Laser microdissection and its application to analyze gene expression in arbuscular mycorrhizal symbiosis

Fixation and Laser Capture Microdissection of Plant Tissue for RNA Extraction and RNASeq Library Preparation

To study the transcriptome of individual plant cells at specific points in time, we developed protocols for fixation, embedding, and sectioning of plant tissue followed by laser capture microdissection (LCM) and processing for RNA recovery. LCM allows the isolation of individual cell types from heterogeneous tissue sections and is particularly suited to plant processing because it does not require the breakdown of cell walls. This approach allows accurate separation of a small volume of cells that can be used to study gene expression profiles in different tissues or cell layers. The technique requires neither separation of cells by enzymatic digestion of any kind nor cell-specific reporter genes, and it allows storage of fixed and embedded tissue for months before capture. The methods for fixation, embedding, sectioning, and capturing of plant cells that we describe yield high-quality RNA suitable for making libraries for RNASeq. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Tissue Preparation for Laser Capture Microdissection Basic Protocol 2: Tissue Sectioning Basic Protocol 3: Laser Capture Microdissection of Embedded Tissue Basic Protocol 4: RNA Extraction from Laser Capture Microdissection Samples.

Plant-Fungal Interactions: Laser Microdissection as a Tool to Study Cell Specificity

In the past 20 years, laser microdissection (LMD) technology has been widely applied to plant tissues, allowing to obtain new information on the role of different cell-type populations during plant development and interactions, including plant-pathogen interactions. The application of a LMD approach allowed verifying the response of plant and pathogen during the progression of the infection in different cell types, focusing both on gene expression in host plants and pathogens. Here, a protocol to apply the LMD approach to study plant and fungal transcript profiles in different cell-type populations is described in detail, from the biological material preparation to RNA extraction and gene expression analyses.

Laser Microdissection as a Useful Tool to Study Gene Expression in Plant and Fungal Partners in AM Symbiosis

Laser microdissection (LMD) technology has been widely applied to plant tissues, offering novel information on the role of different cell-type populations during plant-microbe interactions. In this chapter, protocols to apply the LMD approach to study plant and fungal transcript profiles in different cell-type populations from arbuscular mycorrhizal (AM) roots are described in detail, starting from the biological material preparation to gene expression analyses by RT-PCR and RT-qPCR.

Single-Cell RNA Sequencing of Plant-Associated Bacterial Communities

Plants in soil are not solitary, hence continually interact with and obtain benefits from a community of microbes ("microbiome"). The meta-functional output from the microbiome results from complex interactions among the different community members with distinct taxonomic identities and metabolic capacities. Particularly, the bacterial communities of the root surface are spatially organized structures composed of root-attached biofilms and planktonic cells arranged in complex layers. With the distinct but coordinated roles among the different member cells, bacterial communities resemble properties of a multicellular organism. High throughput sequencing technologies have allowed rapid and large-scale analysis of taxonomic composition and metabolic capacities of bacterial communities. However, these methods are generally unable to reconstruct the assembly of these communities, or how the gene expression patterns in individual cells/species are coordinated within these communities. Single-cell transcriptomes of community members can identify how gene expression patterns vary among members of the community, including differences among different cells of the same species. This information can be used to classify cells based on functional gene expression patterns, and predict the spatial organization of the community. Here we discuss strategies for the isolation of single bacterial cells, mRNA enrichment, library construction, and analysis and interpretation of the resulting single-cell RNA-Seq datasets. Unraveling regulatory and metabolic processes at the single cell level is expected to yield an unprecedented discovery of mechanisms involved in bacterial recruitment, attachment, assembly, organization of the community, or in the specific interactions among the different members of these communities.

Fixation and Laser Capture Microdissection of Plant Tissue for RNA Extraction and RNASeq Library Preparation

In order to study the transcriptome of individual plant cells at specific points in time, we developed protocols for fixation, embedding, and sectioning of plant tissue followed by laser capture microdissection (LCM) and processing for RNA recovery. LCM allows the isolation of individual cell types from heterogeneous tissue sections and is particularly suited to plant processing because it does not require the breakdown of cell walls. This approach allows accurate separation of a small volume of cells that can be used to study gene expression profiles in different tissues or cell layers. The technique does not require separation of cells by enzymatic digestion of any kind, does not require cell-specific reporter genes, and allows storage of fixed and embedded tissue for months before capture. The methods for fixation, embedding, sectioning, and capture of plant cells that we describe yield high-quality RNA suitable for making libraries for RNASeq. © 2018 by John Wiley & Sons, Inc.

