Interactions between hemiparasitic plants and their hosts: the importance of organic carbon transfer

Quantitative Approach for Determining Reproductive Life-History Strategies of Parasitic Plants: A Case Study in Balanophora

Parasitic plants are a diverse and unique polyphyletic assemblage of flowering plants that survive by obtaining resources via direct vascular connections to a host plant. Ecologically important in their native ecosystems, these typically cryptic plants remain understudied and fundamental knowledge of the biology, ecology, and evolution of most species is lacking. This gap limits our understanding of ecosystems and conservation management. We established a multistep protocol to conduct the first investigation of the reproductive life history of root parasite genus Balanophora, testing the hypotheses of perenniality, cryptic perenniality, and plasticity across five geographically isolated populations in Taiwan. A review of 123 Balanophora publications found contradictory determinations, including no determination (87%), perennial (9%), annual (1%), biennial (1%), or a combination (2%). No primary study investigated the question, and no determination was accompanied by reference. Between 2021 and 2024, we tested a hypothesis of perenniality (109 individuals, 135 patches) and cryptic perenniality (73 host samples), monitored population dynamics (whole population), and potential for endophytic/dormant haustorial tissue (101 roots) across five isolated populations of Balanophora fungosa ssp. fungosa in Taiwan. Our results support semelparous annuality. After reproduction, individuals senesce and die, and the following year's population is recruited from newly germinated individuals which together develop in size and number during a vegetative growth period, undergo reproduction, and then themselves senesce and die. Each cycle is completed within a 12-month period. Synthesis: Our study provides the first quantitative determination of a semelparous annual reproductive life-history strategy for any species of Balanophora. This determination is important in our progress toward better understanding the species-and parasitic plants in general-as well as ecological roles within ecosystems and conservation management. Our study further provides a template for future work to expand life-history strategy determination across cryptic root parasitic plants.

Comparative and phylogenetic analyses of Loranthaceae plastomes provide insights into the evolutionary trajectories of plastome degradation in hemiparasitic plants

BACKGROUND

The lifestyle transition from autotrophy to heterotrophy often leads to extensive degradation of plastomes in parasitic plants, while the evolutionary trajectories of plastome degradation associated with parasitism in hemiparasitic plants remain poorly understood. In this study, phylogeny-oriented comparative analyses were conducted to investigate whether obligate Loranthaceae stem-parasites experienced higher degrees of plastome degradation than closely related facultative root-parasites and to explore the potential evolutionary events that triggered the 'domino effect' in plastome degradation of hemiparasitic plants.

RESULTS

Through phylogeny-oriented comparative analyses, the results indicate that Loranthaceae hemiparasites have undergone varying degrees of plastome degradation as they evolved towards a heterotrophic lifestyle. Compared to closely related facultative root-parasites, all obligate stem-parasites exhibited an elevated degree plastome degradation, characterized by increased downsizing, gene loss, and pseudogenization, thereby providing empirical evidence supporting the theoretical expectation that evolution from facultative parasitism to obligate parasitism may result in a higher degree of plastome degradation in hemiparasites. Along with infra-familial divergence in Loranthaceae, several lineage-specific gene loss/pseudogenization events occurred at deep nodes, whereas further independent gene loss/pseudogenization events were observed in shallow branches.

CONCLUSIONS

The findings suggest that in addition to the increasing levels of nutritional reliance on host plants, cladogenesis can be considered as another pivotal evolutionary event triggering the 'domino effect' in plastome degradation of hemiparasitic plants. These findings provide new insights into the evolutionary trajectory of plastome degradation in hemiparasitic plants.

