Progress in parasitic plant biology: host selection and nutrient transfer

Fertilization benefits the facultative parasitic plant Rhamphicarpa fistulosa while gains by the infected host Oryza sativa are marginalized

BACKGROUND AND AIMS

Rhamphicarpa fistulosa (Hochst.) Benth. is an annual facultative parasitic plant adapted to hydromorphic soils. In sub-Saharan Africa it causes high crop losses as a weed in rainfed lowland rice (Oryza sativa L.). Fertilizers are often proposed as a control measure against hemiparasitic weeds, but an understanding of the nutrient effects on R. fistulosa is currently still elusive.

METHODS

In two greenhouse pot experiments, conducted in 2016 in the Netherlands and in 2019 in the UK, host plants (O. sativa, cv IR64) and parasitic plants (R. fistulosa) were grown alone or combined and were subjected to different levels of nutrient availability. Biomass measurements were used to assess whether and how effects of nutrient availability are expressed in the host and parasite.

KEY RESULTS

Compared with parasite-free host plants, the biomass of parasite-infested plants was severely reduced, and nutrient effects on host plant biomass were less pronounced. Conversely, increased nutrient availability did not have an effect on parasitic plants when grown alone, but when grown with a host the parasitic plant biomass increased proportionally. Grown together, the combined biomass of host plant and parasite was substantially lower than that of the host plant grown alone. The ratio of biomass between host plant and parasite was unaffected by nutrient availability.

CONCLUSIONS

Fertilization benefits to rice plants are severely reduced but not completely nullified by R. fistulosa infection. The benefits to production and reproduction accrued by the parasite from increased nutrient availability are restricted to conditions in the presence of a host plant. Host presence and nutrient effects are thus observed to be synergetic; R. fistulosa plants parasitizing a suitable host respond strongly to increasing levels of nutrients. This is associated with an increased root biomass of the parasitic plant itself, but is more likely to result from exploitation of the nutrient uptake capacity of the host plant it parasitizes.

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.

Inhibition of broomrape germination by 2,4-diacetylphloroglucinol produced by environmental Pseudomonas

Parasitic weeds such as broomrapes (Phelipanche ramosa and Orobanche cumana) cause severe damage to crops and their development must be controlled. Given that phloroglucinol compounds (PGCs) produced by environmental Pseudomonas could be toxic towards certain plants, we assessed the potential herbicidal effect of the bacterial model Pseudomonas ogarae F113, a PGCs-producing bacterium, on parasitic weed. By combining the use of a mutagenesis approach and of pure PGCs, we evaluated the in vitro effect of PGC-produced by P. ogarae F113 on broomrape germination and assessed the protective activity of a PGC-producing bacteria on oilseed rape (Brassica napus) against P. ramosa in non-sterile soils. We showed that the inhibition of the germination depends on the PGCs molecular structure and their concentrations as well as the broomrape species and pathovars. This inhibition caused by the PGCs is irreversible, causing a brown coloration of the broomrape seeds. The inoculation of PGCs-producing bacteria limited the broomrape infection of P. ramosa, without affecting the host growth. Moreover, elemental profiling analysis of oilseed rape revealed that neither F113 nor applied PGCs affected the nutrition capacity of the oilseed rape host. Our study expands the knowledge on plant-beneficial Pseudomonas as weed biocontrol agents and opens new avenues for the development of natural bioherbicides to enhance crop yield.

Host shift promotes divergent evolution between closely related holoparasitic species

Distinct hosts have been hypothesized to possess the potential for affecting species differentiation and genome evolution of parasitic organisms. However, what host shift history is experienced by the closely related parasites and whether disparate evolution of their genomes occur remain largely unknown. Here, we screened horizontal gene transfer (HGT) events in a pair of sister species of holoparasitic Boschniakia (Orobanchaceae) having obligate hosts from distinct families to recall the former host-parasite associations and performed a comparative analysis to investigate the difference of their organelle genomes. Except those from the current hosts (Ericaceae and Betulaceae), we identified a number of HGTs from Rosaceae supporting the occurrence of unexpected ancient host shifts. Different hosts transfer functional genes which changed nuclear genomes of this sister species. Likewise, different donors transferred sequences to their mitogenomes, which vary in size due to foreign and repetitive elements rather than other factors found in other parasites. The plastomes are both severely reduced, and the degree of difference in reduction syndrome reaches the intergeneric level. Our findings provide new insights into the genome evolution of parasites adapting to different hosts and extend the mechanism of host shift promoting species differentiation to parasitic plant lineages.

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.

Root hemiparasitic plants are associated with more even communities across North America

Root hemiparasitic plants both compete with and extract resources from host plants. By reducing the abundance of dominant plants and releasing subordinates from competitive exclusion, they can have an outsized impact on plant communities. Most research on the ecological role of hemiparasites is manipulative and focuses on a small number of hemiparasitic taxa. Here, we ask whether patterns in natural plant communities match the expectation that hemiparasites affect the structure of plant communities. Our data were collected on 129 national park units spanning the continental United States. The most common hemiparasite genera were Pedicularis, Castilleja, Krameria, and Comandra. We used null models and linear mixed models to determine whether hemiparasites were associated with changes in community richness and evenness. Hemiparasite presence did not affect community metrics. Hemiparasite abundance was positively associated with increasing evenness of herbaceous species, but not with species richness. The associations that we observed on a continental scale are consistent with evidence that the impacts of root hemiparasitic plants on evenness can be substantial and abundance dependent but that effects on richness are less pronounced. Hemiparasites mediate competitive exclusion in communities to facilitate species coexistence and merit consideration of inclusion in ecological theories of coexistence.

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.

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.

