The Haustorium, a Specialized Invasive Organ in Parasitic Plants

Chromosome-level genome assembly of Pedicularis kansuensis illuminates genome evolution of facultative parasitic plant

Parasitic plants have a heterotrophic lifestyle, in which they withdraw all or part of their nutrients from their host through the haustorium. Despite the release of many draft genomes of parasitic plants, the genome evolution related to the parasitism feature of facultative parasites remains largely unknown. In this study, we present a high-quality chromosomal-level genome assembly for the facultative parasite Pedicularis kansuensis (Orobanchaceae), which invades both legume and grass host species in degraded grasslands on the Qinghai-Tibet Plateau. This species has the largest genome size compared with other parasitic species, and expansions of long terminal repeat retrotransposons accounting for 62.37% of the assembly greatly contributed to the genome size expansion of this species. A total of 42,782 genes were annotated, and the patterns of gene loss in P. kansuensis differed from other parasitic species. We also found many mobile mRNAs between P. kansuensis and one of its host species, but these mobile mRNAs could not compensate for the functional losses of missing genes in P. kansuensis. In addition, we identified nine horizontal gene transfer (HGT) events from rosids and monocots, as well as one single-gene duplication events from HGT genes, which differ distinctly from that of other parasitic species. Furthermore, we found evidence for HGT through transferring genomic fragments from phylogenetically remote host species. Taken together, these findings provide genomic insights into the evolution of facultative parasites and broaden our understanding of the diversified genome evolution in parasitic plants and the molecular mechanisms of plant parasitism.

Macronutrient application rescues performance of tolerant sorghum genotypes when infected by the parasitic plant striga

BACKGROUND AND AIMS

Infection by the hemi-parasitic plant Striga hermonthica causes severe host plant damage and seed production losses. Increased availability of essential plant nutrients reduces infection. Whether, how and to what extent it also reduces striga-induced host plant damage has not been well studied.

METHODS

The effects of improved macro- and micronutrient supply on host plant performance under striga-free and infected conditions were investigated in glasshouse pot assays. One striga-sensitive and two striga-tolerant genotypes were compared. Plants growing in impoverished soils were supplied with (1) 25 % of optimal macro- and micronutrient quantities, (2) 25 % macro- and 100 % micronutrients, (3) 100 % macro- and 25 % micronutrients, or (4) 100 % macro- and micronutrients.

KEY RESULTS

Photosynthesis rates of striga-infected plants of the sensitive genotype increased with improved nutrition (from 12.2 to 22.1 μmol m-2 s-1) but remained below striga-free levels (34.9-38.8 μmol m-2 s-1). For the tolerant genotypes, increased macronutrient supply offset striga-induced photosynthesis losses. Striga-induced relative grain losses of 100 % for the sensitive genotype were reduced to 74 % by increased macronutrients. Grain losses of 80 % in the tolerant Ochuti genotype, incurred at low nutrient supply, were reduced to 5 % by improved nutrient supply.

CONCLUSIONS

Increasing macronutrient supply reduces the impact of striga on host plants but can only restore losses when applied to genotypes with a tolerant background.

Tracking the messengers: Emerging advances in mRNA-based plant communication

Messenger RNAs (mRNAs) are the templates for protein translation but can also act as non-cell-autonomous signaling molecules. Plants input endogenous and exogenous cues to mobile mRNAs and output them to local or systemic target cells and organs to support specific plant responses. Mobile mRNAs form ribonucleoprotein (RNP) complexes with proteins during transport. Components of these RNP complexes could interact with plasmodesmata (PDs), a major mediator of mRNA transport, to ensure mRNA mobility and transport selectivity. Based on advances in the last two to three years, this review summarizes mRNA transport mechanisms in local and systemic signaling from the perspective of RNP complex formation and PD transport. We also discuss the physiological roles of endogenous mRNA transport and the recently revealed roles of non-cell-autonomous mRNAs in inter-organism communication.

The soil microbiome modulates the sorghum root metabolome and cellular traits with a concomitant reduction of Striga infection

Sorghum bicolor is among the most important cereals globally and a staple crop for smallholder farmers in sub-Saharan Africa. Approximately 20% of sorghum yield is lost annually in Africa due to infestation with the root parasitic weed Striga hermonthica. Existing Striga management strategies are not singularly effective and integrated approaches are needed. Here, we demonstrate the functional potential of the soil microbiome to suppress Striga infection in sorghum. We associate this suppression with microbiome-mediated induction of root endodermal suberization and aerenchyma formation and with depletion of haustorium-inducing factors, compounds required for the initial stages of Striga infection. We further identify specific bacterial taxa that trigger the observed Striga-suppressive traits. Collectively, our study describes the importance of the soil microbiome in the early stages of root infection by Striga and pinpoints mechanisms of Striga suppression. These findings open avenues to broaden the effectiveness of integrated Striga management practices.

Global changes in gene expression during compatible and incompatible interactions of faba bean (Vicia faba L.) during Orobanche foetida parasitism

Orobanche foetida Poiret is the main constraint facing faba bean crop in Tunisia. Indeed, in heavily infested fields with this parasitic plant, yield losses may reach 90%, and the recent estimation of the infested area is around 80,000 ha. Identifying genes involved in the Vicia faba/O. foetida interaction is crucial for the development of effective faba bean breeding programs. However, there is currently no available information on the transcriptome of faba bean responding to O. foetida parasitism. In this study, we employed RNA sequencing to explore the global gene expression changes associated with compatible and incompatible V. faba/O. foetida interactions. In this perspective, two faba bean varieties (susceptible and resistant) were examined at the root level across three stages of O. foetida development (Before Germination (BG), After Germination (AG) and Tubercule Stage (TS)). Our analyses presented an exploration of the transcriptomic profile, including comprehensive assessments of differential gene expression and Gene Ontology (GO) enrichment analyses. Specifically, we investigated key pathways revealing the complexity of molecular responses to O. foetida attack. In this study, we detected differential gene expression of pathways associated with secondary metabolites: flavonoids, auxin, thiamine, and jasmonic acid. To enhance our understanding of the global changes in V. faba response to O. foetida, we specifically examined WRKY genes known to play a role in plant host-parasitic plant interactions. Furthermore, considering the pivotal role of parasitic plant seed germination in this interaction, we investigated genes involved in the orobanchol biosynthesis pathway. Interestingly, we detected the gene expression of VuCYP722C homolog, coding for a key enzyme involved in orobanchol biosynthesis, exclusively in the susceptible host. Clearly, this study enriches our understanding of the V. faba/O. foetida interaction, shedding light on the main differences between susceptible and resistant faba bean varieties during O. foetida infestation at the gene expression level.

Comparative secretome analysis of Striga and Cuscuta species identifies candidate virulence factors for two evolutionarily independent parasitic plant lineages

BACKGROUND

Many parasitic plants of the genera Striga and Cuscuta inflict huge agricultural damage worldwide. To form and maintain a connection with a host plant, parasitic plants deploy virulence factors (VFs) that interact with host biology. They possess a secretome that represents the complement of proteins secreted from cells and like other plant parasites such as fungi, bacteria or nematodes, some secreted proteins represent VFs crucial to successful host colonisation. Understanding the genome-wide complement of putative secreted proteins from parasitic plants, and their expression during host invasion, will advance understanding of virulence mechanisms used by parasitic plants to suppress/evade host immune responses and to establish and maintain a parasite-host interaction.