LysM Receptor-Like Kinase and LysM Receptor-Like Protein Families: An Update on Phylogeny and Functional Characterization

Members of plant specific families of receptor-like kinases (RLKs) and receptor-like proteins (RLPs), containing 3 extracellular LysMs have been shown to directly bind and/or to be involved in perception of lipo-chitooligosaccharides (LCO), chitooligosaccharides (CO), and peptidoglycan (PGN), three types of GlcNAc-containing molecules produced by microorganisms. These receptors are involved in microorganism perception by plants and can activate different plant responses leading either to symbiosis establishment or to defense responses against pathogens. LysM-RLK/Ps belong to multigenic families. Here, we provide a phylogeny of these families in eight plant species, including dicotyledons and monocotyledons, and we discuss known or putative biological roles of the members in each of the identified phylogenetic groups. We also report and discuss known biochemical properties of the LysM-RLK/Ps.

Plant carbon nourishment of arbuscular mycorrhizal fungi

Reciprocal nutrient exchange between the majority of land plants and arbucular mycorrhizal (AM) fungi is the cornerstone of a stable symbiosis. To date, a dogma in the comprehension of AM fungal nourishment has been delivery of host organic carbon in the form of sugars. More recently a role for lipids as alternative carbon source or as a signalling molecule during AM symbiosis was proposed. Here we review the symbiotic requirement for carbohydrates and lipids across developmental stages of the AM symbiosis. We present a role for carbohydrate metabolism and signalling to maintain intraradical fungal growth, as opposed to lipid uptake at the arbuscule as an indispensible requirement for completion of the AM fungal life cycle.

Adapted Biotroph Manipulation of Plant Cell Ploidy

Diverse plant biotrophs that establish a sustained site of nutrient acquisition induce localized host endoreduplication. Endoreduplication is a process by which cells successively replicate their genomes without mitosis, resulting in an increase in nuclear DNA ploidy. Elevated ploidy is associated with enhanced cell size, metabolic capacity, and the capacity to differentiate. Localized host endoreduplication induced by adapted plant biotrophs promotes biotroph colonization, development, and/or proliferation. When induced host endoreduplication is limited, biotroph growth and/or development are compromised. Herein, we examine a diverse set of plant-biotroph interactions to identify (a) common host components manipulated to promote induced host endoreduplication and (b) biotroph effectors that facilitate this induced host process. Shared mechanisms to promote host endoreduplication and development of nutrient exchange/feeding sites include manipulation centered on endocycle entry at the G2-M transition as well as yet undefined roles for differentiation regulators (e.g., CLE peptides) and pectin/cell wall modification.

Plant signalling in symbiosis and immunity

Plants encounter a myriad of microorganisms, particularly at the root-soil interface, that can invade with detrimental or beneficial outcomes. Prevalent beneficial associations between plants and microorganisms include those that promote plant growth by facilitating the acquisition of limiting nutrients such as nitrogen and phosphorus. But while promoting such symbiotic relationships, plants must restrict the formation of pathogenic associations. Achieving this balance requires the perception of potential invading microorganisms through the signals that they produce, followed by the activation of either symbiotic responses that promote microbial colonization or immune responses that limit it.

Laser microdissection: A promising tool for exploring microorganisms and their interactions with hosts

Laser microdissection is a method that allows for the isolation of homogenous cell populations from their native niches in tissues for downstream molecular assays. This method is widely used for genomic analysis, gene expression profiling and proteomic and metabolite assays in various fields of biology, but it remains an uncommon approach in microbiological research. In spite of the limited number of publications, laser microdissection was shown to be an extremely useful method for studying host-microorganism interactions in animals and plants, investigating bacteria within biofilms, identifying uncultivated bacteria and performing single prokaryotic cell analysis. The current paper describes the methodological aspects of commercially available laser microdissection instruments and representative examples that demonstrate the advantages of this method for resolving a variety of issues in microbiology.

Flowers and mycorrhizal roots--closer than we think?

