose wall formation. Microgametogenesis resulted in round and atypical generative and vegetative nuclei. Additionally, the heterocolpate, echinate pollen grains, which were shed at the two-celled stage, were seen for the first time in their native state with environmental scanning electron microscopy. This work contributes to the understanding of A. americanum, the genus Arceuthobium, and angiosperms as a whole.

Single-Copy Nuclear Genes Place Haustorial Hydnoraceae within Piperales and Reveal a Cretaceous Origin of Multiple Parasitic Angiosperm Lineages

Extreme haustorial parasites have long captured the interest of naturalists and scientists with their greatly reduced and highly specialized morphology. Along with the reduction or loss of photosynthesis, the plastid genome often decays as photosynthetic genes are released from selective constraint. This makes it challenging to use traditional plastid genes for parasitic plant phylogenetics, and has driven the search for alternative phylogenetic and molecular evolutionary markers. Thus, evolutionary studies, such as molecular clock-based age estimates, are not yet available for all parasitic lineages. In the present study, we extracted 14 nuclear single copy genes (nSCG) from Illumina transcriptome data from one of the “strangest plants in the world”, Hydnora visseri (Hydnoraceae). A ∼15,000 character molecular dataset, based on all three genomic compartments, shows the utility of nSCG for reconstructing phylogenetic relationships in parasitic lineages. A relaxed molecular clock approach with the same multi-locus dataset, revealed an ancient age of ∼91 MYA for Hydnoraceae. We then estimated the stem ages of all independently originated parasitic angiosperm lineages using a published dataset, which also revealed a Cretaceous origin for Balanophoraceae, Cynomoriaceae and Apodanthaceae. With the exception of Santalales, older parasite lineages tend to be more specialized with respect to trophic level and have lower species diversity. We thus propose the “temporal specialization hypothesis” (TSH) implementing multiple independent specialization processes over time during parasitic angiosperm evolution.

Pollination and mating systems of Apodanthaceae and the distribution of reproductive traits in parasitic angiosperms

The most recent reviews of the reproductive biology and sexual systems of parasitic angiosperms were published 17 yr ago and reported that dioecy might be associated with parasitism. We use current knowledge on parasitic lineages and their sister groups, and data on the reproductive biology and sexual systems of Apodanthaceae, to readdress the question of possible trends in the reproductive biology of parasitic angiosperms. • Fieldwork in Zimbabwe and Iran produced data on the pollinators and sexual morph frequencies in two species of Apodanthaceae. Data on pollinators, dispersers, and sexual systems in parasites and their sister groups were compiled from the literature. • With the possible exception of some Viscaceae, most of the ca. 4500 parasitic angiosperms are animal-pollinated, and ca. 10% of parasites are dioecious, but the gain and loss of dioecy across angiosperms is too poorly known to infer a statistical correlation. The studied Apodanthaceae are dioecious and pollinated by nectar- or pollen-foraging Calliphoridae and other flies. • Sister group comparisons so far do not reveal any reproductive traits that evolved (or were lost) concomitant with a parasitic life style, but the lack of wind pollination suggests that this pollen vector may be maladaptive in parasites, perhaps because of host foliage or flowers borne close to the ground.

Fungal endophytes of the obligate parasitic dwarf mistletoe Arceuthobium americanum (Santalaceae) act antagonistically in vitro against the native fungal pathogen Cladosporium (Davidiellaceae) of their host

