Integrating ecology and physiology of root-hemiparasitic interaction: interactive effects of abiotic resources shape the interplay between parasitism and autotrophy

Hemiparasitic plants facilitate ecological restoration of encroached European grasslands

Species-rich grasslands of temperate Europe are threatened by the spread and increasing dominance of the rhizomatous grass Calamagrostis epigejos. Native hemiparasitic Rhinanthus species have been proposed as biocontrol to suppress C. epigejos, but experimental evidence is limited. We conducted a series of experiments at 21 grassland sites in Central Europe encroached by C. epigejos to test the effects of Rhinanthus on C. epigejos and on plant and arthropod communities and compare them to the effects of mowing applied once or twice a year. Rhinanthus suppressed Calamagrostis epigejos significantly more than mowing. Mowing frequency and Rhinanthus sowing had synergistic effects, leading to an average 75% reduction of C. epigejos biomass in Rhinanthus plots that were mown twice during the three-to four-year experimental period. The effect was more pronounced at sites in dry climates. Both Rhinanthus and mowing significantly increased plant diversity. Rhinanthus plots mown twice changed most rapidly towards the target grassland vegetation. The abundance and species richness of arachnids and Auchenorrhyncha were generally higher in abandoned plots than in plots with active management. Rhinanthus plots harboured fewer grass-feeding insects but more forb-associated herbivorous Auchenorrhyncha and Heteroptera species and several heliophilous spiders. Our experiment showed that Rhinanthus spp., in combination with mowing, is an efficient and widely applicable method to reverse the encroachment of C. epigejos into grasslands and restore the diversity of infested plant communities. Active management measures also trigger a species turnover of arthropod communities, reflecting changes in grassland vegetation and arthropod guild species pools.

Hemiparasite Phtheirospermum japonicum growth benefits from a second host and inflicts greater host damage with exogenous N supply

While parasites are likely to connect to multiple host plants in nature, parasitism dynamics under multiple association conditions remain unclear and are difficult to separate from competitive effects. In this study, a five-compartment split root-box was constructed to allow a single facultative root hemiparasite, Phtheirospermum japonicum, to connect to zero, one or two Medicago sativa hosts while maintaining constant plant number and independently controlling nutrient supply. In the first experiment, we found that P. japonicum derived equal, additive benefits from attachment to a second host irrespective of parasite N status. In the second experiment, parasites were grown at four N levels in either parasitic or control conditions. Attachment caused a constant, absolute increase in parasite mass at all N levels, while host damage increased at higher parasite N levels despite an apparent decrease in host to parasite N transfer. Our findings suggest that host damage caused by P. japonicum may be strengthened by exogenous nitrogen supply to the parasite.

Delineating the role of host plants in regulating the water and salinity stress induced changes in sandalwood roots

The interaction of root hemi-parasite (sandalwood) with its hosts is crucial for establishing successful plantations under abiotic stresses. In the present study, we explored the best possible host for sandalwood along with its effect on sandalwood physiology in terms of water and nutrients. Interactive effects of host species (Alternanthera sp., Azadirachta indica, Dalbergia sissoo, Melia dubia, and Aquilaria malaccensis) with sandalwood were observed under eight treatments {100% best available water (BAW); 100% BAW + nutrient medium; 50% water deficit; 50% water deficit + nutrient medium; 100% saline water (ECiw 8ds/m); 100% saline water (ECiw 8ds/m) + nutrient medium; 50% water deficit + saline water (ECiw 8ds/m); and 50% water deficit + saline water (ECiw 8ds/m) + nutrient medium}. A significant change in morpho-physiological traits of sandalwood roots was observed under different stress conditions, which were slightly improved through external supply of nutrient medium. Dalbergia sissoo (Shisham) and Melia dubia (Dek) seemed to be the best host plants providing better environment for sandalwood growth and development, i.e., higher plant height (59.7 and 53.68 cm) and collar diameter (3.24 and 3.07 mm) under stresses by maintaining water and ionic balance. Root length is an important parameter that was reduced by 27.58%, 19.22%, and 36.3% under water deficit, salinity, and combined stress of water deficit and salinity. Sandalwood grown with D. sissoo and M. dubia maintained the lowest Ψw (- 1.38 MPa) and Ψs (- 1.47 and - 1.48 MPa), respectively. In addition, sandalwood cultivated with D. sissoo and A. indica had higher accumulation of soluble proteins (0.48 and 0.42 mg/g) and soluble sugars (98.56 and 91.04 mg/g) in their roots. Results also showed that sandalwood roots had higher K+/Na+ with compatible host, i.e., with A. indica (1.85) and D. sissoo (1.83) than other studied hosts. It was also observed that sandalwood plants could not grow and survive alone under stress conditions even with application of nutrient medium. Based on the morphological traits, it was observed that sandalwood grown with hosts, Dalbergia sissoo and Melia dubia, was able to tolerate stress conditions better than other studied hosts. We can further recommend growing sandalwood with D. sissoo and M. dubia as a viable option to endure adverse environmental conditions.