Roots and flowers are formed at the extreme ends of plants and they differ in almost every aspect of their development and function; even so, they exhibit surprising molecular commonalities. For example, the calcium and calmodulin-dependent protein kinase (CCaMK) plays a central role in root symbioses with fungi and bacteria, but is also highly expressed in developing anthers. Moreover, independent evidence from transcriptomics, phylogenomics, and genetics reveals common developmental elements in root symbioses and reproductive development. We discuss the significance of these overlaps, and we argue that an integrated comparative view of the two phenomena will stimulate research and provide new insight, not only into shared components, but also into the specific aspects of anther development and root symbioses.

Laser capture microdissection in Ectocarpus siliculosus: the pathway to cell-specific transcriptomics in brown algae

Laser capture microdissection (LCM) facilitates the isolation of individual cells from tissue sections, and when combined with RNA amplification techniques, it is an extremely powerful tool for examining genome-wide expression profiles in specific cell-types. LCM has been widely used to address various biological questions in both animal and plant systems, however, no attempt has been made so far to transfer LCM technology to macroalgae. Macroalgae are a collection of widespread eukaryotes living in fresh and marine water. In line with the collective effort to promote molecular investigations of macroalgal biology, here we demonstrate the feasibility of using LCM and cell-specific transcriptomics to study development of the brown alga Ectocarpus siliculosus. We describe a workflow comprising cultivation and fixation of algae on glass slides, laser microdissection, and RNA amplification. To illustrate the effectiveness of the procedure, we show qPCR data and metrics obtained from cell-specific transcriptomes generated from both upright and prostrate filaments of Ectocarpus.

Spatial gene expression analysis in tomato hypocotyls suggests cysteine as key precursor of vascular sulfur accumulation implicated in Verticillium dahliae defense

Verticillium dahliae is a prominent generator of plant vascular wilting disease and sulfur (S)-enhanced defense (SED) mechanisms contribute to its in-planta elimination. The accumulation of S-containing defense compounds (SDCs) including elemental S (S(0) ) has been described based on the comparison of two near-isogenic tomato (Solanum lycopersicum) lines differing in fungal susceptibility. To better understand the effect of S nutrition on V. dahliae resistance both lines were supplied with low, optimal or supraoptimal sulfate-S. An absolute quantification demonstrated a most effective fungal elimination due to luxury plant S nutrition. High-pressure liquid chromatography (HPLC) showed a strong regulation of Cys levels and an S-responsive GSH pool rise in the bulk hypocotyl. High-frequency S peak accumulations were detected in vascular bundles of resistant tomato plants after fungal colonization by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Global transcriptomic analysis suggested that early steps of the primary S metabolism did not promote the SDCs synthesis in the whole hypocotyl as gene expression was downregulated after infection. Enhanced S fertilization mostly alleviated the repressive fungal effect but did not reverse it. Upregulation of glutathione (GSH)-associated genes in bulk hypocotyls but not in vascular bundles indicated a global antioxidative role of GSH. To finally assign the contribution of S metabolism-associated genes to high S(0) accumulations exclusively found in the resistant tomato line, a spatial gene expression approach was applied. Laser microdissection of infected vascular bundles revealed a switch toward transcription of genes connected with cysteine (Cys) synthesis. The upregulation of LeOASTLp1 suggests a role for Cys as key precursor for local S accumulations (possibly S(0) ) in the vascular bundles of the V. dahliae-resistant tomato line.

Gene expression in vessel-associated cells upon xylem embolism repair in Vitis vinifera L. petioles

In this work, the involvement of vessel-associated cells in embolism recovery was investigated by studying leaf petiole hydraulics and expression profiles of aquaporins and genes related to sugar metabolism. Two different stress treatments were imposed onto grapevines to induce xylem embolism: one involved a pressure collar applied to the stems, while the other consisted of water deprivation (drought). Embolism formation and repair were monitored during stress application and release (recovery). At the same time, stomatal conductance (g(s)), leaf water potential (Ψ(leaf)) and leaf abscisic acid (ABA) concentration were measured. For each treatment, gene transcript levels were assessed on vessel-associated cells (isolated from leaf petioles by laser microdissection technique) and whole petioles. Both treatments induced severe xylem embolism formation and drops in g s and Ψ (leaf) at a lesser degree and with faster recovery in the case of application of the pressure collar. Leaf ABA concentration only increased upon drought and subsequent recovery. Transcripts linked to sugar mobilisation (encoding a β-amylase and a glucose-6-P transporter) were over-expressed upon stress or recovery, both in vessel-associated cells and whole petioles. However, two aquaporin genes (VvPIP2;1 and VvPIP2;4N) were activated upon stress or recovery only in vessel-associated cells, suggesting a specific effect on embolism refilling. Furthermore, the latter gene was only activated upon drought and subsequent recovery, suggesting that either severe water stress or ABA is required for its regulation.