Endophytic fungi likely occur in all plants, yet little is known about those of parasitic plants, despite their potential to influence parasite success. Arceuthobium americanum is a parasitic angiosperm that greatly compromises the North American timber industry. We hypothesized that (1) A. americanum hosts fungal endophytes, and (2) these endophytes help A. americanum resist infection by fungal pathogens. • Healthy A. americanum stem and fruit tissues were differentially stained for cellulose and chitin and visualized using fluorescence microscopy. Stem sections (sterilized vs. unsterilized) and seeds were incubated on agar plates to cultivate fungi, both to extract DNA for ITS rDNA sequencing and to observe interactions with native fungi from unsterilized specimens. • Aside from xylem vessel elements, fungal structures were observed in all tissues, including those of the embryo. The ITS sequences of fungi cultured from internal tissues closely matched those of the known endophytes Phoma, Sydowia, and Phacidiopycnis, while those of surface organisms closely matched Cladosporium spp. Cultured fungi from internal tissues (putative endophytes) inhibited the growth of the surface organisms without affecting the other endophytes. • Fungal communities are established in A. americanum stems as well as in fruits and seeds, suggesting vertical transmission. These internally derived fungi act antagonistically toward fungi with pathogenic tendencies. As such, native mistletoe endophytes might protect A. americanum against fungal pathogens in nature. In the future, manipulation of endophytes might be a component of mistletoe control programs.

An antibody against a conserved C-terminal consensus motif from plant alternative oxidase (AOX) isoforms 1 and 2 label plastids in the explosive dwarf mistletoe (Arceuthobium americanum, Santalaceae) fruit exocarp

Dwarf mistletoes, genus Arceuthobium (Santalaceae), are parasitic angiosperms that spread their seeds by an explosive process. As gentle heating triggers discharge in the lab, we wondered if thermogenesis (endogenous heat production) is associated with dispersal. Thermogenesis occurs in many plants and is enabled by mitochondrial alternative oxidase (AOX) activity. The purpose of this study was to probe Arceuthobium americanum fruit (including seed tissues) collected over a 10-week period with an anti-AOX antibody/gold-labeled secondary antibody to determine if AOX could be localized in situ, and if so, quantitatively assess whether label distribution changed during development; immunochemical results were evaluated with Western blotting. No label could be detected in the mitochondria of any fruit or seed tissue, but was observed in fruit exocarp plastids of samples collected in the last 2 weeks of study; plastids collected in week 10 had significantly more label than week 9 (p = 0.002). Western blotting of whole fruit and mitochondrial proteins revealed a signal at 30-36 kD, suggestive of AOX, while blots of whole fruit (but not mitochondrial fraction) proteins showed a second band at 40-45 kD, in agreement with plastid terminal oxidases (PTOXs). AOX enzymes are likely present in the A. americanum fruit, even though they were not labeled in mitochondria. The results strongly indicate that the anti-AOX antibody was labeling PTOX in plastids, probably at a C-terminal region conserved in both enzymes. PTOX in plastids may be involved in fruit ripening, although a role for PTOX in thermogenesis cannot be eliminated.

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.

Plant defense: warding off attack by pathogens, herbivores, and parasitic plants

Plant defense: warding off attack by pathogens, herbivores, and parasitic plants

Rate accelerations in nuclear 18S rDNA of mycoheterotrophic and parasitic angiosperms.

Rate variation in genes from all three genomes has been observed frequently in plant lineages with a parasitic and mycoheterotrophic mode of life. While the loss of photosynthetic ability leads to a relaxation of evolutionary constraints in genes involved in the photosynthetic apparatus, it remains to be determined how prevalent increased substitution rates are in nuclear DNA of non-photosynthetic angiosperms. In this study we infer rates of molecular evolution of 18S rDNA of all parasitic and mycoheterotorphic plant families (except Lauraceae and Polygalaceae) using relative rate tests. In several holoparasitic and mycoheterotrophic plant lineages extremely high substitution rates are observed compared to other photosynthetic angiosperms. The position and frequency of these substitutions have been identified to understand the mutation dynamics of 18S rRNA in achlorophyllous plants. Despite the presence of significantly elevated substitution rates, very few mutations occur in major functional and structural regions of the small ribosomal molecule, providing evidence that the efficiency of the translational apparatus in non-photosynthetic plants has not been affected.

