New strigolactone analogs as plant hormones with low activities in the rhizosphere

Synthesis and Biological Properties of Fluorescent Strigolactone Mimics Derived from 1,8-Naphthalimide

Strigolactones (SLs) have potential to be used in sustainable agriculture to mitigate various stresses that plants have to deal with. The natural SLs, as well as the synthetic analogs, are difficult to obtain in sufficient amounts for practical applications. At the same time, fluorescent SLs would be useful for the mechanistic understanding of their effects based on bio-imaging or spectroscopic techniques. In this study, new fluorescent SL mimics containing a substituted 1,8-naphthalimide ring system connected through an ether link to a bioactive furan-2-one moiety were prepared. The structural, spectroscopic, and biological activity of the new SL mimics on phytopathogens were investigated and compared with previously synthetized fluorescent SL mimics. The chemical group at the C-6 position of the naphthalimide ring influences the fluorescence parameters. All SL mimics showed effects similar to GR24 on phytopathogens, indicating their suitability for practical applications. The pattern of the biological activity depended on the fungal species, SL mimic and concentration, and hyphal order. This dependence is probably related to the specificity of each fungal receptor-SL mimic interaction, which will have to be analyzed in-depth. Based on the biological properties and spectroscopic particularities, one SL mimic could be a good candidate for microscopic and spectroscopic investigations.

Novel Strigolactone Mimics That Modulate Photosynthesis and Biomass Accumulation in Chlorella sorokiniana

In terrestrial plants, strigolactones act as multifunctional endo- and exo-signals. On microalgae, the strigolactones determine akin effects: induce symbiosis formation with fungi and bacteria and enhance photosynthesis efficiency and accumulation of biomass. This work aims to synthesize and identify strigolactone mimics that promote photosynthesis and biomass accumulation in microalgae with biotechnological potential. Novel strigolactone mimics easily accessible in significant amounts were prepared and fully characterized. The first two novel compounds contain 3,5-disubstituted aryloxy moieties connected to the bioactive furan-2-one ring. In the second group of compounds, a benzothiazole ring is connected directly through the cyclic nitrogen atom to the bioactive furan-2-one ring. The novel strigolactone mimics were tested on Chlorella sorokiniana NIVA-CHL 176. All tested strigolactones increased the accumulation of chlorophyll b in microalgae biomass. The SL-F3 mimic, 3-(4-methyl-5-oxo-2,5-dihydrofuran-2-yl)-3H-benzothiazol-2-one (7), proved the most efficient. This compound, applied at a concentration of 10-7 M, determined a significant biomass accumulation, higher by more than 15% compared to untreated control, and improved the quantum yield efficiency of photosystem II. SL-F2 mimic, 5-(3,5-dibromophenoxy)-3-methyl-5H-furan-2-one (4), applied at a concentration of 10-9 M, improved protein production and slightly stimulated biomass accumulation. Potential utilization of the new strigolactone mimics as microalgae biostimulants is discussed.

Strigolactones: diversity, perception, and hydrolysis

Strigolactones (SLs) are a unique and novel class of phytohormones that regulate numerous processes of growth and development in plants. Besides their endogenous functions as hormones, SLs are exuded by plant roots to stimulate critical interactions with symbiotic fungi but can also be exploited by parasitic plants to trigger their seed germination. In the past decade, since their discovery as phytohormones, rapid progress has been made in understanding the SL biosynthesis and signaling pathway. Of particular interest are the diversification of natural SLs and their exact mode of perception, selectivity, and hydrolysis by their dedicated receptors in plants. Here we provide an overview of the emerging field of SL perception with a focus on the diversity of canonical, non-canonical, and synthetic SL probes. Moreover, this review offers useful structural insights into SL perception, the precise molecular adaptations that define receptor-ligand specificities, and the mechanisms of SL hydrolysis and its attenuation by downstream signaling components.

Karrikin perception and signalling

Karrikins (KARs) are a class of butenolide compounds found in smoke that were first identified as seed germination stimulants for fire-following species. Early studies of KARs classified the germination and postgermination responses of many plant species and investigated crosstalk with plant hormones that regulate germination. The discovery that Arabidopsis thaliana responds to KARs laid the foundation for identifying mutants with altered KAR responses. Genetic analysis of KAR signalling revealed an unexpected link to strigolactones (SLs), a class of carotenoid-derived plant hormones. Substantial progress has since been made towards understanding how KARs are perceived and regulate plant growth, in no small part due to advances in understanding SL perception. KAR and SL signalling systems are evolutionarily related and retain a high degree of similarity. There is strong evidence that KARs are natural analogues of an endogenous signal(s), KAI2 ligand (KL), which remains unknown. KAR/KL signalling regulates many developmental processes in plants including germination, seedling photomorphogenesis, and root and root hair growth. KAR/KL signalling also affects abiotic stress responses and arbuscular mycorrhizal symbiosis. Here, we summarise the current knowledge of KAR/KL signalling and discuss current controversies and unanswered questions in this field.

Noncanonical Strigolactone Analogues Highlight Selectivity for Stimulating Germination in Two Phelipanche ramosa Populations

Strigolactones (SLs) are plant hormones exuded in the rhizosphere with a signaling role for the development of arbuscular mycorrhizal (AM) fungi and as stimulants of seed germination of the parasitic weeds Orobanche, Phelipanche, and Striga, the most threatening weeds of major crops worldwide. Phelipanche ramosa is present mainly on rape, hemp, and tobacco in France. P. ramosa 2a preferentially attacks hemp, while P. ramosa 1 attacks rapeseed. The recently isolated cannalactone (14) from hemp root exudates has been characterized as a noncanonical SL that selectively stimulates the germination of P. ramosa 2a seeds in comparison with P. ramosa 1. In the present work, (-)-solanacol (5), a canonical orobanchol-type SL exuded by tobacco and tomato, was established to possess a remarkable selective germination stimulant activity for P. ramosa 2a seeds. Two cannalactone analogues, named (±)-SdL19 and (±)-SdL118, have been synthesized. They have an unsaturated acyclic carbon chain with a tertiary hydroxy group and a methyl or a cyclopropyl group instead of a cyclohexane A-ring, respectively. (±)-SdL analogues are able to selectively stimulate P. ramosa 2a, revealing that these minimal structural elements are key for this selective bioactivity. In addition, (±)-SdL19 is able to inhibit shoot branching in Pisum sativum and Arabidopsis thaliana and induces hyphal branching in the AM fungus Rhizophagus irregularis, like SLs.