RESULTS

We conducted a comparative analysis of the secretomes of root (Striga spp.) and shoot (Cuscuta spp.) parasitic plants, to enable prediction of candidate VFs. Using orthogroup clustering and protein domain analyses we identified gene families/functional annotations common to both Striga and Cuscuta species that were not present in their closest non-parasitic relatives (e.g. strictosidine synthase like enzymes), or specific to either the Striga or Cuscuta secretomes. For example, Striga secretomes were strongly associated with 'PAR1' protein domains. These were rare in the Cuscuta secretomes but an abundance of 'GMC oxidoreductase' domains were found, that were not present in the Striga secretomes. We then conducted transcriptional profiling of genes encoding putatively secreted proteins for the most agriculturally damaging root parasitic weed of cereals, S. hermonthica. A significant portion of the Striga-specific secretome set was differentially expressed during parasitism, which we probed further to identify genes following a 'wave-like' expression pattern peaking in the early penetration stage of infection. We identified 39 genes encoding putative VFs with functions such as cell wall modification, immune suppression, protease, kinase, or peroxidase activities, that are excellent candidates for future functional studies.

CONCLUSIONS

Our study represents a comprehensive secretome analysis among parasitic plants and revealed both similarities and differences in candidate VFs between Striga and Cuscuta species. This knowledge is crucial for the development of new management strategies and delaying the evolution of virulence in parasitic weeds.

Exploring Information Exchange between Thesium chinense and Its Host Prunella vulgaris through Joint Transcriptomic and Metabolomic Analysis

BACKGROUND

Thesium chinense known as the "plant antibiotic" is a facultative root hemi-parasitic herb while Prunella vulgaris can serve as its host. However, the molecular mechanisms underlying the communication between T. chinense and its host remained largely unexplored. The aim of this study was to provide a comprehensive view of transferred metabolites and mobile mRNAs exchanged between T. chinense and P. vulgaris.

RESULTS

The wide-target metabolomic and transcriptomic analysis identified 5 transferred metabolites (ethylsalicylate, eriodictyol-7-O-glucoside, aromadendrin-7-O-glucoside, pruvuloside B, 2-ethylpyrazine) and 50 mobile genes between T. chinense and P. vulgaris, as well as haustoria formation related 56 metabolites and 44 genes. There were 4 metabolites (ethylsalicylate, eriodictyol-7-O-glucoside, aromadendrin-7-O-glucoside and pruvuloside B) that are transferred from P. vulgaris to T. chinense, whereas 2-ethylpyrazine was transferred in the opposite direction. Furthermore, we inferred a regulatory network potentially involved in haustoria formation, where three metabolites (N,N'-Dimethylarginine/SDMA, NG,NG-Dimethyl-L-arginine, 2-Acetoxymethyl-anthraquinone) showed significant positive correlations with the majority of haustoria formation-related genes.

CONCLUSIONS

These results suggested that there was an extensive exchange of information with P. vulgaris including transferred metabolites and mobile mRNAs, which might facilitate the haustoria formation and parasition of T. chinense.

Developing for nutrient uptake: Induced organogenesis in parasitic plants and root nodule symbiosis

Plants have evolved diverse strategies to meet their nutritional needs. Parasitic plants employ haustoria, specialized structures that facilitate invasion of host plants and nutrient acquisition. Legumes have adapted to nitrogen-limited conditions by developing nodules that accommodate nitrogen-fixing rhizobia. The formation of both haustoria and nodules is induced by signals originating from the interacting organisms, namely host plants and rhizobial bacteria, respectively. Emerging studies showed that both organogenesis crucially involves plant hormones such as auxin, cytokinins, and ethylene and also integrate nutrient availability, particularly nitrogen. In this review, we discuss recent advances on hormonal and environmental control of haustoria and nodules development with side-by-side comparison. These underscore the remarkable plasticity of plant organogenesis.

A roadmap of haustorium morphogenesis in parasitic plants

Parasitic plants invade their host through their invasive organ, the haustorium. This organ connects to the vasculature of the host roots and hijacks water and nutrients. Although parasitism has evolved independently in plants, haustoria formation follows a similar mechanism throughout different plant species, highlighting the developmental plasticity of plant tissues. Here, we compare three types of haustoria formed by the root and shoot in the plant parasites Striga and Cuscuta. We discuss mechanisms underlying the interactions with their hosts and how different approaches have contributed to major understanding of haustoria formation and host invasion. We also illustrate the role of auxin and cytokinin in controlling this process.

Invited Review Beyond parasitic convergence: unravelling the evolution of the organellar genomes in holoparasites

BACKGROUND

The molecular evolution of organellar genomes in angiosperms has been studied extensively, with some lineages, such as parasitic ones, displaying unique characteristics. Parasitism has emerged 12 times independently in angiosperm evolution. Holoparasitism is the most severe form of parasitism, and is found in ~10 % of parasitic angiosperms. Although a few holoparasitic species have been examined at the molecular level, most reports involve plastomes instead of mitogenomes. Parasitic plants establish vascular connections with their hosts through haustoria to obtain water and nutrients, which facilitates the exchange of genetic information, making them more susceptible to horizontal gene transfer (HGT). HGT is more prevalent in the mitochondria than in the chloroplast or nuclear compartments.

SCOPE

This review summarizes current knowledge on the plastid and mitochondrial genomes of holoparasitic angiosperms, compares the genomic features across the different lineages, and discusses their convergent evolutionary trajectories and distinctive features. We focused on Balanophoraceae (Santalales), which exhibits extraordinary traits in both their organelles.

CONCLUSIONS

Apart from morphological similarities, plastid genomes of holoparasitic plants also display other convergent features, such as rampant gene loss, biased nucleotide composition and accelerated evolutionary rates. In addition, the plastomes of Balanophoraceae have extremely low GC and gene content, and two unexpected changes in the genetic code. Limited data on the mitochondrial genomes of holoparasitic plants preclude thorough comparisons. Nonetheless, no obvious genomic features distinguish them from the mitochondria of free-living angiosperms, except for a higher incidence of HGT. HGT appears to be predominant in holoparasitic angiosperms with a long-lasting endophytic stage. Among the Balanophoraceae, mitochondrial genomes exhibit disparate evolutionary paths with notable levels of heteroplasmy in Rhopalocnemis and unprecedented levels of HGT in Lophophytum. Despite their differences, these Balanophoraceae share a multichromosomal mitogenome, a feature also found in a few free-living angiosperms.