In vitro hormone-regulated growth and floral induction of Cuscuta reflexa: a parasitic angiosperm

The role of different growth regulators in callus induction, shoot regeneration, floral induction and chlorophyll content of the obligatory parasitic plant Cuscutareflexa has been studied. Callus development was excellent from the nodal part of the shoot explants in modified Murashige and Skoog (MMS) media supplemented with 2 mg L−1 benzyl adenine (MMS1c). Supplementation of 2 mg L−1 naphthalene acetic acid (NAA) along with MMS1c (MMS2c) was responsible for estimable shoot induction and development in callus. 2,4-Dichloro acetic acid (2,4-D) played a crucial role in the floral induction of C. reflexa in vitro. MMS supplemented with 2 mg L−1 NAA and 2 mg L−1 2,4-D (MMS3b) supported floral induction after shooting in vitro. MMS supplemented with 3 mg L−1 2,4-D (MMS4a) rapidly induced flower directly from the stem explants without showing any elongation of shoot. MMS1c along with MMS3b (MMS5a) showed callus proliferation followed by shoot elongation and floral induction. In vitro MMS5a grown plants show a sharp increase in the chlorophyll contents. Cytokinin treatment further increases the chlorophyll level of the plant.

Natural Biocontrol Agents of Aeginetia pedunculata (Roxb.) Wall. (Orobanchaceae), a Root Holoparasitic Angiosperm of Sugarcane

Root holoparasitic angiosperm, Aeginetia pedunculata (Roxb.) Wall. (Orobanchaceae) causes wilt disease in sugarcane around sugar factory areas at Plassey, West Bengal. Periodic surveys in the infected sugarcane plots revealed the occurrence of two insect herbivores, Spilosoma obliqua and Gonocephalum depressum and two fungal pathogens, Fusarium oxysporum and Erysiphe cichoracearum on A. pedunculata . It was estimated from field observations that these insect and fungal species reduced seed capacity of A. pedunculata by 25.1, 88.5, 80.0 and 50.9%, respectively. Though the insect species are polyphagous and the fungal pathogens are polytrophic in nature, these were found to reduce the seed production capacity of the root parasitic angiosperm causing wilt disease in sugarcane. A suitable strategy for enhancing the activities of these agents in nature and studying the specificity of F. oxysporum and E. cichoracearum to A. pedunculata may lead to the management of this parasitic angiosperm in the sugarcane ecosystem.

Enhanced stilbene production in cell cultures of Cayratia trifolia through co-treatment with abiotic and biotic elicitors and sucrose

This report demonstrates the elicitation effect on growth and stilbene accumulation in cell cultures of Cayratia trifolia (Vitaceae) by an extract of the angiosperm parasite Cuscuta reflexa and salicylic acid in combination with sucrose feeding. Cell cultures of C. trifolia, a tropical liana, were maintained in liquid Murashige and Skoog's basal medium containing 0.25 mg l−1 naphthalene acetic acid, 0.2 mg l−1 kinetin with 3% sucrose and 250 mg l−1 casein hydrolysate. The cells treated with Cuscuta elicitor showed increased polyphenol oxidase activity with increasing concentration of the elicitor, while total phenol content remained almost unchanged. Enhanced yield of stilbenes (∼8-fold) was recorded in the cells treated with 200 mg l−1 Cuscuta elicitor for 7 d. Optimum accumulation of stilbenes with a non-significant decrease in cell growth as compared with control was recorded with the addition of 3% sucrose on the seventh day of cell culture. Addition of 3% sucrose with salicylic acid at 500 μM and Cuscuta extract at 200 mg l−1 on the seventh day enhanced total stilbene yield up to 50.1 mg l−1, which was ∼14-fold higher than in control cultures. Piceid content increased ∼200-fold in such cultures.

