Isolation and identification of alectrol as (+)-orobanchyl acetate, a germination stimulant for root parasitic plants

Hyphal Growth and Conidia Germination Are Induced by Phytohormones in the Root Colonizing and Plant Growth Promoting Fungus Metarhizium guizhouense

Beneficial associations are very important for plants and soil-dwelling microorganisms in different ecological niches, where communication by chemical signals is relevant. Among the chemical signals, the release of phytohormones by plants is important to establish beneficial associations with fungi, and a recently described association is that of the entomopathogenic ascomycete fungus Metarhizium with plants. Here, we evaluated the effect of four different phytohormones, synthetic strigolactone (GR24), sorgolactone (SorL), 3-indolacetic acid (IAA) and gibberellic acid (GA3), on the fungus Metarhizium guizhouense strain HA11-2, where the germination rate and hyphal elongation were determined at three different times. All phytohormones had a positive effect on germination, with GA3 showing the greatest effect, and for hyphal length, on average, the group treated with synthetic strigolactone GR24 showed greater average hyphal length at 10 h of induction. This work expands the knowledge of the effect of phytohormones on the fungus M. guizhouense, as possible chemical signals for the rapid establishment of the fungus-plant association.

Plant specialized metabolites in the rhizosphere of tomatoes: secretion and effects on microorganisms

Plants interact with microorganisms in the phyllosphere and rhizosphere. Here the roots exude plant specialized metabolites (PSMs) that have diverse biological and ecological functions. Recent reports have shown that these PSMs influence the rhizosphere microbiome, which is essential for the plant's growth and health. This review summarizes several specialized metabolites secreted into the rhizosphere of the tomato plant (Solanum lycopersicum), which is an important model species for plant research and a commercial crop. In this review, we focused on the effects of such plant metabolites on plant-microbe interactions. We also reviewed recent studies on improving the growth of tomatoes by analyzing and reconstructing the rhizosphere microbiome and discussed the challenges to be addressed in establishing sustainable agriculture.

Structure Elucidation and Biosynthesis of Orobanchol

Strigolactones (SLs), a class of phytohormones that regulate diverse developmental processes, were initially characterized as host-derived germination stimulants for seeds belonging to the genera Striga, Orobanche, and Phelipanche. Orobanchol (1), which is detected in the root exudates of several plants and recognized as a prevalent SL, was first isolated from the root exudates of red clover as a germination stimulant for Orobanche minor in 1998. However, the structure of this stimulant proposed at that time was disputable considering its predicted germination-inducing activity for Striga gesnerioides. The genuine structure of orobanchol was elucidated following a decade-long controversy, which ultimately facilitated the understanding of the importance of SL stereochemistry in Striga seed germination. Recently, studies focusing on clarifying the biosynthesis pathway of orobanchol are being conducted. Cytochrome P450 monooxygenases are involved in orobanchol biosynthesis downstream of carlactonoic acid (CLA) via two pathways: either through 4-deoxyorobanchol or direct conversion from CLA. Substantial progress in the identification of more SL structures and clarification of their biosynthetic mechanisms will further contribute in the comprehension of their structural diversity's functional importance and agricultural applications. Herein, we have reviewed the history leading to the discovery of the genuine structure of orobanchol and the current understanding of its biosynthetic mechanisms.

Characterization of a Chickpea Mutant Resistant to Phelipanche aegyptiaca Pers. and Orobanche crenata Forsk

Chickpea (Cicer arietinum L.) is a major pulse crop in Israel grown on about 3000 ha spread, from the Upper Galilee in the north to the North-Negev desert in the south. In the last few years, there has been a gradual increase in broomrape infestation in chickpea fields in all regions of Israel. Resistant chickpea cultivars would be simple and effective solution to control broomrape. Thus, to develop resistant cultivars we screened an ethyl methanesulfonate (EMS) mutant population of F01 variety (Kabuli type) for broomrape resistance. One of the mutant lines (CCD7M14) was found to be highly resistant to both Phelipanche aegyptiaca and Orobanche crenata. The resistance mechanism is based on the inability of the mutant to produce strigolactones (SLs)-stimulants of broomrape seed germination. LC/MS/MS analysis revealed the SLs orobanchol, orobanchyl acetate, and didehydroorobanchol in root exudates of the wild type, but no SLs could be detected in the root exudates of CCD7M14. Sequence analyses revealed a point mutation (G-to-A transition at nucleotide position 210) in the Carotenoid Cleavage Dioxygenase 7 (CCD7) gene that is responsible for the production of key enzymes in the biosynthesis of SLs. This nonsense mutation resulted in a CCD7 stop codon at position 70 of the protein. The influences of the CCD7M14 mutation on chickpea phenotype and chlorophyll, carotenoid, and anthocyanin content were characterized.

Sunflower Metabolites Involved in Resistance Mechanisms against Broomrape

Several strategies have been proposed to control parasitic plants and these include biological control, the use of herbicides, and suicidal germination or trap crops. In the case of sunflower, the most effective solution is genetic resistance. Nevertheless, the use of resistant cultivars is followed by the appearance of new and more virulent races of the parasite that overcome the existing sources of resistance. For this reason, it is necessary to increase our knowledge of the mechanisms involved in the resistant varieties. In this study, the presence of compounds involved in pre-attachment resistance mechanisms in sunflower varieties was analyzed. The production of stimulators of the germination of parasitic plant seeds and the concentration of the haustorial elongation inhibitor, scopoletin, were measured in exudates of resistant and susceptible sunflower varieties. It was found that dehydrocostuslactone and orobanchyl acetate are not involved in the resistance of the sunflower varieties studied. The results demonstrate that costunolide, heliolactone, and scopoletin are related to sunflower resistance. The sunflower varieties studied do not use all available options to fight parasitic plants. This could be one of the reasons that sunflower resistance is rapidly overcome by broomrape.