Expansion of the Strigolactone Profluorescent Probes Repertory: The Right Probe for the Right Application

Strigolactones (SLs) are intriguing phytohormones that not only regulate plant development and architecture but also interact with other organisms in the rhizosphere as root parasitic plants (Striga, Orobanche, and Phelipanche) and arbuscular mycorrhizal fungi. Starting with a pioneering work in 2003 for the isolation and identification of the SL receptor in parasitic weeds, fluorescence labeling of analogs has proven a major strategy to gain knowledge in SL perception and signaling. Here, we present novel chemical tools for understanding the SL perception based on the enzymatic properties of SL receptors. We designed different profluorescent SL Guillaume Clavé (GC) probes and performed structure-activity relationship studies on pea, Arabidopsis thaliana, and Physcomitrium (formerly Physcomitrella) patens. The binding of the GC probes to PsD14/RMS3, AtD14, and OsD14 proteins was tested. We demonstrated that coumarin-based profluorescent probes were highly bioactive and well-adapted to dissect the enzymatic properties of SL receptors in pea and a resorufin profluorescent probe in moss, contrary to the commercially available fluorescein profluorescent probe, Yoshimulactone Green (YLG). These probes offer novel opportunities for the studies of SL in various plants.

Strigolactones (SLs) modulate the plastochron by regulating KLUH (KLU) transcript abundance in Arabidopsis

The timing of leaf emergence at the shoot apical meristem, or plastochron, is highly regulated in plants. Among the genes known to regulate the plastochron in Arabidopsis (Arabidopsis thaliana), KLUH (KLU), orthologous to the rice (Oryza sativa) PLASTOCHRON1, encodes the cytochrome P450 CYP78A5, and is thought to act through generation of a still unknown mobile signal. As klu mutants display not only a short plastochron but also a branching phenotype reminiscent of strigolactone (SL) mutants, we investigated whether KLU/CYP78A5 is involved in SL biosynthesis. We combined a genetic approach, a parasitic plant seed germination bioassay to test klu root exudates, and analysis of transcript abundances of SL-biosynthesis genes in the Arabidopsis klu mutants. We demonstrate that KLU is not involved in the SL-biosynthesis pathway. Moreover, this work allowed us to uncover a new role for SL during Arabidopsis development in modulating plastochron via a KLU-dependent pathway. Globally our data reveal that KLU is required for plastochron-specific SL responses, a first indication of crosstalk between SL and the KLU-derived signal.

Cannabinoids vs. whole metabolome: Relevance of cannabinomics in analyzing Cannabis varieties

Cannabis sativa has a long history of domestication both for its bioactive compounds and its fibers. This has produced hundreds of varieties, usually characterized in the literature by chemotypes, with Δ⁹-THC and CBD content as the main markers. However, chemotyping could also be done based on minor compounds (phytocannabinoids and others). In this work, a workflow, which we propose to name cannabinomics, combines mass spectrometry of the whole metabolome and statistical analysis to help differentiate C. sativa varieties and deciphering their characteristic markers. By applying this cannabinomics approach to the data obtained from 20 varieties of C. sativa (classically classified as chemotype I, II, or III), we compared the results with those obtained by a targeted quantification of 11 phytocannabinoids. Cannabinomics can be considered as a complementary tool for phenotyping and genotyping, allowing the identification of minor compounds playing a key role as markers of differentiation.

A Phelipanche ramosa KAI2 protein perceives strigolactones and isothiocyanates enzymatically

Phelipanche ramosa is an obligate root-parasitic weed that threatens major crops in central Europe. In order to germinate, it must perceive various structurally divergent host-exuded signals, including isothiocyanates (ITCs) and strigolactones (SLs). However, the receptors involved are still uncharacterized. Here, we identify five putative SL receptors in P. ramosa and show that PrKAI2d3 is involved in the stimulation of seed germination. We demonstrate the high plasticity of PrKAI2d3, which allows it to interact with different chemicals, including ITCs. The SL perception mechanism of PrKAI2d3 is similar to that of endogenous SLs in non-parasitic plants. We provide evidence that PrKAI2d3 enzymatic activity confers hypersensitivity to SLs. Additionally, we demonstrate that methylbutenolide-OH binds PrKAI2d3 and stimulates P. ramosa germination with bioactivity comparable to that of ITCs. This study demonstrates that P. ramosa has extended its signal perception system during evolution, a fact that should be considered for the development of specific and efficient biocontrol methods.

Evaluation of the Effect of Strigolactones and Synthetic Analogs on Fungi

Strigolactones (SLs) are components of root exudates as a consequence of active release from the roots into the soil. Notably, they have been described as stimulants of seed germination in parasitic plants and of the presymbiotic growth in arbuscular mycorrhizal (AM) fungi, which are a crucial component of the plant root beneficial microbiota. SLs have therefore the potential to influence other microbes that proliferate in the soil around the roots and may interact with plants. A direct effect of SL analogs on the in vitro growth of a number of saprotrophic or plant pathogenic fungi was indeed reported.Here we describe a standardized method to evaluate the effect of SLs or their synthetic analogs on AM and filamentous fungi. For AM fungi, we propose a spore germination assay since it is more straightforward than the hyphal branching assay and it does not require deep expertise and skills. For filamentous fungi that can grow in axenic cultures, we describe the assay based on SLs embedded in the solid medium or dissolved in liquid cultures where the fungus is inoculated to evaluate the effect on growth, hyphal branching or conidia germination. These assays are of help to test the activity of natural SLs as well as of newly designed SL analogs for basic and applied research.