Orobanche crenata: A Bibliometric Analysis of a Noxious Parasitic Plant

Orobanche crenata is a parasitic weed representing a serious constraint to crop production in the Mediterranean basin. Here, we aim to evaluate the global scientific research status and trends of O. crenata through a bibliometric analysis to identify prominent research themes, development trends, and major contributors in terms of authors, institutions, countries, and journals. In the span of 53 years, from 1968 to 2021, 274 articles related to this field were retrieved from Scopus database and were analyzed using VOSviewer and BiblioShiny software. Results showed that 70.4% of all articles on O. crenata have been published in the last two decades. "Control methods" was the most prevalent research theme with 55.9% of all articles. Weed Research is the most influential journal. The countries with the highest number of articles were Spain, Egypt, and Italy. The Institute for Sustainable Agriculture is the most involved institution, contributing to 31.7% of all articles, and authors from Spain were the most productive. The latest research literature (5 years) was performed mainly by authors from Spain, Morocco, and Tunisia, emphasizing the persistence of this constraint in these countries. Keyword analysis revealed that "Vicia faba", "germination", and "legumes" are the most researched hotspots. Despite the growing collaborative behavior in this area, cooperation between countries is still deficient and should be extended to countries that are recently affected by this scourge to exchange expertise already acquired by experienced researchers, thus allowing better worldwide control of this parasitic weed.

CRISPR gene editing to improve crop resistance to parasitic plants

Parasitic plants pose a significant threat to global agriculture, causing substantial crop losses and hampering food security. In recent years, CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) gene-editing technology has emerged as a promising tool for developing resistance against various plant pathogens. Its application in combating parasitic plants, however, remains largely unexplored. This review aims to summarise current knowledge and research gaps in utilising CRISPR to develop resistance against parasitic plants. First, we outline recent improvements in CRISPR gene editing tools, and what has been used to combat various plant pathogens. To realise the immense potential of CRISPR, a greater understanding of the genetic basis underlying parasitic plant-host interactions is critical to identify suitable target genes for modification. Therefore, we discuss the intricate interactions between parasitic plants and their hosts, highlighting essential genes and molecular mechanisms involved in defence response and multilayer resistance. These include host resistance responses directly repressing parasitic plant germination or growth and indirectly influencing parasitic plant development via manipulating environmental factors. Finally, we evaluate CRISPR-mediated effectiveness and long-term implications for host resistance and crop improvement, including inducible resistance response and tissue-specific activity. In conclusion, this review highlights the challenges and opportunities CRISPR technology provides to combat parasitic plants and provides insights for future research directions to safeguard global agricultural productivity.

Balanophora genomes display massively convergent evolution with other extreme holoparasites and provide novel insights into parasite-host interactions

Parasitic plants have evolved to be subtly or severely dependent on host plants to complete their life cycle. To provide new insights into the biology of parasitic plants in general, we assembled genomes for members of the sandalwood order Santalales, including a stem hemiparasite (Scurrula) and two highly modified root holoparasites (Balanophora) that possess chimaeric host-parasite tubers. Comprehensive genome comparisons reveal that hemiparasitic Scurrula has experienced a relatively minor degree of gene loss compared with autotrophic plants, consistent with its moderate degree of parasitism. Nonetheless, patterns of gene loss appear to be substantially divergent across distantly related lineages of hemiparasites. In contrast, Balanophora has experienced substantial gene loss for the same sets of genes as an independently evolved holoparasite lineage, the endoparasitic Sapria (Malpighiales), and the two holoparasite lineages experienced convergent contraction of large gene families through loss of paralogues. This unprecedented convergence supports the idea that despite their extreme and strikingly divergent life histories and morphology, the evolution of these and other holoparasitic lineages can be shaped by highly predictable modes of genome reduction. We observe substantial evidence of relaxed selection in retained genes for both hemi- and holoparasitic species. Transcriptome data also document unusual and novel interactions between Balanophora and host plants at the host-parasite tuber interface tissues, with evidence of mRNA exchange, substantial and active hormone exchange and immune responses in parasite and host.

Role of Strigolactones in the Host Specificity of Broomrapes and Witchweeds

Root parasitic plants of the Orobanchaceae, broomrapes and witchweeds, pose a severe problem to agriculture in Europe, Asia and especially Africa. These parasites are totally dependent on their host for survival, and therefore, their germination is tightly regulated by host presence. Indeed, their seeds remain dormant in the soil until a host root is detected through compounds called germination stimulants. Strigolactones (SLs) are the most important class of germination stimulants. They play an important role in planta as a phytohormone and, upon exudation from the root, function in the recruitment of symbiotic arbuscular mycorrhizal fungi. Plants exude mixtures of various different SLs, possibly to evade detection by these parasites and still recruit symbionts. Vice versa, parasitic plants must only respond to the SL composition that is exuded by their host, or else risk germination in the presence of non-hosts. Therefore, parasitic plants have evolved an entire clade of SL receptors, called HTL/KAI2s, to perceive the SL cues. It has been demonstrated that these receptors each have a distinct sensitivity and specificity to the different known SLs, which possibly allows them to recognize the SL-blend characteristic of their host. In this review, we will discuss the molecular basis of SL sensitivity and specificity in these parasitic plants through HTL/KAI2s and review the evidence that these receptors contribute to host specificity of parasitic plants.

Detection of Secreted Effector Proteins from Phelipanche aegyptiaca During Invasion of Melon Roots

Parasites can interact with their host plants through the induction and delivery of secreted effector proteins that facilitate plant colonization by decomposing plant cell walls and inhibiting plant immune response to weaken the defense ability of the host. Yet effectors mediating parasitic plant-host interactions are poorly understood. Phelipanche aegyptiaca is an obligate root parasite plant causing severe yield and economic losses in agricultural fields worldwide. Host resistance against P. aegyptiaca occurred during the attachment period of parasitism. Comparative transcriptomics was used to assess resistant and susceptible interactions simultaneously between P. aegyptiaca and two contrasting melon cultivars. In total, 2,740 secreted proteins from P. aegyptiaca were identified here. Combined with transcriptome profiling, 209 candidate secreted effector proteins (CSEPs) were predicted, with functional annotations such as cell wall degrading enzymes, protease inhibitors, transferases, kinases, and elicitor proteins. A heterogeneous expression system in Nicotiana benthamiana was used to investigate the functions of 20 putatively effector genes among the CSEPs. Cluster 15140.0 can suppress BAX-triggered programmed cell death in N. benthamiana. These findings showed that the prediction of P. aegyptiaca effector proteins based on transcriptomic analysis and multiple bioinformatics software is effective and more accurate, providing insights into understanding the essential molecular nature of effectors and laying the foundation of revealing the parasite mechanism of P. aegyptiaca, which is helpful in understanding parasite-host plant interaction.

Non-volatile metabolites mediate plant interactions with insect herbivores

Non-volatile metabolites constitute the bulk of plant biomass. From the perspective of plant-insect interactions, these structurally diverse compounds include nutritious core metabolites and defensive specialized metabolites. In this review, we synthesize the current literature on multiple scales of plant-insect interactions mediated by non-volatile metabolites. At the molecular level, functional genetics studies have revealed a large collection of receptors targeting plant non-volatile metabolites in model insect species and agricultural pests. By contrast, examples of plant receptors of insect-derived molecules remain sparse. For insect herbivores, plant non-volatile metabolites function beyond the dichotomy of core metabolites, classed as nutrients, and specialized metabolites, classed as defensive compounds. Insect feeding tends to elicit evolutionarily conserved changes in plant specialized metabolism, whereas its effect on plant core metabolism varies widely based the interacting species. Finally, several recent studies have demonstrated that non-volatile metabolites can mediate tripartite communication on the community scale, facilitated by physical connections established through direct root-to-root communication, parasitic plants, arbuscular mycorrhizae and the rhizosphere microbiome. Recent advances in both plant and insect molecular biology will facilitate further research on the role of non-volatile metabolites in mediating plant-insect interactions.

