Conversion of carlactone to carlactonoic acid is a conserved function of MAX1 homologs in strigolactone biosynthesis

Strigolactones: New players in the nitrogen-phosphorus signalling interplay

Nitrogen (N) and phosphorus (P) are among the most important macronutrients for plant growth and development, and the most widely used as fertilizers. Understanding how plants sense and respond to N and P deficiency is essential to optimize and reduce the use of chemical fertilizers. Strigolactones (SLs) are phytohormones acting as modulators and sensors of plant responses to P deficiency. In the present work, we assess the potential role of SLs in N starvation and in the N-P signalling interplay. Physiological, transcriptional and metabolic responses were analysed in wild-type and SL-deficient tomato plants grown under different P and N regimes, and in plants treated with a short-term pulse of the synthetic SL analogue 2'-epi-GR24. The results evidence that plants prioritize N over P status by affecting SL biosynthesis. We also show that SLs modulate the expression of key regulatory genes of phosphate and nitrate signalling pathways, including the N-P integrators PHO2 and NIGT1/HHO. The results support a key role for SLs as sensors during early plant responses to both N and phosphate starvation and mediating the N-P signalling interplay, indicating that SLs are involved in more physiological processes than so far proposed.

Origins and evolution of the dual functions of strigolactones as rhizosphere signaling molecules and plant hormones

Strigolactones (SLs) play roles as a class of plant hormones and rhizosphere signaling chemicals that induce hyphal branching of arbuscular mycorrhizal fungi and seed germination of parasitic plants. Therefore, SLs have dual functions. Recent progress in genome sequencing and genetic studies of bryophytes and algae has begun to shed light on the origin and evolution of these two functions of SLs.

Strigolactone biosynthesis catalyzed by cytochrome P450 and sulfotransferase in sorghum

Root parasitic plants such as Striga, Orobanche, and Phelipanche spp. cause serious damage to crop production world-wide. Deletion of the Low Germination Stimulant 1 (LGS1) gene gives a Striga-resistance trait in sorghum (Sorghum bicolor). The LGS1 gene encodes a sulfotransferase-like protein, but its function has not been elucidated. Since the profile of strigolactones (SLs) that induce seed germination in root parasitic plants is altered in the lgs1 mutant, LGS1 is thought to be an SL biosynthetic enzyme. In order to clarify the enzymatic function of LGS1, we looked for candidate SL substrates that accumulate in the lgs1 mutants and performed in vivo and in vitro metabolism experiments. We found the SL precursor 18-hydroxycarlactonoic acid (18-OH-CLA) is a substrate for LGS1. CYP711A cytochrome P450 enzymes (SbMAX1 proteins) in sorghum produce 18-OH-CLA. When LGS1 and SbMAX1 coding sequences were co-expressed in Nicotiana benthamiana with the upstream SL biosynthesis genes from sorghum, the canonical SLs 5-deoxystrigol and 4-deoxyorobanchol were produced. This finding showed that LGS1 in sorghum uses a sulfo group to catalyze leaving of a hydroxyl group and cyclization of 18-OH-CLA. A similar SL biosynthetic pathway has not been found in other plant species.

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.

The Physcomitrium (Physcomitrella) patens PpKAI2L receptors for strigolactones and related compounds function via MAX2-dependent and -independent pathways