Parasitic Plants: An Overview of Mechanisms by Which Plants Perceive and Respond to Parasites

In contrast to most autotrophic plants, which produce carbohydrates from carbon dioxide using photosynthesis, parasitic plants obtain water and nutrients by parasitizing host plants. Many important crop plants are infested by these heterotrophic plants, leading to severe agricultural loss and reduced food security. Understanding how host plants perceive and resist parasitic plants provides insight into underlying defense mechanisms and the potential for agricultural applications. In this review, we offer a comprehensive overview of the current understanding of host perception of parasitic plants and the pre-attachment and post-attachment defense responses mounted by the host. Since most current research overlooks the role of organ specificity in resistance responses, we also summarize the current understanding and cases of cross-organ parasitism, which indicates nonconventional haustorial connections on other host organs, for example, when stem parasitic plants form haustoria on their host roots. Understanding how different tissue types respond to parasitic plants could provide the potential for developing a universal resistance mechanism in crops against both root and stem parasitic plants.

Striga hermonthica Suicidal Germination Activity of Potent Strigolactone Analogs: Evaluation from Laboratory Bioassays to Field Trials

The obligate hemiparasite Striga hermonthica is one of the major global biotic threats to agriculture in sub-Saharan Africa, causing severe yield losses of cereals. The germination of Striga seeds relies on host-released signaling molecules, mainly strigolactones (SLs). This dependency opens up the possibility of deploying SL analogs as "suicidal germination agents" to reduce the accumulated seed bank of Striga in infested soils. Although several synthetic SL analogs have been developed for this purpose, the utility of these compounds in realizing the suicidal germination strategy for combating Striga is still largely unknown. Here, we evaluated the efficacy of three potent SL analogs (MP3, MP16, and Nijmegen-1) under laboratory, greenhouse, and farmer's field conditions. All investigated analogs showed around a 50% Striga germination rate, equivalent to a 50% reduction in infestation, which was comparable to the standard SL analog GR24. Importantly, MP16 had the maximum reduction of Striga emergence (97%) in the greenhouse experiment, while Nijmegen-1 appeared to be a promising candidate under field conditions, with a 43% and 60% reduction of Striga emergence in pearl millet and sorghum fields, respectively. These findings confirm that the selected SL analogs appear to make promising candidates as simple suicidal agents both under laboratory and real African field conditions, which may support us to improve suicidal germination technology to deplete the Striga seed bank in African agriculture.

LATERAL ORGAN BOUNDARIES DOMAIN 25 functions as a key regulator of haustorium development in dodders

Parasitic plants reduce crop yield worldwide. Dodder (Cuscuta campestris) is a stem parasite that attaches to its host, using haustoria to extract nutrients and water. We analyzed the transcriptome of six C. campestris tissues and identified a key gene, LATERAL ORGAN BOUNDARIES DOMAIN 25 (CcLBD25), as highly expressed in prehaustoria and haustoria. Gene coexpression networks from different tissue types and laser-capture microdissection RNA-sequencing data indicated that CcLBD25 could be essential for regulating cell wall loosening and organogenesis. We employed host-induced gene silencing by generating transgenic tomato (Solanum lycopersicum) hosts that express hairpin RNAs to target and down-regulate CcLBD25 in the parasite. Our results showed that C. campestris growing on CcLBD25 RNAi transgenic tomatoes transited to the flowering stage earlier and had reduced biomass compared with C. campestris growing on wild-type (WT) hosts, suggesting that parasites growing on transgenic plants were stressed due to insufficient nutrient acquisition. We developed an in vitro haustorium system to assay the number of prehaustoria produced on strands from C. campestris. Cuscuta campestris grown on CcLBD25 RNAi tomatoes produced fewer prehaustoria than those grown on WT tomatoes, indicating that down-regulating CcLBD25 may affect haustorium initiation. Cuscuta campestris haustoria growing on CcLBD25 RNAi tomatoes exhibited reduced pectin digestion and lacked searching hyphae, which interfered with haustorium penetration and formation of vascular connections. The results of this study elucidate the role of CcLBD25 in haustorium development and might contribute to developing parasite-resistant crops.

Physiological and ecological warnings that dodders pose an exigent threat to farmlands in Eastern Africa

Invasive holoparasitic plants of the genus Cuscuta (dodder) threaten African ecosystems due to their rapid spread and attack on various host plant species. Most Cuscuta species cannot photosynthesize and hence rely on host plants for nourishment. After attachment through a peg-like organ called a haustorium, the parasites deprive hosts of water and nutrients, which negatively affects host growth and development. Despite their rapid spread in Africa, dodders have attracted limited research attention, although data on their taxonomy, host range, and epidemiology are critical for their management. Here, we combine taxonomy and phylogenetics to reveal the presence of field dodder (Cuscuta campestris) and C. kilimanjari (both either naturalized or endemic to East Africa), in addition to the introduction of the giant dodder (C. reflexa), a south Asian species, in continental Africa. These parasites have a wide host range, parasitizing species across 13 angiosperm orders. We evaluated the possibility of C. reflexa to expand this host range to tea (Camelia sinensis), coffee (Coffea arabica), and mango (Mangifera indica), crops of economic importance to Africa, for which haustorial formation and vascular-bundle connections in all three crops revealed successful parasitism. However, only mango mounted a successful postattachment resistance response. Furthermore, species distribution models predicted high habitat suitability for Cuscuta spp. across major tea- and coffee-growing regions of Eastern Africa, suggesting an imminent risk to these crops. Our findings provide relevant insights into a poorly understood threat to biodiversity and economic wellbeing in Eastern Africa, and provide critical information to guide development of management strategies to avert Cuscuta spp. spread.

Current progress in Striga management

The Striga, particularly S. he rmonthica, problem has become a major threat to food security, exacerbating hunger and poverty in many African countries. A number of Striga control strategies have been proposed and tested during the past decade, however, further research efforts are still needed to provide sustainable and effective solutions to the Striga problem. In this paper, we provide an update on the recent progress and the approaches used in Striga management, and highlight emerging opportunities for developing new technologies to control this enigmatic parasite.