The plant vampire diaries: a historic perspective on Cuscuta research

The angiosperm genus Cuscuta lives as an almost achlorophyllous root- and leafless holoparasite and has therefore occupied scientists for more than a century. The 'evolution' of Cuscuta research started with early studies that established the phylogenetic framework for this unusual genus. It continued to produce groundbreaking cytological, morphological, and physiological insight throughout the second half of the 20th century and culminated in the last two decades in exciting discoveries regarding the molecular basis of Cuscuta parasitism that were facilitated by the modern 'omics' tools and traceable fluorescent marker technologies of the 21st century. This review will show how present activities are inspired by those past breakthroughs. It will describe significant milestones and recurring themes of Cuscuta research and connect these to the remaining as well as newly evolving questions and future directions in this research field that is expected to sustain its strong growth in the future.

Host Resistance to Parasitic Plants-Current Knowledge and Future Perspectives

Parasitic flowering plants represent a diverse group of angiosperms, ranging from exotic species with limited distribution to prominent weeds, causing significant yield losses in agricultural crops. The major damage caused by them is related to the extraction of water and nutrients from the host, thus decreasing vegetative growth, flowering, and seed production. Members of the root parasites of the Orobanchaceae family and stem parasites of the genus Cuscuta are among the most aggressive and damaging weeds, affecting both monocotyledonous and dicotyledonous crops worldwide. Their control and eradication are hampered by the extreme seed longevity and persistence in soil, as well as their taxonomic position, which makes it difficult to apply selective herbicides not damaging to the hosts. The selection of resistant cultivars is among the most promising approaches to deal with this matter, although still not widely employed due to limited knowledge of the molecular mechanisms of host resistance and inheritance. The current review aims to summarize the available information on host resistance with a focus on agriculturally important parasitic plants and to outline the future perspectives of resistant crop cultivar selection to battle the global threat of parasitic plants.

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.

Genomic and Epigenomic Mechanisms of the Interaction between Parasitic and Host Plants

Parasitic plants extract nutrients from the other plants to finish their life cycle and reproduce. The control of parasitic weeds is notoriously difficult due to their tight physical association and their close biological relationship to their hosts. Parasitic plants differ in their susceptible host ranges, and the host species differ in their susceptibility to parasitic plants. Current data show that adaptations of parasitic plants to various hosts are largely genetically determined. However, multiple cases of rapid adaptation in genetically homogenous parasitic weed populations to new hosts strongly suggest the involvement of epigenetic mechanisms. Recent progress in genome-wide analyses of gene expression and epigenetic features revealed many new molecular details of the parasitic plants' interactions with their host plants. The experimental data obtained in the last several years show that multiple common features have independently evolved in different lines of the parasitic plants. In this review we discuss the most interesting new details in the interaction between parasitic and host plants.

Protein Profiling of Psittacanthus calyculatus during Mesquite Infection

Psittacanthus calyculatus is a hemiparasite mistletoe that represents an ecological problem due to the impacts caused to various tree species of ecological and commercial interest. Although the life cycle for the Psittacanthus genus is well established in the literature, the development stages and molecular mechanism implicated in P. calyculatus host infection are poorly understood. In this study, we used a manageable infestation of P. laevigata with P. calyculatus to clearly trace the infection, which allowed us to describe five phenological infective stages of mistletoe on host tree branches: mature seed (T1), holdfast formation (T2), haustorium activation (T3), haustorium penetration (T4), and haustorium connection (T5) with the host tree. Proteomic analyses revealed proteins with a different accumulation and cellular processes in infective stages. Activities of the cell wall-degrading enzymes cellulase and β-1,4-glucosidase were primarily active in haustorium development (T3), while xylanase, endo-glucanase, and peptidase were highly active in the haustorium penetration (T4) and xylem connection (T5). Patterns of auxins and cytokinin showed spatial concentrations in infective stages and moreover were involved in haustorium development. These results are the first evidence of proteins, cell wall-degrading enzymes, and phytohormones that are involved in early infection for the Psittacanthus genus, and thus represent a general infection mechanism for other mistletoe species. These results could help to understand the molecular dialogue in the establishment of P. calyculatus parasitism.

Strigol induces germination of the facultative parasitic plant Phtheirospermum japonicum in the absence of nitrate ions

Root parasitic plants in the family Orobanchaceae, such as Striga and Orobanche spp., infest major crops worldwide, leading to a multibillion-dollar loss annually. Host-derived strigolactones (SLs), recognized by a group of α/β hydrolase receptors (KAI2d) in these parasites, are important determinants for germinating root parasitic plants near the roots of host plants. Phtheirospermum japonicum, a facultative hemiparasitic Orobanchaceae plant, can germinate and grow in the presence or absence of the host and can also exhibit root chemotropism to host-derived SLs that are perceived via KAI2d. However, the importance of SLs in P. japonicum germination remains unclear. In this study, we found that germination of P. japonicum was suppressed in the absence of nitrate ions and that germination of P. japonicum was promoted by exogenous strigol, an SL, under such conditions. We propose a model in which P. japonicum may select either independent living or parasitism in response to ambient nitrogen conditions and host presence.

Silencing of a mannitol transport gene in Phelipanche aegyptiaca by the tobacco rattle virus system reduces the parasite germination on the host root

Root parasitic weed Phelipanche aegyptiaca is an obligate plant parasite that causes severe damage to host crops. Agriculture crops mainly belong to the Brassicaceae, Leguminosae, Cruciferae, and Solanaceae plant families affected by this parasitic weed, leading to the devastating loss of crop yield and economic growth. This root-specific parasitic plant is not able to complete its life cycle without a suitable host and is dependent on the host plant for nutrient uptake and germination. Therefore, selected parasitic genes of P. aegyptiaca which were known to be upregulated upon interaction with the host were chosen. These genes are essential for parasitism, and reduced activity of these genes could affect host-parasitic interaction and provide resistance to the host against these parasitic weeds. To check and examine the role of these parasitic genes which can affect the development of host resistance, we silenced selected genes in the P. aegyptiaca using the tobacco rattle virus (TRV) based virus-induced gene silencing (VIGS) method. Our results demonstrated that the total number of P. aegyptiaca parasite tubercles attached to the root of the host plant Nicotiana benthamiana was substantially decreased in all the silenced plants. However, silencing of the P. aegyptiaca MNT1 gene which encodes the mannitol transporter showed a significantly reduced number of germinated shoots and tubercles. Thus, our study indicates that the mannitol transport gene of P. aegyptiaca plays a crucial role in parasitic germination, and silencing of the PaMNT1 gene abolishes the germination of parasites on the host roots.