In angiosperms, the α/β hydrolase DWARF14 (D14), along with the F-box protein MORE AXILLARY GROWTH2 (MAX2), perceives strigolactones (SL) to regulate developmental processes. The key SL biosynthetic enzyme CAROTENOID CLEAVAGE DIOXYGENASE8 (CCD8) is present in the moss Physcomitrium patens, and PpCCD8-derived compounds regulate moss extension. The PpMAX2 homolog is not involved in the SL response, but 13 PpKAI2LIKE (PpKAI2L) genes homologous to the D14 ancestral paralog KARRIKIN INSENSITIVE2 (KAI2) encode candidate SL receptors. In Arabidopsis thaliana, AtKAI2 perceives karrikins and the elusive endogenous KAI2-Ligand (KL). Here, germination assays of the parasitic plant Phelipanche ramosa suggested that PpCCD8-derived compounds are likely noncanonical SLs. (+)-GR24 SL analog is a good mimic for PpCCD8-derived compounds in P. patens, while the effects of its enantiomer (-)-GR24, a KL mimic in angiosperms, are minimal. Interaction and binding assays of seven PpKAI2L proteins pointed to the stereoselectivity toward (-)-GR24 for a single clade of PpKAI2L (eu-KAI2). Enzyme assays highlighted the peculiar behavior of PpKAI2L-H. Phenotypic characterization of Ppkai2l mutants showed that eu-KAI2 genes are not involved in the perception of PpCCD8-derived compounds but act in a PpMAX2-dependent pathway. In contrast, mutations in PpKAI2L-G, and -J genes abolished the response to the (+)-GR24 enantiomer, suggesting that PpKAI2L-G, and -J proteins are receptors for moss SLs.

Harmony but Not Uniformity: Role of Strigolactone in Plants

Strigolactones (SLs) represent an important new plant hormone class marked by their multifunctional roles in plants and rhizosphere interactions, which stimulate hyphal branching in arbuscular mycorrhizal fungi (AMF) and seed germination of root parasitic plants. SLs have been broadly implicated in regulating root growth, shoot architecture, leaf senescence, nodulation, and legume-symbionts interaction, as well as a response to various external stimuli, such as abiotic and biotic stresses. These functional properties of SLs enable the genetic engineering of crop plants to improve crop yield and productivity. In this review, the conservation and divergence of SL pathways and its biological processes in multiple plant species have been extensively discussed with a particular emphasis on its interactions with other different phytohormones. These interactions may shed further light on the regulatory networks underlying plant growth, development, and stress responses, ultimately providing certain strategies for promoting crop yield and productivity with the challenges of global climate and environmental changes.

Strigolactones, how are they synthesized to regulate plant growth and development?

Strigolactones (SLs) are multifunctional plant metabolites working not only as allelochemicals in the rhizosphere, but also as a novel class of hormones regulating growth and development in planta. To date, more than 30 SLs have been characterized, but the reason why plants produce structurally diverse SLs and the details of their biosynthetic pathway remain elusive. Recent studies using transcriptomics and reverse genetic techniques have paved the way to clarify the entire biosynthetic pathway of structurally diverse SLs. In this review, we discuss how various SLs are synthesized and what SL structural diversity means for plant growth and development.

Specific methylation of (11R)-carlactonoic acid by an Arabidopsis SABATH methyltransferase

An Arabidopsis S-adenosyl-L-methionine-dependent methyltransferase belonging to the SABATH family catalyzes the specific carboxymethylation of (11R)-carlactonoic acid. Methyl carlactonoate (MeCLA), found in Arabidopsis (Arabidopsis thaliana) as a non-canonical strigolactone (SL), may be a biosynthetic intermediate of various non-canonical SLs and biologically active as a plant hormone. MeCLA is formed from carlactonoic acid (CLA), but the methyltransferases (MTs) converting CLA to MeCLA remain unclear. Previous studies have demonstrated that the carboxymethylation of acidic plant hormones is catalyzed by the same protein family, the SABATH family (Wang et al. in Evol Bioinform 15:117693431986086. https://doi.org/10.1177/1176934319860864 , 2019). In the present study, we focused on the At4g36470 gene, an Arabidopsis SABATH MT gene co-expressed with the MAX1 gene responsible for CLA formation for biochemical characterization. The recombinant At4g36470 protein expressed in Escherichia coli exhibited exclusive activity against naturally occurring (11R)-CLA among the substrates, including CLA enantiomers and a variety of acidic plant hormones. The apparent K_m value for (11R)-CLA was 1.46 μM, which was relatively smaller than that of the other Arabidopsis SABATH MTs responsible for the carboxymethylation of acidic plant hormones. The strict substrate specificity and high affinity of At4g36470 suggested it is an (11R)-CLA MT. We also confirmed the function of the identified gene by reconstructing MeCLA biosynthesis using transient expression in Nicotiana benthamiana. Phylogenetic analysis demonstrated that At4g36470 and its orthologs in non-canonical SL-producing plants cluster together in an exclusive clade, suggesting that the SABATH MTs of this clade may be involved in the carboxymethylation of CLA and the biosynthesis of non-canonical SLs.