Plant apocarotenoids: from retrograde signaling to interspecific communication

Carotenoids are isoprenoid compounds synthesized by all photosynthetic and some non-photosynthetic organisms. They are essential for photosynthesis and contribute to many other aspects of a plant's life. The oxidative breakdown of carotenoids gives rise to the formation of a diverse family of essential metabolites called apocarotenoids. This metabolic process either takes place spontaneously through reactive oxygen species or is catalyzed by enzymes generally belonging to the CAROTENOID CLEAVAGE DIOXYGENASE family. Apocarotenoids include the phytohormones abscisic acid and strigolactones (SLs), signaling molecules and growth regulators. Abscisic acid and SLs are vital in regulating plant growth, development and stress response. SLs are also an essential component in plants' rhizospheric communication with symbionts and parasites. Other apocarotenoid small molecules, such as blumenols, mycorradicins, zaxinone, anchorene, β-cyclocitral, β-cyclogeranic acid, β-ionone and loliolide, are involved in plant growth and development, and/or contribute to different processes, including arbuscular mycorrhiza symbiosis, abiotic stress response, plant-plant and plant-herbivore interactions and plastid retrograde signaling. There are also indications for the presence of structurally unidentified linear cis-carotene-derived apocarotenoids, which are presumed to modulate plastid biogenesis and leaf morphology, among other developmental processes. Here, we provide an overview on the biology of old, recently discovered and supposed plant apocarotenoid signaling molecules, describing their biosynthesis, developmental and physiological functions, and role as a messenger in plant communication.

Analytical methods in strigolactone research

Strigolactones (SLs) are important plant hormones that are produced via the carotenoid biosynthetic pathway and occur at extremely low concentrations in various plant species. They regulate root development, play important roles in symbioses between higher plants and mycorrhizal fungi, and stimulate germination of plant-parasitic Orobanche and Striga species. Chemical analysis is central to research on the biochemistry of SLs and their roles in developmental biology and plant physiology. Here we summarize key issues relating to the identification and quantification of SLs isolated from plant tissues and exudates. The advantages and drawbacks of different protocols used for strigolactone analysis are discussed, and guidelines for selecting a procedure that will minimize losses during isolation and purification prior to final analysis are proposed. Hyphenated techniques suitable for SL analysis such as GC-MS and LC-MS/MS are also discussed, and newer ambient techniques such as HR-DART-MS and DESI-MS are highlighted as tools with considerable potential in SL research. A key advantage of these methods is that they require only simply sample preparation.

Recent progress in the chemistry and biochemistry of strigolactones

Strigolactones (SLs) are plant secondary metabolites derived from carotenoids. SLs play important roles in the regulation of plant growth and development in planta and coordinate interactions between plants and other organisms including root parasitic plants, and symbiotic and pathogenic microbes in the rhizosphere. In the 50 years since the discovery of the first SL, strigol, our knowledge about the chemistry and biochemistry of SLs has advanced explosively, especially over the last two decades. In this review, recent advances in the chemistry and biology of SLs are summarized and possible future outcomes are discussed.

The Many Models of Strigolactone Signaling

Strigolactones (SLs) are a class of plant hormones involved in several biological processes that are of great agricultural concern. While initiating plant-fungal symbiosis, SLs also trigger germination of parasitic plants that pose a major threat to farming. In vascular plants, SLs control shoot branching, which is linked to crop yield. SL research has been a fascinating field that has produced a variety of different signaling models, reflecting a complex picture of hormone perception. Here, we review recent developments in the SL field and the crystal structures that gave rise to various models of receptor activation. We also highlight the increasing number of discovered SL molecules, reflecting the existence of cross-kingdom SL communication.

Allelochemicals and Signaling Chemicals in Plants

Plants abound with active ingredients. Among these natural constituents, allelochemicals and signaling chemicals that are released into the environments play important roles in regulating the interactions between plants and other organisms. Allelochemicals participate in the defense of plants against microbial attack, herbivore predation, and/or competition with other plants, most notably in allelopathy, which affects the establishment of competing plants. Allelochemicals could be leads for new pesticide discovery efforts. Signaling chemicals are involved in plant neighbor detection or pest identification, and they induce the production and release of plant defensive metabolites. Through the signaling chemicals, plants can either detect or identify competitors, herbivores, or pathogens, and respond by increasing defensive metabolites levels, providing an advantage for their own growth. The plant-organism interactions that are mediated by allelochemicals and signaling chemicals take place both aboveground and belowground. In the case of aboveground interactions, mediated air-borne chemicals are well established. Belowground interactions, particularly in the context of soil-borne chemicals driving signaling interactions, are largely unknown, due to the complexity of plant-soil interactions. The lack of effective and reliable methods of identification and clarification their mode of actions is one of the greatest challenges with soil-borne allelochemicals and signaling chemicals. Recent developments in methodological strategies aim at the quality, quantity, and spatiotemporal dynamics of soil-borne chemicals. This review outlines recent research regarding plant-derived allelochemicals and signaling chemicals, as well as their roles in agricultural pest management. The effort represents a mechanistically exhaustive view of plant-organism interactions that are mediated by allelochemicals and signaling chemicals and provides more realistic insights into potential implications and applications in sustainable agriculture.