Spatial aggregation patterns in four mistletoe species: ecological and environmental determinants

Plant spatial distribution is an important topic in ecology as it determines species coexistence and biodiversity dynamics. Usually, plants show clustered distributions in nature. Mistletoes are a good example of aggregated distributions, as they form dense aggregations due to several factors (availability of competent hosts, seed dispersal vectors, microclimate conditions). We analysed four native mistletoe species with divergent life histories and host ranges: Desmaria mutabilis and Tristerix corymbosus from the temperate rainforests of southern Chile; and Tristerix aphyllus and Tristerix verticillatus from the northern semi-desert zone. While T. corymbosus and T. verticillatus have a wide host range, T. aphyllus and D. mutabilis are specialists that can parasitize only a few plant species. We hypothesized that specialized species would be more aggregated due to ecological and environmental restrictions. We used heterogeneous Poisson models to quantify spatial aggregation. Three of the four mistletoe species were spatially clustered at both environments, with aggregation being stronger in the temperate rainforest of southern Chile and particularly in the host-specialist species. Our results suggest that environmental constraints are more important than ecological constraints (host range) in shaping mistletoe spatial structure. Mistletoe aggregated spatial distribution depends primarily on the environment that they inhabit, which conditions host spatial availability, and arrangement.

Water status and macronutrient concentrations, but not carbon status, of Viscum album ssp. album are determined by its hosts: a study across nine mistletoe-host pairs in central Switzerland

Introduction

European mistletoe, Viscum album L., is a hemiparasite that can infect various tree species, yet our understanding of its physiological interactions with host species is limited.

Methods

Nine mistletoe-host pairs (i.e. V. album ssp. album growing on nine different broadleaf tree species) under different growth conditions in central Switzerland were selected to examine the carbon, water and nutrient relationships between mistletoe and its hosts. We measured leaf morphological traits, isotopic compositions (δ13C and δ15N), concentrations of non-structural carbohydrates (NSC) and specific compounds (i.e. mobile sugars and starch), and macronutrients (i.e. N, P, K, Ca, Mg, S) in leaf and xylem tissues of both mistletoe and its hosts.

Results and Discussion

There were only non-significant relationships between NSC concentrations in mistletoe and in its host species across the nine mistletoe-host pairs, suggesting the carbon condition of V. album ssp. album is determined by both the heterotrophic carbon transfer and self-photosynthetic capacity among different mistletoe-host pairs. However, mistletoe leaf morphological traits (single leaf area and mass, and leaf mass per unit leaf area) did not change across the nine mistletoe-host pairs, and mistletoe leaf δ13C, water content and macronutrient concentrations were linearly correlated with those in the host leaves. Macronutrients showed accumulations in mistletoe across the nine pairs. Further, tissue N concentrations were significantly higher in mistletoe grown on N-fixing hosts than on non-N-fixing hosts. Finally, leaf N:P in mistletoe was significantly correlated with the ratio in the host across the nine mistletoe-host pairs. Overall, our results indicate strong relationships between mistletoe and its hosts for water- and nutrient-related traits, but not for carbon-related traits, which demonstrates that V. album ssp. album can adjust its physiology to survive on different deciduous tree species hosts and under different site conditions.

The Angiosperm Stem Hemiparasitic Genus Cassytha (Lauraceae) and Its Host Interactions: A Review

Cassytha, also known as laurel dodder or love vine, is a stem hemiparasite of the Lauraceae family. It has long been used for medicinal purposes in many countries and has increasingly influenced agricultural and natural ecosystems by its effects on a wide range of host species. Previous studies have focused on the taxonomy and evolutionary position of different Cassytha, with the pan-tropical species Cassytha filiformis being the most widely studied. However, Cassytha-host interactions have never been reviewed, which is an essential issue related to the understanding of mechanisms underlying plant hemiparasitic and the assessment of benefits and damage caused by aerial parasitic plants. This review explores the parasitic habits, worldwide distribution, and host range of Cassytha, and examines its impacts on the biology of host plants and the overall influence of environmental changes on Cassytha-host associations. We also comment on areas of future research directions that require to better understanding Cassytha-host interactions. It appeared that some traits, such as flowering phenology, facilitated Cassytha's widespread distribution and successful parasitism and that Cassytha preferred woody species rather than herbaceous species as a host, and preferred species from certain families as hosts, such as Fabaceae and Myrtaceae. Cassytha often decreased biomass and impacted the physiology of host species and global environmental changes seemed to intensify the negative impacts of Cassytha on their hosts. Cassytha was not only a noxious weed, but can also function as a biocontrol agent to mitigate alien plant invasion.