Protein actors sustaining arbuscular mycorrhizal symbiosis: underground artists break the silence

The roots of most land plants can enter a relationship with soil-borne fungi belonging to the phylum Glomeromycota. This symbiosis with arbuscular mycorrhizal (AM) fungi belongs to the so-called biotrophic interactions, involving the intracellular accommodation of a microorganism by a living plant cell without causing the death of the host. Although profiling technologies have generated an increasing depository of plant and fungal proteins eligible for sustaining AM accommodation and functioning, a bottleneck exists for their functional analysis as these experiments are difficult to carry out with mycorrhiza. Nonetheless, the expansion of gene-to-phenotype reverse genetic tools, including RNA interference and transposon silencing, have recently succeeded in elucidating some of the plant-related protein candidates. Likewise, despite the ongoing absence of transformation tools for AM fungi, host-induced gene silencing has allowed knockdown of fungal gene expression in planta for the first time, thus unlocking a technological limitation in deciphering the functional pertinence of glomeromycotan proteins during mycorrhizal establishment. This review is thus intended to draw a picture of our current knowledge about the plant and fungal protein actors that have been demonstrated to be functionally implicated in sustaining AM symbiosis mostly on the basis of silencing approaches.

Laser microdissection and microarray analysis of Tuber melanosporum ectomycorrhizas reveal functional heterogeneity between mantle and Hartig net compartments

The ectomycorrhizal (ECM) symbiosis, a mutualistic plant-fungus association, plays a fundamental role in forest ecosystems by enhancing plant growth and by providing host protection from root diseases. The cellular complexity of the symbiotic organ, characterized by the differentiation of structurally specialized tissues (i.e. the fungal mantle and the Hartig net), is the major limitation to study fungal gene expression in such specific compartments. We investigated the transcriptional landscape of the ECM fungus Tuber melanosporum during the major stages of its life cycle and we particularly focused on the complex symbiotic stage by combining the use of laser capture microdissection and microarray gene expression analysis. We isolated the fungal/soil (i.e. the mantle) and the fungal/plant (i.e. the Hartig net) interfaces from transverse sections of T. melanosporum/Corylus avellana ectomycorrhizas and identified the distinct genetic programmes associated with each compartment. Particularly, nitrogen and water acquisition from soil, synthesis of secondary metabolites and detoxification mechanisms appear to be important processes in the fungal mantle. In contrast, transport activity is enhanced in the Hartig net and we identified carbohydrate and nitrogen-derived transporters that might play a key role in the reciprocal resources' transfer between the host and the symbiont.

Functional genomics of a generalist parasitic plant: laser microdissection of host-parasite interface reveals host-specific patterns of parasite gene expression

BACKGROUND

Orobanchaceae is the only plant family with members representing the full range of parasitic lifestyles plus a free-living lineage sister to all parasitic lineages, Lindenbergia. A generalist member of this family, and an important parasitic plant model, Triphysaria versicolor regularly feeds upon a wide range of host plants. Here, we compare de novo assembled transcriptomes generated from laser micro-dissected tissues at the host-parasite interface to uncover details of the largely uncharacterized interaction between parasitic plants and their hosts.

RESULTS

The interaction of Triphysaria with the distantly related hosts Zea mays and Medicago truncatula reveals dramatic host-specific gene expression patterns. Relative to above ground tissues, gene families are disproportionally represented at the interface including enrichment for transcription factors and genes of unknown function. Quantitative Real-Time PCR of a T. versicolor β-expansin shows strong differential (120x) upregulation in response to the monocot host Z. mays; a result that is concordant with our read count estimates. Pathogenesis-related proteins, other cell wall modifying enzymes, and orthologs of genes with unknown function (annotated as such in sequenced plant genomes) are among the parasite genes highly expressed by T. versicolor at the parasite-host interface.

CONCLUSIONS

Laser capture microdissection makes it possible to sample the small region of cells at the epicenter of parasite host interactions. The results of our analysis suggest that T. versicolor's generalist strategy involves a reliance on overlapping but distinct gene sets, depending upon the host plant it is parasitizing. The massive upregulation of a T. versicolor β-expansin is suggestive of a mechanism for parasite success on grass hosts. In this preliminary study of the interface transcriptomes, we have shown that T. versicolor, and the Orobanchaceae in general, provide excellent opportunities for the characterization of plant genes with unknown functions.