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

INTRODUCTION

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

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

Target-based selectivity of strigolactone agonists and antagonists in plants and their potential use in agriculture

Strigolactones (SLs) are small carotenoid-derived molecules that possess a wide spectrum of functions, including plant hormonal activities and chemical mediation of rhizosphere communication with both root parasitic plants and symbiotic arbuscular mycorrhizal fungi. Chemicals that regulate the functions of SLs may therefore have the potential to become widely used in agricultural applications. For example, various SL analogs and mimics have been developed to reduce the seed banks of root parasites in the field. Other analogs and mimics act selectively to suppress branching, with weak, or no stimulation, of germination in root parasites. In addition, some antagonists for SL receptors have been developed based on the mechanisms of SL perception. A better understanding of the modes of action of SL perception by various receptors will help to support the design of SL analogs, mimics, and antagonists with high activity and selectivity. Here, we review the compounds reported so far from the viewpoint of their selectivity to their targets, and the possibilities for their use in agriculture.

Structural diversity in the strigolactones

Strigolactones (SLs) are a class of signalling molecules secreted by the roots of plants into the rhizosphere. On the one hand, they serve as the signal for recruiting arbuscular mycorrhizal fungi which have a symbiotic relationship with plants. On the other hand, they are also host detection signals for the non-symbiotic, pathogenic, root parasitic plants, which use the SLs as germination stimulants. Finally, recently the SLs were discovered to be a new class of plant hormones that regulate processes such as branching/tillering and root architecture. Intriguingly, >25 different SLs have already been discovered that all have the so-called D-ring but otherwise display many differences in structure and functional groups. In this review, we will critically discuss the structural diversity in the SLs. How are they synthesized in plants; how has this structural diversity possibly evolved; what is the biological relevance of this diversity; and what does this imply for the perception of the SLs by receptors in the plant itself and in other organisms? Finally, we conclude that only little is known about the biological significance of this structural diversity, and we will give an outlook on how to elucidate their importance further.

Phenotypical and biochemical characterisation of resistance for parasitic weed (Orobanche foetida Poir.) in radiation-mutagenised mutants of chickpea

BACKGROUND

Some radiation-mutagenised chickpea mutants potentially resistant to the broomrape, Orobanche foetida Poir., were selected through field trials. The objectives of this work were to confirm resistance under artificial infestation, in pots and mini-rhizotron systems, and to determine the developmental stages of broomrape affected by resistance and the relevant resistance mechanisms induced by radiation mutagenesis.

RESULTS

Among 30 mutants tested for resistance to O. foetida, five shared strong resistance in both pot experiments and mini-rhizotron systems. Resistance was not complete, but the few individuals that escaped resistance displayed high disorders of shoot development. Results demonstrated a 2-3-fold decrease in stimulatory activity of root exudates towards broomrape seed germination in resistant mutants in comparison with non-irradiated control plants and susceptible mutants. Resistance was associated with an induction of broomrape necrosis early during infection. When infested, most of the resistant mutants shared enhanced levels of soluble phenolic contents, phenylalanine ammonia lyase activity, guaiacol peroxidase activity and polyphenol oxidase activity, in addition to glutathione and notably ascorbate peroxidase gene expression in roots.

CONCLUSION

Results confirmed enhanced resistance in chickpea radiation-mutagenised mutants, and demonstrated that resistance is based on alteration of root exudation, presumed cell-wall reinforcement and change in root oxidative status in response to infection. © 2016 Society of Chemical Industry.

Structural diversity of strigolactones and their distribution in the plant kingdom

Strigolactones (SLs) are plant secondary metabolites that were first identified as germination stimulants for the root parasitic weeds witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.). In the rhizosphere, SLs also promote root colonization by arbuscular mycorrhizal fungi. In plants, SLs as a novel class of plant hormones regulate various aspects of plant growth and development. Herein I discuss structural diversity of naturally occurring SLs and their distribution in the plant kingdom.

A concise synthesis of optically active solanacol, the germination stimulant for seeds of root parasitic weeds

Solanacol, isolated from tobacco (Nicotiana tabacum L.), is a germination stimulant for seeds of root parasitic weeds. A concise synthesis of optically active solanacol has been achieved by employing enzymatic resolution as a key step.

Strigolactone involvement in root development, response to abiotic stress, and interactions with the biotic soil environment

Strigolactones, recently discovered as plant hormones, regulate the development of different plant parts. In the root, they regulate root architecture and affect root hair length and density. Their biosynthesis and exudation increase under low phosphate levels, and they are associated with root responses to these conditions. Their signaling pathway in the plant includes protein interactions and ubiquitin-dependent repressor degradation. In the root, they lead to changes in actin architecture and dynamics as well as localization of the PIN-FORMED auxin transporter in the plasma membrane. Strigolactones are also involved with communication in the rhizosphere. They are necessary for germination of parasitic plant seeds, they enhance hyphal branching of arbuscular mycorrhizal fungi of the Glomus and Gigaspora spp., and they promote rhizobial symbiosis. This review focuses on the role played by strigolactones in root development, their response to nutrient deficiency, and their involvement with plant interactions in the rhizosphere.