Cytochrome P450 Superfamily: Evolutionary and Functional Divergence in Sorghum (Sorghum bicolor) Stress Resistance

Cytochrome P450 (CYP) genes encode enzymes that catalyze various growth-, development-, and stress-related reactions. Sorghum (Sorghum bicolor) is a type of C4 plant and an important cash crop. However, systematic identification and analysis of functional differentiation and evolution of CYP genes have not been carried out in this species. In the present study, we revealed that the sorghum genome contains 351 CYP genes, which can be divided into nine classes. These genes are from ancestors and repeated segments, rather than tandem repeats. Based on collinearity results, a large number of CYPs were extended before cotyledon differentiation, during the emergence of Gramineae, suggesting that genomewide duplication events and stress adaptation processes were important for the expansion of CYP genes. Their gene structure and motifs contain conserved regions and include various changes and loci. The expression characteristics and functional annotation of CYP genes indicated tissue specificity and selective expression. Overall, we identified all CYP genes in the sorghum genome and preliminarily explored their naming, structure, evolution, expression, and functional differentiation. The results advanced our understanding of plant gene family evolution and functional differentiation.

Establishment of strigolactone-producing bacterium-yeast consortium

Strigolactones (SLs) are a class of phytohormones playing diverse roles in plant growth and development, yet the limited access to SLs is largely impeding SL-based foundational investigations and applications. Here, we developed Escherichia coli–Saccharomyces cerevisiae consortia to establish a microbial biosynthetic platform for the synthesis of various SLs, including carlactone, carlactonoic acid, 5-deoxystrigol (5DS; 6.65 ± 1.71 μg/liter), 4-deoxyorobanchol (3.46 ± 0.28 μg/liter), and orobanchol (OB; 19.36 ± 5.20 μg/liter). The SL-producing platform enabled us to conduct functional identification of CYP722Cs from various plants as either OB or 5DS synthase. It also allowed us to quantitatively compare known variants of plant SL biosynthetic enzymes in the microbial system. The titer of 5DS was further enhanced through pathway engineering to 47.3 μg/liter. This work provides a unique platform for investigating SL biosynthesis and evolution and lays the foundation for developing SL microbial production process.

Major components of the KARRIKIN INSENSITIVE2-dependent signaling pathway are conserved in the liverwort Marchantia polymorpha

KARRIKIN INSENSITIVE2 (KAI2) was first identified as a receptor of karrikins, smoke-derived germination stimulants. KAI2 is also considered a receptor of an unidentified endogenous molecule called the KAI2 ligand. Upon KAI2 activation, signals are transmitted through the degradation of D53/SMXL proteins via MAX2-dependent ubiquitination. Although components in the KAI2-dependent signaling pathway, namely MpKAI2A and MpKAI2B, MpMAX2, and MpSMXL, exist in the genome of the liverwort Marchantia polymorpha, their functions remain unknown. Here, we show that early thallus growth is retarded and gemma dormancy in the dark is suppressed in Mpkai2a and Mpmax2 loss-of-function mutants. These defects are counteracted in Mpkai2a Mpsmxl and Mpmax2 Mpsmxl double mutants indicating that MpKAI2A, MpMAX2, and MpSMXL act in the same genetic pathway. Introduction of MpSMXLd53, in which a domain required for degradation is mutated, into wild-type plants mimicks Mpkai2a and Mpmax2 plants. In addition, the detection of citrine fluorescence in Nicotiana benthamiana cells transiently expressing a SMXL-Citrine fusion protein requires treatment with MG132, a proteasome inhibitor. These findings imply that MpSMXL is subjected to degradation, and that the degradation of MpSMXL is crucial for MpKAI2A-dependent signaling in M. polymorpha. Therefore, we claim that the basic mechanisms in the KAI2-dependent signaling pathway are conserved in M. polymorpha.