Photosynthesis, ionomics and metabolomics of the host-hemiparasite association Acacia gerrardii-Viscum schimperi

Viscum schimperi is an evergreen hemiparasitic plant that can grow on stems and branches of several tree species. It penetrates the host tissues and forms a vascular bridge (haustorium) to withdraw the nutritive resources. Its relationships with hosts remain unknown. This study aimed to investigate the physiological and biochemical attributes of the host-hemiparasite association Acacia gerrardii -Viscum schimperi . The hemiparasite exhibited 2.4- and 3.0-fold lower photosynthetic activity and water use efficiency, and 1.2- and 4.1-fold higher transpiration rate and stomatal conductance. Equally, it displayed 4.9- and 2.6-fold greater water potential and osmotic potential, and in least 3.0times more accumulated 39 K, 85 Rb and 51 V, compared to the host. Nevertheless, it had no detrimental effect on photosynthetic activity, water status and multi-element accumulations in the host. Based on metabolome profiling, V. schimperi could use xanthurenic acid and propylparaben to acquire potassium from the host, and N -1-naphthylacetamide and N -Boc-hydroxylamine to weaken or kill the distal part of the infected branch and to receive the total xylem contents. In contrast, A. gerrardii could used N -acetylserotonin, arecoline, acetophenone and 6-methoxymellein to defend against V. schimperi infection.

Host age affects the performance of the root hemiparasitic plant Rhinanthus alectorolophus

Interactions between root hemiparasitic plants and their hosts are strongly affected by host identity, but may also depend on the condition of the host. An important determinant of host quality could be host age, as it may influence host size, allocation patterns, responses to infection, and the strength of competition for light between parasite and host. We investigated the effects of host species identity, host age and above-ground separation of hemiparasite and host on the interactions between the hemiparasite Rhinanthus alectorolophus and five host species in a factorial experiment. The host species were planted at six different times, from 10 weeks before the parasite was planted to 4 weeks after. Host age strongly influenced the performance of the parasite, but these effects also varied among host species. Parasites grew largest with hosts planted at the same time or 2 weeks earlier, but their performance strongly declined both with increasing host age and with the time they grew autotrophically. A large part of the variation due to host age but not of that due to host species identity could be related to the negative influence of host size at the likely time of parasite attachment. The low quality of older hosts was not due to light competition, suggesting that effective exploitation of these hosts was prevented by other factors like harder roots, stronger defense against parasite attack or competition for resources taken up by the host roots. Suppression of host growth by the parasites declined with increasing host age. The results indicate that the choice of host age may influence the results of studies on hemiparasites. They also highlight the importance for annual root hemiparasites of attachment in early spring, that is, at a time when their mostly perennial hosts produce fresh roots but are still poorly developed above ground.

Diverse Host Spectrum and the Parasitic Process in the Pantropical Hemiparasite Cassytha filiformis L. (Lauraceae) in China

Many hemiparasites attach to a range of different host species, resulting in complex parasite–host interactions. Comprehensive molecular phylogenies allow the investigation of evolutionary relationships between these host plants. We surveyed the hosts of the laurel dodder (Cassytha filiformis, Lauraceae) in China, representing 184 species from 146 genera, 67 families, and spanning flowering plants, conifers, and ferns, using host phylogenetic relationships to investigate the susceptibility to attack by this hemiparasitic plant among the vascular plants. The process of produced well-formed haustoria by C. filiformis was also observed in detail for six different hosts. Our results show that C. filiformis grows mainly on trees and shrubs from phylogenetically divergent members of the rosid and asterid eudicot clades, often attacking multiple adjacent hosts simultaneously, and forming extensive colonies. However, whether and to what extent transitions between C. filiformis and host plants occur remain unclear. Physiological evidence for the complex parasite–host species interactions need to be studied in the future.

Host root exudates initiate a foraging preference by the root parasite Santalum album

Haustoria of root-parasitic plants draw nutrients from the roots of host species. While recent studies have assessed host preferences of parasitic plants, how root-exuded chemicals can mediate host tropism and selection by root-parasitic plants is poorly understood. Under greenhouse conditions, we performed two pot experiments to determine whether the root parasite Santalum album selectively forages for superior hosts (N2-fixing Acacia confusa Merr. or Dalbergia odorifera T. Chen) rather than for inferior hosts (non-N2-fixing Bischofia polycarpa (levl.) Airy Shaw or Dracontomelon duperreranum Pierre), and whether S. album uses host root exudates and/or specific chemicals in these root exudates to locate and trigger haustorium formation. Lateral roots and haustoria of S. album seedlings exhibited greater growth in the direction of D. odorifera roots than toward roots from the other three hosts. Comparative metabolic analysis revealed that D. odorifera root exudates were enriched in isoflavonoid, flavonoid and flavone/flavonol biosynthesis pathways, and that the relative contents of flavonoids were significantly greater in the root exudates of D. odorifera than in those of the other three hosts. Root exudates from D. odorifera significantly promoted S. album root growth, haustorium formation and reactive oxygen species accumulation in haustoria. Our results demonstrate that the key step in plant parasitism by S. album is based on root exudation by a host plant; the exudates function as a metabolite signal that activate lateral root growth and haustorium formation. Our results also indicate that flavonoids in the root exudates could play an important role in S. album foraging activity. Information on the responses of root parasites to host root exudates and/or haustorium-inducing chemicals may be useful for selecting superior host species to plant with valuable species of root parasites.