Strigolactones and the control of plant development: lessons from shoot branching

Strigolactones (SLs) were originally identified through their activities as root exudates in the rhizosphere; however, it is now clear that they have many endogenous signalling roles in plants. In this review we discuss recent progress in understanding SL action in planta, particularly in the context of the regulation of shoot branching, one of the best-characterized endogenous roles for SLs. Rapid progress has been made in understanding SL biosynthesis, but many questions remain unanswered. There are hints of as yet unidentified sources of SL, as well as unknown SL-like molecules with important signalling functions. SL signalling is even more enigmatic. Although a likely receptor has been identified, along with some candidate immediate downstream targets, our understanding of how these targets mediate SL signalling is limited. There is still considerable uncertainty about whether the targets of SL signalling are primarily transcriptional or not. There is at least one non-transcriptional target, because a rapid primary response to SL is the removal of PIN1 auxin exporter proteins from the plasma membrane in vascular-associated cells of the stem. We discuss how the various early events in SL signalling could result in the observed changes in shoot branching.

Structural requirements of strigolactones for germination induction and inhibition of Striga gesnerioides seeds

Structure-activity relationship studies of strigolactones and Striga gesnerioides seed germination revealed strict structural requirements for germination induction and a new function of the plant hormones as germination inhibitors. Stereoisomers of the naturally occurring strigolactones, strigol, sorgolactone, orobanchol, sorgomol and 5-deoxystrigol, 36 in total, were prepared and screened for the ability to induce and/or inhibit the germination of Striga hermonthica and Striga gesnerioides seeds collected from mature plants that parasitized on sorghum and cowpea, respectively. All of the compounds induced S. hermonthica seed germination, albeit displayed differential activities. On the other hand, only a limited number of the compounds induced significant germination in S. gesnerioides, thus indicating strict structural requirements. Strigolactones inducing high germination in S. gesnerioides induced low germination in S. hermonthica. Strigolactones with the same configuration at C3a, C8b and C2' as that in 5-deoxystrigol (9a) induced high germination of S. hermonthica seeds, but most of them inhibited the germination of S. gesnerioides. The differential response of S. gesnerioides to strigolactones may play an important role in the survival of the species. However, the compounds could be used as means of control if mixed cropping of cowpea and sorghum is adopted.

Some soybean cultivars have ability to induce germination of sunflower broomrape

Sunflower broomrape is a noxious parasitic weed which has caused severe damage to crop ecosystems. Trap crops can release a mixture of allelochemicals to induce the germination of sunflower broomrape. We studied the allelopathic effects of soybean on sunflower broomrape. Fourteen common soybean cultivars were grown in pots. Samples were collected from soybean plants and rhizosphere soil at five growth stages (V1, V3, V5, R2, and R4). The allelopathic effects of soybean reached highest at the V3 stage. Methanolic extracts of soybean roots induced higher broomrape germination than methanolic extracts of stems or leaves. The germination rates induced by root extracts (10-fold dilution) were positively correlated with germination rates induced by stem (10-fold dilution) and leaf extracts (10-fold dilution). The broomrape germination rates induced by root extracts were also positively correlated with soybean nodule diameter and dry weight. The results indicated that soybeans could induce sunflower broomrape germination. We conclude that soybean has the potential to be used as a trap crop for sunflower broomrape.

Structure and activity of strigolactones: new plant hormones with a rich future

Strigolactones (SLs) constitute a new class of plant hormones which are active as germination stimulants for seeds of parasitic weeds of Striga, Orobanche, and Pelipanchi spp, in hyphal branching of arbuscular mycorrhizal (AM) fungi and as inhibitors of shoot branching. In this review, the focus is on molecular features of these SLs. The occurrence of SLs in root exudates of host plants is described. The naming protocol for SL according to the International Union of Pure and Applied Chemistry (IUPAC) rules and the 'at a glance' method is explained. The total synthesis of some natural SLs is described with details for all eight stereoisomers of strigol. The problems encountered with assigning the correct structure of natural SLs are analyzed for orobanchol, alectrol, and solanacol. The structure-activity relationship of SLs as germination stimulants leads to the identification of the bioactiphore of SLs. Together with a tentative mechanism for the mode of action, a model has been derived that can be used to design and prepare active SL analogs. This working model has been used for the preparation of a series of new SL analogs such as Nijmegen-1, and analogs derived from simple ketones, keto enols, and saccharine. The serendipitous finding of SL mimics which are derived from the D-ring in SLs (appropriately substituted butenolides) is reported. For SL mimics, a mode of action is proposed as well. Recent new results support this proposal. The stability of SLs and SL analogs towards hydrolysis is described and some details of the mechanism of hydrolysis are discussed as well. The attempted isolation of the protein receptor for germination and the current status concerning the biosynthesis of natural SLs are briefly discussed. Some non-SLs as germinating agents are mentioned. The structure-activity relationship for SLs in hyphal branching of AM fungi and in repression of shoot branching is also analyzed. For each of the principle functions, a working model for the design of new active SL analogs is described and its applicability and implications are discussed. It is shown that the three principal functions use a distinct perception system. The importance of stereochemistry for bioactivity has been described for the various functions.