Strigolactone mimic 2-nitrodebranone is highly active in Arabidopsis growth and development

Strigolactones play crucial roles in regulating plant architecture and development, as endogenous hormones, and orchestrating symbiotic interactions with fungi and parasitic plants, as components of root exudates. rac-GR24 is currently the most widely used strigolactone analog and serves as a reference compound in investigating the action of strigolactones. In this study, we evaluated a suite of debranones and found that 2-nitrodebranone (2NOD) exhibited higher biological activity than rac-GR24 in various aspects of plant growth and development in Arabidopsis, including hypocotyl elongation inhibition, root hair promotion and senescence acceleration. The enhanced activity of 2NOD in promoting AtD14-SMXL7 and AtD14-MAX2 interactions indicates that the molecular structure of 2NOD is a better match for the ligand perception site pocket of D14. Moreover, 2NOD showed lower activity than rac-GR24 in promoting Orobanche cumana seed germination, suggesting its higher ability to control plant architecture than parasitic interactions. In combination with the improved stability of 2NOD, these results demonstrate that 2NOD is a strigolactone analog that can specifically mimic the activity of strigolactones and that 2NOD exhibits strong potential as a tool for studying the strigolactone signaling pathway in plants.

Current progress in Striga management

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

On improving strigolactone mimics for induction of suicidal germination of the root parasitic plant Striga hermonthica

Strigolactones (SLs) are plant hormones that regulate the branching of plants and seed germination stimulants of root parasitic plants. As root parasites are a great threat to agricultural production, the use of SL agonists could be anticipated to provide an efficient method for regulating root parasites as suicidal germination inducers. A series of phenoxyfuranone-type SL mimics, termed debranones, has been reported to show potent bioactivities, including reduction of the tiller number on rice, and stimulation of seed germination in the root parasite Striga hermonthica. To exert both activities, two substituents on the phenyl ring of the molecules were important but at least a substituent at the 2-position must be an electron-withdrawing group. However, little is known about the effect of the properties of the substituents at the 2-position on bioactivities. Here, we found that different substituents at the 2-position give different preferences for bioactivities. Halogenated debranones were more effective than the others and SL agonist GR24 for inhibiting rice tiller but far less effective in the induction of S. hermonthica germination. Meanwhile, nitrile and methyl derivatives clearly stimulated the germination of S. hermonthica seeds. Although their IC₅₀ values were 100 times higher than that of GR24 in the receptor competitive binding assay, their physiological activities were approximately 1/10 of GR24. These differences could be due to their uptake in plants and/or their physicochemical stability under our experimental conditions. These findings could support the design of more potent and selective SL agonists that could contribute to solving big agricultural issues.

Deconvoluting apocarotenoid-mediated retrograde signaling networks regulating plastid translation and leaf development

Signals originating within plastids modulate organelle differentiation by transcriptionally regulating nuclear-encoded genes. These retrograde signals are also integral regulators of plant development, including leaf morphology. The clb5 mutant displays severe leaf morphology defects due to Apocarotenoid Signal 1 (ACS1) accumulation in the developmentally arrested plastid. Transcriptomic analysis of clb5 validates that ACS1 accumulation deregulates hundreds of nuclear genes, including the suppression of most genes encoding plastid ribosomal proteins. Herein, we order the molecular events causing the leaf phenotype associated with the accumulation of ACS1, which includes two consecutive retrograde signaling cascades. Firstly, ACS1 originating in the plastid drives inhibition of plastid translation (IPT) via nuclear transcriptome remodeling of chlororibosomal proteins, requiring light as an essential component. Subsequently, IPT results in leaf morphological defects via a GUN1-dependent pathway shared with seedlings undergoing chemical IPT treatments and is restricted to an early window of the leaf development. Collectively, this work advances our understanding of the complexity within plastid retrograde signaling exemplified by sequential signal exchange and consequences that in a particular temporal and spatial context contribute to the modulation of leaf development.

Synthesis of Profluorescent Strigolactone Probes for Biochemical Studies

In this chapter, we will describe a method we set up to synthesize two profluorescent strigolactone (SL) mimic probes (GC240 and GC242) and the optimized protocols developed to study the enzymatic properties of various strigolactone receptors. The Arabidopsis AtD14 SL receptor is used here as a model for this purpose.

Efficient Mimics for Elucidating Zaxinone Biology and Promoting Agricultural Applications

Zaxinone is an apocarotenoid regulatory metabolite required for normal rice growth and development. In addition, zaxinone has a large application potential in agriculture, due to its growth-promoting activity and capability to alleviate infestation by the root parasitic plant Striga through decreasing strigolactone (SL) production. However, zaxinone is poorly accessible to the scientific community because of its laborious organic synthesis that impedes its further investigation and utilization. In this study, we developed easy-to-synthesize and highly efficient mimics of zaxinone (MiZax). We performed a structure-activity relationship study using a series of apocarotenoids distinguished from zaxinone by different structural features. Using the obtained results, we designed several phenyl-based compounds synthesized with a high-yield through a simple method. Activity tests showed that MiZax3 and MiZax5 exert zaxinone activity in rescuing root growth of a zaxinone-deficient rice mutant, promoting growth, and reducing SL content in roots and root exudates of wild-type plants. Moreover, these compounds were at least as efficient as zaxinone in suppressing transcript level of SL biosynthesis genes and in alleviating Striga infestation under greenhouse conditions, and did not negatively impact mycorrhization. Taken together, MiZax are a promising tool for elucidating zaxinone biology and investigating rice development, and suitable candidates for combating Striga and increasing crop growth.

Translation of Strigolactones from Plant Hormone to Agriculture: Achievements, Future Perspectives, and Challenges

Strigolactones (SLs) control plant development, enhance symbioses, and act as germination stimulants for some of the most destructive species of parasitic weeds, making SLs a potential tool to improve crop productivity and resilience. Field trials demonstrate the potential use of SLs as agrochemicals or genetic targets in breeding programs, with applications in improving drought tolerance, increasing yields, and controlling parasitic weeds. However, for effective translation of SLs into agriculture, understanding and exploiting SL diversity and the development of economically viable sources of SL analogs will be critical. Here we review how manipulation of SL signaling can be used when developing new tools and crop varieties to address some critical challenges, such as nutrient acquisition, resource allocation, stress tolerance, and plant-parasite interactions.