Broomrape Species Parasitizing Odontarrhena lesbiaca (Brassicaceae) Individuals Act as Nickel Hyperaccumulators

The elemental defense hypothesis supports that metal hyperaccumulation in plant tissues serves as a mechanism underpinning plant resistance to herbivores and pathogens. In this study, we investigate the interaction between Odontarrhena lesbiaca and broomrape parasitic species, in the light of the defense hypothesis of metal hyperaccumulation. Plant and soil samples collected from three serpentine sites in Lesbos, Greece were analyzed for Ni concentrations. Phelipanche nowackiana and Phelipanche nana were found to infect O. lesbiaca. In both species, Ni concentration decreased gradually from tubercles to shoots and flowers. Specimens of both species with shoot nickel concentrations above 1000 mg.kg^-1 were found, showing that they act as nickel hyperaccumulators. Low values of parasite to O. lesbiaca leaf or soil nickel quotients were observed. Orobanche pubescens growing on a serpentine habitat but not in association with O. lesbiaca had very low Ni concentrations in its tissues analogous to excluder plants growing on serpentine soils. Infected O. lesbiaca individuals showed lower leaf nickel concentrations relative to the non-infected ones. Elevated leaf nickel concentration of O. lesbiaca individuals did not prevent parasitic plants to attack them and to hyperaccumulate metals to their tissues, contrary to predictions of the elemental defense hypothesis.

Kin Recognition in the Parasitic Plant Triphysaria versicolor Is Mediated Through Root Exudates

Triphysaria is a facultative parasitic plant in the Orobanchaceae that parasitizes the roots of a wide range of host plants including Arabidopsis, Medicago, rice and maize. The important exception to this broad host range is that Triphysaria rarely parasitize other Triphysaria. We explored self and kin recognition in Triphysaria versicolor and showed that exudates collected from roots of host species, Arabidopsis thaliana and Medicago truncatula, induced haustorium development when applied to the roots of Triphysaria seedlings in vitro while those collected from Triphysaria did not. In mixed exudate experiments, Triphysaria exudates did not inhibit the haustorium-inducing activity of those from host roots. Interestingly, when roots of Triphysaria seedlings were treated with either horseradish peroxidase or fungal laccase, the extracts showed haustorium-inducing factor (HIF) activity, suggesting that Triphysaria roots contain the proper substrates for producing HIFs. Transgenic Triphysaria roots overexpressing a fungal laccase gene TvLCC1 showed an increased responsiveness to a known HIF, 2,6-dimethoxy benzoquinone (DMBQ), in developing haustoria. Our results indicate kin recognition in Triphysaria is associated with the lack of active HIFs in root exudates. Treatment of Triphysaria roots with enzymatic oxidases activates or releases molecules that are HIFs. This study shows that exogenously applied oxidases can activate HIFs in Triphysaria roots that had no previous HIF activity. Further studies are necessary to determine if differential oxidase activities in host and parasite roots account for the kin recognition in haustorium development.

Responses of Rhizospheric Microbial Communities of Native and Alien Plant Species to Cuscuta Parasitism

Parasitic plants have major impacts on host fitness. In the case of species of the holoparasitic Cuscuta genus, these impacts were shown to be particularly strong in some invasive alien plants, which has raised interest in the underlying mechanism. We hypothesized that Cuscuta parasitization may exert strong influence in shaping the diversity patterns in the host rhizosphere microbiome and that this may vary between native (coevolved) and alien (non-coevolved) plants. Here, we report on a field study exploring the effect of parasitization by Cuscuta australis on the rhizosphere microbiota (16S and ITS rDNA) of four plant species sharing and three plant species not sharing the parasite's native range. Despite a predominant role of the host species in shaping the rhizosphere microbiota, the role of host origin and of parasitization still appeared important in structuring microbial communities and their associated functions. Bacterial communities were more strongly influenced than fungi by the native range of the host plant, while fungi were slightly more affected than bacteria by parasitization. About 7% of bacterial phylotypes and 11% of fungal phylotypes were sensitive to Cuscuta parasitization. Parasitization also reduced the abundance of arbuscular mycorrhizal fungi by ca. 18% and of several genes related to plant growth promoting functions (e.g., nitrogen metabolism and quorum sensing). Both fungi and bacteria differentially responded to host parasitization depending on host origin, and the extent of these shifts suggests that they may have more dramatic consequences for alien than for native plants.

Effects of enhanced UV-B radiation on the interaction between rice and Magnaporthe oryzae in Yuanyang terrace

Enhanced ultraviolet-B (UV-B) radiation affected the growth of rice and Magnaporthe oryzae, and changed the interactions between them. Increased UV-B radiation (5.0 kJ m-2 d-1) on rice leaves in a Yuanyang terrace was conducted before, during, and after infection of the leaves with Magnaporthe oryzae. The relationship between rice blast and UV-B radiation on the disease resistance of rice and the pathogenicity of M. oryzae was studied, and the effects of enhanced UV-B radiation on the interactions between rice and M. oryzae were analysed. The results indicated the following: (1) enhanced UV-B radiation significantly reduced the rice blast disease index, but as infection progressed, the inhibitory effect of UV-B radiation on the disease was weakened. (2) UV-B radiation treatment before infection with M. oryzae (UV-B + M.) significantly increased the activity of the enzymes related to disease resistance (phenylalanine ammonia lyase, lipoxygenase, chitinase, and β-1,3-glucanase), and the plants exhibited light-induced resistance. (3) Exposure to UV-B radiation after M. oryzae infection (M. + UV-B) did not induce disease course-related protein (PR) activity, but the content of soluble sugar increased. The osmotic stress caused by pathogenic fungi infection was alleviated by active accumulation of soluble sugar; due to this lack of nutrients, it was difficult for the rice blast fungus to grow. (4) Enhanced UV-B radiation significantly inhibited the production of conidia by M. oryzae, and the expression of the pathogenic genes Chitinase, MGP1, MAGB, and CPKA was significantly downregulated. The pathogenicity of M. oryzae was reduced by UV-B radiation. The resistance of rice leaves was weakened by simultaneous exposure to UV-B radiation and M. oryzae (UV-B/M.). Hence, UV-B radiation can weaken the infectivity of M. oryzae, improve the resistance of traditional rice, and contain the disease.