Structure-function relations of strigolactone analogs: activity as plant hormones and plant interactions

Strigolactones (SLs) have several functions as signaling molecules in their interactions with symbiotic arbuscular mycorrhizal (AM) fungi and the parasitic weeds Orobanche and Striga. SLs are also a new class of plant hormone regulating plant development. In all three organisms, a specific and sensitive receptor-mediated perception system is suggested. By comparing the activity of synthetic SL analogs on Arabidopsis root-hair elongation, Orobanche aegyptiaca seed germination, and hyphal branching of the AM fungus Glomus intraradices, we found that each of the tested organisms differs in its response to the various examined synthetic SL analogs. Structure-function relations of the SL analogs suggest substitutions on the A-ring as the cause of this variation. Moreover, the description of competitive antagonistic analogs suggests that the A-ring of SL can affect not only affinity to the receptor, but also the molecule's ability to activate it. The results support the conclusion that Arabidopsis, Orobanche, and AM fungi possess variations in receptor sensitivity to SL analogs, probably due to variation in SL receptors among the different species.

Structure-activity relationship studies of strigolactone-related molecules for branching inhibition in garden pea: molecule design for shoot branching

Initially known for their role in the rhizosphere in stimulating the seed germination of parasitic weeds such as the Striga and Orobanche species, and later as host recognition signals for arbuscular mycorrhizal fungi, strigolactones (SLs) were recently rediscovered as a new class of plant hormones involved in the control of shoot branching in plants. Herein, we report the synthesis of new SL analogs and, to our knowledge, the first study of SL structure-activity relationships for their hormonal activity in garden pea (Pisum sativum). Comparisons with their action for the germination of broomrape (Phelipanche ramosa) are also presented. The pea rms1 SL-deficient mutant was used in a SL bioassay based on axillary bud length after direct SL application on the bud. This assay was compared with an assay where SLs were fed via the roots using hydroponics and with a molecular assay in which transcript levels of BRANCHED1, the pea homolog of the maize TEOSINTE BRANCHED1 gene were quantified in axillary buds only 6 h after application of SLs. We have demonstrated that the presence of a Michael acceptor and a methylbutenolide or dimethylbutenolide motif in the same molecule is essential. It was established that the more active analog 23 with a dimethylbutenolide as the D-ring could be used to control the plant architecture without strongly favoring the germination of P. ramosa seeds. Bold numerals refer to numbers of compounds.

Germination stimulants of Phelipanche ramosa in the rhizosphere of Brassica napus are derived from the glucosinolate pathway

Phelipanche ramosa is a major parasitic weed of Brassica napus. The first step in a host-parasitic plant interaction is stimulation of parasite seed germination by compounds released from host roots. However, germination stimulants produced by B. napus have not been identified yet. In this study, we characterized the germination stimulants that accumulate in B. napus roots and are released into the rhizosphere. Eight glucosinolate-breakdown products were identified and quantified in B. napus roots by gas chromatography-mass spectrometry. Two (3-phenylpropanenitrile and 2-phenylethyl isothiocyanate [2-PEITC]) were identified in the B. napus rhizosphere. Among glucosinolate-breakdown products, P. ramosa germination was strongly and specifically triggered by isothiocyanates, indicating that 2-PEITC, in particular, plays a key role in the B. napus-P. ramosa interaction. Known strigolactones were not detected by ultraperformance liquid chromatography-tandem mass spectrometry, and seed of Phelipanche and Orobanche spp. that respond to strigolactones but not to isothiocyanates did not germinate in the rhizosphere of B. napus. Furthermore, both wild-type and strigolactone biosynthesis mutants of Arabidopsis thaliana Atccd7 and Atccd8 induced similar levels of P. ramosa seed germination, suggesting that compounds other than strigolactone function as germination stimulants for P. ramosa in other Brassicaceae spp. Our results open perspectives on the high adaptation potential of root-parasitic plants under host-driven selection pressures.

How do nitrogen and phosphorus deficiencies affect strigolactone production and exudation?

Plants exude strigolactones (SLs) to attract symbiotic arbuscular mycorrhizal fungi in the rhizosphere. Previous studies have demonstrated that phosphorus (P) deficiency, but not nitrogen (N) deficiency, significantly promotes SL exudation in red clover, while in sorghum not only P deficiency but also N deficiency enhances SL exudation. There are differences between plant species in SL exudation under P- and N-deficient conditions, which may possibly be related to differences between legumes and non-legumes. To investigate this possibility in detail, the effects of N and P deficiencies on SL exudation were examined in Fabaceae (alfalfa and Chinese milk vetch), Asteraceae (marigold and lettuce), Solanaceae (tomato), and Poaceae (wheat) plants. In alfalfa as expected, and unexpectedly in tomato, only P deficiency promoted SL exudation. In contrast, in Chinese milk vetch, a leguminous plant, and in the other non-leguminous plants examined, N deficiency as well as P deficiency enhanced SL exudation. Distinct reductions in shoot P levels were observed in plants grown under N deficiency, except for tomato, in which shoot P level was increased by N starvation, suggesting that the P status of the shoot regulates SL exudation. There seems to be a correlation between shoot P levels and SL exudation across the species/families investigated.

How do nitrogen and phosphorus deficiencies affect strigolactone production and exudation?

