Transformation and degradation of tebuconazole and its metabolites in constructed wetlands with arbuscular mycorrhizal fungi colonization

Arbuscular mycorrhizal fungi (AMF) colonization has been used in constructed wetlands (CWs) to enhance treatment performance. However, its role in azole (fungicide) degradation and microbial community changes is not well understood. This study aims to explore the impact of AMF on the degradation of tebuconazole and its metabolites in CWs. Total organic carbon levels were consistently higher with the colonization of AMF (AMF+; 9.63- 16.37 mg/L) compared to without the colonization of AMF (AMF-; 8.79-14.48 mg/L) in CWs. Notably, tebuconazole removal was swift, occurring within one day in both treatments (p = 0.885), with removal efficiencies ranging from 94.10 % to 97.83 %. That's primarily due to rapid substrate absorption at the beginning, while degradation follows with a longer time. Four metabolites were reported in CWs first time: tebuconazole hydroxy, tebuconazole lactone, tebuconazole carboxy acid, and tebuconazole dechloro. AMF decreased the abundance of tebuconazole dechloro in the liquid phase, suggesting an inhibitory effect of AMF on dechlorination processes. Furthermore, tebuconazole carboxy acid and hydroxy were predominantly found in plant roots, with a higher abundance observed in AMF+ treatments. Metagenomic analysis highlighted an increasing abundance in bacterial community structure in favor of beneficial microorganisms (xanthomonadales, xanthomonadaceae, and lysobacter), along with a notable presence of functional genes like codA, NAD, and deaD in AMF+ treatments. These findings highlight the positive influence of AMF on tebuconazole stress resilience, microbial community modification, and the enhancement of bioremediation capabilities in CWs.

Unveiling the complexity of strigolactones: exploring structural diversity, biosynthesis pathways, and signaling mechanisms

Strigolactone is the collective name for compounds containing a butenolide as a part of their structure, first discovered as compounds that induce seed germination of root parasitic plants. They were later found to be rhizosphere signaling molecules that induce hyphal branching of arbuscular mycorrhizal fungi, and, finally, they emerged as a class of plant hormones. Strigolactones are found in root exudates, where they display a great variability in their chemical structure. Their structure varies among plant species, and multiple strigolactones can exist in one species. Over 30 strigolactones have been identified, yet the chemical structure of the strigolactone that functions as an endogenous hormone and is found in the above-ground parts of plants remains unknown. We discuss our current knowledge of the synthetic pathways of diverse strigolactones and their regulation, as well as recent progress in identifying strigolactones as plant hormones. Strigolactone is perceived by the DWARF14 (D14), receptor, an α/β hydrolase which originated by gene duplication of KARRIKIN INSENSITIVE 2 (KAI2). D14 and KAI2 signaling pathways are partially overlapping paralogous pathways. Progress in understanding the signaling mechanisms mediated by two α/β hydrolase receptors as well as remaining challenges in the field of strigolactone research are reviewed.

Identification and expression of strigolactone biosynthesis and signaling genes and the in vitro effects of strigolactones in olive (Olea europaea L.)

Strigolactones (SLs), synthesized in plant roots, play a dual role in modulating plant growth and development, and in inducing the germination of parasitic plant seeds and arbuscular mycorrhizal fungi in the rhizosphere. As phytohormones, SLs are crucial in regulating branching and shaping plant architecture. Despite the significant impact of branching strategies on the yield performance of fruit crops, limited research has been conducted on SLs in these crops. In our study, we identified the transcript sequences of SL biosynthesis and signaling genes in olive (Olea europaea L.) using rapid amplification of cDNA ends. We predicted the corresponding protein sequences, analyzed their characteristics, and conducted molecular docking with bioinformatics tools. Furthermore, we quantified the expression levels of these genes in various tissues using quantitative real-time PCR. Our findings demonstrate the predominant expression of SL biosynthesis and signaling genes (OeD27, OeMAX3, OeMAX4, OeMAX1, OeD14, and OeMAX2) in roots and lateral buds, highlighting their importance in branching. Treatment with rac-GR24, an SL analog, enhanced the germination frequency of olive seeds in vitro compared with untreated embryos. Conversely, inhibition of SL biosynthesis with TIS108 increased lateral bud formation in a hard-to-root cultivar, underscoring the role of SLs as phytohormones in olives. These results suggest that modifying SL biosynthesis and signaling pathways could offer novel approaches for olive breeding, with potential applicability to other fruit crops.

AtMYBS1 negatively regulates heat tolerance by directly repressing the expression of MAX1 required for strigolactone biosynthesis in Arabidopsis

Heat stress caused by global warming requires the development of thermotolerant crops to sustain yield. It is necessary to understand the molecular mechanisms that underlie heat tolerance in plants. Strigolactones (SLs) are a class of carotenoid-derived phytohormones that regulate plant development and responses to abiotic or biotic stresses. Although SL biosynthesis and signaling processes are well established, genes that directly regulate SL biosynthesis have rarely been reported. Here, we report that the MYB-like transcription factor AtMYBS1/AtMYBL, whose gene expression is repressed by heat stress, functions as a negative regulator of heat tolerance by directly inhibiting SL biosynthesis in Arabidopsis. Overexpression of AtMYBS1 led to heat hypersensitivity, whereas atmybs1 mutants displayed increased heat tolerance. Expression of MAX1, a critical enzyme in SL biosynthesis, was induced by heat stress and downregulated in AtMYBS1-overexpression (OE) plants but upregulated in atmybs1 mutants. Overexpression of MAX1 in the AtMYBS1-OE background reversed the heat hypersensitivity of AtMYBS1-OE plants. Loss of MAX1 function in the atmyb1 background reversed the heat-tolerant phenotypes of atmyb1 mutants. Yeast one-hybrid assays, chromatin immunoprecipitation‒qPCR, and transgenic analyses demonstrated that AtMYBS1 directly represses MAX1 expression through the MYB binding site in the MAX1 promoter in vivo. The atmybs1d14 double mutant, like d14 mutants, exhibited hypersensitivity to heat stress, indicating the necessary role of SL signaling in AtMYBS1-regulated heat tolerance. Our findings provide new insights into the regulatory network of SL biosynthesis, facilitating the breeding of heat-tolerant crops to improve crop production in a warming world.