Transcriptome Profiling of a Common Mistletoe Species Parasitizing Four Typical Host Species in Urban Southwest China

Comparing gene expressions among parasitic plants infecting different host species can have significant implications for understanding host–parasite interactions. Taxillus nigrans is a common hemiparasitic species in Southwest China that parasitizes a variety of host species. However, a lack of nucleotide sequence data to date has hindered transcriptome-level research on T. nigrans. In this study, the transcriptomes of T. nigrans individuals parasitizing four typical host species (Broussonetia papyrifera (Bpap), a broad-leaved tree species; Cryptomeria fortunei (Cfor), a coniferous tree species; Cinnamomum septentrionale (Csep), an evergreen tree species; and Ginkgo biloba (Gbil), a deciduous-coniferous tree species) were sequenced, and the expression profiles and metabolic pathways were compared among hosts. A total of greater than 400 million reads were generated in nine cDNA libraries. These were de novo assembled into 293823 transcripts with an N50 value of 1790 bp. A large number of differentially expressed genes (DEGs) were identified when comparing T. nigrans individuals on different host species: Bpap vs. Cfor (1253 DEGs), Bpap vs. Csep (864), Bpap vs. Gbil (517), Cfor vs. Csep (259), Cfor vs. Gbil (95), and Csep vs. Gbil (40). Four hundred and fifteen unigenes were common to all six pairwise comparisons; these were primarily associated with Cytochrome P450 and environmental adaptation, as determined in a KEGG enrichment analysis. Unique unigenes were also identified, specific to Bpap vs. Cfor (808 unigenes), Bpap vs. Csep (329 unigenes), Bpap vs. Gbil (87 unigenes), Cfor vs. Csep (108 unigenes), Cfor vs. Gbil (32 unigenes), and Csep vs. Gbil comparisons (23 unigenes); partial unigenes were associated with the metabolism of terpenoids and polyketides regarding plant hormone signal transduction. Weighted gene co-expression network analysis (WGCNA) revealed four modules that were associated with the hosts. These results provide a foundation for further exploration of the detailed molecular mechanisms involved in plant parasitism.

Distinguishing carbon gains from photosynthesis and heterotrophy in C3-hemiparasite-C3-host pairs

BACKGROUND AND AIMS

Previous carbon stable isotope (13C) analyses have shown for very few C3-hemiparasites utilizing C4- or CAM-hosts the use of two carbon sources, autotrophy and heterotrophy. This 13C approach, however, failed for the frequently occurring C3-C3 parasite-host pairs. Thus, we used hydrogen stable isotope (2H) natural abundances as a substitute for 13C within a C3-Orobanchaceae sequence graded by haustoria complexity and C3-Santalaceae.

METHODS

Parasitic plants and their real or potential host plants as references were collected in Central European lowland and alpine mountain meadows and forests. Parasitic plants included the xylem-feeding holoparasite Lathraea squamaria parasitizing on the same carbon nutrient source (xylem-transported organic carbon compounds) as potentially Pedicularis, Rhinanthus, Bartsia, Melampyrum and Euphrasia hemiparasites. Reference plants were used for an autotrophy-only isotope baseline. A multi-element stable isotope natural abundance approach was applied.

KEY RESULTS

Species-specific heterotrophic carbon gain ranging from 0 to 51 % was estimated by a 2H mixing-model. The sequence in heterotrophic carbon gain mostly met the morphological grading by haustoria complexity: Melampyrum- < Rhinanthus- < Pedicularis-type.

CONCLUSION

Due to higher transpiration and lower water-use efficiency, depletion in 13C, 18O and 2H compared to C3-host plants should be expected for tissues of C3-hemiparasites. However, 2H is counterbalanced by transpiration (2H-depletion) and heterotrophy (2H-enrichment). Progressive 2H-enrichment can be used as a proxy to evaluate carbon gains from hosts.

Approaching a revolution in hemiparasitic plant biology? A commentary on 'Distinguishing carbon gains from photosynthesis and heterotrophy in C3-hemiparasite-C3-host-pairs'

This article comments on: Philipp Giesemann and Gerhard Gebauer, Distinguishing carbon gains from photosynthesis and heterotrophy in C₃-hemiparasite–C₃-host pairs, Annals of Botany, Volume 129, Issue 6, 12 May 2022, Pages 647–656 https://doi.org/10.1093/aob/mcab153

Performance of generalist hemiparasitic Euphrasia across a phylogenetically diverse host spectrum