Plant cytochrome P450 plasticity and evolution

The superfamily of cytochrome P450 (CYP) enzymes plays key roles in plant evolution and metabolic diversification. This review provides a status on the CYP landscape within green algae and land plants. The 11 conserved CYP clans known from vascular plants are all present in green algae and several green algae-specific clans are recognized. Clan 71, 72, and 85 remain the largest CYP clans and include many taxa-specific CYP (sub)families reflecting emergence of linage-specific pathways. Molecular features and dynamics of CYP plasticity and evolution are discussed and exemplified by selected biosynthetic pathways. High substrate promiscuity is commonly observed for CYPs from large families, favoring retention of gene duplicates and neofunctionalization, thus seeding acquisition of new functions. Elucidation of biosynthetic pathways producing metabolites with sporadic distribution across plant phylogeny reveals multiple examples of convergent evolution where CYPs have been independently recruited from the same or different CYP families, to adapt to similar environmental challenges or ecological niches. Sometimes only a single or a few mutations are required for functional interconversion. A compilation of functionally characterized plant CYPs is provided online through the Plant P450 Database (erda.dk/public/vgrid/PlantP450/).

Strigolactones regulate sepal senescence in Arabidopsis

Flower sepals are critical for flower development and vary greatly in life span depending on their function post-pollination. Very little is known about what controls sepal longevity. Using a sepal senescence mutant screen, we identified two Arabidopsis mutants with delayed senescence directly connecting strigolactones with senescence regulation in a novel floral context that hitherto has not been explored. The mutations were in the strigolactone biosynthetic gene MORE AXILLARY GROWTH1 (MAX1) and in the strigolactone receptor gene DWARF14 (AtD14). The mutation in AtD14 changed the catalytic Ser97 to Phe in the enzyme active site, which is the first mutation of its kind in planta. The lesion in MAX1 was in the haem-iron ligand signature of the cytochrome P450 protein, converting the highly conserved Gly469 to Arg, which was shown in a transient expression assay to substantially inhibit the activity of MAX1. The two mutations highlighted the importance of strigolactone activity for driving to completion senescence initiated both developmentally and in response to carbon-limiting stress, as has been found for the more well-known senescence-associated regulators ethylene and abscisic acid. Analysis of transcript abundance in excised inflorescences during an extended night suggested an intricate relationship among sugar starvation, senescence, and strigolactone biosynthesis and signalling.

Targeted mutagenesis of two homologous ATP-binding cassette subfamily G (ABCG) genes in tomato confers resistance to parasitic weed Phelipanche aegyptiaca

Phelipanche aegyptiaca and Orobanche spp. are obligate plant root-parasitic weeds that cause extensive damage in agricultural crop plants. Their germination requires exposure to strigolactones (SLs) exuded by the host plant roots. Here we studied genes in the host plant tomato involved in SL exudation and their impact on parasitic weeds. We provide evidence that CRISPR/Cas9-mediated targeted mutagenesis of two homologous ATP-binding cassette subfamily G (ABCG) genes, ABCG44 (Solyc08g067610) and ABCG45 (Solyc08g067620), in tomato significantly reduces SLs in the root exudate and abolishes germination of the root-parasitic weed P. aegyptiaca. Based on genome sequence similarity between ABCG44 and ABCG45, a 20-bp target sequence in their exon region was selected to design single guide RNA targeting both genes using CRISPR/Cas9. The plant binary vector constructs harboring the specific Cas9 and single guide RNA were transformed into tomato. Selected T₀ mutated tomato plants showed different types of deletions at both gene loci. Genotype analysis of T₁ plants suggested stable inheritance of the introduced mutations without any potential off-target effects. The phenotype of Cas9-mutated plants included increased shoot branching and growth of axillary buds, and reduced length of primary stems. Interestingly, reduced germination of P. aegyptiaca resulted from a decrease in the SL orobanchol in the root exudate of Cas9-mutated plants; however, orobanchol content in the root extract was unchanged compared to control plants. Moreover, in single and double ABCG mutants, expression of the SL-biosynthesis genes CCD8 and MAX1 decreased. The current study offers insights into CRISPR-mediated mutagenesis of ABCG genes, which could serve as an efficient control method to prevent root-parasitic weed germination.