Activation of Strigolactone Biosynthesis by the DWARF14-LIKE/KARRIKIN-INSENSITIVE2 Pathway in Mycorrhizal Angiosperms, but Not in Arabidopsis, a Non-mycorrhizal Plant

Strigolactones (SLs) are a class of plant hormones that regulate many aspects of plant growth and development. SLs also improve symbiosis with arbuscular mycorrhizal fungi (AMF) in the rhizosphere. Recent studies have shown that the DWARF14-LIKE (D14L)/KARRIKIN-INSENSITIVE2 (KAI2) family, paralogs of the SL receptor D14, are required for AMF colonization in several flowering plants, including rice. In this study, we found that (-)-GR5, a 2'S-configured enantiomer of a synthetic SL analog (+)-GR5, significantly activated SL biosynthesis in rice roots via D14L. This result is consistent with a recent report, showing that the D14L pathway positively regulates SL biosynthesis in rice. In fact, the SL levels tended to be lower in the roots of the d14l mutant under both inorganic nutrient-deficient and -sufficient conditions. We also show that the increase in SL levels by (-)-GR5 was observed in other mycorrhizal plant species. In contrast, the KAI2 pathway did not upregulate the SL level and the expression of SL biosynthetic genes in Arabidopsis, a non-mycorrhizal plant. We also examined whether the KAI2 pathway enhances SL biosynthesis in the liverwort Marchantia paleacea, where SL functions as a rhizosphere signaling molecule for AMF. However, the SL level and SL biosynthetic genes were not positively regulated by the KAI2 pathway. These results imply that the activation of SL biosynthesis by the D14L/KAI2 pathway has been evolutionarily acquired after the divergence of bryophytes to efficiently promote symbiosis with AMF, although we cannot exclude the possibility that liverworts have specifically lost this regulatory system.

Strigolactones and Shoot Branching: What Is the Real Hormone and How Does It Work?

There have been substantial advances in our understanding of many aspects of strigolactone regulation of branching since the discovery of strigolactones as phytohormones. These include further insights into the network of phytohormones and other signals that regulate branching, as well as deep insights into strigolactone biosynthesis, metabolism, transport, perception and downstream signaling. In this review, we provide an update on recent advances in our understanding of how the strigolactone pathway co-ordinately and dynamically regulates bud outgrowth and pose some important outstanding questions that are yet to be resolved.

Synthesis and Biological Profiling of Novel Strigolactone Derivatives for Arabidopsis Growth and Development

The artificially synthesized strigolactone (SL) analogue GR24 is currently the most widely used reference compound in studying the biological functions of SLs. To elucidate the structure-activity relationship and find more promising derivatives with unique molecular profiles, we design and synthesized three series of novel GR24 derivatives and explored their activities in hypocotyl and root development of Arabidopsis. Among the 50 synthesized compounds, A11a, A12a, and A20d were found to have high activities comparable to GR24 for hypocotyl and/or primary root elongation inhibition in Arabidopsis. Some new analogues have been discovered to exhibit unique activities: (1) A20c, A21e, and A21o are specific inhibitors in primary root elongation; (2) A21c, A26c, and A27a exhibit a high promotion effect on Arabidopsis primary root elongation; and (3) A27e possesses the most unique profiles completely opposite to GR24 that promotes both hypocotyl elongation and primary root development. Moreover, we revealed that the AtD14 receptor does not affect the inhibitory effect of SL analogues in Arabidopsis root development. The ligand-receptor interactions for the most representative analogues A11a and A27e were deciphered with a long time scale molecular dynamics simulation study, which provides the molecular basis of their distinct functions, and may help scientists design novel phytohormones.

Fertilization controls tiller numbers via transcriptional regulation of a MAX1-like gene in rice cultivation

Fertilization controls various aspects of cereal growth such as tiller number, leaf size, and panicle size. However, despite such benefits, global chemical fertilizer use must be reduced to achieve sustainable agriculture. Here, based on field transcriptome data from leaf samples collected during rice cultivation, we identify fertilizer responsive genes and focus on Os1900, a gene orthologous to Arabidopsis thaliana MAX1, which is involved in strigolactone biosynthesis. Elaborate genetic and biochemical analyses using CRISPR/Cas9 mutants reveal that Os1900 together with another MAX1-like gene, Os5100, play a critical role in controlling the conversion of carlactone into carlactonoic acid during strigolactone biosynthesis and tillering in rice. Detailed analyses of a series of Os1900 promoter deletion mutations suggest that fertilization controls tiller number in rice through transcriptional regulation of Os1900, and that a few promoter mutations alone can increase tiller numbers and grain yields even under minor-fertilizer conditions, whereas a single defective os1900 mutation does not increase tillers under normal fertilizer condition. Such Os1900 promoter mutations have potential uses in breeding programs for sustainable rice production.

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.

Desmethyl type germinone, a specific agonist for the HTL/KAI2 receptor, induces the Arabidopsis seed germination in a gibberellin-independent manner

DWARF14 (D14) and HTL/KAI2 (KAI2) are paralogous receptors in the α/β-hydrolase superfamily. D14 is the receptor for a class of plant hormones, strigolactones (SLs), and KAI2 is the receptor for the smoke-derived seed germination inducer, Karrikin (KAR), in Arabidopsis. Germinone (Ger) was previously reported as a KAI2 agonist with germination-inducing activity for thermo-inhibited Arabidopsis seed. However, Ger was not specific to KAI2, and could also bind to D14. It was reported that SL analogs with a desmethyl-type D-ring structure are specifically recognized by KAI2. On the basis of this observation, we synthesized a desmethyl-type germinone (dMGer). We found that dMGer is highly specific to KAI2. Moreover, dMGer induced Arabidopsis seed germination more effectively than did Ger. In addition, dMGer induced the seed germination of Arabidopsis in a manner independently of GA, a well-known germination inducer in plants.