Plants exude strigolactones (SLs) to attract symbiotic arbuscular mycorrhizal fungi in the rhizosphere. Previous studies have demonstrated that phosphorus (P) deficiency, but not nitrogen (N) deficiency, significantly promotes SL exudation in red clover, while in sorghum not only P deficiency but also N deficiency enhances SL exudation. There are differences between plant species in SL exudation under P- and N-deficient conditions, which may possibly be related to differences between legumes and non-legumes. To investigate this possibility in detail, the effects of N and P deficiencies on SL exudation were examined in Fabaceae (alfalfa and Chinese milk vetch), Asteraceae (marigold and lettuce), Solanaceae (tomato), and Poaceae (wheat) plants. In alfalfa as expected, and unexpectedly in tomato, only P deficiency promoted SL exudation. In contrast, in Chinese milk vetch, a leguminous plant, and in the other non-leguminous plants examined, N deficiency as well as P deficiency enhanced SL exudation. Distinct reductions in shoot P levels were observed in plants grown under N deficiency, except for tomato, in which shoot P level was increased by N starvation, suggesting that the P status of the shoot regulates SL exudation. There seems to be a correlation between shoot P levels and SL exudation across the species/families investigated.

Induction of seed germination in Orobanche spp. by extracts of traditional Chinese medicinal herbs

The co-evolution of Orobanche spp. and their hosts within the same environment has resulted in a high degree of adaptation and effective parasitism whereby the host releases parasite germination stimulants, which are likely to be unstable in the soil. Our objective was to investigate whether extracts from non-host plants, specifically, Chinese medicinal plants, could stimulate germination of Orobanche spp. Samples of 606 Chinese medicinal herb species were extracted with deionized water and methanol. The extracts were used to induce germination of three Orobanche species; Orobanche minor, Orobanche cumana, and Orobanche aegyptiaca. O. minor exhibited a wide range of germination responses to the various herbal extracts. O. cumana and O. aegyptiaca exhibited an intermediate germination response to the herbal extracts. O. minor, which has a narrow host spectrum, showed higher germination rates in response to different herbal extracts compared with those of O. cumana and O. aegyptiaca, which have a broader host spectrum. Methanolic extracts of many Chinese herbal species effectively stimulated seed germination among the Orobanche spp., even though they were not the typical hosts. The effective herbs represent interesting examples of potential trap crops. Different countries can also screen extracts from indigenous herbaceous plants for their ability to induce germination of Orobanche spp. seeds. The use of such species as trap plants could diminish the global soil seed bank of Orobanche.

Ent-2'-epi-Orobanchol and its acetate, as germination stimulants for Striga gesnerioides seeds isolated from cowpea and red clover

Striga gesnerioides is a root parasitic weed of economic significance to cowpea (Vigna unguiculata) crops in Western Africa. Seeds of the parasite germinate in response to cowpea root exudates. Germination stimulants for the seeds were isolated from the hydroponic culture filtrate of cowpea, and their structures were unambiguously determined as (-)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchol and (+)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchyl acetate, on the basis of mass, CD, and (1)H NMR spectra; optical rotatory power; and chromatographic behavior on HPLC. The alcohol was first isolated and identified from the cowpea root exudates, and the acetate may be the same compound that had been previously isolated from the exudates and designated as alectrol. Identity of the stimulants produced by cowpea to those produced by red clover (Trifolium pratense) was confirmed.

Structural requirements of strigolactones for germination induction of Striga gesnerioides seeds

Strigolactones are highly potent germination stimulants for seeds of the parasitic weeds Striga and Orobanche spp. 4-Hydroxy-GR24 and 4-acetoxy-GR24 were prepared and their abilities to induce seed germination of Striga gesnerioides evaluated. Optically active (8bR,2'R)-isomers induced germination, although the racemic diastereomers were inactive. In contrast, the stereoisomer of GR24 with the same configuration induced negligible germination. Some stereoisomers of GR24 and its analogues acted as effective antagonists for induction of seed germination by cowpea root exudates. These results suggest that both an oxygenated substituent at C-4 and the configuration of the tricyclic lactone and the D-ring are essential structural requirements for induction of germination in S. gesnerioides seeds.

Strigolactones affect lateral root formation and root-hair elongation in Arabidopsis

Strigolactones (SLs) have been proposed as a new group of plant hormones, inhibiting shoot branching, and as signaling molecules for plant interactions. Here, we present evidence for effects of SLs on root development. The analysis of mutants flawed in SLs synthesis or signaling suggested that the absence of SLs enhances lateral root formation. In accordance, roots grown in the presence of GR24, a synthetic bioactive SL, showed reduced number of lateral roots in WT and in max3-11 and max4-1 mutants, deficient in SL synthesis. The GR24-induced reduction in lateral roots was not apparent in the SL signaling mutant max2-1. Moreover, GR24 led to increased root-hair length in WT and in max3-11 and max4-1 mutants, but not in max2-1. SLs effect on lateral root formation and root-hair elongation may suggest a role for SLs in the regulation of root development; perhaps, as a response to growth conditions.