The mechanism of host-induced germination in root parasitic plants

Chemical signals known as strigolactones (SLs) were discovered more than 50 years ago as host-derived germination stimulants of parasitic plants in the Orobanchaceae. Strigolactone-responsive germination is an essential adaptation of obligate parasites in this family, which depend upon a host for survival. Several species of obligate parasites, including witchweeds (Striga, Alectra spp.) and broomrapes (Orobanche, Phelipanche spp.), are highly destructive agricultural weeds that pose a significant threat to global food security. Understanding how parasites sense SLs and other host-derived stimulants will catalyze the development of innovative chemical and biological control methods. This review synthesizes the recent discoveries of strigolactone receptors in parasitic Orobanchaceae, their signaling mechanism, and key steps in their evolution.

Adaptation of the parasitic plant lifecycle: germination is controlled by essential host signaling molecules

Parasitic plants are plants that connect with a haustorium to the vasculature of another, host, plant from which they absorb water, assimilates, and nutrients. Because of this parasitic lifestyle, parasitic plants need to coordinate their lifecycle with that of their host. Parasitic plants have evolved a number of host detection/host response mechanisms of which the germination in response to chemical host signals in one of the major families of parasitic plants, the Orobanchaceae, is a striking example. In this update review, we discuss these germination stimulants. We review the different compound classes that function as germination stimulants, how they are produced, and in which host plants. We discuss why they are reliable signals, how parasitic plants have evolved mechanisms that detect and respond to them, and whether they play a role in host specificity. The advances in the knowledge underlying this signaling relationship between host and parasitic plant have greatly improved our understanding of the evolution of plant parasitism and are facilitating the development of more effective control measures in cases where these parasitic plants have developed into weeds.

The P450 multigene family of Fontainea and insights into diterpenoid synthesis

BACKGROUND

Cytochrome P450s (P450s) are enzymes that play critical roles in the biosynthesis of physiologically important compounds across all organisms. Although they have been characterised in a large number of plant species, no information relating to these enzymes are available from the genus Fontainea (family Euphorbiaceae). Fontainea is significant as the genus includes species that produce medicinally significant epoxy-tigliane natural products, one of which has been approved as an anti-cancer therapeutic.

RESULTS

A comparative species leaf metabolome analysis showed that Fontainea species possess a chemical profile different from various other plant species. The diversity and expression profiles of Fontainea P450s were investigated from leaf and root tissue. A total of 103 and 123 full-length P450 genes in Fontainea picrosperma and Fontainea venosa, respectively (and a further 127/125 partial-length) that were phylogenetically classified into clans, families and subfamilies. The majority of P450 identified are most active within root tissue (66.2% F. picrosperma, 65.0% F. venosa). Representatives within the CYP71D and CYP726A were identified in Fontainea that are excellent candidates for diterpenoid synthesis, of which CYP726A1, CYP726A2 and CYP71D1 appear to be exclusive to Fontainea species and were significantly more highly expressed in root tissue compared to leaf tissue.

CONCLUSION

This study presents a comprehensive overview of the P450 gene family in Fontainea that may provide important insights into the biosynthesis of the medicinally significant epoxy-tigliane diterpenes found within the genus.