Chemotactic Host-Finding Strategies of Plant Endoparasites and Endophytes

Plants interact with microorganisms in the environment during all stages of their development and in most of their organs. These interactions can be either beneficial or detrimental for the plant and may be transient or long-term. In extreme cases, microorganisms become endoparastic or endophytic and permanently reside within a plant, while the host plant undergoes developmental reprogramming and produces new tissues or organs as a response to the invasion. Events at the cellular and molecular level following infection have been extensively described, however the mechanisms of how these microorganisms locate their plant hosts via chemotaxis remain largely unknown. In this review, we summarize recent findings concerning the signalling molecules that regulate chemotaxis of endoparasitic/endophytic bacteria, fungi, and nematodes. In particular, we will focus on the molecules secreted by plants that are most likely to act as guidance cues for microorganisms. These compounds are found in a wide range of plant species and show a variety of secondary effects. Interestingly, these compounds show different attraction potencies depending on the species of the invading organism, suggesting that cues perceived in the soil may be more complex than anticipated. However, what the cognate receptors are for these attractants, as well as the mechanism of how these attractants influence these organisms, remain important outstanding questions. Host-targeting marks the first step of plant-microorganism interactions, therefore understanding the signalling molecules involved in this step plays a key role in understanding these interactions as a whole.

Populations of the Parasitic Plant Phelipanche ramosa Influence Their Seed Microbiota

Seeds of the parasitic weed Phelipanche ramosa are well adapted to their hosts because they germinate and form haustorial structures to connect to roots in response to diverse host-derived molecular signals. P. ramosa presents different genetic groups that are preferentially adapted to certain hosts. Since there are indications that microbes play a role in the interaction especially in the early stages of the interaction, we studied the microbial diversity harbored by the parasitic seeds with respect to their host and genetic group. Twenty-six seed lots from seven cropping plots of three different hosts-oilseed rape, tobacco, and hemp-in the west of France were characterized for their bacterial and fungal communities using 16S rRNA gene and ITS (Internal transcribed spacer) sequences, respectively. First seeds were characterized genetically using twenty microsatellite markers and phenotyped for their sensibility to various germination stimulants including strigolactones and isothiocyanates. This led to the distinction of three P. ramosa groups that corresponded to their host of origin. The observed seed diversity was correlated to the host specialization and germination stimulant sensitivity within P. ramosa species. Microbial communities were both clustered by host and plot of origin. The seed core microbiota was composed of seventeen species that were also retrieved from soil and was in lower abundances for bacteria and similar abundances for fungi compared to seeds. The host-related core microbiota of parasitic seeds was limited and presumably well adapted to the interaction with its hosts. Two microbial candidates of Sphingobacterium species and Leptosphaeria maculans were especially identified in seeds from oilseed rape plots, suggesting their involvement in host recognition and specialization as well as seed fitness for P. ramosa by improving the production of isothiocyanates from glucosinolates in the rhizosphere of oilseed rape.

Initiation of arbuscular mycorrhizal symbiosis involves a novel pathway independent from hyphal branching

The arbuscular mycorrhizal symbiosis is a very common association between plant roots and soil fungi, which greatly contributes to plant nutrition. Root-exuded compounds known as strigolactones act as symbiotic signals stimulating the fungus prior to root colonization. Strigolactones also play an endogenous role in planta as phytohormones and contribute to the regulation of various developmental traits. Structure-activity relationship studies have revealed both similarities and differences between the structural features required for bioactivity in plants and arbuscular mycorrhizal fungi. In the latter case, bioassays usually measured a stimulation of hyphal branching on isolated fungi of the Gigaspora genus, grown in vitro. Here, we extended these investigations with a bioassay that evaluates the bioactivity of strigolactone analogs in a symbiotic situation and the use of the model mycorrhizal fungus Rhizophagus irregularis. Some general structural requirements for bioactivity reported previously for Gigaspora were confirmed. We also tested additional strigolactone analogs bearing modifications on the conserved methylbutenolide ring, a key element of strigolactone perception by plants. A strigolactone analog with an unmethylated butenolide ring could enhance the ability of R. irregularis to colonize host roots. Surprisingly, when applied to the isolated fungus in vitro, this compound stimulated germ tube elongation but inhibited hyphal branching. Therefore, this compound was able to act on the fungal and/or plant partner to facilitate initiation of the arbuscular mycorrhizal symbiosis, independently from hyphal branching and possibly from the strigolactone pathway.

A New Series of Carlactonoic Acid Based Strigolactone Analogs for Fundamental and Applied Research

Strigolactones (SLs) are a group of carotenoid derived plant hormones that play a key role in establishing plant architecture and adapting it to environmental changes, and are involved in plants response to biotic and abiotic stress. SLs are also released into the soil to serve as a chemical signal attracting beneficial mycorrhizal fungi. However, this signal also induces seed germination in root parasitic weeds that represent a major global threat for agriculture. This wide spectrum of biological functions has made SL research one of the most important current topics in fundamental and applied plant science. The availability of SLs is crucial for investigating SL biology as well as for agricultural application. However, natural SLs are produced in very low amounts, and their organic synthesis is quite difficult, which creates a need for efficient and easy-to-synthesize analogs and mimics. Recently, we have generated a set of SL analogs, Methyl Phenlactonoates (MPs), which resemble the non-canonical SL carlactonoic acid. In this paper, we describe the development and characterization of a new series of easy-to-synthesize MPs. The new analogs were assessed with respect to regulation of shoot branching, impact on leaf senescence, and induction of seed germination in different root parasitic plants species. Some of the new analogs showed higher efficiency in inhibiting shoot branching as well as in triggering parasitic seed germination, compared to the commonly used GR24. MP16 was the most outstanding analog showing high activity in different SL biological functions. In summary, our new analogs series contains very promising candidates for different applications, which include the usage in studies for understanding different aspects of SL biology as well as large scale field application for combating root parasitic weeds, such as Striga hermonthica that devastates cereal yields in sub-Saharan Africa.