The Karrikin Receptor Karrikin Insensitive2 Positively Regulates Heat Stress Tolerance in Arabidopsis thaliana

In this study, we investigated the potential role of the karrikin receptor KARRIKIN INSENSITIVE2 (KAI2) in the response of Arabidopsis seedlings to high-temperature stress. We performed phenotypic, physiological and transcriptome analyses of Arabidopsis kai2 mutants and wild-type (WT) plants under control (kai2_C and WT_C, respectively) and 6- and 24-h heat stress conditions (kai2_H6, kai2_H24, WT_H6 and WT_H24, respectively) to understand the basis for KAI2-regulated heat stress tolerance. We discovered that the kai2 mutants exhibited hypersensitivity to high-temperature stress relative to WT plants, which might be associated with a more highly increased leaf surface temperature and cell membrane damage in kai2 mutant plants. Next, we performed comparative transcriptome analysis of kai2_C, kai2_H6, kai2_H24, WT_C, WT_H6 and WT_H24 to identify transcriptome differences between WT and kai2 mutants in response to heat stress. K-mean clustering of normalized gene expression separated the investigated genotypes into three clusters based on heat-treated and non-treated control conditions. Within each cluster, the kai2 mutants were separated from WT plants, implying that kai2 mutants exhibited distinct transcriptome profiles relative to WT plants. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses showed a repression in 'misfolded protein binding', 'heat shock protein binding', 'unfolded protein binding' and 'protein processing in endoplasmic reticulum' pathways, which was consistent with the downregulation of several genes encoding heat shock proteins and heat shock transcription factors in the kai2 mutant versus WT plants under control and heat stress conditions. Our findings suggest that chemical or genetic manipulation of KAI2 signaling may provide a novel way to improve heat tolerance in plants.

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.

Nanosensor Applications in Plant Science

Plant science is a major research topic addressing some of the most important global challenges we face today, including energy and food security. Plant science has a role in the production of staple foods and materials, as well as roles in genetics research, environmental management, and the synthesis of high-value compounds such as pharmaceuticals or raw materials for energy production. Nanosensors-selective transducers with a characteristic dimension that is nanometre in scale-have emerged as important tools for monitoring biological processes such as plant signalling pathways and metabolism in ways that are non-destructive, minimally invasive, and capable of real-time analysis. A variety of nanosensors have been used to study different biological processes; for example, optical nanosensors based on Förster resonance energy transfer (FRET) have been used to study protein interactions, cell contents, and biophysical parameters, and electrochemical nanosensors have been used to detect redox reactions in plants. Nanosensor applications in plants include nutrient determination, disease assessment, and the detection of proteins, hormones, and other biological substances. The combination of nanosensor technology and plant sciences has the potential to be a powerful alliance and could support the successful delivery of the 2030 Sustainable Development Goals. However, a lack of knowledge regarding the health effects of nanomaterials and the high costs of some of the raw materials required has lessened their commercial impact.

A New Series of Strigolactone Analogs Derived From Cinnamic Acids as Germination Inducers for Root Parasitic Plants

Root parasitic plants such as Striga and Orobanche cause significant damage on crop production, particularly in sub-Saharan Africa. Their seeds germinate by sensing host root-derived signaling molecules called strigolactones (SLs). SL mimics can be used as suicidal germination inducers for root parasitic plants. Previous attempts to develop such chemicals have revealed that the methylbutenolide ring (D-ring), a common substructure in all the naturally occurring SLs, is critical for SL agonistic activity, suggesting that it should be possible to generate new SL mimics simply by coupling a D-ring with another molecule. Because structural information regarding SLs and their receptor interaction is still limited, such an approach might be an effective strategy to develop new potent SL agonists. Here, we report development of a series of new SL analogs derived from cinnamic acid (CA), the basis of a class of phenylpropanoid natural products that occur widely in plants. CA has an aromatic ring and a double-bond side-chain structure, which are advantageous for preparing structurally diverse derivatives. We prepared SL analogs from cis and trans configuration CA, and found that all the cis-CA-derived SL analogs had stronger activities as seed germination inducers for the root parasitic plants, Orobanche minor and Striga hermonthica, compared with the corresponding trans-CA-derived analogs. Moreover, introduction of a substitution at the C-4 position increased the germination-stimulating activity. We also found that the SL analogs derived from cis-CA were able to interact directly with SL receptor proteins more effectively than the analogs derived from trans-CA. The cis isomer of CA was previously reported to have a growth promoting effect on non-parasitic plants such as Arabidopsis. We found that SL analogs derived from cis-CA also showed growth promoting activity toward Arabidopsis, suggesting that these new SL agonists might be useful not only as suicidal germination inducers for root parasitic weeds, but also as plant growth promoters for the host plants.

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.

Asymbiotic mass production of the arbuscular mycorrhizal fungus Rhizophagus clarus

Arbuscular mycorrhizal (AM) symbiosis is a mutually beneficial interaction between fungi and land plants and promotes global phosphate cycling in terrestrial ecosystems. AM fungi are recognised as obligate symbionts that require root colonisation to complete a life cycle involving the production of propagules, asexual spores. Recently, it has been shown that Rhizophagus irregularis can produce infection-competent secondary spores asymbiotically by adding a fatty acid, palmitoleic acid. Furthermore, asymbiotic growth can be supported using myristate as a carbon and energy source for their asymbiotic growth to increase fungal biomass. However, the spore production and the ability of these spores to colonise host roots were still limited compared to the co-culture of the fungus with plant roots. Here we show that a combination of two plant hormones, strigolactone and jasmonate, induces the production of a large number of infection-competent spores in asymbiotic cultures of Rhizophagus clarus HR1 in the presence of myristate and organic nitrogen. Inoculation of asymbiotically-generated spores promoted the growth of host plants, as observed for spores produced by symbiotic culture system. Our findings provide a foundation for the elucidation of hormonal control of the fungal life cycle and the development of inoculum production schemes.