The genus Cuscuta (Convolvolaceac): An updated review on indigenous uses, phytochemistry, and pharmacology

Cuscuta, commonly known as dodder, is a genus of family convolvolaceace. Approximately 170 species of Cuscuta are extensively distributed in temperate and subtropical areas of the world. Species of this genus are widely used as essential constituents in functional foods and traditional medicinal systems. Various parts of many members of Cuscuta have been found efficacious against a variety of diseases. Phytochemical investigations have confirmed presence of biologically active moieties such as flavonoids, alkaloids, lignans, saponines, phenolics, tannins, and fatty acids. Pharmacological studies and traditional uses of these plants have proved that they are effective antibacterial, antioxidant, antiostioporotic, hepatoprotective, anti-inflammatory, antitumor, antipyretic, antihypertensive, analgesic, anti hair fall, and antisteriogenic agents.

Risk versus reward: host dependent parasite mortality rates and phenotypes in the facultative generalist Triphysaria versicolor

BACKGROUND

Parasitic plants engage in a complex molecular dialog with potential host plants to identify a host and overcome host defenses to initiate development of the parasitic feeding organ, the haustorium, invade host tissues, and withdraw water and nutrients. While one of two critical signaling events in the parasitic plant life cycle (germination via stimulant chemicals) has been relatively well-studied, the signaling event that triggers haustorium formation remains elusive. Elucidation of this poorly understood molecular dialogue will shed light on plant-plant communication, parasitic plant physiology, and the evolution of parasitism in plants.

RESULTS

Here we present an experimental framework that develops easily quantifiable contrasts for the facultative generalist parasitic plant, Triphysaria, as it feeds across a broad range of diverse flowering plants. The contrasts, including variable parasite growth form and mortality when grown with different hosts, suggest a dynamic and host-dependent molecular dialogue between the parasite and host. Finally, by comparing transcriptome datasets from attached versus unattached parasites we gain insight into some of the physiological processes that are altered during parasitic behavior including shifts in photosynthesis-related and stress response genes.

CONCLUSIONS

This work sheds light on Triphysaria's parasitic life habit and is an important step towards understanding the mechanisms of haustorium initiation factor perception, a unique form of plant-plant communication.

Assessment of Plant Growth Promoting and Abiotic Stress Tolerance Properties of Wheat Endophytic Fungi

The aims of the present work were to isolate and characterize fungal endophytic communities associated with healthy wheat (Triticum aestivum L.) plants, collected from the North China. Segregated endophytes were screened for their PGP traits, abiotic stresses (heavy metals, salinity, drought, and temperature), and antibiotic sensitivity. A total of 16 endophytic fungi were isolated using the culture-dependent approach from different tissue parts of wheat plants. Based upon their internal transcribed spacer (ITS) rDNA gene sequencing, 15 out of 16 isolates were selected for further analysis. In the contemporary investigation, a number of the tested endophytes exhibited fairly good 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD) (0.03±0.011 to 1.43±0.01 µmol α-KB mg⁻¹ protein hr⁻¹), indole acetic acid (IAA) (1.125±0.04 to36.12±0.004µgml⁻¹), and phosphate solubilizing index (PSI) (2.08±0.03to5.16±0.36) activities. More than 30% isolates gave positive result for siderophore and ammonia tests, whereas all exhibited catalase activity but only 2 (582PDA1 and 582PDA11) produced hydrogen cyanide. Trichoderma strains showed salt, heavy metals, and drought tolerance at high levels and also exhibited resistance to all the tested antibiotics. Strain 582PDA4 was found to be the most temperature (55°C) tolerant isolate. The findings of this study indicated that the microbial endophytes isolated from wheat plants possessing a crucial function to improve plant growth could be utilized as biofertilizers or bioagents to establish a sustainable crop production system.

Salinity-associated microRNAs and their potential roles in mediating salt tolerance in rice colonized by the endophytic root fungus Piriformospora indica

Piriformospora indica (P. indica), an endophytic root fungus, supports the growth and enhanced tolerance of plants to biotic and abiotic stresses. Several recent studies showed the significant role of small RNA (sRNA) molecules including microRNAs (miRNAs) in plant adaption to environmental stress, but little is known concerning the symbiosis-mediated salt stress tolerance regulated at miRNAs level. The overarching goal of this research is to elucidate the impact of miRNAs in regulating the P. indica-mediated salt tolerance in rice. Applying sRNA-seq analysis led to identify a set of 547 differentially abundant miRNAs in response to P. indica inoculation and salt stress. These included 206 rice-specific and 341 previously known miRNAs from other plant species. In silico analysis of miRNAs predictions of the differentially abundant miRNAs led to identifying of 193 putatively target genes, most of which were encoded either genes or transcription factors involved in nutrient uptake, sodium ion transporters, growth regulators, and auxin- responsive proteins. The rice-specific miRNAs targeted the transcription factors involved in the import of potassium ions into the root cells, the export of sodium ions, and plant growth and development. Interestingly, P. indica affected the differential abundance of miRNAs regulated genes and transcription factors linked to salt stress tolerance. Our data helps to understand the molecular basis of salt stress tolerance mediated by symbionts in plant and the potential impact of miRNAs for genetic improvement of rice varieties for tolerance to salt stress.

A new UHPLC-MS/MS method for the direct determination of strigolactones in root exudates and extracts

INTRODUCTION

Strigolactones (SLs) are the most representative germination stimulants for seeds of root parasitic plants, and they show activity even at concentrations below 10^-10  M. The low amounts of stimulants produced by the host and their rapid degradability make it crucial to develop analytical methods with very low limits of quantification.