Combining laser microdissection and RNA-seq to chart the transcriptional landscape of fungal development

BACKGROUND

During sexual development, filamentous ascomycetes form complex, three-dimensional fruiting bodies for the protection and dispersal of sexual spores. Fruiting bodies contain a number of cell types not found in vegetative mycelium, and these morphological differences are thought to be mediated by changes in gene expression. However, little is known about the spatial distribution of gene expression in fungal development. Here, we used laser microdissection (LM) and RNA-seq to determine gene expression patterns in young fruiting bodies (protoperithecia) and non-reproductive mycelia of the ascomycete Sordaria macrospora.

RESULTS

Quantitative analysis showed major differences in the gene expression patterns between protoperithecia and total mycelium. Among the genes strongly up-regulated in protoperithecia were the pheromone precursor genes ppg1 and ppg2. The up-regulation was confirmed by fluorescence microscopy of egfp expression under the control of ppg1 regulatory sequences. RNA-seq analysis of protoperithecia from the sterile mutant pro1 showed that many genes that are differentially regulated in these structures are under the genetic control of transcription factor PRO1.

CONCLUSIONS

We have generated transcriptional profiles of young fungal sexual structures using a combination of LM and RNA-seq. This allowed a high spatial resolution and sensitivity, and yielded a detailed picture of gene expression during development. Our data revealed significant differences in gene expression between protoperithecia and non-reproductive mycelia, and showed that the transcription factor PRO1 is involved in the regulation of many genes expressed specifically in sexual structures. The LM/RNA-seq approach will also be relevant to other eukaryotic systems in which multicellular development is investigated.

Revealing natural relationships among arbuscular mycorrhizal fungi: culture line BEG47 represents Diversispora epigaea, not Glomus versiforme

Background

Understanding the mechanisms underlying biological phenomena, such as evolutionarily conservative trait inheritance, is predicated on knowledge of the natural relationships among organisms. However, despite their enormous ecological significance, many of the ubiquitous soil inhabiting and plant symbiotic arbuscular mycorrhizal fungi (AMF, phylum Glomeromycota) are incorrectly classified.

Methodology/Principal Findings

Here, we focused on a frequently used model AMF registered as culture BEG47. This fungus is a descendent of the ex-type culture-lineage of Glomus epigaeum, which in 1983 was synonymised with Glomus versiforme. It has since then been used as ‘G. versiforme BEG47’. We show by morphological comparisons, based on type material, collected 1860–61, of G. versiforme and on type material and living ex-type cultures of G. epigaeum, that these two AMF species cannot be conspecific, and by molecular phylogenetics that BEG47 is a member of the genus Diversispora.

Conclusions

This study highlights that experimental works published during the last >25 years on an AMF named ‘G. versiforme’ or ‘BEG47’ refer to D. epigaea, a species that is actually evolutionarily separated by hundreds of millions of years from all members of the genera in the Glomerales and thus from most other commonly used AMF ‘laboratory strains’. Detailed redescriptions substantiate the renaming of G. epigaeum (BEG47) as D. epigaea, positioning it systematically in the order Diversisporales, thus enabling an evolutionary understanding of genetical, physiological, and ecological traits, relative to those of other AMF. Diversispora epigaea is widely cultured as a laboratory strain of AMF, whereas G. versiforme appears not to have been cultured nor found in the field since its original description.

Stars and symbiosis: microRNA- and microRNA*-mediated transcript cleavage involved in arbuscular mycorrhizal symbiosis

The majority of plants are able to form the arbuscular mycorrhizal (AM) symbiosis in association with AM fungi. During symbiosis development, plant cells undergo a complex reprogramming resulting in profound morphological and physiological changes. MicroRNAs (miRNAs) are important components of the regulatory network of plant cells. To unravel the impact of miRNAs and miRNA-mediated mRNA cleavage on root cell reprogramming during AM symbiosis, we carried out high-throughput (Illumina) sequencing of small RNAs and degradome tags of Medicago truncatula roots. This led to the annotation of 243 novel miRNAs. An increased accumulation of several novel and conserved miRNAs in mycorrhizal roots suggest a role of these miRNAs during AM symbiosis. The degradome analysis led to the identification of 185 root transcripts as mature miRNA and also miRNA*-mediated mRNA cleavage targets. Several of the identified miRNA targets are known to be involved in root symbioses. In summary, the increased accumulation of specific miRNAs and the miRNA-mediated cleavage of symbiosis-relevant genes indicate that miRNAs are an important part of the regulatory network leading to symbiosis development.