Biological Functions of Strigolactones and Their Crosstalk With Other Phytohormones

Phytohormones are small chemicals critical for plant development and adaptation to a changing environment. Strigolactones (SLs), carotenoid-derived small signalling molecules and a class of phytohormones, regulate multiple developmental processes and respond to diverse environmental signals. SLs also coordinate adjustments in the balance of resource distribution by strategic modification of the plant development, allowing plants to adapt to nutrient deficiency. Instead of operating independently, SL interplays with abscisic acid, cytokinin, auxin, ethylene, and some other plant phytohormones, forming elaborate signalling networks. Hormone signalling crosstalk in plant development and environmental response may occur in a fully concerted manner or as a cascade of sequential events. In many cases, the exact underlying mechanism is unclear because of the different effects of phytohormones and the varying backgrounds of their actions. In this review, we systematically summarise the synthesis, signal transduction, and biological functions of SLs and further highlight the significance of crosstalk between SLs and other phytohormones during plant development and resistance to ever-changing environments.

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.

Strigolactone biosynthesis, transport and perception

Strigolactones (SLs) are plant hormones that regulate diverse developmental processes and environmental responses. They are also known to be root-derived chemical signals that regulate symbiotic and parasitic interactions with arbuscular mycorrhizal fungi and root parasitic plants, respectively. Since the discovery of the hormonal function of SLs in 2008, there has been much progress in the SL research field. In particular, a number of breakthroughs have been achieved in our understanding of SL biosynthesis, transport and perception. The discovery of the hormonal function of SL was quite valuable not only as the identification of a new class of plant hormones, but also as the discovery of the long-sought-after SL biosynthetic and response mutants. These mutants in several plant species provided us the genetic resources to address fundamental questions regarding SL biosynthesis and perception. Such mutants were further characterized later, and biochemical analyses of these genetically identified factors have uncovered the outline of SL biosynthesis and perception so far. Moreover, new genes involved in SL transport have been discovered through reverse genetic analyses. In this review, we summarize recent advances in SL research with a focus on biosynthesis, transport and perception.

Abundance and Extracellular Release of Phytohormones in Aero-terrestrial Microalgae (Trebouxiophyceae, Chlorophyta) As a Potential Chemical Signaling Source1

Phytohormones are pivotal signaling compounds in higher plants, in which they exert their roles intracellularly, but are also released for cell-to-cell communication. In unicellular organisms, extracellularly released phytohormones can be involved in chemical crosstalk with other organisms. However, compared to higher plants, hardly any knowledge is available on the roles of phytohormones in green algae. Here, we studied phytohormone composition and extracellular release in aero-terrestrial Trebouxiophyceae. We investigated (a) which phytohormones are produced and if they are released extracellularly, and if extracellular phytohormone levels are (b) affected by environmental stimuli, and (c) differ between lichen-forming and non-lichen-forming species. Three free-living microalgae (Apatococcus lobatus, Chloroidium ellipsoideum, and Myrmecia bisecta) and three lichen-forming microalgae (Asterochloris glomerata, Trebouxia decolorans, and Trebouxia sp.) were studied. Algae were grown on solid media and the following cellular phytohormones were identified by ultra-high-performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS): indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), abscisic acid (ABA), gibberellin A4 (GA4 ), and zeatin (ZT). Furthermore, IAA, IBA, ABA, jasmonic acid (JA), gibberellin A3 (GA3 ), and GA4 were found to be released extracellularly. IAA and ABA were released by all six species, and IAA was the most concentrated. Phytohormone release was affected by light and water availability, especially IAA in A. glomerata, Trebouxia sp., and C. ellipsoideum. No clear patterns were observed between lichen-forming and non-lichen-forming species. The results are envisaged to contribute valuable baseline information for further studies into the roles of phytohormones in microalgae.

An improved strategy to analyse strigolactones in complex sample matrices using UHPLC-MS/MS

BACKGROUND

Strigolactones represent the most recently described group of plant hormones involved in many aspects of plant growth regulation. Simultaneously, root exuded strigolactones mediate rhizosphere signaling towards beneficial arbuscular mycorrhizal fungi, but also attract parasitic plants. The seed germination of parasitic plants induced by host strigolactones leads to serious agricultural problems worldwide. More insight in these signaling molecules is hampered by their extremely low concentrations in complex soil and plant tissue matrices, as well as their instability. So far, the combination of tailored isolation-that would replace current unspecific, time-consuming and labour-intensive processing of large samples-and a highly sensitive method for the simultaneous profiling of a broad spectrum of strigolactones has not been reported.

RESULTS

Depending on the sample matrix, two different strategies for the rapid extraction of the seven structurally similar strigolactones and highly efficient single-step pre-concentration on polymeric RP SPE sorbent were developed and validated. Compared to conventional methods, controlled temperature during the extraction and the addition of an organic modifier (acetonitrile, acetone) to the extraction solvent helped to tailor strigolactone isolation from low initial amounts of root tissue (150 mg fresh weight, FW) and root exudate (20 ml), which improved both strigolactone stability and sample purity. We have designed an efficient UHPLC separation with sensitive MS/MS detection for simultaneous analysis of seven natural strigolactones including their biosynthetic precursors-carlactone and carlactonoic acid. In combination with the optimized UHPLC-MS/MS method, attomolar detection limits were achieved. The new method allowed successful profiling of seven strigolactones in small exudate and root tissue samples of four different agriculturally important plant species-sorghum, rice, pea and tomato.