OBJECTIVE

To develop a sensitive and validated analytical method for the simultaneous quantification of seven SLs [7-oxoorobanchyl acetate (1), solanacol (2), orobanchol (4), strigol (5), fabacyl acetate (6), orobanchyl acetate (7), and 5-deoxystrigol (8)].

METHODS

SLs were analysed using ultra-high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS), with (±)-GR24 (3) employed as internal standard (IS). Validation was based on selectivity, linearity, precision of the peak areas (repeatability and intermediate precision), detection and quantification limits, and stability.

RESULTS

A simple, rapid and reliable UHPLC-MS/MS method has been validated for the routine analysis of seven SLs and has been successfully applied to quantify them in exudates and extracts from tomato roots (Solanum lycopersicum). The limits of quantifications range from 0.05 μg/L for 5-deoxystrigol to 0.96 μg/L for solanacol.

CONCLUSION

The method provides a useful tool for research in all the fields related to SLs, both for studies related to their function as hormones, and signalling molecules in the rhizosphere, without sample preparation required for extracts and root exudates in less than 11 minutes.

Ecological Relevance of the Major Allelochemicals in Lycopersicon esculentum Roots and Exudates

Stigmasterol, bergapten, and α-tomatine were isolated from tomato roots. The preliminary phytotoxic activities of stigmasterol and α-tomatine were evaluated in a wheat-coleoptile bioassay, and α-tomatine was more active than stigmasterol. To confirm its phytotoxic activity, α-tomatine was tested on Lactuca sativa and two weeds ( Lolium perenne and Echinochloa crus-galli), and it was active in all cases. The stimulatory activities of α-tomatine and stigmasterol on parasitic-plant germination were also evaluated, and α-tomatine was found to be active on Phelipanche ramosa, a parasitic plant of tomato. α-Tomatine was identified in root exudates by LC-MS/MS. This confirms that α-tomatine is exuded by roots into the environment, where it could act as both an allelochemical and a stimulator of P. ramosa, a parasitic plant of tomato.

Resource-directed foraging of the Neotropical mistletoe Struthanthus flexicaulis (Loranthaceae)

Some parasitic plants are capable of vegetative propagation, which allows them access to new hosts and improves nutrient availability. We aimed to determine what factors positively influence this propagation in the xylem-tapping Struthanthus flexicaulis, focusing on the use of directed foraging as a means of optimising access to resources. The study site was a rupicolous plant community in southeast Brazil. We evaluated how the success of branch propagation (by contagion) of the mistletoe is influenced by the crown height and density of its main host, the legume Mimosa calodendron. Oriented foraging was tested through comparing the response of mistletoe branches toward a potential host (resources) or a plastic net (no resources). Successful contagion was related to high density and the departure height of a branch from a host crown. In a high-density area, 89% of branches reached a new host versus 21% in a low-density area. Formation of an appressorium on either the plastic nets or hosts elicited branch growth reorientation towards it, as well as development of new branches. These responses were significantly stronger towards hosts than nets, and attributed to effective xylem tapping. The foraging strategy of S. flexicaulis benefits from branch response (growth and reorientation) to the development of appressoria and their resulting haustoria. The growth of appressoria on non-resource substrates and the ensuing response optimises the mistletoe spread by allowing these substrates to act as physical support for spreading branches and to infect a host that was previously out of reach.

Piriformospora indica: Potential and Significance in Plant Stress Tolerance

Owing to its exceptional ability to efficiently promote plant growth, protection and stress tolerance, a mycorrhiza like endophytic Agaricomycetes fungus Piriformospora indica has received a great attention over the last few decades. P. indica is an axenically cultiviable fungus which exhibits its versatility for colonizing/hosting a broad range of plant species through directly manipulating plant hormone-signaling pathway during the course of mutualism. P. indica-root colonization leads to a better plant performance in all respect, including enhanced root proliferation by indole-3-acetic acid production which in turn results into better nutrient-acquisition and subsequently to improved crop growth and productivity. Additionally, P. indica can induce both local and systemic resistance to fungal and viral plant diseases through signal transduction. P. indica-mediated stimulation in antioxidant defense system components and expressing stress-related genes can confer crop/plant stress tolerance. Therefore, P. indica can biotize micropropagated plantlets and also help these plants to overcome transplantation shock. Nevertheless, it can also be involved in a more complex symbiotic relationship, such as tripartite symbiosis and can enhance population dynamic of plant growth promoting rhizobacteria. In brief, P. indica can be utilized as a plant promoter, bio-fertilizer, bioprotector, bioregulator, and biotization agent. The outcome of the recent literature appraised herein will help us to understand the physiological and molecular bases of mechanisms underlying P. indica-crop plant mutual relationship. Together, the discussion will be functional to comprehend the usefulness of crop plant-P. indica association in both achieving new insights into crop protection/improvement as well as in sustainable agriculture production.

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.

Leaf morphophysiology of a Neotropical mistletoe is shaped by seasonal patterns of host leaf phenology

Several mistletoe species are able to grow and reproduce on both deciduous and evergreen hosts, suggesting a degree of plasticity in their ability to cope with differences in intrinsic host functions. The aim of this study was to investigate the influence of host phenology on mistletoe water relations and leaf gas exchange. Mistletoe Passovia ovata parasitizing evergreen (Miconia albicans) hosts and P. ovata parasitizing deciduous (Byrsonima verbascifolia) hosts were sampled in a Neotropical savanna. Photosynthetic parameters, diurnal cycles of stomatal conductance, pre-dawn and midday leaf water potential, and stomatal anatomical traits were measured during the peak of the dry and wet seasons, respectively. P. ovata showed distinct water-use strategies that were dependent on host phenology. For P. ovata parasitizing the deciduous host, water use efficiency (WUE; ratio of photosynthetic rate to transpirational water loss) was 2-fold lower in the dry season than in the wet season; in contrast, WUE was maintained at the same level during the wet and dry seasons in P. ovata parasitizing the evergreen host. Generally, mistletoe and host diurnal cycles of stomatal conductance were linked, although there were clear differences in leaf water potential, with mistletoe showing anisohydric behaviour and the host showing isohydric behaviour. Compared to mistletoes attached to evergreen hosts, those parasitizing deciduous hosts had a 1.4-fold lower stomatal density and 1.2-fold wider stomata on both leaf surfaces, suggesting that the latter suffered less intense drought stress. This is the first study to show morphophysiological differences in the same mistletoe species parasitizing hosts of different phenological groups. Our results provide evidence that phenotypical plasticity (anatomical and physiological) might be essential to favour the use of a greater range of hosts.