Generalist hemiparasites may attach to many different host species and experience complex parasite-host interactions. How these parasite-host interactions impact on the fitness of hemiparasitic plants remain largely unknown. We used experimentally tractable eyebrights (Euphrasia, Orobanchaceae) to understand parasite-host species interactions affecting the performance of a generalist hemiparasitic plant. Common garden experiments were carried out measuring Euphrasia performance across 45 diverse hosts and in different parasite-host combinations. We show that variation in hemiparasite performance can be attributed mainly to host species and host phylogenetic relationships (λ = 0.82; 0.17-1.00 CI). When variation in performance is considered temporally, annual host species cause earlier flowering, and lead to poorer performance late in the season. While Euphrasia species typically perform similarly on a given host species, some eyebrights show more specialized parasite-host species interactions. Our results show that generalist hemiparasites only benefit from attaching to a limited, but phylogenetically divergent, subset of hosts. The conserved responses of divergent Euphrasia species suggest hemiparasite performance is affected by common host attributes. However, evidence for more complex parasite-host species interactions show that a generalist hemiparasite can potentially respond to individual host selection pressures and may adapt to local host communities.

Closely related parasitic plants have similar host requirements and related effects on hosts

The performance of root hemiparasites depends strongly on host species identity, but it remains unknown whether there exist general patterns in the quality of species as hosts for hemiparasites and in their sensitivity to parasitism. In a comparative approach, the model root hemiparasites Rhinanthus minor and R. alectorolophus were grown with 25 host species (grasses, forbs, and legumes) at two nutrient levels. Hosts grown without parasites served as a control. Host species identity strongly influenced parasite biomass and other traits, and both parasites grew better with legumes and grasses than with forbs. The biomass of R. alectorolophus was much higher than that of R. minor with all host plants and R. alectorolophus responded much more strongly to higher nutrient availability than R. minor. The performance of the two species of Rhinanthus with individual hosts was strongly correlated, and it was also correlated with that of R. alectorolophus and the related Odontites vulgaris in previous experiments with many of the same hosts, but only weakly with that of the less closely related Melampyrum arvense. The negative effect of R. minor on host biomass was less strong than that of R. alectorolophus, but stronger relative to its own biomass, suggesting that it is more parasitic. The impact of the two parasites on individual hosts did not depend on nutrient level and was correlated. Several legumes and grasses were tolerant of parasitism. While R. minor slightly reduced mean overall productivity, R. alectorolophus increased it with several species, indicating that the loss of host biomass was more than compensated by that of the parasite. The results show that closely related parasites have similar host requirements and correlated negative effects on individual hosts, but that there are also specific interactions between pairs of parasitic plants and their hosts.

Does Epichloë Endophyte Enhance Host Tolerance to Root Hemiparasite?

Epichloë endophytes have been shown to be mutualistic symbionts of cool-season grasses under most environmental conditions. Although pairwise interactions between hemiparasites and their hosts are heavily affected by host-associated symbiotic microorganisms, little attention has been paid to the effects of microbe-plant interactions, particularly endophytic symbiosis, in studies examining the effects of parasitic plants on host performance. In this study, we performed a greenhouse experiment to examine the effects of hereditary Epichloë endophyte symbiosis on the growth of two host grasses (Stipa purpurea and Elymus tangutorum) in the presence or absence of a facultative root hemiparasite (Pedicularis kansuensis Maxim). We observed parasitism of both hosts by P. kansuensis: when grown with a host plant, the hemiparasite decreased the performance of the host while improving its own biomass and survival rate of the hemiparasite. Parasitized endophyte-infected S. purpurea plants had higher biomass, tillers, root:shoot ratio, and photosynthetic parameters and a lower number of functional haustoria than the endophyte-free S. purpurea conspecifics. By contrast, parasitized endophyte-infected E. tangutorum had a lower biomass, root:shoot ratio, and photosynthetic parameters and a higher number of haustoria and functional haustoria than their endophyte-free counterparts. Our results reveal that the interactions between the endophytes and the host grasses are context dependent and that plant-plant interactions can strongly affect their mutualistic interactions. Endophytes originating from S. purpurea alleviate the host biomass reduction by P. kansuensis and growth depression in the hemiparasite. These findings shed new light on using grass-endophyte symbionts as biocontrol methods for the effective and sustainable management of this weedy hemiparasite.

Intraspecific competition for host resources in a parasite

Intraspecific competition among parasites should, in theory, increase virulence, but we lack clear evidence of this from nature.^1-3 Parasitic plants, which are sessile and acquire carbon-based resources through both autotrophy (photosynthesis) and heterotrophy (obtaining carbon from the host), provide a unique opportunity to experimentally study the role of intraspecific competition for nutrients in shaping the biology of both parasite and host.^4-6 Here, we manipulated the spatial position of naturally occurring individuals of desert mistletoe (Phoradendron californicum), a xylem hemiparasite, by removing parasites from co-infected branches of a common nitrogen-fixing host, velvet mesquite (Prosopsis velutina), in the Sonoran Desert. We measured physiological performance of both host and parasite individuals under differing competitive environments-parasite location along the xylem stream-through time. Performance was determined by measuring resource availability and use, given that resource demand changed with competitor removal and monsoon-driven amelioration of seasonal drought. Our principal finding was that intraspecific competition exists for xylem resources between mistletoe individuals, including host carbon. Host performance and seasonal climate variation altered the strength of competition and virulence. Hemiparasitic desert mistletoes demonstrated high heterotrophy, yet experimental removals revealed density- and location-dependent effects on the host through feedbacks that reduced mistletoe autotrophy and improved resource availability for the remaining mistletoe individual. Trophic flexibility tempered intraspecific competition for resources and reduced virulence. Mistletoe co-infections might therefore attenuate virulence to maintain access to resources in particularly stressful ecological environments. In summary, experimental field manipulations revealed evidence for intraspecific competition in a parasite species.