Mineral nutrient concentration influences sunflower infection by broomrape (Orobanche cumana)

Orobanche cumana Wallr., a root parasitic angiosperm, causes severe yield losses in Helianthus annuus L. (sunflower) in Europe. Until now, the only effective method of controlling this parasite has been the use of resistant sunflower genotypes. Broomrape resistance is, however, poorly understood even though previous studies have revealed several defence mechanisms. The study of a susceptible (2603) and a resistant (LR1) sunflower genotype in hydroponic co-culture showed that the degree of infection by broomrape is influenced by the concentration of nutrients in the growth medium. For the susceptible genotype, an increase in broomrape necrosis was observed when the nutrient concentration was increased. In the resistant genotype LR1, the rate of infection was reduced by increasing the concentration of mineral nutrients, measured as a decrease in broomrape attachments and a lack of underground stem development. When sunflowers were cultivated in full-strength medium, these findings correlated with a lower 14C incorporation in broomrape and a change in carbon allocation to host plant organs with a reinforced “shoot apex sink strength”. Results demonstrated that in controlled conditions, the nutrient concentration directly affects sunflower resistance potential towards broomrape.

Purification and Characterization of a β-Glucosidase from the Root Parasitic PlantOrobanche minorSm.

The beta-glucosidase of a root parasitic angiosperm, Orobanche minor Sm., was purified and characterized. The optimum pH and temperature for activity of the enzyme were 5.0 and 50 degrees C. The beta-glucosidase was stable at up to 50 degrees C at pH 4.0-10.0. The M(r) was estimated to be 33 kD by SDS-PAGE. The enzyme hydrolyzed p-nitrophenyl-beta-D-glucopyranoside and salicin, but not the cell wall of O. minor or cellohexaose.

Specific developmental pathways underlie host specificity in the parasitic plant Orobanche

Parasitic angiosperms are an ecologically and economically important group of plants. However our understanding of the basis for host specificity in these plants is embryonic. Recently we investigated host specificity in the parasitic angiosperm Orobanche minor, and demonstrated that this host generalist parasite comprises genetically defined races that are physiologically adapted to specific hosts. Populations occurring naturally on red clover (Trifolium pratense) and sea carrot (Daucus carota subsp. gummifer) respectively, showed distinct patterns of host specificity at various developmental stages, and a higher fitness on their natural hosts, suggesting these races are locally adapted. Here we discuss the implications of our findings from a broader perspective. We suggest that differences in signal responsiveness and perception by the parasite, as well as qualitative differences in signal production by the host, may elicit host specificity in this parasitic plant. Together with our earlier demonstration that these O. minor races are genetically distinct based on molecular markers, our recent data provide a snapshot of speciation in action, driven by host specificity. Indeed, host specificity may be an underestimated catalyst for speciation in parasitic plants generally. We propose that identifying host specific races using physiological techniques will complement conventional molecular marker-based approaches to provide a framework for delineating evolutionary relationships among cryptic host-specific parasitic plants.

Studies on the Host Recognition Mechanism of Root Parasitic Plants

Among parasitic angiosperms, witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.) are the two most devastating root parasitic plants, causing enormous crop losses around the world. These root parasites have evolved special strategies to ensure their survival; the seeds germinate only when they are within the host rhizosphere so that after germination the seedlings can reach and parasitize the host roots. To locate host roots, root parasites exploit strigolactones (SLs), host-derived chemical signals that are released by host plants for symbiotic arbuscular mycorrhizal (AM) fungi. In addition to these functions as rhizosphere signaling molecules, SLs act as a class of plant hormones regulating shoot branching. Using high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS), SLs in root exudates from various plants have been characterized. During studies, in addition to the known SLs, more than 10 novel SLs have been isolated and their structures determined. Plants produce and exude mixtures of SLs, and nutrient availability affects SL production and exudation. These results suggest that quantitative and qualitative differences in SL production and/or exudation are important in host recognition of root parasitic plants.