Carotenoid inhibitors reduce strigolactone production and Striga hermonthica infection in rice

The strigolactones are internal and rhizosphere signalling molecules in plants that are biosynthesised through carotenoid cleavage. They are secreted by host roots into the rhizosphere where they signal host-presence to the symbiotic arbuscular mycrorrhizal (AM) fungi and the parasitic plants of the Orobanche, Phelipanche and Striga genera. The seeds of these parasitic plants germinate after perceiving these signalling molecules. After attachment to the host root, the parasite negatively affects the host plant by withdrawing water, nutrients and assimilates through a direct connection with the host xylem. In many areas of the world these parasites are a threat to agriculture but so far very limited success has been achieved to minimize losses due to these parasitic weeds. Considering the carotenoid origin of the strigolactones, in the present study we investigated the possibilities to reduce strigolactone production in the roots of plants by blocking carotenoid biosynthesis using carotenoid inhibitors. Hereto the carotenoid inhibitors fluridone, norflurazon, clomazone and amitrole were applied to rice either through irrigation or through foliar spray. Irrigation application of all carotenoid inhibitors and spray application of amitrole significantly decreased strigolactone production, Striga hermonthica germination and Striga infection, also in concentrations too low to affect growth and development of the host plant. Hence, we demonstrate that the application of carotenoid inhibitors to plants can affect S. hermonthica germination and attachment indirectly by reducing the strigolactone concentration in the rhizosphere. This finding is useful for further studies on the relevance of the strigolactones in rhizosphere signalling. Since these inhibitors are available and accessible, they may represent an efficient technology for farmers, including poor subsistence farmers in the African continent, to control these harmful parasitic weeds.

Structural requirements of strigolactones for hyphal branching in AM fungi

Strigolactones are a group of terpenoid lactones that act as a host-derived signal in the rhizosphere communication of plants with arbuscular mycorrhizal (AM) fungi and root parasitic weeds as well as an endogenous plant hormone regulating shoot branching in plants. Strigolactones induce hyphal branching in AM fungi at very low concentrations, suggesting a highly sensitive perception system for strigolactones present in AM fungi. However, little is known about the structural requirements of strigolactones for hyphal branching in AM fungi. Here, we tested a series of natural and synthetically modified strigolactones as well as non-strigolactone-type germination stimulants for hyphal branching-inducing activity in germinating spores of the AM fungus Gigaspora margarita. All tested compounds with a tricyclic lactone coupled to a methylbutenolide via an enol ether bond showed activity, but differed in the active concentration and in the branching pattern of hyphae. Truncation of the A- and AB-rings in the tricyclic ABC lactone of strigolactones resulted in a drastic reduction in hyphal branching activity. Although the connection of the C-ring in the tricyclic lactone to the methylbutenolide D-ring was shown to be essential for hyphal branching, the bridge structure in the C-D part was found not necessarily to be enol ether, being replaceable with either alkoxy or imino ethers. These structural requirements in AM fungi are very similar but not identical to those observed in root parasitic weeds, especially with respect to the enol ether bridge in the C-D part.

A tomato strigolactone-impaired mutant displays aberrant shoot morphology and plant interactions

Strigolactones are considered a new group of plant hormones. Their role as modulators of plant growth and signalling molecules for plant interactions first became evident in Arabidopsis, pea, and rice mutants that were flawed in strigolactone production, release, or perception. The first evidence in tomato (Solanum lycopersicon) of strigolactone deficiency is presented here. Sl-ORT1, previously identified as resistant to the parasitic plant Orobanche, had lower levels of arbuscular mycorrhizal fungus (Glomus intraradices) colonization, possibly as a result of its reduced ability to induce mycorrhizal hyphal branching. Biochemical analysis of mutant root extracts suggested that it produces only minute amounts of two of the tomato strigolactones: solanacol and didehydro-orobanchol. Accordingly, the transcription level of a key enzyme (CCD7) putatively involved in strigolactone synthesis in tomato was reduced in Sl-ORT1 compared with the wild type (WT). Sl-ORT1 shoots exhibited increased lateral shoot branching, whereas exogenous application of the synthetic strigolactone GR24 to the mutant restored the WT phenotype by reducing the number of lateral branches. Reduced lateral shoot branching was also evident in grafted plants which included a WT interstock, which was grafted between the mutant rootstock and the scion. In roots of these grafted plants, the CCD7 transcription level was not significantly induced, nor was mycorrhizal sensitivity restored. Hence, WT-interstock grafting, which restores mutant shoot morphology to WT, does not restore mutant root properties to WT. Characterization of the first tomato strigolactone-deficient mutant supports the putative general role of strigolactones as messengers of suppression of lateral shoot branching in a diversity of plant species.

The strigolactone story

Strigolactones (SLs) were originally isolated from plant root exudates as germination stimulants for root parasitic plants of the family Orobanchaceae, including witchweeds (Striga spp.), broomrapes (Orobanche and Phelipanche spp.), and Alectra spp., and so were regarded as detrimental to the producing plants. Their role as indispensable chemical signals for root colonization by symbiotic arbuscular mycorrhizal fungi was subsequently unveiled, and SLs then became recognized as beneficial plant metabolites. In addition to these functions in the rhizosphere, it has been recently shown that SLs or their metabolites are a novel class of plant hormones that inhibit shoot branching. Furthermore, SLs are suggested to have other biological functions in rhizosphere communications and in plant growth and development.

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: structures and biological activities

Strigolactones released from plant roots induce seed germination of root parasitic weeds, witchweeds (Striga spp.) and broomrapes (Orobanche spp.), and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. In addition to these functions in the rhizosphere, strigolactones have recently been shown to be a novel class of plant hormones regulating shoot outgrowth. The natural strigolactones identified so far have the common C-D ring moiety, which is thought to be the essential structure for exhibiting biological activity. The introduction of substitutions on the A-B ring moiety of 5-deoxystrigol, the basic strigolactone, affords various strigolactones, e.g. hydroxylation on C-4, C-5 and C-9 leads to orobanchol, strigol and sorgomol respectively. Then, acetylation and probably other derivatisations of these hydroxy-strigolactones would occur. Although the C-2'-(R) stereochemistry was thought to be an important structural feature for potent germination stimulation activity, 2'-epi-strigolactones were found in root exudates of tobacco, rice, pea and other plant species, indicating that at least some plants produce both epimers.