The combined effects of water and nitrogen on the relationship between a native hemiparasite and its invasive host

Stem hemiparasites are dependent on their hosts for water and nitrogen. Most studies, however, have assessed the influence of one factor on parasite : host associations, thus limiting our mechanistic understanding of their performance in nature. We investigated the combined effects of water and nitrogen (N) availability on both a host (Ulex europaeus) and its parasite (Cassytha pubescens). Parasite infection significantly decreased host shoot biomass and shoot : root ratio more severely in high water than low water, irrespective of N supply. Parasite stem [N] was significantly higher in high water than low water treatments, regardless of N supply, but parasite biomass did not vary among treatments. Irrespective of water and N supply, infected plants had significantly lower total, root and nodule biomass, predawn and midday quantum yields, maximum electron transport rates, water potentials and nitrogen concentration [N]. Parasite δ¹³ C was significantly higher than that of the host. Our results suggested that stem hemiparasites can better extract resources from hosts when water availability is high, resulting in a greater impact on the host under these conditions. When hemiparasitic plants are being investigated as a biocontrol for invasive weeds, they may be more effective in wetter habitats than in drier ones.

Distribution Dynamics and Roles of Starch in Non-photosynthetic Vegetative Organs of Santalum album Linn., a Hemiparasitic Tree

Allocation dynamics of stored starch plays essential roles in the development and growth of trees. Previous studies focused on the dynamics and the characteristics of starch in autotrophic trees. However, although starch granules have been detected in the organs or tissues of some parasitic plants, studies on the allocation dynamics and roles of storage starch in them are limited. Therefore, we determined and estimated the allocation dynamics and roles of starch in Santalum album Linn., a hemiparasitic tree, using morphological and physiological methods. Our findings showed abundant starch in the stem and root of S. album at the early seedling stage. Although S. album seedlings attached to the host showed no significant changes in starch levels throughout the experiment, unattached and host-removed seedlings exhibited a gradual decrease in the starch content over time. When the starch content of unattached seedlings was less than 1%, they started to die. Starch accumulated to high levels in developing and active haustoria; however, starch levels were low in the inactive haustoria. The present study suggests that starch may provide energy to seedlings that have no host, allowing them to survive during the unattached phase, thus increasing their chance to attach to host roots by extending their survival duration. In addition, we speculate that storage starch is potentially involved in the development of haustoria and in the physiological processes of S. album related to the absorption and transportation of water and nutrients from its host.

Effects of drought and N level on the interactions of the root hemiparasite Rhinanthus alectorolophus with a combination of three host species

Increasing nitrogen deposition and more frequent drought events are likely to change plant interactions in natural grasslands. Both factors may also influence the interactions between hemiparasitic plants, regarded as keystone species in many grasslands, and their host species. We grew a combination of three suitable hosts, a grass, a forb and a legume, with and without the hemiparasite Rhinanthus alectorolophus at three levels of nitrogen (N) and two levels of water availability in a factorial design. Biomass of the hemiparasite and host community increased with N level and was reduced by drought to a similar degree. Larger plants in fertilised pots started to wilt earlier, and the presence of a hemiparasite further increased drought sensitivity. The hemiparasite strongly reduced biomass of the host community and overall productivity, and affected the competitive balance among host plants because it particularly reduced biomass of the dominant grass. These effects were the opposite of those of high N. The hemiparasite increased the root mass fraction of the hosts at all levels of N and water availability, indicating that the effect of the hemiparasite on the hosts was mainly due to loss of belowground resources. Our results indicate that hemiparasites will not always respond more strongly to increased N availability and drought than autotrophic plants, and that hemiparasites can have similarly strong effects on grassland communities as soil fertility and drought. By preferentially attacking dominant species the hemiparasites might alleviate the negative effects of nutrient enrichment on grassland diversity.