Effect of D-ring C-3' methylation of strigolactone analogs on their transcription regulating activity in rice

Strigolactones (SLs) are a well-known class of plant hormones, which are involved in a number of developmental and adaptation processes and mediate different interspecific interactions. In spite of the growing knowledge on SL biosynthesis and signal transduction, effects of structural modifications on the activity and efficiency of SLs and their analogs remain largely elusive. SLs are characterized by the presence of a lactone ring (D-ring) that is connected by an enol ether bridge to a second moiety. In this study, we investigated the effect of additional D-ring methylation of SL analogs on their transcription regulating activity. For this purpose, we compared the SL analogs MP13 and AR8, which differ only by the presence of a methyl group at the C-3' atom in the latter. Transcription regulating activity was determined by quantitative real-time PCR measurement of transcript levels of SL-dependent, feed-back regulated genes in treated wild type and ccd7 mutant rice seedlings. Results obtained indicate that C-3' methylation reduces the transcription regulating activity, as shown by the more pronounced suppression of the SL biosynthesis genes DWARF27 (D27) and CAROTENOID CLEAVAGE DIOXYGENASES (CCD7 and CCD8) and higher induction of the SL signaling repressor gene DWARF53 (D53) in MP13 treated seedlings. These results are consistent with a recent study on the biological activities of MP13 and AR8.

Strigolactones: Plant Hormones with Promising Features

Almost 80 years after the discovery of the first plant hormone, auxin, a few years ago a new class of plant hormones, the strigolactones, was discovered. These molecules have unprecedented biological activity in a number of highly important biological processes in plants but also outside the plant in the rhizosphere, the layer of soil surrounding the roots of plants and teeming with life. The exploitation of this amazing biological activity is not without challenges: the synthesis of strigolactones is complicated and designing the desired activity a difficult task. This minireview describes the current state of knowledge about the strigolactones and how synthetic analogs can be developed that can potentially contribute to the development of a sustainable agriculture.

Design and visualization of second-generation cyanoisoindole-based fluorescent strigolactone analogs

Strigolactones (SLs) are a family of terpenoid allelochemicals that were recognized as plant hormones only a decade ago. They influence a myriad of both above- and below-ground developmental processes, and are an important survival strategy for plants in nutrient-deprived soils. A rapidly emerging approach to gain knowledge on hormone signaling is the use of traceable analogs. A unique class of labeled SL analogs was constructed, in which the original tricyclic lactone moiety of natural SLs is replaced by a fluorescent cyanoisoindole ring system. Biological evaluation as parasitic seed germination stimulant and hypocotyl elongation repressor proved the potency of the cyanoisoindole strigolactone analogs (CISAs) to be comparable to the commonly accepted standard GR24. Additionally, via a SMXL6 protein degradation assay, we provided molecular evidence that the compounds elicit SL-like responses through the natural signaling cascade. All CISAs were shown to exhibit fluorescent properties, and the high quantum yield and Stokes shift of the pyrroloindole derivative CISA-7 also enabled in vivo visualization in plants. In contrast to the previously reported fluorescent analogs, CISA-7 displays a large similarity in shape and structure with natural SLs, which renders the analog a promising tracer to investigate the spatiotemporal distribution of SLs in plants and fungi.

New hybrid type strigolactone mimics derived from plant growth regulator auxin

Strigolactones (SLs) constitute a new class of plant hormones of increasing importance in plant science. The structure of natural SLs is too complex for ready access by synthesis. Therefore, much attention is being given to design of SL analogues and mimics with a simpler structure but with retention of bioactivity. Here new hybrid type SL mimics have been designed derived from auxins, the common plant growth regulators. Auxins were simply coupled with the butenolide D-ring using bromo (or chloro) butenolide. D-rings having an extra methyl group at the vicinal C-3' carbon atom, or at the C-2' carbon atom, or at both have also been studied. The new hybrid type SL mimics were bioassayed for germination activity of seeds of the parasitic weeds S. hermonthica, O. minor and P. ramosa using the classical method of counting germinated seeds and a colorimetric method. For comparison SL mimics derived from phenyl acetic acid were also investigated. The bioassays revealed that mimics with a normal D-ring had appreciable to good activity, those with an extra methyl group at C-2' were also appreciably active, whereas those with a methyl group in the vicinal C-3' position were inactive (S. hermonthica) or only slightly active. The new hybrid type mimics may be attractive as potential suicidal germination agents in agronomic applications.

Methylation at the C-3' in D-Ring of Strigolactone Analogs Reduces Biological Activity in Root Parasitic Plants and Rice

Strigolactones (SLs) regulate plant development and induce seed germination in obligate root parasitic weeds, e.g. Striga spp. Because organic synthesis of natural SLs is laborious, there is a large need for easy-to-synthesize and efficient analogs. Here, we investigated the effect of a structural modification of the D-ring, a conserved structural element in SLs. We synthesized and investigated the activity of two analogs, MP13 and MP26, which differ from previously published AR8 and AR36 only in the absence of methylation at C-3'. The de-methylated MP13 and MP26 were much more efficient in regulating plant development and inducing Striga seed germination, compared with AR8. Hydrolysis assays performed with purified Striga SL receptor and docking of AR8 and MP13 to the corresponding active site confirmed and explained the higher activity. Field trials performed in a naturally Striga-infested African farmer's field unraveled MP13 as a promising candidate for combating Striga by inducing germination in host's absence. Our findings demonstrate that methylation of the C-3' in D-ring in SL analogs has a negative impact on their activity and identify MP13 and, particularly, MP26 as potent SL analogs with simple structures, which can be employed to control Striga, a major threat to global food security.

Resorcinol-Type Strigolactone Mimics as Potent Germinators of the Parasitic Plants Striga hermonthica and Phelipanche ramosa

Strigolactones are a particular class of plant metabolites with diverse biological functions starting from the stimulation of parasitic seed germination to phytohormonal activity. The expansion of parasitic weeds in the fields of developing countries is threatening the food supply and calls for simple procedures to combat these weeds. Strigolactone analogues represent a promising approach for such control through suicidal germination, i.e., parasitic seed germination without the presence of the host causing parasite death. In the present work, the synthesis of resorcinol-type strigolactone mimics related to debranones is reported. These compounds were highly stable even at alkaline pH levels and able to induce seed germination of parasitic plants Striga hermonthica and Phelipanche ramosa at low concentrations, EC₅₀ ≈ 2 × 10^-7 M ( Striga) and EC₅₀ ≈ 2 × 10^-9 M ( Phelipanche). On the other hand, the mimics had no significant effect on root architecture of Arabidopsis plants, suggesting a selective activity for parasitic seed germination, making them a primary target as suicidal germinators.