There Is No Carbon Transfer Between Scots Pine and Pine Mistletoe but the Assimilation Capacity of the Hemiparasite Is Constrained by Host Water Use Under Dry Conditions

Pine mistletoe is a hemiparasitic shrub that can produce its own photosynthates. There is a lack of knowledge about the interaction of mistletoe and host under varying environmental condition that might influence carbon gain and allocation. In a 13C-pulse labeling experiment with mature Pinus sylvestris (pine) infected by mistletoes grown in naturally dry or irrigated conditions, (1) mistletoe clusters were shielded from 13CO2 added, and (2) mistletoes or host needles were removed to manipulate the local assimilate and water availability. No 13C signal was found in shielded mistletoes, indicating no carbon transfer from the host to the mistletoe. When the pine needles were removed from girdled branches, no 13C signal was found in the host tissues, implying no carbon transfer from mistletoe to the host. However, mistletoes on needle-removed pine trees accumulated more labeled assimilates and had higher non-structural carbohydrate (NSC) concentrations only under naturally dry conditions but not in irrigated plots. Our results suggest that mistletoes show full carbon autonomy, as they neither receive carbon from nor provide carbon resource to the host trees. Moreover, the high assimilation capacity of mistletoes seems to be constrained by the host water use under dry conditions, suggesting that drought stress is not only negatively impacting trees but also mistletoes. Therefore, we conclude that the hemiparasites live on their own in terms of carbon gain which, however, depends on the water provided by the host tree.

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.

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.

Partial mycoheterotrophy is common among chlorophyllous plants with Paris-type arbuscular mycorrhiza

BACKGROUND AND AIMS

An arbuscular mycorrhiza is a mutualistic symbiosis with plants as carbon providers for fungi. However, achlorophyllous arbuscular mycorrhizal species are known to obtain carbon from fungi, i.e. they are mycoheterotrophic. These species all have the Paris type of arbuscular mycorrhiza. Recently, two chlorophyllous Paris-type species proved to be partially mycoheterotrophic. In this study, we explore the frequency of this condition and its association with Paris-type arbuscular mycorrhiza.

METHODS

We searched for evidence of mycoheterotrophy in all currently published 13C, 2H and 15N stable isotope abundance patterns suited for calculations of enrichment factors, i.e. isotopic differences between neighbouring Paris- and Arum-type species. We found suitable data for 135 plant species classified into the two arbuscular mycorrhizal morphotypes.

KEY RESULTS

About half of the chlorophyllous Paris-type species tested were significantly enriched in 13C and often also enriched in 2H and 15N, compared with co-occurring Arum-type species. Based on a two-source linear mixing model, the carbon gain from the fungal source ranged between 7 and 93 % with ferns > horsetails > seed plants. The seed plants represented 13 families, many without a previous record of mycoheterotrophy. The 13C-enriched chlorophyllous Paris-type species were exclusively herbaceous perennials, with a majority of them thriving on shady forest ground.

CONCLUSIONS

Significant carbon acquisition from fungi appears quite common and widespread among Paris-type species, this arbuscular mycorrhizal morphotype probably being a pre-condition for developing varying degrees of mycoheterotrophy.

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.

Comparative Plastome Analysis of Root- and Stem-Feeding Parasites of Santalales Untangle the Footprints of Feeding Mode and Lifestyle Transitions

In plants, parasitism triggers the reductive evolution of plastid genomes (plastomes). To disentangle the molecular evolutionary associations between feeding on other plants below- or aboveground and general transitions from facultative to obligate parasitism, we analyzed 34 complete plastomes of autotrophic, root- and stem-feeding hemiparasitic, and holoparasitic Santalales. We observed inexplicable losses of housekeeping genes and tRNAs in hemiparasites and dramatic genomic reconfiguration in holoparasitic Balanophoraceae, whose plastomes have exceptionally low GC contents. Genomic changes are related primarily to the evolution of hemi- or holoparasitism, whereas the transition from a root- to a stem-feeding mode plays no major role. In contrast, the rate of molecular evolution accelerates in a stepwise manner from autotrophs to root- and then stem-feeding parasites. Already the ancestral transition to root-parasitism coincides with a relaxation of selection in plastomes. Another significant selectional shift in plastid genes occurs as stem-feeders evolve, suggesting that this derived form coincides with trophic specialization despite the retention of photosynthetic capacity. Parasitic Santalales fill a gap in our understanding of parasitism-associated plastome degeneration. We reveal that lifestyle-genome associations unfold interdependently over trophic specialization and feeding mode transitions, where holoparasitic Balanophoraceae provide a system for exploring the functional realms of plastomes.