A native parasitic plant and soil microorganisms facilitate a native plant co-occurrence with an invasive plant

Invasive plants often interact with antagonists that include native parasitic plants and pathogenic soil microbes, which may reduce fitness of the invaders. However, to date, most of the studies on the ecological consequences of antagonistic interactions between invasive plants and the resident biota focused only on pairwise interactions. A full understanding of invasion dynamics requires studies that test the effects of multiple antagonists on fitness of invasive plants and co-occurring native plants. Here, we used an invasive plant Mikania micrantha, a co-occurring native plant Coix lacryma-jobi, and a native holoparasitic plant Cuscuta campestris to test whether parasitism on M. micrantha interacts with soil fungi and bacteria to reduce fitness of the invader and promote growth of the co-occurring native plant. In a factorial setup, M. micrantha and C. lacryma-jobi were grown together in pots in the presence versus absence of parasitism on M. micrantha by C. campestris and in the presence versus absence of full complements of soil bacteria and fungi. Fungicide and bactericide were used to suppress soil fungi and bacteria, respectively. Findings show that heavy parasitism by C. campestris caused the greatest reduction in M. micrantha biomass when soil fungi and bacteria were suppressed. In contrast, the co-occurring native plant C. lacryma-jobi experienced the greatest increase in biomass when grown with heavily parasitized M. micrantha and in the presence of a full complement of soil fungi and bacteria. Taken together, our results suggest that selective parasitism on susceptible invasive plants by native parasitic plants and soil microorganisms may diminish competitive ability of invasive plants and facilitate native plant coexistence with invasive plants.

Coming undone: hemiparasite presence and effects in a prairie grassland diminish over time

Root hemiparasites acquire resources from neighboring plants' vascular systems and can limit host growth, depress community productivity, and exert keystone effects. The strength of these effects is posited to be greater where hosts are nutrient-stressed but studies of annual hemiparasites show effects to be short-lived and variable. We conducted a 10-year experiment testing whether fertilizer addition alters the impact of the clonal, perennial hemiparasite Pedicularis canadensis on a prairie community and examine whether short-term trends reflect longer-term effects on community dynamics. Hemiparasite removal in 1-m² plots increased productivity over the first three field seasons, but later the difference between removal and non-removal plots diminished as P. canadensis disappeared from 24 of the 48 non-removal plots. Effects of hemiparasite removal were context independent relative to fertilizer and shade treatments, but fertilizer initially increased, and then subsequently suppressed P. canadensis biomass. In non-removal plots, hemiparasite biomass was negatively associated with total community dry mass, which was greater in fertilized plots. Initially, fertilizer promoted graminoids, but after seven more field seasons, non-legume forbs responded most strongly. Measures of biodiversity tended to increase with hemiparasite cover. Demographic data collected at two different times for P. canadensis show high survivorship of established plants, high seed input, with seedling survival greater in taller vegetation. Unlike annual hemiparasite populations, well-established P. canadensis buffer populations against large demographic swings. At the scale of a few square meters, this keystone species produces significant heterogeneity in a prairie, but its presence at that scale is transient over approximately one decade.

Hemiparasitic plants increase alpine plant richness and evenness but reduce arbuscular mycorrhizal fungal colonization in dominant plant species

Hemiparasitic plants increase plant biodiversity by reducing the abundance of dominant plant species, allowing for the establishment of subordinate species. Hemiparasites reduce host resources by directly removing nutrients from hosts, competing for light and space, and may indirectly reduce host resources by disrupting plant associations with symbiotic root fungi, like arbuscular mycorrhizal fungi and dark-septate endophytes. Here, we explored how a generalist hemiparasite, Castilleja, influences plant richness, evenness, community composition, and mycorrhizal colonization patterns across a ∼1,000 m elevational gradient in the North American Rocky Mountains. We hypothesized that the presence of Castilleja would be associated with increased plant richness and evenness, shaping plant community composition, and would reduce mycorrhizal colonization within dominant plant taxa. However, the magnitude of the effects would be contingent upon climate contexts, that is, elevation. Overall, we found that the presence of Castilleja was associated with an 11% increase in plant richness and a 5% increase in plant evenness, regardless of elevation. However, we found that the presence of Castilleja influenced plant composition at only two of the five sites and at the remaining three of five sites, plot pairing was the only predictor that influenced composition. Additionally, we found that the presence of Castilleja reduced mycorrhizal fungal colonization within dominant plant species by ∼20%, regardless of elevation. Taken together, our results suggest that hemiparasites regulate plant diversity, evenness, and interactions with mycorrhizal fungi independent of abiotic and biotic contexts occurring at the site, although overall effect on community composition is likely driven by site-level factors.

A native parasitic plant affects the performance of an introduced host regardless of environmental variation across field sites

Increasing evidence from glasshouse studies shows that native hemiparasitic plants can significantly impact the performance and growth of introduced host plants. We investigated the effect of the native Australian hemiparasite Cassytha pubescens R.Br. on the introduced shrub Ulex europaeus L. at three field sites in South Australia. Parasite infection significantly decreased midday PSII efficiency (ΦPSII) and the maximum electron transport rates (ETRmax) of U. europaeus across sites. The impact of C. pubescens on the photosynthetic performance of U. europaeus may have been caused by infected plants having significantly lower N and K, but higher Fe and Al than uninfected plants at all sites. Significant Al and Fe enrichment in infected plants may be possibly due to the parasite indirectly inducing rhizosphere acidification. At two sites, C. pubescens significantly affected host Fv/Fm, indicating chronic photoinhibition in response to infection. The impact of infection on Fv/Fm was greatest at the wettest site, in line with an experiment where C. pubescens had more impact under high water availability. At this site, infected plants also had the highest foliar Fe and Al. The C isotope (δ13C) of infected plants was significantly lower than that of uninfected plants at only one site. Unusually, the δ13C of the parasite was the same as or significantly higher than that of the hosts. There were no site effects on parasite Fv/Fm or ΦPSII; however, ETRmax and δ13C varied across sites. The results suggest that this native parasite has negative effects on U. europaeus in the field, as was found for glasshouse studies. The abundance of this introduced weed in Australia could be negatively affected by C. pubescens infection.