Regulation of hematopoietic cell clusters in the placental niche through SCF/Kit signaling in embryonic mouse

Hematopoietic stem cells (HSCs) emerge from and expand in the mouse placenta at mid-gestation. To determine their compartment of origin and define extrinsic signals governing their commitment to this lineage, we identified hematopoietic cell (HC) clusters in mouse placenta, defined as cells expressing the embryonic HSC markers CD31, CD34 and Kit, by immunohistochemistry. HC clusters were first observed in the placenta at 9.5 days post coitum (dpc). To determine their origin, we tagged the allantoic region with CM-DiI at 8.25 dpc, prior to placenta formation, and cultured embryos in a whole embryo culture (WEC) system. CM-DiI-positive HC clusters were observed 42 hours later. To determine how clusters are extrinsically regulated, we isolated niche cells using laser capture micro-dissection and assayed them for expression of genes encoding hematopoietic cytokines. Among a panel of candidates assayed, only stem cell factor (SCF) was expressed in niche cells. To define niche cells, endothelial and mesenchymal cells were sorted by flow cytometry from dissociated placenta and hematopoietic cytokine gene expression was investigated. The endothelial cell compartment predominantly expressed SCF mRNA and protein. To determine whether SCF/Kit signaling regulates placental HC cluster proliferation, we injected anti-Kit neutralizing antibody into 10.25 dpc embryos and assayed cultured embryos for expression of hematopoietic transcription factors. Runx1, Myb and Gata2 were downregulated in the placental HC cluster fraction relative to controls. These observations demonstrate that placental HC clusters originate from the allantois and are regulated by endothelial niche cells through SCF/Kit signaling.

Laser capture microdissection of uredinia formed by Melampsora larici-populina revealed a transcriptional switch between biotrophy and sporulation

The foliar rust caused by the basidiomycete Melampsora larici-populina is the main disease affecting poplar plantations in Europe. The biotrophic status of rust fungi is a major limitation to study gene expression of cell or tissue types during host infection. At the uredinial stage, infected poplar leaves contain distinct rust tissues such as haustoria, infection hyphae, and uredinia with sporogenous hyphae and newly formed asexual urediniospores. Laser capture microdissection (LCM) was used to isolate three areas corresponding to uredinia and subjacent zones in the host mesophyll for expression analysis with M. larici-populina whole-genome exon oligoarrays. Optimization of tissue preparation prior to LCM allowed isolation of RNA of good integrity for genome-wide expression profiling. Our results indicate that the poplar rust uredinial stage is marked by distinct genetic programs related to biotrophy in the host palisade mesophyll and to sporulation in the uredinium. A strong induction of transcripts encoding small secreted proteins, likely containing rust effectors, is observed in the mesophyll, suggesting a late maintenance of suppression of host defense in the tissue containing haustoria and infection hyphae. On the other hand, cell cycle and cell defense rescue transcripts are strongly accumulated in the sporulation area. This combined LCM-transcriptomic approach brings new insights on the molecular mechanisms underlying urediniospore formation in rust fungi.

Control--the Striga conundrum

There is a wide range of existing and potential control options for Striga. This paper describes and discusses many of the control options, with a focus on technology limitations, adoption limitations (real or potential) and, in the case of novel technologies, development limitations. The paper addresses the question as to why, after many years of research, control method testing, piloting and technology dissemination, the wide-scale effective control of Striga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze is so elusive. Limitations, including variable technology reliability, poor access to control technology, costs (monetary, labour, skills) associated with control technology, limited practicality of methods and poor information, all hamper the adoption and impact of existing control methods. Some of the same issues may impact upon novel control technologies, and this needs careful consideration. Additional issues surround other potential technologies, especially so in the case of transgenic approaches. Suggestions are made as to how the impasse of effective Striga control can be overcome. More effective use of integrated control approaches, improved crop germplasm phenotyping, enhanced understanding of the host/non-host--parasite interaction and better integration and communication among the parasitic plant research, development and extension community are among the suggestions made.