Parasitism modifies the direct effects of warming on a hemiparasite and its host

Climate change is affecting interactions among species, including host-parasite interactions. The effects of warming are of particular interest for interactions in which parasite and host physiology are intertwined, such as those between parasitic plants and their hosts. However, little is known about how warming will affect plant parasitic interactions, hindering our ability to predict how host and parasite species will respond to climate change. Here, we test how warming affects aboveground and belowground biomass of a hemiparasitic species (Castilleja sulphurea) and its host (Bouteloua gracilis), asking whether the effects of warming depend on the interaction between these species. We also measured how warming affected the number of haustorial connections between parasite and host. We grew each species alone and together under ambient and warmed conditions. Hosts produced more belowground biomass under warming. However, host biomass was reduced when plants were grown with a hemiparasite. Thus, parasitism negated the benefit of warming on belowground growth of the host. Host resource allocation to roots versus shoots also changed in response to both interaction with the parasite and warming, with hosts producing more root biomass relative to shoot biomass when grown with a parasite and when warmed. As expected, hemiparasite biomass was greater when grown with a host. Warmed parasites had lower root:shoot ratios but only when grown with a host. Under elevated temperatures, hemiparasite aboveground biomass was marginally greater, and plants produced significantly more haustoria. These findings indicate that warming can influence biomass production, both by modifying the interaction between host plants and hemiparasites and by affecting the growth of each species directly. To predict how species will be affected, it is important to understand not only the direct effects of warming but also the indirect effects that are mediated by species interactions. Ultimately, understanding how climate change will affect species interactions is key to understanding how it will affect individual species.

Picky carnivorous plants? Investigating preferences for preys' trophic levels - a stable isotope natural abundance approach with two terrestrial and two aquatic Lentibulariaceae tested in Central Europe

BACKGROUND AND AIMS

Stable isotope two-source linear mixing models are frequently used to calculate the nutrient-uptake efficiency of carnivorous plants from pooled prey. This study aimed to separate prey into three trophic levels as pooled prey limits statements about the contribution of a specific trophic level to the nutrition of carnivorous plants. Phytoplankton were used as an autotrophic reference for aquatic plants as the lack of suitable reference plants impedes calculation of their efficiency.

METHODS

Terrestrial (Pinguicula) and aquatic (Utricularia) carnivorous plants alongside autotrophic reference plants and potential prey from six sites in Germany and Austria were analysed for their stable isotope natural abundances (δ15N, δ13C). A two-source linear mixing model was applied to calculate the nutrient-uptake efficiency of carnivorous plants from pooled prey. Prey preferences were determined using a Bayesian inference isotope mixing model.

KEY RESULTS

Phytophagous prey represented the main contribution to the nutrition of Pinguicula (approx. 55 %), while higher trophic levels contributed a smaller amount (diverse approx. 27 %, zoophagous approx. 17 %). As well as around 48 % nitrogen, a small proportion of carbon (approx. 9 %) from prey was recovered in the tissue of plants. Aquatic Utricularia australis received 29 % and U. minor 21 % nitrogen from zooplankton when applying phytoplankton as the autotrophic reference.

CONCLUSIONS

The separation of prey animals into trophic levels revealed a major nutritional contribution of lower trophic level prey (phytophagous) for temperate Pinguicula species. Naturally, prey of higher trophic levels (diverse, zoophagous) are rarer, resulting in a smaller chance of being captured. Phytoplankton represents an adequate autotrophic reference for aquatic systems to estimate the contribution of zooplankton-derived nitrogen to the tissue of carnivorous plants. The autonomous firing of Utricularia bladders results in the additional capture of phytoplankton, calling for new aquatic references to determine the nutritional importance of phytoplankton for aquatic carnivorous plants.