Effect of the strigolactone analogs methyl phenlactonoates on spore germination and root colonization of arbuscular mycorrhizal fungi

Strigolactones (SLs), a novel class of plant hormones, are key regulator of plant architecture and mediator of biotic interactions in the rhizosphere. Root-released SLs initiate the establishment of arbuscular mycorrhizal (AM) symbiosis by inducing spore germination and hyphal branching in AM fungi (AMF). However, these compounds also trigger the germination of root parasitic weeds, paving the way for deleterious infestation. Availability of SLs is required for investigating of their functions and also for application in agriculture. However, natural SLs are difficult to synthesize due to their complex structure and cannot be isolated at large scale, as they are released at very low concentrations. Therefore, there is a need for synthetic SL analogs. Recently, we reported on the development of simple SL analogs, methyl phenlactonoates (MPs), which show high SL activity in plants. Here, we investigate the effect of MP1, MP3 and the widely used SL-analog GR24 on AMF spore germination and host root colonization. Our results show that MP1 and MP3 inhibit AMF spore germination, but promote the intra-radical root colonization, both more efficiently than GR24. These results indicate that field application of MP1 and MP3 does not have negative impact on mycorrhizal fungi. In conclusion, our data together with the previously reported simple synthesis, high activity in regulating plant architecture and inducing Striga seed germination, demonstrate the utility of MP1 and MP3 as for field application in combating root parasitic weeds by inducing germination in host's absence.

Chemical regulators of plant hormones and their applications in basic research and agriculture*

Plant hormones are small molecules that play versatile roles in regulating plant growth, development, and responses to the environment. Classic methodologies, including genetics, analytic chemistry, biochemistry, and molecular biology, have contributed to the progress in plant hormone studies. In addition, chemical regulators of plant hormone functions have been important in such studies. Today, synthetic chemicals, including plant growth regulators, are used to study and manipulate biological systems, collectively referred to as chemical biology. Here, we summarize the available chemical regulators and their contributions to plant hormone studies. We also pose questions that remain to be addressed in plant hormone studies and that might be solved with the help of chemical regulators.

Structure-activity relationships of strigolactones via a novel, quantitative in planta bioassay

Strigolactones (SLs) are plant hormones with various functions in development, responses to stress, and interactions with (micro)organisms in the rhizosphere, including with seeds of parasitic plants. Their perception for hormonal functions requires an α,β-hydrolase belonging to the D14 clade in higher plants; perception of host-produced SLs by parasitic seeds relies on similar but phylogenetically distinct proteins (D14-like). D14 and D14-like proteins are peculiar receptors, because they cleave SLs before undergoing a conformational change that elicits downstream events. Structure-activity relationship data show that the butenolide D-ring is crucial for bioactivity. We applied a bioisosteric approach to the structure of SLs by synthetizing analogues and mimics of natural SLs in which the D-ring was changed from a butenolide to a lactam and then evaluating their bioactivity. This was done by using a novel bioassay based on Arabidopsis transgenic lines expressing AtD14 fused to firefly luciferase, in parallel with the quantification of germination-inducing activity on parasitic seeds. The results obtained showed that the in planta bioassay is robust and quantitative, and thus can be confidently added to the SL-survey toolbox. The results also showed that modification of the butenolide ring into a lactam one significantly hampers the biological activity exhibited by SLs possessing a canonical lactonic D-ring.

Methyl phenlactonoates are efficient strigolactone analogs with simple structure

Strigolactones (SLs) are a new class of phytohormones that also act as germination stimulants for root parasitic plants, such as Striga spp., and as branching factors for symbiotic arbuscular mycorrhizal fungi. Sources for natural SLs are very limited. Hence, efficient and simple SL analogs are needed for elucidating SL-related biological processes as well as for agricultural applications. Based on the structure of the non-canonical SL methyl carlactonoate, we developed a new, easy to synthesize series of analogs, termed methyl phenlactonoates (MPs), evaluated their efficacy in exerting different SL functions, and determined their affinity for SL receptors from rice and Striga hermonthica. Most of the MPs showed considerable activity in regulating plant architecture, triggering leaf senescence, and inducing parasitic seed germination. Moreover, some MPs outperformed GR24, a widely used SL analog with a complex structure, in exerting particular SL functions, such as modulating Arabidopsis roots architecture and inhibiting rice tillering. Thus, MPs will help in elucidating the functions of SLs and are promising candidates for agricultural applications. Moreover, MPs demonstrate that slight structural modifications clearly impact the efficiency in exerting particular SL functions, indicating that structural diversity of natural SLs may mirror a functional specificity.

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.

Strigolactones: new plant hormones in the spotlight

The development and growth of plants are regulated by interplay of a plethora of complex chemical reactions in which plant hormones play a pivotal role. In recent years, a group of new plant hormones, namely strigolactones (SLs), was discovered and identified. The first SL, strigol, was isolated in 1966, but it took almost 20 years before the details of its structure were fully elucidated. At present, two families of SLs are known, one having the stereochemistry of (+)-strigol and the other that of (-)-orobanchol, the most abundant naturally occurring SL. The most well-known bioproperty of SLs is the germination of seeds of the parasitic weeds Striga and Orobanche. It is evident that SLs are going to play a prominent role in modern molecular botany. In this review, relevant molecular and bioproperties of SLs are discussed. Items of importance are the effect of stereochemistry, structure-activity relationships, design and synthesis of analogues with a simple structure, but with retention of bioactivity, introduction of fluorescent labels into SLs, biosynthetic origin of SLs, mode of action in plants, application in agriculture for the control of parasitic weeds, stimulation of the branching of arbuscular mycorrhizal (AM) fungi, and the control of plant architecture. The future potential of SLs in molecular botany is highlighted.