Fabacyl acetate, a germination stimulant for root parasitic plants from Pisum sativum

A germination stimulant, fabacyl acetate, was purified from root exudates of pea (Pisum sativum L.) and its structure was determined as ent-2'-epi-4a,8a-epoxyorobanchyl acetate [(3aR,4R,4aR,8bS,E)-4a,8a-epoxy-8,8-dimethyl-3-(((R)-4-methyl-5-oxo-2,5-dihydrofuran-2-yloxy)methylene)-2-oxo-3,3a,4,5,6,7,8,8b-decahydro-2H-indeno[1,2-b]furan-4-yl acetate], by 1D and 2D NMR spectroscopic, ESI- and EI-MS spectrometric, X-ray crystallographic analyses, and by comparing the (1)H NMR spectroscopic data and relative retention times (RR(t)) in LC-MS and GC-MS with those of synthetic standards prepared from (+)-orobanchol and (+)-2'-epiorobanchol. The (1)H NMR spectroscopic data and RR(t) of fabacyl acetate were identical with those of an isomer prepared from (+)-2'-epiorobanchol except for the opposite sign in CD spectra. This is the first natural ent-strigolactone containing an epoxide group. Fabacyl acetate was previously detected in root exudates of other Fabaceae plants including faba bean (Vicia faba L.) and alfalfa (Medicago sativa L.).

Strigolactone inhibition of shoot branching

A carotenoid-derived hormonal signal that inhibits shoot branching in plants has long escaped identification. Strigolactones are compounds thought to be derived from carotenoids and are known to trigger the germination of parasitic plant seeds and stimulate symbiotic fungi. Here we present evidence that carotenoid cleavage dioxygenase 8 shoot branching mutants of pea are strigolactone deficient and that strigolactone application restores the wild-type branching phenotype to ccd8 mutants. Moreover, we show that other branching mutants previously characterized as lacking a response to the branching inhibition signal also lack strigolactone response, and are not deficient in strigolactones. These responses are conserved in Arabidopsis. In agreement with the expected properties of the hormonal signal, exogenous strigolactone can be transported in shoots and act at low concentrations. We suggest that endogenous strigolactones or related compounds inhibit shoot branching in plants. Furthermore, ccd8 mutants demonstrate the diverse effects of strigolactones in shoot branching, mycorrhizal symbiosis and parasitic weed interaction.

Strigolactones, host recognition signals for root parasitic plants and arbuscular mycorrhizal fungi, from Fabaceae plants

Both root parasitic plants and arbuscular mycorrhizal (AM) fungi take advantage of strigolactones, released from plant roots as signal molecules in the initial communication with host plants, in order to commence parasitism and mutualism, respectively. In this study, strigolactones in root exudates from 12 Fabaceae plants, including hydroponically grown white lupin (Lupinus albus), a nonhost of AM fungi, were characterized by comparing retention times of germination stimulants on reverse-phase high-performance liquid chromatography (HPLC) with those of standards and by using tandem mass spectrometry (LC/MS/MS). All the plant species examined were found to exude known strigolactones, such as orobanchol, orobanchyl acetate, and 5-deoxystrigol, suggesting that these strigolactones are widely distributed in the Fabaceae. It should be noted that even the nonmycotrophic L. albus exuded orobanchol, orobanchyl acetate, 5-deoxystrigol, and novel germination stimulants. By contrast to the mycotrophic Fabaceae plant Trifolium pratense, in which phosphorus deficiency promoted strigolactone exudation, neither phosphorus nor nitrogen deficiency increased exudation of these strigolactones in L. albus. Therefore, the regulation of strigolactone production and/or exudation seems to be closely related to the nutrient acquisition strategy of the plants.

Biosynthetic considerations could assist the structure elucidation of host plant produced rhizosphere signalling compounds (strigolactones) for arbuscular mycorrhizal fungi and parasitic plants

Parasitic plants cause devastating losses to crop yields in several parts of the world. The root parasites, Striga and Orobanche species, use chemical signalling molecules that are exuded by the roots of plants in extremely low concentrations, and that can induce germination of the seeds of these parasites, to detect the vicinity of a suitable host. The majority of the so far identified germination stimulants belong to the strigolactones. It was recently discovered that this class of compounds can also induce hyphal branching in the symbiotic arbuscular mycorrhizal fungi, a process involved in root colonisation. The elucidation of the structure of new strigolactones is hindered by their low abundance and instability. In the present paper, we have used existing knowledge on the structure of strigolactones and combined it with recently obtained insight in the biosynthetic origin of these signalling compounds. This enabled us to postulate structures for strigolactones that have been isolated but for which so far the structure has not been elucidated, but also to propose structures of strigolactones that may be discovered in the future. Considering the strongly increased importance of the strigolactones, we expect that more groups will look for these compounds and also in systems so far not exploited. This could lead to the discovery of new strigolactones for which we expect the present biogenetic considerations will facilitate identification and structure elucidation.