Genome-wide identification of MST, SUT and SWEET family sugar transporters in root parasitic angiosperms and analysis of their expression during host parasitism

BACKGROUND

Root parasitic weeds are a major constraint to crop production worldwide causing significant yearly losses in yield and economic value. These parasites cause their destruction by attaching to their hosts with a unique organ, the haustorium, that allows them to obtain the nutrients (sugars, amino acids, etc.) needed to complete their lifecycle. Parasitic weeds differ in their nutritional requirements and degree of host dependency and the differential expression of sugar transporters is likely to be a critical component in the parasite's post-attachment survival.

RESULTS

We identified gene families encoding monosaccharide transporters (MSTs), sucrose transporters (SUTs), and SWEETs (Sugars Will Eventually be Exported Transporters) in three root-parasitic weeds differing in host dependency: Triphysaria versicolor (facultative hemiparasite), Phelipanche aegyptiaca (holoparasite), and Striga hermonthica (obligate hemiparasite). The phylogenetic relationship and differential expression profiles of these genes throughout parasite development were examined to uncover differences existing among parasites with different levels of host dependence. Differences in estimated gene numbers are found among the three parasites, and orthologs within the different sugar transporter gene families are found to be either conserved among the parasites in their expression profiles throughout development, or to display parasite-specific differences in developmentally-timed expression. For example, MST genes in the pGLT clade express most highly before host connection in Striga and Triphysaria but not Phelipanche, whereas genes in the MST ERD6-like clade are highly expressed in the post-connection growth stages of Phelipanche but highest in the germination and reproduction stages in Striga. Whether such differences reflect changes resulting from differential host dependence levels is not known.

CONCLUSIONS

While it is tempting to speculate that differences in estimated gene numbers and expression profiles among members of MST, SUT and SWEET gene families in Phelipanche, Striga and Triphysaria reflect the parasites' levels of host dependence, additional evidence that altered transporter gene expression is causative versus consequential is needed. Our findings identify potential targets for directed manipulation that will allow for a better understanding of the nutrient transport process and perhaps a means for controlling the devastating effects of these parasites on crop productivity.

Absence of Complex I Is Associated with Diminished Respiratory Chain Function in European Mistletoe

Parasitism is a life history strategy found across all domains of life whereby nutrition is obtained from a host. It is often associated with reductive evolution of the genome, including loss of genes from the organellar genomes [1, 2]. In some unicellular parasites, the mitochondrial genome (mitogenome) has been lost entirely, with far-reaching consequences for the physiology of the organism [3, 4]. Recently, mitogenome sequences of several species of the hemiparasitic plant mistletoe (Viscum sp.) have been reported [5, 6], revealing a striking loss of genes not seen in any other multicellular eukaryotes. In particular, the nad genes encoding subunits of respiratory complex I are all absent and other protein-coding genes are also lost or highly diverged in sequence, raising the question what remains of the respiratory complexes and mitochondrial functions. Here we show that oxidative phosphorylation (OXPHOS) in European mistletoe, Viscum album, is highly diminished. Complex I activity and protein subunits of complex I could not be detected. The levels of complex IV and ATP synthase were at least 5-fold lower than in the non-parasitic model plant Arabidopsis thaliana, whereas alternative dehydrogenases and oxidases were higher in abundance. Carbon flux analysis indicates that cytosolic reactions including glycolysis are greater contributors to ATP synthesis than the mitochondrial tricarboxylic acid (TCA) cycle. Our results describe the extreme adjustments in mitochondrial functions of the first reported multicellular eukaryote without complex I.

Chemical Traits of Hemiparasitism in Odontites luteus

The study of the monoterpene glycosides content of Odontites luteus has shown the presence of a total of fifteen iridoid glucosides. The presence of compounds 1 - 5 and 7 - 10 is perfectly on-line with both the biogenetic pathway for iridoids biosynthesis in Lamiales and the current botanical classification of the species. On the other side, the presence of compounds like agnuside (6), adoxosidic acid (11), monotropein (12), 6,7-dihydromonotropein (13), methyl oleoside (14) and methyl glucooleoside (15) is of high interest because, first of all, they have never been reported before in Lamiales. In second instance, the majority of the last compounds are formally derived from a different biogenetic pathway which involves deoxyloganic acid/loganin and led to the formation of decarboxylated iridoid showing the 8β-configuration. Furthermore, a second abnormality was found during our study and this regards compounds 14 and 15 which are seco-iriodids and thus not typical for this family. The presence of these unusual compounds, biogenetically not related to species belonging to Lamiales, is a clear evidence of the metabolites transfer from the hosts. In fact, the collection area is also populated by species belonging to Oleaceae and Ericaceae which could be the possible hosts since the biosynthesis of seco-iridoids and or iridoids related to deoxyloganic acid/loganin pathway, with the 8β-configuration, is well documented in these species.