Strigolactones: mediators of osmotic stress responses with a potential for agrochemical manipulation of crop resilience

After quickly touching upon general aspects of strigolactone biology and functions, including structure, synthesis, and perception, this review focuses on the role and regulation of the strigolactone pathway during osmotic stress, in light of the most recent research developments. We discuss available data on organ-specific dynamics of strigolactone synthesis and interaction with abscisic acid in the acclimatization response, with emphasis on the ecophysiological implications of the effects on the stomatal closure process. We highlight the importance of considering roots and shoots separately as well as combined versus individual stress treatments; and of performing reciprocal grafting experiments to work out organ contributions and long-distance signalling events and components under more realistic conditions. Finally, we elaborate on the question of if and how synthetic or natural strigolactones, alone or in combination with crop management strategies such as grafting, hold potential to maximize crop resilience to abiotic stresses.

Which are the major players, canonical or non-canonical strigolactones?

Strigolactones (SLs) can be classified into two structurally distinct groups: canonical and non-canonical SLs. Canonical SLs contain the ABCD ring system, and non-canonical SLs lack the A, B, or C ring but have the enol ether-D ring moiety, which is essential for biological activities. The simplest non-canonical SL is the SL biosynthetic intermediate carlactone. In plants, carlactone and its oxidized metabolites, such as carlactonoic acid and methyl carlactonoate, are present in root and shoot tissues. In some plant species, including black oat (Avena strigosa), sunflower (Helianthus annuus), and maize (Zea mays), non-canonical SLs in the root exudates are major germination stimulants. Various plant species, such as tomato (Solanum lycopersicum), Arabidopsis, and poplar (Populus spp.), release carlactonoic acid into the rhizosphere. These observations suggest that both canonical and non-canonical SLs act as host-recognition signals in the rhizosphere. In contrast, the limited distribution of canonical SLs in the plant kingdom, and the structure-specific and stereospecific transportation of canonical SLs from roots to shoots, suggest that plant hormones inhibiting shoot branching are not canonical SLs but, rather, are non-canonical SLs.

Inhibition of strigolactone receptors by N-phenylanthranilic acid derivatives: Structural and functional insights

The strigolactone (SL) family of plant hormones regulates a broad range of physiological processes affecting plant growth and development and also plays essential roles in controlling interactions with parasitic weeds and symbiotic fungi. Recent progress elucidating details of SL biosynthesis, signaling, and transport offers many opportunities for discovering new plant-growth regulators via chemical interference. Here, using high-throughput screening and downstream biochemical assays, we identified N-phenylanthranilic acid derivatives as potent inhibitors of the SL receptors from petunia (DAD2), rice (OsD14), and Arabidopsis (AtD14). Crystal structures of DAD2 and OsD14 in complex with inhibitors further provided detailed insights into the inhibition mechanism, and in silico modeling of 19 other plant strigolactone receptors suggested that these compounds are active across a large range of plant species. Altogether, these results provide chemical tools for investigating SL signaling and further define a framework for structure-based approaches to design and validate optimized inhibitors of SL receptors for specific plant targets.

Strigolactones, karrikins and beyond

The plant hormones strigolactones are synthesized from carotenoids and signal via the α/β hydrolase DWARF 14 (D14) and the F-box protein MORE AXILLARY GROWTH 2 (MAX2). Karrikins, molecules produced upon fire, share MAX2 for signalling, but depend on the D14 paralog KARRIKIN INSENSITIVE 2 (KAI2) for perception with strong evidence that the MAX2-KAI2 protein complex might also recognize so far unknown plant-made karrikin-like molecules. Thus, the phenotypes of the max2 mutants are the complex consequence of a loss of both D14-dependent and KAI2-dependent signalling, hence, the reason why some biological roles, attributed to strigolactones based on max2 phenotypes, could never be observed in d14 or in the strigolactone-deficient max3 and max4 mutants. Moreover, the broadly used synthetic strigolactone analog rac-GR24 has been shown to mimic strigolactone as well as karrikin(-like) signals, providing an extra level of complexity in the distinction of the unique and common roles of both molecules in plant biology. Here, a critical overview is provided of the diverse biological processes regulated by strigolactones and/or karrikins. These two growth regulators are considered beyond their boundaries, and the importance of the yet unknown karrikin-like molecules is discussed as well.

New Strigolactone Mimics as Exogenous Signals for Rhizosphere Organisms

The importance of strigolactones in plant biology prompted us to synthesize simplified strigolactone mimics effective as exogenous signals for rhizosphere organisms. New strigolactone mimics easily derived from simple and available starting materials in significant amounts were prepared and fully characterized. These compounds contain an aromatic or heterocyclic ring, usually present in various bioactive molecules, connected by an ether link to a furan-2-one moiety. The new synthesized strigolactone mimics were confirmed to be active on plant pathogenic fungi and parasitic weed seeds.

Triazolide Strigolactone Mimics Influence Root Development in Arabidopsis

Strigolactones are the most recently recognized class of phytohormones, which are also known to establish plant symbiosis with arbuscular mycorhizal fungi or induce germination of parasitic plants. Their relatively complex structures and low stability urgently calls for simple derivatives with maintained biological function. We have prepared a series of triazolide strigolactone mimics and studied their ability to affect root development of Arabidopsis thaliana. The strigolactone mimics significantly induced root elongation and lateral root formation while resembling the effect of the reference compound GR24.

Strigolactone Signaling and Evolution

Strigolactones are a structurally diverse class of plant hormones that control many aspects of shoot and root growth. Strigolactones are also exuded by plants into the rhizosphere, where they promote symbiotic interactions with arbuscular mycorrhizal fungi and germination of root parasitic plants in the Orobanchaceae family. Therefore, understanding how strigolactones are made, transported, and perceived may lead to agricultural innovations as well as a deeper knowledge of how plants function. Substantial progress has been made in these areas over the past decade. In this review, we focus on the molecular mechanisms, core developmental roles, and evolutionary history of strigolactone signaling. We also propose potential translational applications of strigolactone research to agriculture.

Strigolactones: how far is their commercial use for agricultural purposes?