De novo assembly and characterization of the transcriptome of the parasitic weed dodder identifies genes associated with plant parasitism

Parasitic flowering plants are one of the most destructive agricultural pests and have major impact on crop yields throughout the world. Being dependent on finding a host plant for growth, parasitic plants penetrate their host using specialized organs called haustoria. Haustoria establish vascular connections with the host, which enable the parasite to steal nutrients and water. The underlying molecular and developmental basis of parasitism by plants is largely unknown. In order to investigate the process of parasitism, RNAs from different stages (i.e. seed, seedling, vegetative strand, prehaustoria, haustoria, and flower) were used to de novo assemble and annotate the transcriptome of the obligate plant stem parasite dodder (Cuscuta pentagona). The assembled transcriptome was used to dissect transcriptional dynamics during dodder development and parasitism and identified key gene categories involved in the process of plant parasitism. Host plant infection is accompanied by increased expression of parasite genes underlying transport and transporter categories, response to stress and stimuli, as well as genes encoding enzymes involved in cell wall modifications. By contrast, expression of photosynthetic genes is decreased in the dodder infective stages compared with normal stem. In addition, genes relating to biosynthesis, transport, and response of phytohormones, such as auxin, gibberellins, and strigolactone, were differentially expressed in the dodder infective stages compared with stems and seedlings. This analysis sheds light on the transcriptional changes that accompany plant parasitism and will aid in identifying potential gene targets for use in controlling the infestation of crops by parasitic weeds.

Growth but not photosynthesis response of a host plant to infection by a holoparasitic plant depends on nitrogen supply

Parasitic plants can adversely influence the growth of their hosts by removing resources and by affecting photosynthesis. Such negative effects depend on resource availability. However, at varied resource levels, to what extent the negative effects on growth are attributed to the effects on photosynthesis has not been well elucidated. Here, we examined the influence of nitrogen supply on the growth and photosynthesis responses of the host plant Mikania micrantha to infection by the holoparasite Cuscuta campestris by focusing on the interaction of nitrogen and infection. Mikania micrantha plants fertilized at 0.2, 1 and 5 mM nitrate were grown with and without C. campestris infection. We observed that the infection significantly reduced M. micrantha growth at each nitrate fertilization and more severely at low than at high nitrate. Such alleviation at high nitrate was largely attributed to a stronger influence of infection on root biomass at low than at high nitrate fertilization. However, although C. campestris altered allometry and inhibited host photosynthesis, the magnitude of the effects was independent of nitrate fertilizations. The infection reduced light saturation point, net photosynthesis at saturating irradiances, apparent quantum yield, CO2 saturated rate of photosynthesis, carboxylation efficiency, the maximum carboxylation rate of Rubisco, and maximum light-saturated rate of electron transport, and increased light compensation point in host leaves similarly across nitrate levels, corresponding to a similar magnitude of negative effects of the parasite on host leaf soluble protein and Rubisco concentrations, photosynthetic nitrogen use efficiency and stomatal conductance across nitrate concentrations. Thus, the more severe inhibition in host growth at low than at high nitrate supplies cannot be attributed to a greater parasite-induced reduction in host photosynthesis, but the result of a higher proportion of host resources transferred to the parasite at low than at high nitrate levels.

Filial mistletoes: the functional morphology of moss sporophytes

BACKGROUND

A moss sporophyte inherits a haploid set of genes from the maternal gametophyte to which it is attached and another haploid set of genes from a paternal gametophyte. Evolutionary conflict is expected between genes of maternal and paternal origin that will be expressed as adaptations of sporophytes to extract additional resources from maternal gametophytes and adaptations of maternal gametophytes to restrain sporophytic demands.

INTERPRETATION

The seta and stomata of peristomate mosses are interpreted as sporophytic devices for increasing nutrient transfer. The seta connects the foot, where nutrients are absorbed, to the developing capsule, where nutrients are needed for sporogenesis. Its elongation lifts stomata of the apophysis above the boundary layer, into the zone of turbulent air, thereby increasing the transpirational pull that draws nutrients across the haustorial foot. The calyptra is interpreted as a gametophytic device to reduce sporophytic demands. The calyptra fits tightly over the intercalary meristem of the sporophytic apex and prevents lateral expansion of the meristem. While intact, the calyptra delays the onset of transpiration.

PREDICTIONS

Nutrient transfer across the foot, stomatal number and stomatal aperture are predicted to be particular arenas of conflict between sporophytes and maternal gametophytes, and between maternal and paternal genomes of sporophytes.

Strigolactones and root infestation by plant-parasitic Striga, Orobanche and Phelipanche spp

Strigolactones are signaling molecules that play a role in host recognition by parasitic plants of the Striga, Orobanche and Phelipanche genera which are among the most detrimental weeds in agriculture. The same class of molecules is also involved in the establishment of the symbiosis of plants with arbuscular mycorrhizal (AM) fungi. In addition, the strigolactones are being shown to be involved in an increasing number of physiological processes in plants, such as the regulation of plant architecture and the response to abiotic factors such as nutrient availability and light. These new findings suggest that biosynthesis and perception of strigolactones are conserved throughout the plant kingdom. The structural variation in the strigolactones discovered so far and its possible role in host recognition by the parasites and AM fungi as well as the evolution of strigolactone-dependent-germination in parasitic plants will be discussed. Finally, due to the recent advance in strigolactone research, new insights are emerging on the relation between parasitic and host plants which may result in new strategies to control parasitic plant infestation that will be discussed in this review.