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.

Host shoot clipping depresses the growth of weedy hemiparasitic Pedicularis kansuensis

Root hemiparasitic plants show optimal growth when attached to a suitable host by abstracting water and nutrients. Despite the fact that damage to host plants in the wild occurs frequently in various forms (e.g. grazing), effects of host damage on growth and physiological performance of root hemiparasites remain unclear. In this study, host shoot clipping was conducted to determine the influence of host damage on photosynthetic and growth performance of a weedy root hemiparasite, Pedicularis kansuensis, and its interaction with a host, Elymus nutans. Photosynthetic capacity, tissue mineral nutrient content and plant biomass of P. kansuensis were significantly improved when attached to a host plant. Host clipping had no effect on quantum efficiency (ΦPSII), but significantly reduced the growth rate and biomass of P. kansuensis. In contrast, clipping significantly improved photosynthetic capacity and accumulation of potassium in E. nutans. No significant decrease in biomass was observed in clipped host plants. By changing nutrient absorption and allocation, clipping affected the interaction between P. kansuensis and its host. Our results showed that host clipping significantly suppressed the growth of weedy P. kansuensis, but did not affect biomass accumulation in E. nutans. We propose that grazing (a dominant way of causing host damage in the field) may have a potential in the control against the weedy hemiparasite.

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.

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

Root hemiparasites are green photosynthetic plants, which parasitically acquire resources from host xylem. Mineral nutrients and water, two principal below-ground abiotic resources, were assumed to affect the interaction between hemiparasites and their hosts. The shape of these effects and the underlying physiological mechanisms have, however, remained unclear. We conducted a glasshouse experiment with root-hemiparasitic Rhinanthus alectorolophus, in which we manipulated the availability of mineral nutrients and water. Biomass production and Chl fluorescence of the hemiparasites and hosts were recorded, together with proportion of host-derived carbon in hemiparasite biomass. The abiotic resources had profound interactive effects on the performance of both the hemiparasite and the hosts, as well as the balance of above-ground biomass between them. These effects were mainly based on an increase of growth and photosynthetic efficiency under high nutrient concentrations, on the hemiparasite's ability to induce strong water stress on the hosts if water is limiting, and on release of the host from parasitism by simultaneous abundance of both resources. Hemiparasitism is a highly variable interaction, in which environmental conditions affect both the parasitic and autotrophic (and thus competitive) components. A hemiparasite's own photosynthesis plays a crucial role in the assimilation of parasitized mineral resources and their transformation into growth and fitness.

Water and nutrient relationships between a mistletoe and its mangrove host under saline conditions

Xylem-tapping mistletoes are known to have generally higher transpiration rate (Tr), lower CO2 assimilation rate (A) and therefore lower water-use efficiency (WUE) than their hosts. There are long-standing contradictions in water relations and nitrogen use in photosynthesis. Gas exchange, chlorophyll fluorescence and nutrition components were investigated in a special mistletoe-host pair, Viscum ovalifolium-Sonneratia caseolaris, as the host was a mangrove growing in a saline environment. Our results show that both plants had high foliar N content, therefore it was consistent with the N-parasitism hypothesis, although the mistletoe had a lower Tr than its mangrove host. It was suggested that the mistletoe reduces its Tr under salt stress with N sufficient conditions. The mistletoe had a fundamental limitation of photosynthesis, and was photoinhibited with regard to high salinity, but it developed more photoprotection to thermal radiation. Additionally, both stomatal conductance (gs) and mesophyll conductance (gm) limitations on photosynthesis dominated in the mistletoe under salt stress even though it had a high foliar N content similar to the host.

Ultrastructure of hydathode trichomes of hemiparasitic Rhinanthus alectorolophus and Odontites vernus: how important is their role in physiology and evolution of parasitism in Orobanchaceae?