Mixotrophy in Land Plants: Why To Stay Green?

Mixotrophic plants combine photosynthesis and heterotrophic nutrition. Recent research suggests mechanisms explaining why mixotrophy is so common in terrestrial ecosystems. First, mixotrophy overcomes nutrient limitation and/or seedling establishment constraints. Second, although genetic drift may push mixotrophs to full heterotrophy, the role of photosynthesis in reproduction stabilizes mixotrophy.

Water-stress physiology of Rhinanthus alectorolophus, a root-hemiparasitic plant

Root-hemiparasitic plants of the genus Rhinanthus acquire resources through a water-wasting physiological strategy based on high transpiration rate mediated by the accumulation of osmotically active compounds and constantly open stomata. Interestingly, they were also documented to withstand moderate water stress which agrees with their common occurrence in rather dry habitats. Here, we focused on the water-stress physiology of Rhinanthus alectorolophus by examining gas exchange, water relations, stomatal density, and biomass production and its stable isotope composition in adult plants grown on wheat under contrasting (optimal and drought-inducing) water treatments. We also tested the effect of water stress on the survival of Rhinanthus seedlings, which were watered either once (after wheat sowing), twice (after wheat sowing and the hemiparasite planting) or continuously (twice and every sixth day after that). Water shortage significantly reduced seedling survival as well as the biomass production and gas exchange of adult hemiparasites. In spite of that drought-stressed and even wilted plants from both treatments still considerably photosynthesized and transpired. Strikingly, low-irrigated plants exhibited significantly elevated photosynthetic rate compared with high-irrigated plants of the same water status. This might relate to biochemical adjustments of these plants enhancing the resource uptake from the host. Moreover, low-irrigated plants did not acclimatize to water stress by lowering their osmotic potential, perhaps due to the capability to tolerate drought without such an adjustment, as their osmotic potential at full turgor was already low. Contrary to results of previous studies, hemiparasites seem to close their stomata in response to severe drought stress and this happens probably passively after turgor is lost in guard cells. The physiological traits of hemiparasites, namely the low osmotic potential associated with their parasitic lifestyle and the ability to withstand drought and recover from the wilting likely enable them to grow in dry habitats. However, the absence of osmotic adjustment of adults and sensitivity of seedlings to severe drought stress demonstrated here may result in a substantial decline of the hemiparasitic species with ongoing climate change.

Thesium linophyllon parasitizes and suppresses expansive Calamagrostis epigejos

Root-hemiparasitic interaction between the dominant grass Calamagrostis epigejos and the hemiparasite Thesium linophyllon was studied to assess the potential of the parasite to regulate dominance of the grass, which is expanding into species-rich steppe grasslands. First, we aimed to identify physiological links between the two species as a principal indicator of the parasitic relationship. Second, we analysed the dynamics of the two species in the vegetation of a steppe grassland at the foot of the Bükk Mountains, Hungary, where their joint presence is recorded in a long-term permanent plot monitoring dataset to detect patterns associated with the parasitic ecological interaction. Numerous well-developed functional haustoria of Th. linophyllon were identified on the root systems of C. epigejos. The joint dynamics of C. epigejos and Th. linophyllon displayed clear signs of the parasitic interaction: (1) the dynamics of Th. linophyllon frequency was positively associated with the initial cover of C. epigejos; (2) maximum recorded cover values of the two species were strongly positively correlated; and (3) the extent of C. epigejos decrease in the vegetation was significantly positively associated with maximum Th. linophyllon cover recorded throughout the monitoring period. We demonstrate that C. epigejos can be parasitized by Th. linophyllon, which restricts abundance of the grass. Th. linophyllon thus has potential to act as a native biological control of C. epigejos in steppe grasslands.

Does nitrogen affect the interaction between a native hemiparasite and its native or introduced leguminous hosts?

Associations between plants and nitrogen (N)-fixing rhizobia intensify with decreasing N supply and come at a carbon cost to the host. However, what additional impact parasitic plants have on their leguminous hosts' carbon budget in terms of effects on host physiology and growth is unknown. Under glasshouse conditions, Ulex europaeus and Acacia paradoxa either uninfected or infected with the hemiparasite Cassytha pubescens were supplied (high nitrogen (HN)) or not (low nitrogen (LN)) with extra N. The photosynthetic performance and growth of the association were measured. Cassytha pubescens significantly reduced the maximum electron transport rates and total biomass of U. europaeus but not those of A. paradoxa, regardless of N. Infection significantly decreased the root biomass of A. paradoxa only at LN, while the significant negative effect of infection on roots of U. europaeus was less severe at LN. Infection had a significant negative impact on host nodule biomass. Ulex europaeus supported significantly greater parasite biomass (also per unit host biomass) than A. paradoxa, regardless of N. We concluded that rhizobia do not influence the effect of a native parasite on overall growth of leguminous hosts. Our results suggest that C. pubescens will have a strong impact on U. europaeus but not A. paradoxa, regardless of N in the field.