Attack on all fronts: functional relationships between aerial and root parasitic plants and their woody hosts and consequences for ecosystems

This review discusses how understanding of functional relationships between parasitic plants and their woody hosts have benefited from a range of approaches to their study. Gross comparisons of nutrient content between infected and uninfected hosts, or parts of hosts, have been widely used to infer basic differences or similarities between hosts and parasites. Coupling of nutrient information with additional evidence of key processes such as transpiration, respiration and photosynthesis has helped elucidate host-parasite relationships and, in some cases, the anatomical nature of their connection and even the physiology of plants in general. For example, detailed analysis of xylem sap from hosts and parasites has increased our understanding of the spatial and temporal movement of solutes within plants. Tracer experiments using natural abundance or enriched application of stable isotopes ((15)N, (13)C, (18)O) have helped us to understand the extent and form of heterotrophy, including the effect of the parasite on growth and functioning of the host (and its converse) as well as environmental effects on the parasite. Nutritional studies of woody hosts and parasites have provided clues to the distribution of parasitic plants and their roles in ecosystems. This review also provides assessment of several corollaries to the host-parasite association.

Contrasting effects of hemiparasites on ecosystem processes: can positive litter effects offset the negative effects of parasitism?

Hemiparasites are known to influence community structure and ecosystem functioning, but the underlying mechanisms are not well studied. Variation in the impacts of hemiparasites on diversity and production could be due to the difference in the relative strength of two interacting pathways: direct negative effects of parasitism and positive effects on N availability via litter. Strong effects of parasitism should result in substantial changes in diversity and declines in productivity. Conversely, strong litter effects should result in minor changes in diversity and increased productivity. We conducted field-based surveys to determine the association of Castilleja occidentalis with diversity and productivity in the alpine tundra. To examine litter effects, we compared the decomposition of Castilleja litter with litter of four other abundant plant species, and examined the decomposition of those four species when mixed with Castilleja. Castilleja was associated with minor changes in diversity but almost a twofold increase in productivity and greater foliar N in co-occurring species. Our decomposition trials suggest litter effects are due to both the rapid N loss of Castilleja litter and the effects of mixing Castilleja litter with co-occurring species. Castilleja produces litter that accelerates decomposition in the alpine tundra, which could accelerate the slow N cycle and boost productivity. We speculate that these positive effects of litter outweigh the effects of parasitism in nutrient-poor systems with long-lived hemiparasites. Determining the relative importance of parasitism and litter effects of this functional group is crucial to understand the strong but variable roles hemiparasites play in affecting community structure and ecosystem processes.

Significance of Cuscutain, a cysteine protease from Cuscuta reflexa, in host-parasite interactions

BACKGROUND

Plant infestation with parasitic weeds like Cuscuta reflexa induces morphological as well as biochemical changes in the host and the parasite. These modifications could be caused by a change in protein or gene activity. Using a comparative macroarray approach Cuscuta genes specifically upregulated at the host attachment site were identified.

RESULTS

One of the infestation specific Cuscuta genes encodes a cysteine protease. The protein and its intrinsic inhibitory peptide were heterologously expressed, purified and biochemically characterized. The haustoria specific enzyme was named cuscutain in accordance with similar proteins from other plants, e.g. papaya. The role of cuscutain and its inhibitor during the host parasite interaction was studied by external application of an inhibitor suspension, which induced a significant reduction of successful infection events.

CONCLUSIONS

The study provides new information about molecular events during the parasitic plant--host interaction. Inhibition of cuscutain cysteine proteinase could provide means for antagonizing parasitic plants.

The ant-pollination system of Cytinus hypocistis (Cytinaceae), a Mediterranean root holoparasite

BACKGROUND AND AIMS

The genus Cytinus is composed of rootless, stemless and leafless parasites whose flowers are only visible during the reproductive period when they arise from the host tissues. Most of the taxa occur in Madagascar and South Africa, where mammal pollination has been suggested for one species. There is only one species in the Mediterranean region, and its pollination system has been unknown. Here, a long-term field observation study is combined with experimental pollination treatments in order to assess the pollination biology and reproductive system in the Mediterranean species Cytinus hypocistis.

METHODS

Field studies were carried out in six populations in southern Spain over 4 years. Temporal and spatial patterns of variation in the composition and behaviour of floral visitors were characterized. Pollen loads and pollen viability were observed, and exclusion and controlled-pollination treatments were also conducted.

KEY RESULTS

Cytinus hypocistis is a self-compatible monoecious species that relies on insects for seed production. Ants were the main visitors, accounting for 97.4 % of total floral visits, and exclusion experiments showed that they act as true pollinators. They consistently touched reproductive organs, carried large pollen loads and transported viable pollen, although the different ant species observed in the flowers differed in their pollination effectiveness. The abundance of flying visitors was surprisingly low, and only the fly Oplisa aterrima contributed to fruit production and cross-pollination.

CONCLUSIONS

Mutualistic services by ant are essential for the pollination of Cytinus hypocistis. Although this parasite does not exhibit typical features of the 'ant-pollination syndrome', many other characteristics indicate that it is evolving to a more specialized ant-pollination system. The striking interspecific differences in the pollination systems of Mediterranean Cytinus (ant-pollinated) and some South African Cytinus (mammal-pollinated) make this genus an excellent model to investigate the divergent evolution of pollination systems in broadly disjunct areas.