Vesicular cells of the lodgepole pine dwarf mistletoe (Arceuthobium americanum) fruit: development, cytochemistry, and lipid analysis

The lodgepole pine dwarf mistletoe, Arceuthobium americanum Nutt. ex Engelm., is a parasitic angiosperm that infects conifers in western Canadian forests. A striking feature of the Arceuthobium life cycle is explosive seed dispersal, triggered by hydrostatic build-up within the fruit, localized in a tissue called “viscin.” Viscin is composed of two cell types: “viscin cells” that accrue pressure in their hydrated mucilage, and “vesicular cells” with a previously undetermined function. The objective of this work was to investigate the development and composition of vesicular cells using microscopy, cytochemistry, and gas chromatography – mass spectroscopy. We found that vesicular cells are initiated from mesocarp cells lining the viscin cells. As development of the seed progresses, vesicular cells form a single layer, enlarge, and become isodiametric. Later, the cells make up all of the remaining 3–4 outer mesocarp layers. Concomitant with enlargement, intracellular triacylglycerides (TAGs) largely composed of 16-carbon and 18-carbon fatty acids, accumulate. Immediately prior to discharge, vesicular cell boundaries become indistinct, and the cell contents become confluent, creating a lipid mass between the viscin cells and the exocarp. Vesicular cells and their resulting lipid mass likely function in discharge by acting as a hydrophobic barrier to water loss and (or) as a repellant, resistant layer.

Parasitic angiosperms, semagenesis and general strategies for plant–plant signaling in the rhizosphere

In addition to their roles in eukaryotic defense and development, reactive oxygen species (ROS) have recently been identified as critical for host attachment by the parasitic angiosperms. In a process known as semagenesis, ROS generated at the root tip of Striga asiatica (L.) Kuntze (Scrophulariaceae) function together with host peroxidases to oxidize monolignols at the host root surface. As a result, para-benzoquinone products accumulate as both necessary and sufficient components for inducing development of the host attachment organ, the haustorium. This event constitutes the critical vegetative/pathogenic transition in the parasite. New evidence is presented that semagenesis occurs broadly in plant-plant signaling. Eudicotyledenous seedlings are more sensitive to the xenognostic benzoquinones than monocots, but general root development, including root elongation, root hair initiation and root hair growth, is impacted in both clades. Specific inhibitors of haustorial development in S. asiatica also inhibit benzoquinone-mediated root development in the non-parasites. These results suggest a common mechanism for benzoquinone perception. Semagenesis enriches our understanding of the mechanisms available for small-molecule underground information exchange among plants. Critical differences in this process, as used by parasitic plants, are beginning to emerge and point towards new strategies for managing parasitic angiosperms in agricultural settings.

Parasitism of host trees by the Loranthaceae in the region of Douala (Cameroon)

The Loranthaceae constitute the most important parasite plants that cause variable damages: morphological, technological, ecological and socio- economic. These numerous and damaging effects make some parasitic angiosperms true agronomic threats, especially in developing countries. The Loranthaceae is represented in Cameroon by 26 species gathered into 7 genus. The study area includes four sites: a traditional plantation of Cola nitida situated at Penja (70 km Nord of Douala), the main road to Douala airport, one quarter (Logbessou) and Makondo village (80 km East of Douala) in an orchard dominated by Citrus . A total of 2643 individuals of DBH (diameter at breast height) greater or equal to 5 cm were inventoried. 637 individuals were parasitized. Eight known species were identified. Phragmanthera capitata is more frequent and more abundant (76.14%). The infested host trees belong to 16 species gathered into 12 genus and 10 families. The most parasitized host family is Sterculiaceae . The most sensitive host species to the parasitism of Loranthaceae is P. americana (21.51%), followed by C. nitida (17.27%) and Terminalia mantaly (13.65%). Lauraceae is infested by 7 Loranthaceae out of 8 parasites investigated. The consequences of the parasitism of the Loranthaceae demonstrate the need for establishing comprehensive ecosystem management programs. Key words : Host trees, inventory, Loranthaceae , parasites plants.