The Rhinanthoid clade of the family Orobanchaceae comprises plants displaying a hemiparasitic or holoparasitic strategy of resource acquisition. Some of its species (mainly Rhinanthus spp.) are often used as models for studies of hemiparasite physiology. Although there is a well-developed concept covering their physiological processes, most recent studies have neglected the existence of hydathode trichomes present on leaves of these hemiparasitic plants. As a first step for the proposed integration of these structures in the theory of physiological processes of the hemiparasites, we described the outer micromorphology and ultrastructure of the hydathode trichomes on leaves of hemiparasitic Rhinanthus alectorolophus and Odontites vernus with scanning and transmission electron microscopy (SEM and TEM, respectively). The TEM inspections of both types of trichome revealed typical ultrastructural features: labyrinthine cell wall, high content of cytoplasm in cells with numerous mitochondria and presence of plasmodesmata. All these features indicate high metabolic activity complying with their function as glandular trichomes actively secreting water. The active secretion of water by the hydathode trichomes (evidence for which is summarised here) also presents a possible mechanism explaining results of previous gas exchange measurements detecting high dark respiration and transpiration rates and a tight inter-correlation between them in hemiparasitic Orobanchaceae. In addition, this process is hypothesised to have allowed multiple evolutionary transitions from facultative to obligate hemiparasitism and unique xylem-feeding holoparasitism of Lathraea with a long-lived underground stage featuring a rhizome covered by scales of leaf origin.

How does elevated grassland productivity influence populations of root hemiparasites? Commentary on Borowicz and Armstrong (Oecologia 2012)

In their recent study, Borowicz and Armstrong (Oecologia 169:783-792, 2012) investigated effects of nutrient availability and competition for light on a perennial root hemiparasite Pedicularis canadensis. Their study showed a reduction of community productivity as a result of hemiparasite infection independently of a clear positive effect of increased nutrients. In contrast, there was a minimal effect of increased competition for light on growth of the parasite. Here, we summarize the available data on the influence of nutrient availability (closely related to productivity) on temperate grassland root hemiparasites thus expanding the discussion presented by Borowicz and Armstrong (Oecologia 169:783-792, 2012). Most studies show that root hemiparasites are highly sensitive to elevated competition for light in productive environments, which is manifested as an increase in mortality coupled to a decrease in population density. Such responses reflect increased mortality of hemiparasite seedlings that are physiologically inefficient in terms of photosynthesis and nutrient acquisition owing to a limited root network and consequently, are highly sensitive to competition for light. However, the susceptibility of hemiparasites to competition for light tends to decrease for individuals that survive the critical seedling stage. Moreover, survivors benefit from elevated nutrient availability, resulting in increased growth and fecundity. Elevated productivity can thus have opposing effects on the survival and growth of hemiparasites depending on life stage. We conclude that the findings by Borowicz and Armstrong (Oecologia 169:783-792, 2012) are not in conflict with this general view that root hemiparasite population ecology is strongly influenced by competition for light in highly productive environments.

The role of heterotrophic carbon acquisition by the hemiparasitic plant Rhinanthus alectorolophus in seedling establishment in natural communities: a physiological perspective

• Heterotrophic acquisition of substantial amounts of organic carbon by hemiparasitic plants was clearly demonstrated by numerous studies. Many hemiparasites are, however, also limited by competition for light preventing the establishment of their populations on highly productive sites. • In a growth-chamber experiment, we investigated the effects of competition for light, simulated by shading, on growth and heterotrophic carbon acquisition by the hemiparasite Rhinanthus alectorolophus attached to C(3) and C(4) hosts using analyses of biomass production and stable isotopes of carbon. • Shading had a detrimental effect on biomass production and vertical growth of the hemiparasites shaded from when they were seedlings, while shading imposed later caused only a moderate decrease of biomass production and had no effect on the height. Moreover, shading increased the proportion of host-derived carbon in hemiparasite biomass (up to 50% in shaded seedlings). • These results demonstrate that host-derived carbon can play a crucial role in carbon budget of hemiparasites, especially if they grow in a productive environment with intense competition for light. The heterotrophic carbon acquisition can allow hemiparasite establishment in communities of moderate productivity, helping well-attached hemiparasites to escape from the critical seedling stage.