Common themes in nutrient acquisition by plant symbiotic microbes, described by the Gene Ontology

A critical function for symbionts is the acquisition of nutrients from their host. Relationships between hosts and symbionts range from biotrophic mutualism to necrotrophic parasitism, with a corresponding range of structures to facilitate nutrient flow between host and symbiont. Here, we review common themes among the nutrient acquisition strategies of a range of plant symbiotic microorganisms, including mutualistic symbionts, biotrophic pathogens that feed from living tissue, necrotrophic pathogens that kill host tissue, and hemibiotrophic pathogens that switch from biotrophy to necrotrophy. We show how Gene Ontology (GO) terms developed by the Plant-Associated Microbe Gene Ontology (PAMGO) Consortium can be used for describing commonalities in nutrient acquisition among diverse plant symbionts. Where appropriate, parallels found among animal symbionts are also highlighted.

Multiple layers of incompatibility to the parasitic witchweed, Striga hermonthica

* Witchweeds (Striga spp.) are major agricultural pests that infest important crops in sub-Saharan Africa. Striga hermonthica parasitizes gramineous plants including sorghum, maize and rice, but not dicots. To understand host recognition mechanisms of S. hermonthica, we investigated its interaction with nonhost dicots including Arabidopsis, cowpea, Lotus japonicus and Phtheirospermum japonicum, a hemiparasite. * Striga hermonthica seeds were pretreated with strigol, a germination stimulant, and allowed to germinate next to a potential host root. We characterized the histological phenotype of the interactions. Moreover, we monitored the infection of a host rice and the nonhost P. japonicum by S. hermonthica using time-lapse photography. * All nonhost dicots tested did not support S. hermonthica shoot growth beyond the six leaf-pair stage; however, the arrest of parasite development occurred at different stages. Striga hermonthica haustoria were able to reach the steles of Arabidopsis and cowpea, while L. japonicus blocked S. hermonthica infection in the root cortex. Striga hermonthica often failed to penetrate P. japonicum roots. * Our analysis indicates that there are at least four types of incompatible interaction to S. hermonthica. Combinations of these different incompatibility mechanisms contribute to the total resistance to S. hermonthica.

Susceptibility of the tomato mutant high pigment-2dg (hp-2dg) to Orobanche spp. infection

The consumption of natural products with potential health benefits has been continuously growing, and enhanced pigmentation is of major economic importance in fruits and vegetables. The tomato hp-2 ( dg ) is an important mutant line that has been introgressed into commercial tomato cultivars marketed as lycopene rich tomatoes (LRT) because of their enhanced fruit pigmentation, attributed to higher levels of carotenoids, including lycopene. Strigolactones are signaling compounds that mediate host finding in root parasitic plants and are biosynthetically derived from carotenoids. Considering the high carotenoid content of the hp-2 ( dg ) mutant, we studied its susceptibility to the root parasite Orobanche. In a field experiment, the average number of Orobanche aegyptiaca plants growing on hp-2 ( dg ) was surprisingly significantly reduced compared with its isogenic wild-type counterpart. In vitro assays and LC-MS/MS analysis showed that this reduction was associated with a lower production of strigolactones, which apparently renders the high-carotenoid hp-2 ( dg ) mutant less susceptible to Orobanche.

Tomato strigolactones are derived from carotenoids and their biosynthesis is promoted by phosphate starvation

* Strigolactones are rhizosphere signalling compounds that mediate host location in arbuscular mycorrhizal (AM) fungi and parasitic plants. Here, the regulation of the biosynthesis of strigolactones is studied in tomato (Solanum lycopersicum). * Strigolactone production under phosphate starvation, in the presence of the carotenoid biosynthesis inhibitor fluridone and in the abscisic acid (ABA) mutant notabilis were assessed using a germination bioassay with seeds of Orobanche ramosa; a hyphal branching assay with Gigaspora spp; and by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) analysis. * The root exudates of tomato cv. MoneyMaker induced O. ramosa seed germination and hyphal branching in AM fungi. Phosphate starvation markedly increased, and fluridone strongly decreased, this activity. Exudates of notabilis induced approx. 40% less germination than the wild-type. The LC-MS/MS analysis confirmed that the biological activity and changes therein were due to the presence of several strigolactones; orobanchol, solanacol and two or three didehydro-orobanchol isomers. * These results show that the AM branching factors and parasitic plant germination stimulants in tomato root exudate are strigolactones and that they are biosynthetically derived from carotenoids. The dual activity of these signalling compounds in attracting beneficial AM fungi and detrimental parasitic plants is further strengthened by environmental conditions such as phosphate availability.

The anatomy of Santalum album (Sandalwood) haustoria

Structural attributes of Santalum album L. (Sandalwood) haustoria have been long overlooked in the literature. This is surprising since successful haustorial formation is key to the survival of individuals of this ecologically and economically important plant. We investigated the morphology of haustoria formed by S. album attached to one of its principal hosts Tithonia diversifolia (Hemsley) A. Gray. The bell-shaped mature haustoria were composed of a peripheral hyaline body and a centrally located penetration peg. The parasite penetration peg can penetrate the host by means of direct pressure and the secretion of cell-wall-degrading enzymes when forming a successful graft union. The latter mechanism is supported by this study as we observed no evidence of collapsed host cells as the result of parasite applied pressure. Upon reaching the xylem tissue of the host root, the penetration peg formed a thin ellipsoidal disc and the host–parasite interface was almost entirely composed of parenchymatous tissue. Luminal continuities were absent between the xylem conducting tissues of the partners, thus suggesting mass flow of solutes is unlikely to occur in this association. High densities of contact parenchyma were found at the host–parasite interface; thus it is probable that these are the principal structures formed by the parasite that facilitate the acquisition of host-derived xylem resources. This study therefore concludes that haustorial anatomy of S. album supports cross membrane (potentially selective) uptake of host-derived solutes as opposed to mass flow via vascular continuity.