Large-scale mRNA transfer between Haloxylon ammodendron (Chenopodiaceae) and herbaceous root holoparasite Cistanche deserticola (Orobanchaceae)

Exchanges of mRNA were shown between host and stem parasites but not root parasites. Cistanche deserticola (Orobanchaceae) is a holoparasitic herb which parasitizes on the roots of woody plant Haloxylon ammodendron (Chenopodiaceae). We used transcriptome sequencing and bioinformatic analyses to identify nearly ten thousand mobile mRNAs. Transcript abundance appears to be a driving force for transfer event and mRNA exchanges occur through haustorial junction. Mobility of selected mRNAs was confirmed in situ and in sunflower-Orobanche cumana heterologous parasitic system. Four C. deserticola →H. ammodendron mobile mRNAs appear to facilitate haustorium development. Of interest, two mobile mRNAs of putative resistance genes CdNLR1 and CdNLR2 cause root-specific hypersensitive response and retard parasite development, which might contribute to parasitic equilibrium. The present study provides evidence for the large-scale mRNA transfer event between a woody host and a root parasite, and demonstrates the functional relevance of six C. deserticola genes in host-parasite interactions.

Cuscuta species: Model organisms for haustorium development in stem holoparasitic plants

Parasitic plants are notorious for causing serious agricultural losses in many countries. Specialized intrusive organs, haustoria, confer on parasitic plants the ability to acquire water and nutrients from their host plants. Investigating the mechanism involved in haustorium development not only reveals the fascinating mystery of how autotrophic plants evolved parasitism but also provides the foundation for developing more effective methods to control the agricultural damage caused by parasitic plants. Cuscuta species, also known as dodders, are one of the most well-known and widely spread stem holoparasitic plants. Although progress has been made recently in understanding the evolution and development of haustoria in root parasitic plants, more and more studies indicate that the behaviors between root and stem haustorium formation are distinct, and the mechanisms involved in the formation of these organs remain largely unknown. Unlike most endoparasites and root holoparasitic plants, which have high host-specificity and self- or kin-recognition to avoid forming haustoria on themselves or closely related species, auto-parasitism and hyper-parasitism are commonly observed among Cuscuta species. In this review, we summarize the current understanding of haustorium development in dodders and the unique characteristics of their parasitizing behaviors. We also outline the advantages of using Cuscuta species as model organisms for haustorium development in stem holoparasitic plants, the current unknown mysteries and limitations in the Cuscuta system, and potential future research directions to overcome these challenges.

Phenolic signals for prehaustorium formation in Striga hermonthica

Striga hermonthica is a root parasitic plant that causes considerable crop yield losses. To parasitize host plants, parasitic plants develop a specialized organ called the haustorium that functions in host invasion and nutrient absorption. The initiation of a prehaustorium, the primitive haustorium structure before host invasion, requires the perception of host-derived compounds, collectively called haustorium-inducing factors (HIFs). HIFs comprise quinones, phenolics, flavonoids and cytokinins for S. hermonthica; however, the signaling pathways from various HIFs leading to prehaustorium formation remain largely uncharacterized. It has been proposed that quinones serve as direct signaling molecules for prehaustorium induction and phenolic compounds originating from the host cell wall are the oxidative precursors, but the overlap and distinction of their downstream signaling remain unknown. Here we show that quinone and phenolic-triggered prehaustorium induction in S. hermonthica occurs through partially divergent signaling pathways. We found that ASBr, an inhibitor of acetosyringone in virulence gene induction in the soil bacterium Agrobacterium, compromised prehaustorium formation in S. hermonthica. In addition, LGR-991, a competitive inhibitor of cytokinin receptors, inhibited phenolic-triggered but not quinone-triggered prehaustorium formation, demonstrating divergent signaling pathways of phenolics and quinones for prehaustorium formation. Comparisons of genome-wide transcriptional activation in response to either phenolic or quinone-type HIFs revealed markedly distinct gene expression patterns specifically at the early initiation stage. While quinone DMBQ triggered rapid and massive transcriptional changes in genes at early stages, only limited numbers of genes were induced by phenolic syringic acid. The number of genes that are commonly upregulated by DMBQ and syringic acid is gradually increased, and many genes involved in oxidoreduction and cell wall modification are upregulated at the later stages by both HIFs. Our results show kinetic and signaling differences in quinone and phenolic HIFs, providing useful insights for understanding how parasitic plants interpret different host signals for successful parasitism.

Mitigating the Mistletoe Menace: Biotechnological and Smart Management Approaches

Mistletoes have been considered a keystone resource for biodiversity, as well as a remarkable source of medicinal attributes that attract pharmacologists. Due to their hemiparasitic nature, mistletoes leach water and nutrients, including primary and secondary metabolites, through the vascular systems of their plant hosts, primarily trees. As a result of intense mistletoe infection, the hosts suffer various growth and physiological detriments, which often lead to tree mortality. Because of their easy dispersal and widespread tropism, mistletoes have become serious pests for commercial fruit and timber plantations. A variety of physical and chemical treatment methods, along with silvicultural practices, have shaped conventional mistletoe management. Others, however, have either failed to circumvent the growing range and tropism of these parasitic plants or present significant environmental and public health risks. A biocontrol approach that could sidestep these issues has never achieved full proof of concept in real-field applications. Our review discusses the downsides of conventional mistletoe control techniques and explores the possibilities of biotechnological approaches using biocontrol agents and transgenic technologies. It is possible that smart management options will pave the way for technologically advanced solutions to mitigate mistletoes that are yet to be exploited.

Cytokinins Induce Prehaustoria Coordinately with Quinone Signals in the Parasitic Plant Striga hermonthica

Orobanchaceae parasitic plants are major threats to global food security, causing severe agricultural damage worldwide. Parasitic plants derive water and nutrients from their host plants through multicellular organs called haustoria. The formation of a prehaustorium, a primitive haustorial structure, is provoked by host-derived haustorium-inducing factors (HIFs). Quinones, including 2,6-dimethoxy-p-benzoquinone (DMBQ), are of the most potent HIFs for various species in Orobanchaceae, but except non-photosynthetic holoparasites, Phelipanche and Orobanche spp. Instead, cytokinin (CK) phytohormones were reported to induce prehaustoria in Phelipanche ramosa. However, little is known about whether CKs act as HIFs in the other parasitic species to date. Moreover, the signaling pathways for quinones and CKs in prehaustorium induction are not well understood. This study shows that CKs act as HIFs in the obligate parasite Striga hermonthica but not in the facultative parasite Phtheirospermum japonicum. Using chemical inhibitors and marker gene expression analysis, we demonstrate that CKs activate prehaustorium formation through a CK-specific signaling pathway that overlaps with the quinone HIF pathway at downstream in S. hermonthica. Moreover, host root exudates activated S. hermonthica CK biosynthesis and signaling genes, and DMBQ and CK inhibitors perturbed the prehaustorium-inducing activity of exudates, indicating that host root exudates include CKs. Our study reveals the importance of CKs for prehaustorium formation in obligate parasitic plants.