Strigolactones are a class of natural and synthetic compounds that in the past decade have been exciting the scientific community not only for their intriguing biological properties but also for their potential applications in agriculture. These applications range from their use as hormones to modify and/or manage plant architecture, to their use as stimulants to induce seed germination of parasitic weeds and thus control their infestation by a reduced seed bank, to their use as 'biostimulants' of plant root colonisation by arbuscular mycorrhizal fungi, improving plant nutritional capabilities, to other still unknown effects on microbial soil communities. More recently, these compounds have also been attracting the interest of agrochemical companies. In spite of their biological attractiveness, practical applications are still greatly hampered by the low product yields obtainable by plant root exudates, by the costs of their synthesis, by the lack of knowledge of their off-target effects and by the not yet specified or properly identified legislation that could regulate the use of these compounds, depending on the agricultural purposes. The aim of this article is to discuss, in the light of current knowledge, the different scenarios that might play out in the near future with regard to the practical application of strigolactones. © 2016 Society of Chemical Industry.

Chemical modification of a phenoxyfuranone-type strigolactone mimic for selective effects on rice tillering or Striga hermonthica seed germination

BACKGROUND

We previously reported that a series of phenoxyfuranone compounds, designated 'debranones', mimic strigolactone (SL) activity. 4-Bromodebranone (4BD) is a functionally selective SL mimic that reduces the number of shoot branches on rice more potently than GR24, a typical synthetic SL analogue, but does not induce seed germination in the root-parasitic plant Striga hermonthica. To enhance the selective activity of debranones in stimulating the seed germination of root-parasitic plants, we prepared several analogues of 4BD in which the chlorine atom was substituted with an H atom at the o-, m- or p-position on the phenyl ring (designated 2-, 3-, or 4-chlorodebranone, respectively) or had a bicyclic group instead of the phenyl ring. We evaluated the biological activities of the compounds with rice tillering assays and S. hermonthica seed germination assays.

RESULTS

Both assays showed that the substituent position affected debranone efficiency, and among the monochlorodebranones, 2-chlorodebranone was more effective than the other two isomers in both assays. When the activities of the bicyclic debranones were compared in the same two assays, one was more active than GR24 in the rice tillering assay. This debranone also stimulated the germination of S. hermonthica seeds. Thus, some debranone derivatives induced the germination of S. hermonthica seeds, although their activities were still ∼1/20 that of GR24.

CONCLUSION

These results strongly suggest that further and rigorous structure-activity relationship studies of the debranones will identify derivatives that more potently stimulate the suicidal germination of S. hermonthica seeds. © 2016 Society of Chemical Industry.

An histidine covalent receptor and butenolide complex mediates strigolactone perception

Strigolactone plant hormones control plant architecture and are key players in both symbiotic and parasitic interactions. They contain an ABC tricyclic lactone connected to a butenolide group, the D ring. The DWARF14 (D14) strigolactone receptor belongs to the superfamily of α/β-hydrolases, and is known to hydrolyze the bond between the ABC lactone and the D ring. Here we characterized the binding and catalytic functions of RAMOSUS3 (RMS3), the pea (Pisum sativum) ortholog of rice (Oryza sativa) D14 strigolactone receptor. Using new profluorescent probes with strigolactone-like bioactivity, we found that RMS3 acts as a single-turnover enzyme that explains its apparent low enzymatic rate. We demonstrated the formation of a covalent RMS3-D-ring complex, essential for bioactivity, in which the D ring was attached to histidine 247 of the catalytic triad. These results reveal an undescribed mechanism of plant hormone reception in which the receptor performs an irreversible enzymatic reaction to generate its own ligand.

Shaping Small Bioactive Molecules to Untangle Their Biological Function: A Focus on Fluorescent Plant Hormones

Modern biology overlaps with chemistry in explaining the structure and function of all cellular processes at the molecular level. Plant hormone research is perfectly located at the interface between these two disciplines, taking advantage of synthetic and computational chemistry as a tool to decipher the complex biological mechanisms regulating the action of plant hormones. These small signaling molecules regulate a wide range of developmental processes, adapting plant growth to ever changing environmental conditions. The synthesis of small bioactive molecules mimicking the activity of endogenous hormones allows us to unveil many molecular features of their functioning, giving rise to a new field, plant chemical biology. In this framework, fluorescence labeling of plant hormones is emerging as a successful strategy to track the fate of these challenging molecules inside living organisms. Thanks to the increasing availability of new fluorescent probes as well as advanced and innovative imaging technologies, we are now in a position to investigate many of the dynamic mechanisms through which plant hormones exert their action. Such a deep and detailed comprehension is mandatory for the development of new green technologies for practical applications. In this review, we summarize the results obtained so far concerning the fluorescent labeling of plant hormones, highlighting the basic steps leading to the design and synthesis of these compelling molecular tools and their applications.

Strigolactones: new plant hormones in action

MAIN CONCLUSION

The key step in the mode of action of strigolactones is the enzymatic detachment of the D-ring. The thus formed hydroxy butenolide induces conformational changes of the receptor pocket which trigger a cascade of reactions in the signal transduction. Strigolactones (SLs) constitute a new class of plant hormones which are of increasing importance in plant science. For the last 60 years, they have been known as germination stimulants for parasitic plants. Recently, several new bio-properties of SLs have been discovered such as the branching factor for arbuscular mycorrhizal fungi, regulation of plant architecture (inhibition of bud outgrowth and of shoot branching) and the response to abiotic factors, etc. To broaden horizons and encourage new ideas for identifying and synthesising new and structurally simple SLs, this review is focused on molecular aspects of this new class of plant hormones. Special attention has been given to structural features, the mode of action of these phytohormones in various biological actions, the design of SL analogs and their applications.

Strigolactones: new plant hormones in action

The key step in the mode of action of strigolactones is the enzymatic detachment of the D-ring. The thus formed hydroxy butenolide induces conformational changes of the receptor pocket which trigger a cascade of reactions in the signal transduction. Strigolactones (SLs) constitute a new class of plant hormones which are of increasing importance in plant science. For the last 60 years, they have been known as germination stimulants for parasitic plants. Recently, several new bio-properties of SLs have been discovered such as the branching factor for arbuscular mycorrhizal fungi, regulation of plant architecture (inhibition of bud outgrowth and of shoot branching) and the response to abiotic factors, etc. To broaden horizons and encourage new ideas for identifying and synthesising new and structurally simple SLs, this review is focused on molecular aspects of this new class of plant hormones. Special attention has been given to structural features, the mode of action of these phytohormones in various biological actions, the design of SL analogs and their applications.