The Haustorium, a Specialized Invasive Organ in Parasitic Plants

Parasitic plants thrive by infecting other plants. Flowering plants evolved parasitism independently at least 12 times, in all cases developing a unique multicellular organ called the haustorium that forms upon detection of haustorium-inducing factors derived from the host plant. This organ penetrates into the host stem or root and connects to its vasculature, allowing exchange of materials such as water, nutrients, proteins, nucleotides, pathogens, and retrotransposons between the host and the parasite. In this review, we focus on the formation and function of the haustorium in parasitic plants, with a specific emphasis on recent advances in molecular studies of root parasites in the Orobanchaceae and stem parasites in the Convolvulaceae.

Does light influence the relationship between a native stem hemiparasite and a native or introduced host?

BACKGROUND AND AIMS

There have been very few studies investigating the influence of light on the effects of hemiparasitic plants on their hosts, despite the fact that hemiparasites are capable of photosynthesis but also access carbon (C) from their host. In this study we manipulated light availability to limit photosynthesis in an established hemiparasite and its hosts, and determined whether this affected the parasite's impact on growth and performance of two different hosts. We expected that limiting light and reducing autotrophic C gain in the parasite (and possibly increasing its heterotrophic C gain) would lead to an increased impact on host growth and/or host photosynthesis in plants grown in low (LL) relative to high light (HL).

METHODS

The Australian native host Leptospermum myrsinoides and the introduced host Ulex europaeus were either infected or not infected with the native stem hemiparasite Cassytha pubescens and grown in either HL or LL. Photosynthetic performance, nitrogen status and growth of hosts and parasite were quantified. Host water potentials were also measured.

KEY RESULTS

In situ midday electron transport rates (ETRs) of C. pubescens on both hosts were significantly lower in LL compared with HL, enabling us to investigate the impact of the reduced level of parasite autotrophy on growth of hosts. Despite the lower levels of photosynthesis in the parasite, the relative impact of infection on host biomass was the same in both LL and HL. In fact, biomass of L. myrsinoides was unaffected by infection in either HL or LL, while biomass of U. europaeus was negatively affected by infection in both treatments. This suggests that although photosynthesis of the parasite was lower in LL, there was no additional impact on host biomass in LL. In addition, light did not affect the amount of parasite biomass supported per unit host biomass in either host, although this parameter was slightly lower in LL than HL for U. europaeus (P = 0·073). We also found no significant enhancement of host photosynthesis in response to infection in either host, regardless of light treatment.

CONCLUSIONS

Despite lower photosynthetic rates in LL, C. pubescens did not increase its dependency on host C to the point where it affected host growth or photosynthesis. The impact of C. pubescens on host growth would be similar in areas of high and low light availability in the field, but the introduced host is more negatively affected by infection.

Hydathode trichomes actively secreting water from leaves play a key role in the physiology and evolution of root-parasitic rhinanthoid Orobanchaceae

BACKGROUND AND AIMS

Root hemiparasites from the rhinanthoid clade of Orobanchaceae possess metabolically active glandular trichomes that have been suggested to function as hydathode trichomes actively secreting water, a process that may facilitate resource acquisition from the host plant's root xylem. However, no direct evidence relating the trichomes to water secretion exists, and carbon budgets associated with this energy-demanding process have not been determined.

METHODS

Macro- and microscopic observations of the leaves of hemiparasitic Rhinanthus alectorolophus were conducted and night-time gas exchange was measured. Correlations were examined among the intensity of guttation, respiration and transpiration, and analysis of these correlations allowed the carbon budget of the trichome activity to be quantified. We examined the intensity of guttation, respiration and transpiration, correlations among which indicate active water secretion.

KEY RESULTS

Guttation was observed on the leaves of 50 % of the young, non-flowering plants that were examined, and microscopic observations revealed water secretion from the glandular trichomes present on the abaxial leaf side. Night-time rates of respiration and transpiration and the presence of guttation drops were positively correlated, which is a clear indicator of hydathode trichome activity. Subsequent physiological measurements on older, flowering plants indicated neither intense guttation nor the presence of correlations, which suggests that the peak activity of hydathodes is in the juvenile stage.

CONCLUSIONS

This study provides the first unequivocal evidence for the physiological role of the hydathode trichomes in active water secretion in the rhinanthoid Orobanchaceae. Depending on the concentration of organic elements calculated to be in the host xylem sap, the direct effect of water secretion on carbon balance ranges from close to neutral to positive. However, it is likely to be positive in the xylem-only feeding holoparasites of the genus Lathraea, which is closely related to Rhinanthus. Thus, water secretion by the hydathodes might be viewed as a physiological pre-adaptation in the evolution of holoparasitism in the rhinanthoid lineage of Orobanchaceae.