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

BACKGROUND

In addition to their roles in eukaryotic defense and development, reactive oxygen species (ROS) have recently been identified as critical for host attachment by the parasitic angiosperms. In a process known as semagenesis, ROS generated at the root tip of Striga asiatica (L.) Kuntze (Scrophulariaceae) function together with host peroxidases to oxidize monolignols at the host root surface. As a result, para-benzoquinone products accumulate as both necessary and sufficient components for inducing development of the host attachment organ, the haustorium. This event constitutes the critical vegetative/pathogenic transition in the parasite.

RESULTS

New evidence is presented that semagenesis occurs broadly in plant-plant signaling. Eudicotyledenous seedlings are more sensitive to the xenognostic benzoquinones than monocots, but general root development, including root elongation, root hair initiation and root hair growth, is impacted in both clades. Specific inhibitors of haustorial development in S. asiatica also inhibit benzoquinone-mediated root development in the non-parasites. These results suggest a common mechanism for benzoquinone perception.

CONCLUSION

Semagenesis enriches our understanding of the mechanisms available for small-molecule underground information exchange among plants. Critical differences in this process, as used by parasitic plants, are beginning to emerge and point towards new strategies for managing parasitic angiosperms in agricultural settings.

Breeding approaches for crenate broomrape (Orobanche crenata Forsk.) management in pea (Pisum sativum L.)

BACKGROUND

Pea cultivation is strongly hampered in Mediterranean and Middle East farming systems by the occurrence of Orobanche crenata Forsk. Strategies of control have been developed, but only marginal successes have been achieved. Most control methods are either unfeasible, uneconomical, hard to achieve or result in incomplete protection. The integration of several control measures is the most desirable strategy.

RESULTS

[corrected] Recent developments in control are presented and re-evaluated in light of recent developments in crop breeding and molecular genetics. These developments are placed within a framework that is compatible with current agronomic practices.

CONCLUSION

The current focus in applied breeding is leveraging biotechnological tools to develop more and better markers to speed up the delivery of improved cultivars to the farmer. To date, however, progress in marker development and delivery of useful markers has been slow. The application of knowledge gained from basic genomic research and genetic engineering will contribute to more rapid pea improvement for resistance against O. crenata and/or the herbicide.

Structure and function of natural and synthetic signalling molecules in parasitic weed germination

The structures of naturally occurring germination stimulants for seeds of the parasitic weeds Striga spp. and Orobanche spp. are described. The bioactiphore in this strigolactone family of stimulants is deduced from a structure-activity relationship and shown to reside in the CD part of the stimulant molecule. A molecular mechanism for the initial stages of seed germination is proposed. The influence of stereochemistry on the stimulant activity is significant. Combining this molecular information leads to a model for the design of synthetic strigolactones. Nijmegen-1 is a typical example of a highly active, newly designed synthetic stimulant. The occurrence of natural stimulants not belonging to the strigolactone family, such as cotylenin and parthenolide, is briefly described. The biosynthesis of natural strigolactones from beta-carotene is analysed in terms of isolated and predicted stimulants. This scheme will be helpful in the search for new strigolactones from root exudates. Protein fishing experiments to isolate and characterise the receptor protein using biotin-labelled GR 24 are described. A receptor protein of 60 kD was identified by this method. Nijmegen-1 has been tested as a suicidal germination agent in field trials on tobacco infested by Orobanche ramosa L. The preliminary results are highly rewarding. Finally, some future challenges in synthesis are described. These include synthesising new natural and synthetic stimulants and establishing the molecular connection between strigolactones as germination stimulants, as the branching factor for arbuscular mycorrhizal fungi and as an inhibitor of shoot branching.

Strigolactones: ecological significance and use as a target for parasitic plant control

Parasitic weeds cause severe damage to important agricultural crops. Although some promising control methods against these parasitic plants have been developed, new strategies continue to be relevant in integrated approaches. The life cycle for root parasitic weeds is intimately associated with their host and is a suitable target for such new control strategies, particularly when directed at the early stages of the host-parasite interaction. Here, the authors focus on knowledge of the germination stimulants-strigolactones-for the root parasitic plants Striga and Orobanche spp. and discuss their biosynthetic origin, ecological significance and physiological and biochemical regulation. In addition, the existing and possible new control strategies that are based on this knowledge, and that could lead to more efficient control methods against these root parasitic weeds, are reviewed.