Why are some plant species missing from restorations? A diagnostic tool for temperate grassland ecosystems

The U.N. Decade on Ecosystem Restoration aims to accelerate actions to prevent, halt, and reverse the degradation of ecosystems, and re-establish ecosystem functioning and species diversity. The practice of ecological restoration has made great progress in recent decades, as has recognition of the importance of species diversity to maintaining the long-term stability and functioning of restored ecosystems. Restorations may also focus on specific species to fulfill needed functions, such as supporting dependent wildlife or mitigating extinction risk. Yet even in the most carefully planned and managed restoration, target species may fail to germinate, establish, or persist. To support the successful reintroduction of ecologically and culturally important plant species with an emphasis on temperate grasslands, we developed a tool to diagnose common causes of missing species, focusing on four major categories of filters, or factors: genetic, biotic, abiotic, and planning & land management. Through a review of the scientific literature, we propose a series of diagnostic tests to identify potential causes of failure to restore target species, and treatments that could improve future outcomes. This practical diagnostic tool is meant to strengthen collaboration between restoration practitioners and researchers on diagnosing and treating causes of missing species in order to effectively restore them.

Genome-enabled discovery of candidate virulence loci in Striga hermonthica, a devastating parasite of African cereal crops

Parasites have evolved proteins, virulence factors (VFs), that facilitate plant colonisation, however VFs mediating parasitic plant-host interactions are poorly understood. Striga hermonthica is an obligate, root-parasitic plant of cereal hosts in sub-Saharan Africa, causing devastating yield losses. Understanding the molecular nature and allelic variation of VFs in S. hermonthica is essential for breeding resistance and delaying the evolution of parasite virulence. We assembled the S. hermonthica genome and identified secreted proteins using in silico prediction. Pooled sequencing of parasites growing on a susceptible and a strongly resistant rice host allowed us to scan for loci where selection imposed by the resistant host had elevated the frequency of alleles contributing to successful colonisation. Thirty-eight putatively secreted VFs had very different allele frequencies with functions including host cell wall modification, protease or protease inhibitor and kinase activities. These candidate loci had significantly higher Tajima's D than the genomic background, consistent with balancing selection. Our results reveal diverse strategies used by S. hermonthica to overcome different layers of host resistance. Understanding the maintenance of variation at virulence loci by balancing selection will be critical to managing the evolution of virulence as part of a sustainable control strategy.

Parasitic plant small RNA analyses unveil parasite-specific signatures of microRNA retention, loss, and gain

Parasitism is a successful life strategy that has evolved independently in several families of vascular plants. The genera Cuscuta and Orobanche represent examples of the two profoundly different groups of parasites: one parasitizing host shoots and the other infecting host roots. In this study, we sequenced and described the overall repertoire of small RNAs from Cuscuta campestris and Orobanche aegyptiaca. We showed that C. campestris contains a number of novel microRNAs (miRNAs) in addition to a conspicuous retention of miRNAs that are typically lacking in other Solanales, while several typically conserved miRNAs seem to have become obsolete in the parasite. One new miRNA appears to be derived from a horizontal gene transfer event. The exploratory analysis of the miRNA population (exploratory due to the absence of a full genomic sequence for reference) from the root parasitic O. aegyptiaca also revealed a loss of a number of miRNAs compared to photosynthetic species from the same order. In summary, our study shows partly similar evolutionary signatures in the RNA silencing machinery in both parasites. Our data bear proof for the dynamism of this regulatory mechanism in parasitic plants.

Horizontal gene transfer from genetically modified plants - Regulatory considerations

Gene technology regulators receive applications seeking permission for the environmental release of genetically modified (GM) plants, many of which possess beneficial traits such as improved production, enhanced nutrition and resistance to drought, pests and diseases. The regulators must assess the risks to human and animal health and to the environment from releasing these GM plants. One such consideration, of many, is the likelihood and potential consequence of the introduced or modified DNA being transferred to other organisms, including people. While such gene transfer is most likely to occur to sexually compatible relatives (vertical gene transfer), horizontal gene transfer (HGT), which is the acquisition of genetic material that has not been inherited from a parent, is also a possibility considered during these assessments. Advances in HGT detection, aided by next generation sequencing, have demonstrated that HGT occurrence may have been previously underestimated. In this review, we provide updated evidence on the likelihood, factors and the barriers for the introduced or modified DNA in GM plants to be horizontally transferred into a variety of recipients. We present the legislation and frameworks the Australian Gene Technology Regulator adheres to with respect to the consideration of risks posed by HGT. Such a perspective may generally be applicable to regulators in other jurisdictions as well as to commercial and research organisations who develop GM plants.

Bidirectional mRNA transfer between Cuscuta australis and its hosts

The holoparasitic dodder (Cuscuta spp.) is able to transfer mRNA and certain plant pathogens (e.g., viruses and bacteria) from the host plant. "Candidatus Liberibacter asiaticus," the phloem-limited causative agent of citrus Huanglongbing, can be transferred from citrus to periwinkle (Catharanthus roseus) mediated by dodder. However, characterization of mRNA transport between dodder and citrus/periwinkle remains unclear. In this study, we sequenced transcriptomes of dodder and its parasitizing host, sweet orange (Citrus sinensis "Newhall") and periwinkle (Catharanthus roseus), to identify and characterize mRNA transfer between dodder and the host plant during parasitism. The mRNA transfer between dodder and citrus/periwinkle was bidirectional and most of the transfer events occurred in the interface tissue. Compared with the citrus-dodder system, mRNA transfer in the periwinkle-dodder system was more frequent. Function classification revealed that a large number of mRNAs transferred between dodder and citrus/periwinkle were involved in secondary metabolism and stress response. Dodder transcripts encoding proteins associated with microtubule-based processes and cell wall biogenesis were transferred to host tissues. In addition, transcripts involved in translational elongation, plasmodesmata, and the auxin-activated signaling pathway were transmitted between dodder and citrus/periwinkle. In particular, transcripts involved in shoot system development and flower development were transferred between the host and dodder in both directions. The high abundance of dodder-origin transcripts, encoding MIP aquaporin protein, and S-adenosylmethionine synthetase 1 protein, in citrus and periwinkle tissues indicated they could play an important biological role in dodder-host interaction. In addition, the uptake of host mRNAs by dodder, especially those involved in seed germination and flower development, could be beneficial for the reproduction of dodder. The results of this study provide new insights into the RNA-based interaction between dodder and host plants.

Strigolactones are chemoattractants for host tropism in Orobanchaceae parasitic plants

Parasitic plants are worldwide threats that damage major agricultural crops. To initiate infection, parasitic plants have developed the ability to locate hosts and grow towards them. This ability, called host tropism, is critical for parasite survival, but its underlying mechanism remains mostly unresolved. To characterise host tropism, we used the model facultative root parasite Phtheirospermum japonicum, a member of the Orobanchaceae. Here, we show that strigolactones (SLs) function as host-derived chemoattractants. Chemotropism to SLs is also found in Striga hermonthica, a parasitic member of the Orobanchaceae, but not in non-parasites. Intriguingly, chemotropism to SLs in P. japonicum is attenuated in ammonium ion-rich conditions, where SLs are perceived, but the resulting asymmetrical accumulation of the auxin transporter PIN2 is diminished. P. japonicum encodes putative receptors that sense exogenous SLs, whereas expression of a dominant-negative form reduces its chemotropic ability. We propose a function for SLs as navigators for parasite roots.