CONCLUSION

The established method provides efficient strigolactone extraction with aqueous mixtures of less nucleophilic organic solvents from small root tissue and root exudate samples, in combination with rapid single-step pre-concentration. This method improves strigolactone stability and eliminates the co-extraction and signal of matrix-associated contaminants during the final UHPLC-MS/MS analysis with an electrospray interface, which dramatically increases the overall sensitivity of the analysis. We show that the method can be applied to a variety of plant species.

Comparative functional analyses of DWARF14 and KARRIKIN INSENSITIVE 2 in drought adaptation of Arabidopsis thaliana

Functional analyses of various strigolactone-deficient mutants have demonstrated that strigolactones enhance drought resistance; however, the mechanistic involvement of the strigolactone receptor DWARF14 (D14) in this trait remains elusive. In this study, loss-of-function analysis of the D14 gene in Arabidopsis thaliana revealed that d14 mutant plants were more drought-susceptible than wild-type plants, which was associated with their larger stomatal aperture, slower abscisic acid (ABA)-mediated stomatal closure, lower anthocyanin content and delayed senescence under drought stress. Transcriptome analysis revealed a consistent alteration in the expression levels of many genes related to the observed physiological and biochemical changes in d14 plants when compared with the wild type under normal and dehydration conditions. A comparative drought resistance assay confirmed that D14 plays a less critical role in Arabidopsis drought resistance than its paralog karrikin receptor KARRIKIN INSENSITIVE 2 (KAI2). In-depth comparative analyses of the single mutants d14 and kai2 and the double mutant d14 kai2, in relation to various drought resistance-associated mechanisms, revealed that D14 and KAI2 exhibited a similar effect on stomatal closure. On the other hand, D14 had a lesser role in the maintenance of cell membrane integrity, leaf cuticle structure and ABA-induced leaf senescence, but a greater role in drought-induced anthocyanin biosynthesis, than KAI2. Interestingly, a possible additive relationship between D14 and KAI2 could be observed in regulating cell membrane integrity and leaf cuticle development. In addition, our findings also suggest the existence of a complex interaction between the D14 and ABA signaling pathways in the adaptation of Arabidopsis to drought.

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.

There and back again: An evolutionary perspective on long-distance coordination of plant growth and development

Vascular plants, unlike bryophytes, have a strong root-shoot dichotomy in which the tissue systems are mutually interdependent; roots are completely dependent on shoots for photosynthetic sugars, and shoots are completely dependent on roots for water and mineral nutrients. Long-distance communication between shoot and root is therefore critical for the growth, development and survival of vascular plants, especially with regard to variable environmental conditions. However, this long-distance signalling does not appear an ancestral feature of land plants, and has likely arisen in vascular plants to service the radical alterations in body-plan seen in this taxon. In this review, we examine the defined hormonal root-to-shoot and shoot-to-root signalling pathways that coordinate the growth of vascular plants, with a particular view to understanding how these pathways may have evolved. We highlight the completely divergent roles of isopentenyl-adenine and trans-zeatin cytokinin species in long-distance signalling, and ask whether cytokinin can really be considered as a single class of hormones in the light of recent research. We also discuss the puzzlingly sparse evidence for auxin as a shoot-to-root signal, the evolutionary re-purposing of strigolactones and gibberellins as hormonal signals, and speculate on the possible role of sugars as long-distance signals. We conclude by discussing the 'design principles' of long-distance signalling in vascular plants.

Chemical identification of 18-hydroxycarlactonoic acid as an LjMAX1 product and in planta conversion of its methyl ester to canonical and non-canonical strigolactones in Lotus japonicus

Strigolactones (SLs) are a group of plant apocarotenoids that act as rhizosphere signaling molecules for both arbuscular mycorrhizal fungi and root parasitic plants. They also regulate plant architecture as phytohormones. The model legume Lotus japonicus (synonym of Lotus corniculatus) produces canonical 5-deoxystrigol (5DS) and non-canonical lotuslactone (LL). The biosynthesis pathways of the two SLs remain elusive. In this study, we characterized the L. japonicus MAX1 homolog, LjMAX1, found in the Lotus japonicus genome assembly build 2.5. The L. japonicus max1 LORE1 insertion mutant was deficient in 5DS and LL production. A recombinant LjMAX1 protein expressed in yeast microsomes converted carlactone (CL) to 18-hydroxycarlactonoic acid (18-OH-CLA) via carlactonoic acid (CLA). Identity of 18-OH-CLA was confirmed by comparison of the methyl ester derivative of the MAX1 product with chemically synthesized methyl 18-hydroycarlactonoate (18-OH-MeCLA) using LC-MS/MS. (11R)-CL was detected as an endogenous compound in the root of L. japonicus.¹³C-labeled CL, CLA, and 18-OH-MeCLA were converted to [¹³C]-5DS and LL in plant feeding experiments using L. japonicus WT. These results showed that LjMAX1 is the crucial enzyme in the biosynthesis of Lotus SLs and that 18-hydroxylated carlactonoates are possible precursors for SL biosynthesis in L. japonicus.

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.

Flexibility of the petunia strigolactone receptor DAD2 promotes its interaction with signaling partners

Strigolactones (SLs) are terpenoid-derived plant hormones that regulate various developmental processes, particularly shoot branching, root development, and leaf senescence. The SL receptor has an unusual mode of action. Upon binding SL, it hydrolyzes the hormone, and then covalently binds one of the hydrolytic products. These initial events enable the SL receptor DAD2 (in petunia) to interact with the F-box protein PhMAX2A of the Skp-Cullin-F-box (SCF) complex and/or a repressor of SL signaling, PhD53A. However, it remains unclear how binding and hydrolysis structurally alters the SL receptor to enable its engagement with signaling partners. Here, we used mutagenesis to alter DAD2 and affect SL hydrolysis or DAD2's ability to interact with its signaling partners. We identified three DAD2 variants whose hydrolytic activity had been separated from the receptor's interactions with PhMAX2A or PhD53A. Two variants, DAD2N242I and DAD2F135A, having substitutions in the core α/β hydrolase-fold domain and the hairpin, exhibited hormone-independent interactions with PhMAX2A and PhD53A, respectively. Conversely, the DAD2D166A variant could not interact with PhMAX2A in the presence of SL, but its interaction with PhD53A remained unaffected. Structural analyses of DAD2N242I and DAD2D166A revealed only small differences compared with the structure of the WT receptor. Results of molecular dynamics simulations of the DAD2N242I structure suggested that increased flexibility is a likely cause for its SL-independent interaction with PhMAX2A. Our results suggest that PhMAX2A and PhD53A have distinct binding sites on the SL receptor and that its flexibility is a major determinant of its interactions with these two downstream regulators.