Association of TaD14-4D, a Gene Involved in Strigolactone Signaling, with Yield Contributing Traits in Wheat

Tillering is a crucial agronomic trait of wheat; it determines yield and plant architecture. Strigolactones (SLs) have been reported to inhibit plant branching. D14, a receptor of SLs, has been described to affect tillering in rice, yet it has seldomly been studied in wheat. In this study, three TaD14 homoeologous genes, TaD14-4A, TaD14-4B, and TaD14-4D, were identified. TaD14-4A, TaD14-4B, and TaD14-4D were constitutively expressed, and TaD14-4D had a higher expression level in most tissues. TaD14 proteins were localized in both cytoplasm and nucleus. An SNP and a 22 bp insertion/deletion (Indel) at the exon regions of TaD14-4D were detected, forming three haplotypes, namely 4D-HapI, 4D-HapII, and 4D-HapIII. Due to the frameshift mutation in the coding region of 4D-HapII, the interaction of 4D-HapII with TaMAX2 and TaD53 was blocked, which led to the blocking of SL signal transduction. Based on the two variation sites, two molecular markers, namely dCAPS-250 and Indel-747, were developed. Association analysis suggested that haplotypes of TaD14-4D were associated with effective tillering number (ETN) and thousand kernel weight (TKW) simultaneously in four environments. The favorable haplotype 4D-HapIII underwent positive selection in global wheat breeding. This study provides insights into understanding the function of natural variations of TaD14-4D and develops two useful molecular markers for wheat breeding.

Identification and characterization of sorgomol synthase in sorghum strigolactone biosynthesis

Strigolactones (SLs), first identified as germination stimulants for root parasitic weeds, act as endogenous phytohormones regulating shoot branching and as root-derived signal molecules mediating symbiotic communications in the rhizosphere. Canonical SLs typically have an ABCD ring system and can be classified into orobanchol- and strigol-type based on the C-ring stereochemistry. Their simplest structures are 4-deoxyorobanchol (4DO) and 5-deoxystrigol (5DS), respectively. Diverse canonical SLs are chemically modified with one or more hydroxy or acetoxy groups introduced into the A- and/or B-ring of these simplest structures, but the biochemical mechanisms behind this structural diversity remain largely unexplored. Sorgomol in sorghum (Sorghum bicolor [L.] Moench) is a strigol-type SL with a hydroxy group at C-9 of 5DS. In this study, we characterized sorgomol synthase. Microsomal fractions prepared from a high-sorgomol-producing cultivar of sorghum, Sudax, were shown to convert 5DS to sorgomol. A comparative transcriptome analysis identified SbCYP728B subfamily as candidate genes encoding sorgomol synthase. Recombinant SbCYP728B35 catalyzed the conversion of 5DS to sorgomol in vitro. Substrate specificity revealed that the C-8bS configuration in the C-ring of 5DS stereoisomers was essential for this reaction. The overexpression of SbCYP728B35 in Lotus japonicus hairy roots, which produce 5DS as an endogenous SL, also resulted in the conversion of 5DS to sorgomol. Furthermore, SbCYP728B35 expression was not detected in nonsorgomol-producing cultivar, Abu70, suggesting that this gene is responsible for sorgomol production in sorghum. Identification of the mechanism modifying parental 5DS of strigol-type SLs provides insights on how plants biosynthesize diverse SLs.

CRISPR/Cas9 mediated mutagenesis of MORE AXILLARY GROWTH 1 in tomato confers resistance to root parasitic weed Phelipanche aegyptiaca