Dodder-transmitted mobile systemic signals activate a salt-stress response characterized by a transcriptome change in Citrus sinensis

Citrus is an essential horticultural fruit whose yield and quality are affected by salinity all over the world. The recognition and adaptive regulation of citrus against salt stress are important areas for cultivar improvement, but the vascular system signal transduction mechanism of the plant response to salt stress remains elusive. In this study, we constructed a dodder (Cuscuta spp.) linked Hamlin sweet orange (Citrus sinensis) plant community in which deliver a vascular signal through the dodder in response to salt stress. RNA-seq technology was used to analyze the gene expression profile of citrus leaves after salt treatment. The results showed that a vascular signal was transmitted to a dodder-linked host plant, triggering a transcriptional response to salt stress. However, the phenotypic and transudative ability of the dodder changed after 24 h. The salt treatment group (Group S) and the dodder-linked group (Group D) respectively contained 1,472 and 557 differentially expressed genes (DEGs). 454 of which were common to both groups. The results of our analysis revealed that the gene expression categories in Group D represented a highly consistent trend compared to the group S plants, indicating that the dodder-bridged vascular signals activated the stress-response of citrus leaves for transcriptomic reconfiguration. The KEGG pathway database and an analysis of key drivers revealed that phenylpropanoid biosynthesis, photosynthesis-antenna proteins, starch and sucrose metabolism, plant hormone signal transduction, circadian rhythm, and MAPK signaling pathways were significantly enriched as the critical genes during salt stress. A systemic signal in the dodder-bridged host significantly regulated abiotic stress-related secondary metabolic pathways, including those for phenylpropanoids, lignin, and lignans. The physiological indexes of photosynthetic intensity, respiration, and attractiveness among communities supported the transcriptional changes. Thus, our results indicate that salt stress-induced vascular system signals can be transmitted through the vascular system of a dodder linking citrus plants, revealing the genetic regulation and physiological changes of citrus leaves responding to plant stress signal transmission.

Integrated metabolomic and transcriptomic analyses of the parasitic plant Cuscuta japonica Choisy on host and non-host plants

BACKGROUND

Cuscuta japonica Choisy (Japanese dodder) is a parasitic weed that damages many plants and affects agricultural production. The haustorium of C. japonica plays a key role during parasitism in host plants; in contrast, some non-host plants effectively inhibit its formation. However, the metabolic differences between normal dodder in host plants and dodder inhibition in non-host plants are largely unknown. Here, we utilized an integrative analysis of transcriptomes and metabolomes to compare the differential regulatory mechanisms between C. japonica interacting with the host plant Ficus microcarpa and the non-host plant Mangifera indica.

RESULTS

After parasitization for 24 h and 72 h, the differentially abundant metabolites between these two treatments were enriched in pathways associated with α-linolenic acid metabolism, linoleic acid metabolism, phenylpropanoid biosynthesis, and pyrimidine metabolism. At the transcriptome level, the flavor biosynthesis pathway was significantly enriched at 24 h, whereas the plant-pathogen interaction, arginine and proline metabolism, and MARK signaling-plant pathways were significantly enriched at 72 h, based on the differentially expressed genes between these two treatments. Subsequent temporal analyses identified multiple genes and metabolites that showed different trends in dodder interactions between the host and non-host plants. In particular, the phenylpropanoid biosynthesis pathway showed significant differential regulation between C. japonica in host and non-host plants.

CONCLUSIONS

These results provide insights into the metabolic mechanisms of dodder-host interactions, which will facilitate future plant protection from C. japonica parasitism.

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.

Comparative genomics of orobanchaceous species with different parasitic lifestyles reveals the origin and stepwise evolution of plant parasitism

Orobanchaceae is the largest family of parasitic plants, containing autotrophic and parasitic plants with all degrees of parasitism. This makes it by far the best family for studying the origin and evolution of plant parasitism. Here we provide three high-quality genomes of orobanchaceous plants, the autotrophic Lindenbergia luchunensis and the holoparasitic plants Phelipanche aegyptiaca and Orobanche cumana. Phylogenomic analysis of these three genomes together with those previously published and the transcriptomes of other orobanchaceous species created a robust phylogenetic framework for Orobanchaceae. We found that an ancient whole-genome duplication (WGD; about 73.48 million years ago), which occurred earlier than the origin of Orobanchaceae, might have contributed to the emergence of parasitism. However, no WGD events occurred in any lineage of orobanchaceous parasites except for Striga after divergence from their autotrophic common ancestor, suggesting that, in contrast with previous speculations, WGD is not associated with the emergence of holoparasitism. We detected evident convergent gene loss in all parasites within Orobanchaceae and between Orobanchaceae and dodder Cuscuta australis. The gene families in the orobanchaceous parasites showed a clear pattern of recent gains and expansions. The expanded gene families are enriched in functions related to the development of the haustorium, suggesting that recent gene family expansions may have facilitated the adaptation of orobanchaceous parasites to different hosts. This study illustrates a stepwise pattern in the evolution of parasitism in the orobanchaceous parasites and will facilitate future studies on parasitism and the control of parasitic plants in agriculture.

A PLETHORA/PIN-FORMED/auxin network mediates prehaustorium formation in the parasitic plant Striga hermonthica

The parasitic plant Striga (Striga hermonthica) invades the host root through the formation of a haustorium and has detrimental impacts on cereal crops. The haustorium results from the prehaustorium, which is derived directly from the differentiation of the Striga radicle. The molecular mechanisms leading to radicle differentiation shortly after germination remain unclear. In this study, we determined the developmental programs that regulate terminal prehaustorium formation in S. hermonthica at cellular resolution. We showed that shortly after germination, cells in the root meristem undergo multiplanar divisions. During growth, the meristematic activity declines and associates with reduced expression of the stem cell regulator PLETHORA1 and the cell cycle genes CYCLINB1 and HISTONE H4. We also observed a basal localization of the PIN-FORMED (PIN) proteins and a decrease in auxin levels in the meristem. Using the structural layout of the root meristem and the polarity of outer-membrane PIN proteins, we constructed a mathematical model of auxin transport that explains the auxin distribution patterns observed during S. hermonthica root growth. Our results reveal a fundamental molecular and cellular framework governing the switch of S. hermonthica roots to form the invasive prehaustoria.