Structural Basis of Karrikin and Non-natural Strigolactone Perception in Physcomitrella patens

In plants, strigolactones are perceived by the dual receptor-hydrolase DWARF14 (D14). D14 belongs to the superfamily of α/β hydrolases and is structurally similar to the karrikin receptor KARRIKIN INSENSITIVE 2 (KAI2). The moss Physcomitrella patens is an ideal model system for studying this receptor family, because it includes 11 highly related family members with unknown ligand specificity. We present the crystal structures of three Physcomitrella D14/KAI2-like proteins and describe a loop-based mechanism that leads to a permanent widening of the hydrophobic substrate gorge. We have identified protein clades that specifically perceive the karrikin KAR₁ and the non-natural strigolactone isomer (-)-5-deoxystrigol in a highly stereoselective manner.

SMAX1/SMXL2 regulate root and root hair development downstream of KAI2-mediated signalling in Arabidopsis

Karrikins are smoke-derived compounds presumed to mimic endogenous signalling molecules (KAI2-ligand, KL), whose signalling pathway is closely related to that of strigolactones (SLs), important regulators of plant development. Both karrikins/KLs and SLs are perceived by closely related α/β hydrolase receptors (KAI2 and D14 respectively), and signalling through both receptors requires the F-box protein MAX2. Furthermore, both pathways trigger proteasome-mediated degradation of related SMAX1-LIKE (SMXL) proteins, to influence development. It has previously been suggested in multiple studies that SLs are important regulators of root and root hair development in Arabidopsis, but these conclusions are based on phenotypes observed in the non-specific max2 mutants and by use of racemic-GR24, a mixture of stereoisomers that activates both D14 and KAI2 signalling pathways. Here, we demonstrate that the majority of the effects on Arabidopsis root development previously attributed to SL signalling are actually mediated by the KAI2 signalling pathway. Using mutants defective in SL or KL synthesis and/or perception, we show that KAI2-mediated signalling alone regulates root hair density and root hair length as well as root skewing, straightness and diameter, while both KAI2 and D14 pathways regulate lateral root density and epidermal cell length. We test the key hypothesis that KAI2 signals by a non-canonical receptor-target mechanism in the context of root development. Our results provide no evidence for this, and we instead show that all effects of KAI2 in the root can be explained by canonical SMAX1/SMXL2 activity. However, we do find evidence for non-canonical GR24 ligand-receptor interactions in D14/KAI2-mediated root hair development. Overall, our results demonstrate that the KAI2 signalling pathway is an important new regulator of root hair and root development in Arabidopsis and lay an important basis for research into a molecular understanding of how very similar and partially overlapping hormone signalling pathways regulate different phenotypic outputs.

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.

Strigolactone perception and deactivation by a hydrolase receptor DWARF14

The perception mechanism for the strigolactone (SL) class of plant hormones has been a subject of debate because their receptor, DWARF14 (D14), is an α/β-hydrolase that can cleave SLs. Here we show via time-course analyses of SL binding and hydrolysis by Arabidopsis thaliana D14, that the level of uncleaved SL strongly correlates with the induction of the active signaling state. In addition, we show that an AtD14D218A catalytic mutant that lacks enzymatic activity is still able to complement the atd14 mutant phenotype in an SL-dependent manner. We conclude that the intact SL molecules trigger the D14 active signaling state, and we also describe that D14 deactivates bioactive SLs by the hydrolytic degradation after signal transmission. Together, these results reveal that D14 is a dual-functional receptor, responsible for both the perception and deactivation of bioactive SLs.

Strigolactone perception and deactivation by a hydrolase receptor DWARF14

The perception mechanism for the strigolactone (SL) class of plant hormones has been a subject of debate because their receptor, DWARF14 (D14), is an α/β-hydrolase that can cleave SLs. Here we show via time-course analyses of SL binding and hydrolysis by Arabidopsis thaliana D14, that the level of uncleaved SL strongly correlates with the induction of the active signaling state. In addition, we show that an AtD14^D218A catalytic mutant that lacks enzymatic activity is still able to complement the atd14 mutant phenotype in an SL-dependent manner. We conclude that the intact SL molecules trigger the D14 active signaling state, and we also describe that D14 deactivates bioactive SLs by the hydrolytic degradation after signal transmission. Together, these results reveal that D14 is a dual-functional receptor, responsible for both the perception and deactivation of bioactive SLs.

Functional Insights into the Roles of Hormones in the Dendrobium officinale-Tulasnella sp. Germinated Seed Symbiotic Association

Dendrobium is one of the largest genera in the Orchidaceae, and D. officinale is used in traditional medicine, particularly in China. D. officinale seeds are minute and contain limited energy reserves, and colonization by a compatible fungus is essential for germination under natural conditions. When the orchid mycorrhizal fungi (OMF) initiates symbiotic interactions with germination-driven orchid seeds, phytohormones from the orchid or the fungus play key roles, but the details of the possible biochemical pathways are still poorly understood. In the present study, we established a symbiotic system between D. officinale and Tulasnella sp. for seed germination. RNA-Seq was used to construct libraries of symbiotic-germinated seeds (DoTc), asymbiotic-germinated seeds (Do), and free-living OMF (Tc) to investigate the expression profiles of biosynthesis and metabolism pathway genes for three classes of endogenous hormones: JA (jasmonic acid), ABA (abscisic acid) and SLs (strigolactones), in D. officinale seeds and OMF under symbiotic and asymbiotic conditions. Low concentrations of endogenous JA, ABA, or SLs were detected in the D. officinale-Tulasnella symbiont compared with the asymbiotic tissues. Gene annotation results suggest that the expression of DEGs (differentially expressed genes) related to JA and ABA biosynthesis from D. officinale were down-regulated, while most of the key DEGs related to SL biosynthesis from D. officinale were up-regulated in the symbiotic germinated seeds compared with the asymbiotic germinated seeds. Moreover, in the OMF, we found a significantly up-regulated differential expression of the JA and ABA biosynthesis-related genes in the symbiotic interaction, with the opposite expression trends to those found in Dendrobium. This indicates that Dendrobium seed symbiotic germination may be stimulated by the apparent involvement of the OMF in the production of hormones, and relatively low concentrations of endogenous JA, ABA, or SLs might be maintained to promote the growth of the D. officinale-Tulasnella symbiotic protocorm-like body. These results will increase our understanding of the possible roles played by endogenous hormones in the regulation of the orchid-fungus symbiosis.

An allelic series at the KARRIKIN INSENSITIVE 2 locus of Arabidopsis thaliana decouples ligand hydrolysis and receptor degradation from downstream signalling

Karrikins are butenolide compounds present in post-fire environments that can stimulate seed germination in many species, including Arabidopsis thaliana. Plants also produce endogenous butenolide compounds that serve as hormones, namely strigolactones (SLs). The receptor for karrikins (KARRIKIN INSENSITIVE 2; KAI2) and the receptor for SLs (DWARF14; D14) are homologous proteins that share many similarities. The mode of action of D14 as a dual enzyme receptor protein is well established, but the nature of KAI2-dependent signalling and its function as a receptor are not fully understood. To expand our knowledge of how KAI2 operates, we screened ethyl methanesulphonate (EMS)-mutagenized populations of A. thaliana for mutants with kai2-like phenotypes and isolated 13 new kai2 alleles. Among these alleles, kai2-10 encoded a D184N protein variant that was stable in planta. Differential scanning fluorimetry assays indicated that the KAI2 D184N protein could interact normally with bioactive ligands. We developed a KAI2-active version of the fluorescent strigolactone analogue Yoshimulactone Green to show that KAI2 D184N exhibits normal rates of ligand hydrolysis. KAI2 D184N degraded in response to treatment with exogenous ligands, suggesting that receptor degradation is a consequence of ligand binding and hydrolysis, but is insufficient for signalling activity. Remarkably, KAI2 D184N degradation was hypersensitive to karrikins, but showed a normal response to strigolactone analogues, implying that these butenolides may interact differently with KAI2. These results demonstrate that the enzymatic and signalling functions of KAI2 can be decoupled, and provide important insights into the mechanistic events that underpin butenolide signalling in plants.

Bioconversion of 5-deoxystrigol stereoisomers to monohydroxylated strigolactones by plants

The bioconversion of 5-deoxystrigol (5DS) and 4-deoxyorobanchol (4DO), the simplest canonical strigolactones (SLs), into monohydroxylated SLs such as strigol, sorgomol and orobanchol was confirmed by administering of stable isotope-labeled substrates to hydroponically grown plants. Liquid chromatography-mass spectrometry analyses established that 5DS was stereoselectively converted into strigol and sorgomol by cotton (Gossypium hirsutum) and Chinese milk vetch (Astragalus sinicus), respectively. 4DO was converted into orobanchol by rice (Oryza sativa). However, the red bell pepper (Capsicum annuum), red clover (Trifolium pratense), and pea (Pisum sativum) negligibly converted 4DO into orobanchol. The red bell pepper converted ent-4DO into 2',8-bisepi-sorgomol. These results suggest that some plants generate orobanchol without passing through 4DO.

Rationally Designed Strigolactone Analogs as Antagonists of the D14 Receptor

Strigolactones (SLs) are plant hormones that inhibit shoot branching and act as signals in communications with symbiotic fungi and parasitic weeds in the rhizosphere. SL signaling is mediated by DWARF14 (D14), which is an α/β-hydrolase that cleaves SLs into an ABC tricyclic lactone and a butenolide group (i.e. D-ring). This cleavage reaction (hydrolysis and dissociation) is important for inducing the interaction between D14 and its target proteins, including D3 and D53. In this study, a hydrolysis-resistant SL analog was predicted to inhibit the activation of the D14 receptor, thereby disrupting the SL signaling pathway. To test this prediction, carba-SL compounds, in which the ether oxygen of the D-ring or the phenol ether oxygen of the SL agonist (GR24 or 4-bromo debranone) was replaced with a methylene group, were synthesized as novel D14 antagonists. Subsequent biochemical and physiological studies indicated that carba-SLs blocked the interaction between D14 and D53 by inhibiting D14 hydrolytic activity. They also suppressed the SL-induced inhibition of rice tiller outgrowths. Additionally, carba-SLs antagonized the SL response in a Striga parasitic weed species. Structural analyses revealed that the D-ring of 7'-carba-4BD was hydrolyzed by D14 but did not dissociate from the 4BD skeleton. Thus, 7'-carba-4BD functioned as an antagonist rather than an agonist. Thus, the hydrolysis of the D-ring of SLs may be insufficient for activating the receptor. This study provides data relevant to designing SL receptor antagonists.