Comparative Proteomic Analysis Provides New Insights into the Development of Haustorium in Taxillus chinensis (DC.) Danser

Taxillus chinensis is an important medicinal and parasitic plant that attacks other plants for living. The development of haustorium is a critical process, imperative for successful parasitic invasion. To reveal the mechanisms underlying haustorium development, we performed an iTRAQ-based proteomics analysis which led to the identification of several differentially abundant proteins (DAPs) in fresh seeds (CK), baby (FB), and adult haustoria (FD). A total of 563 and 785 DAPs were identified and quantified in the early and later developmental stages, respectively. Pathway enrichment analysis revealed that the DAPs are mainly associated with metabolic pathways, ribosome, phenylpropanoid biosynthesis, and photosynthesis. In addition, DAPs associated with the phytohormone signaling pathway changed markedly. Furthermore, we evaluated the content of various phytohormones during different stages of haustoria development. These results indicated that phytohormones are very important for haustorium development. qRT-PCR results validated that the mRNA expression levels were consistent with the expression of proteins, suggesting that our results are reliable. This is the first report on haustoria proteomes in the parasitic plant, Taxillus chinensis, to the best of our knowledge. Our findings will enhance our understanding of the molecular mechanism of haustoria development.

Transcriptional Basis for Haustorium Formation and Host Establishment in Hemiparasitic Psittacanthus schiedeanus Mistletoes

The mistletoe Psittacanthus schiedeanus, a keystone species in interaction networks between plants, pollinators, and seed dispersers, infects a wide range of native and non-native tree species of commercial interest. Here, using RNA-seq methodology we assembled the whole circularized quadripartite structure of P. schiedeanus chloroplast genome and described changes in the gene expression of the nuclear genomes across time of experimentally inoculated seeds. Of the 140,467 assembled and annotated uniGenes, 2,000 were identified as differentially expressed (DEGs) and were classified in six distinct clusters according to their expression profiles. DEGs were also classified in enriched functional categories related to synthesis, signaling, homoeostasis, and response to auxin and jasmonic acid. Since many orthologs are involved in lateral or adventitious root formation in other plant species, we propose that in P. schiedeanus (and perhaps in other rootless mistletoe species), these genes participate in haustorium formation by complex regulatory networks here described. Lastly, and according to the structural similarities of P. schiedeanus enzymes with those that are involved in host cell wall degradation in fungi, we suggest that a similar enzymatic arsenal is secreted extracellularly and used by mistletoes species to easily parasitize and break through tissues of the host.

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.

A host-free transcriptome for haustoriogenesis in Cuscuta campestris: Signature gene expression identifies markers of successive development stages

The development of the infection organ of the parasitic angiosperm genus Cuscuta is a dynamic process that is normally obscured from view as it happens endophytically in its host. We artificially induced haustoriogenesis in Cuscuta campestris by far-red light to define specific morphologically different stages and analyze their transcriptional patterns. This information enabled us to extract sets of high-confidence housekeeping and marker genes for the different stages, validated in a natural infection setting on a compatible host. This study provides a framework for more reproducible investigations of haustoriogenesis and the processes governing host-parasite interactions in shoot parasites, with C. campestris as a model species.

Biomechanics of the parasite-host interaction of the European mistletoe

The European mistletoe (Viscum album) is an epiphytic hemiparasite that attaches to its host by an endophytic system. Two aspects are essential for its survival: the structural integrity of the host-parasite interface must be maintained during host growth and the functional integrity of the interface must be maintained during ontogeny and under mechanical stress. We investigated the mechanical properties of the mistletoe-host interaction. Intact and sliced mistletoe-host samples, with host wood as reference, were subjected to tensile tests up to failure. We quantified the rough fractured surface by digital microscopy and analysed local surface strains by digital image correlation. Tensile strength and deformation energy were independent of mistletoe age but exhibited markedly lower values than host wood samples. Cracks initiated at sites with a major strain of about 30%, especially along the mistletoe-host interface. The risk of sudden failure was counteracted by various sinkers and a lignification gradient that smooths the differences in the mechanical properties between the two species. Our results improve the understanding of the key mechanical characteristics of the host-mistletoe interface and show that the mechanical connection between the mistletoe and its host is age-independent. Thus, functional and structural integrity is ensured over the lifetime of the mistletoe.

Investigating Host and Parasitic Plant Interaction by Tissue-Specific Gene Analyses on Tomato and Cuscuta campestris Interface at Three Haustorial Developmental Stages

Parasitic weeds cause billions of dollars in agricultural losses each year worldwide. Cuscuta campestris (C. campestris), one of the most widespread and destructive parasitic plants in the United States, severely reduces yield in tomato plants. Reducing the spread of parasitic weeds requires understanding the interaction between parasites and hosts. Several studies have identified factors needed for parasitic plant germination and haustorium induction, and genes involved in host defense responses. However, knowledge of the mechanisms underlying the interactions between host and parasitic plants, specifically at the interface between the two organisms, is relatively limited. A detailed investigation of the crosstalk between the host and parasite at the tissue-specific level would enable development of effective parasite control strategies. To focus on the haustorial interface, we used laser-capture microdissection (LCM) with RNA-seq on early, intermediate and mature haustorial stages. In addition, the tomato host tissue that immediately surround the haustoria was collected to obtain tissue- resolution RNA-Seq profiles for C. campestris and tomato at the parasitism interface. After conducting RNA-Seq analysis and constructing gene coexpression networks (GCNs), we identified CcHB7, CcPMEI, and CcERF1 as putative key regulators involved in C. campestris haustorium organogenesis, and three potential regulators, SlPR1, SlCuRe1-like, and SlNLR, in tomatoes that are involved in perceiving signals from the parasite. We used host-induced gene silencing (HIGS) transgenic tomatoes to knock-down the candidate genes in C. campestris and produced CRISPR transgenic tomatoes to knock out candidate genes in tomatoes. The interactions of C. campestris with these transgenic lines were tested and compared with that in wild-type tomatoes. The results of this study reveal the tissue-resolution gene regulatory mechanisms at the parasitic plant-host interface and provide the potential of developing a parasite-resistant system in tomatoes.

Interfamily grafting capacity of petunia

In grafting, an agricultural technique for propagating flower species and fruit trees, two plants are combined to exploit their beneficial characteristics, such as rootstock disease tolerance and vigor. Grafting incompatibility has been observed, however, between distantly related plant combinations, which limits the availability of plant resources. A high grafting capacity has been found in Nicotiana, belonging to Solanaceae, but not in Ipomoea nil, a Convolvulaceae species. Here, we found that Petunia hybrida, another solanaceous species, has similar ability of interfamily grafting, which indicates that interfamily grafting capability in Solanaceae is not limited to the genus Nicotiana. RNA sequencing-based comparative time-series transcriptomic analyses of Nicotiana benthamiana, I. nil, and P. hybrida revealed that N. benthamiana and P. hybrida share a common gene expression pattern, with continued elevated expression of the β-1,4-glucanase subclade gene GH9B3 observed after interfamily grafting. During self-grafting, GH9B3 expression in each species was similarly elevated, thus suggesting that solanaceous plants have altered regulatory mechanisms for GH9B3 gene expression that allow tissue fusion even with other species. Finally, we tested the effect of the β-1,4-glucanase inhibitor D-glucono-1,5-lactone, using glucose as a control, on the interfamily grafting usability of P. hybrida with Arabidopsis rootstock. Strong inhibition of graft establishment was observed only with D-glucono-1,5-lactone, thus suggesting the important role of GH9B3 in P. hybrida grafting. The newly discovered grafting compatibility of Petunia with different families enhances the propagation techniques and the production of flower plants.