Rice DWARF14 acts as an unconventional hormone receptor for strigolactone

Strigolactones (SLs) act as an important class of phytohormones to regulate plant shoot branching, and also serve as rhizosphere signals to mediate interactions of host plants with soil microbes and parasitic weeds. SL receptors in dicots, such as DWARF14 in Arabidopsis (AtD14), RMS3 in pea, and ShHTL7 in Striga, serve as unconventional receptors that hydrolyze SLs into a D-ring-derived intermediate CLIM and irreversibly bind CLIM to trigger SL signal transduction. Here, we show that D14 from the monocot rice can complement Arabidopsis d14 mutant and interact with the SL signaling components in Arabidopsis. Our results further reveal that rice D14, similar to SL receptors in dicots, also serves as an unconventional hormone receptor that generates and irreversibly binds the active form of SLs. These findings uncover the conserved functions of D14 proteins in monocots and dicots.

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.

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.

Upregulation of DWARF27 is associated with increased strigolactone levels under sulfur deficiency in rice

Plants produce strigolactones (SLs) in roots in response to nitrogen or phosphate deficiency. To evaluate SL levels under other mineral deficiencies in rice, we cultivated rice seedlings in hydroponic media without nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and iron. Tiller bud outgrowth was stimulated under calcium deficiency because of low SL levels. SL levels increased under sulfur deficiency, in addition to phosphate, and nitrogen deficiencies. To explore which genes are key regulators of SL production under sulfur deficiency, we analyzed the expression of SL-related genes in sulfur-sufficient and sulfur-deficient conditions. An SL biosynthesis gene, DWARF27 (D27), was strongly expressed under sulfur deficiency, and its expression was decreased by sulfur supply. The levels of D10, D17, and OsMAX1 transcripts did not differ between sulfur-sufficient and sulfur-deficient conditions. These results suggest that the increased SL levels under sulfur deficiency are due to a high expression of D27. A combination of nitrogen, phosphorus, and sulfur deficiencies had no additive synergistic effect on SL production. Under combined phosphorus and sulfur deficiency, the expression levels of most SL biosynthesis genes were elevated. The number of tiller buds in the d27 mutant was higher than in the wild type, but lower than in other d mutants. Under sulfur deficiency, the chlorophyll content of d27 was lower than those of other d mutants. These results indicate that D27 plays an important role in adaptation to sulfur deficiency in rice.

The karrikin receptor KAI2 promotes drought resistance in Arabidopsis thaliana

Drought causes substantial reductions in crop yields worldwide. Therefore, we set out to identify new chemical and genetic factors that regulate drought resistance in Arabidopsis thaliana. Karrikins (KARs) are a class of butenolide compounds found in smoke that promote seed germination, and have been reported to improve seedling vigor under stressful growth conditions. Here, we discovered that mutations in KARRIKIN INSENSITIVE2 (KAI2), encoding the proposed karrikin receptor, result in hypersensitivity to water deprivation. We performed transcriptomic, physiological and biochemical analyses of kai2 plants to understand the basis for KAI2-regulated drought resistance. We found that kai2 mutants have increased rates of water loss and drought-induced cell membrane damage, enlarged stomatal apertures, and higher cuticular permeability. In addition, kai2 plants have reduced anthocyanin biosynthesis during drought, and are hyposensitive to abscisic acid (ABA) in stomatal closure and cotyledon opening assays. We identified genes that are likely associated with the observed physiological and biochemical changes through a genome-wide transcriptome analysis of kai2 under both well-watered and dehydration conditions. These data provide evidence for crosstalk between ABA- and KAI2-dependent signaling pathways in regulating plant responses to drought. A comparison of the strigolactone receptor mutant d14 (DWARF14) to kai2 indicated that strigolactones also contributes to plant drought adaptation, although not by affecting cuticle development. Our findings suggest that chemical or genetic manipulation of KAI2 and D14 signaling may provide novel ways to improve drought resistance.

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.

Strigolactones Biosynthesis and Their Role in Abiotic Stress Resilience in Plants: A Critical Review

Strigolactones (SLs), being a new class of plant hormones, play regulatory roles against abiotic stresses in plants. There are multiple hormonal response pathways, which are adapted by the plants to overcome these stressful environmental constraints to reduce the negative impact on overall crop plant productivity. Genetic modulation of the SLs could also be applied as a potential approach in this regard. However, endogenous plant hormones play central roles in adaptation to changing environmental conditions, by mediating growth, development, nutrient allocation, and source/sink transitions. In addition, the hormonal interactions can fine-tune the plant response and determine plant architecture in response to environmental stimuli such as nutrient deprivation and canopy shade. Considerable advancements and new insights into SLs biosynthesis, signaling and transport has been unleashed since the initial discovery. In this review we present basic overview of SL biosynthesis and perception with a detailed discussion on our present understanding of SLs and their critical role to tolerate environmental constraints. The SLs and abscisic acid interplay during the abiotic stresses is particularly highlighted. Main Conclusion: More than shoot branching Strigolactones have uttermost capacity to harmonize stress resilience.

Chemical genetics and strigolactone perception

Strigolactones (SLs) are a collection of related small molecules that act as hormones in plant growth and development. Intriguingly, SLs also act as ecological communicators between plants and mycorrhizal fungi and between host plants and a collection of parasitic plant species. In the case of mycorrhizal fungi, SLs exude into the soil from host roots to attract fungal hyphae for a beneficial interaction. In the case of parasitic plants, however, root-exuded SLs cause dormant parasitic plant seeds to germinate, thereby allowing the resulting seedling to infect the host and withdraw nutrients. Because a laboratory-friendly model does not exist for parasitic plants, researchers are currently using information gleaned from model plants like Arabidopsis in combination with the chemical probes developed through chemical genetics to understand SL perception of parasitic plants. This work first shows that understanding SL signaling is useful in developing chemical probes that perturb SL perception. Second, it indicates that the chemical space available to probe SL signaling in both model and parasitic plants is sizeable. Because these parasitic pests represent a major concern for food insecurity in the developing world, there is great need for chemical approaches to uncover novel lead compounds that perturb parasitic plant infections.