Root parasitic weeds infect numerous economically important crops, affecting total yield quantity and quality. A lack of an efficient control method limits our ability to manage newly developing and more virulent races of root parasitic weeds. To control the parasite induced damage in most host crops, an innovative biotechnological approach is urgently required. Strigolactones (SLs) are plant hormones derived from carotenoids via a pathway involving the Carotenoid Cleavage Dioxygenase (CCD) 7, CCD8 and More Axillary Growth 1 (MAX1) genes. SLs act as branching inhibitory hormones and strictly required for the germination of root parasitic weeds. Here, we demonstrate that CRISPR/Cas9-mediated targted editing of SL biosynthetic gene MAX1, in tomato confers resistance against root parasitic weed Phelipanche aegyptiaca. We designed sgRNA to target the third exon of MAX1 in tomato plants using the CRISPR/Cas9 system. The T0 plants were edited very efficiently at the MAX1 target site without any non-specific off-target effects. Genotype analysis of T1 plants revealed that the introduced mutations were stably passed on to the next generation. Notably, MAX1-Cas9 heterozygous and homozygous T1 plants had similar morphological changes that include excessive growth of axillary bud, reduced plant height and adventitious root formation relative to wild type. Our results demonstrated that, MAX1-Cas9 mutant lines exhibit resistance against root parasitic weed P. aegyptiaca due to reduced SL (orobanchol) level. Moreover, the expression of carotenoid biosynthetic pathway gene PDS1 and total carotenoid level was altered, as compared to wild type plants. Taking into consideration, the impact of root parasitic weeds on the agricultural economy and the obstacle to prevent and eradicate them, the current study provides new aspects into the development of an efficient control method that could be used to avoid germination of root parasitic weeds.

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.

A Unique Sulfotransferase-Involving Strigolactone Biosynthetic Route in Sorghum

LOW GERMINATION STIMULANT 1 (LGS1) plays an important role in strigolactones (SLs) biosynthesis and Striga resistance in sorghum, but the catalytic function remains unclear. Using the recently developed SL-producing microbial consortia, we examined the activities of sorghum MORE AXILLARY GROWTH1 (MAX1) analogs and LGS1. Surprisingly, SbMAX1a (cytochrome P450 711A enzyme in sorghum) synthesized 18-hydroxy-carlactonoic acid (18-hydroxy-CLA) directly from carlactone (CL) through four-step oxidations. The further oxidated product orobanchol (OB) was also detected in the microbial consortium. Further addition of LGS1 led to the synthesis of both 5-deoxystrigol (5DS) and 4-deoxyorobanchol (4DO). Further biochemical characterization found that LGS1 functions after SbMAX1a by converting 18-hydroxy-CLA to 5DS and 4DO possibly through a sulfonation-mediated pathway. The unique functions of SbMAX1 and LGS1 imply a previously unknown synthetic route toward SLs.

Plant apocarotenoids: from retrograde signaling to interspecific communication

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

Evaluation and Quantification of Natural Strigolactones from Root Exudates

Strigolactones (SLs) in the root exudates can be detected by germination assays with root parasitic weed seeds, but precise and accurate evaluation and quantification are possible only by chemical analysis with the liquid chromatography-tandem mass spectrometry (LC-MS/MS). Here we describe methods for root exudate collection, sample preparation, and LC-MS/MS analysis of SLs.

ζ-Carotene Isomerase Suppresses Tillering in Rice through the Coordinated Biosynthesis of Strigolactone and Abscisic Acid

Rice tillering is an important agronomic trait affecting grain yield. Here, we identified a high-tillering mutant tillering20 (t20), which could be restored to the wild type by treatment with the strigolactone (SL) analog rac-GR24. T20 encodes a chloroplast ζ-carotene isomerase (Z-ISO), which is involved in the biosynthesis of carotenoids and their metabolites, SL and abscisic acid (ABA). The t20 mutant has reduced SL and ABA, raising the question of how SL and ABA biosynthesis is coordinated, and whether they have overlapping functions in tillering. We discovered that rac-GR24 stimulated T20 expression and enhanced all-trans-β-carotene biosynthesis. Importantly, rac-GR24 also stimulated expression of Oryza sativa 9-CIS-EPOXYCAROTENOID DIOXYGENASE 1 (OsNCED1) through induction of Oryza sativa HOMEOBOX12 (OsHOX12), promoting ABA biosynthesis in shoot base. On the other hand, ABA treatment significantly repressed SL biosynthesis and the ABA biosynthetic mutants displayed elevated SL biosynthesis. ABA treatment reduced the number of basal tillers in both t20 and wild-type plants. Furthermore, while ABA-deficient mutants aba1 and aba2 had the same number of basal tillers as wild type, they had more unproductive upper tillers at maturity. This work demonstrates complex interactions in the biosynthesis of carotenoid, SLs and ABA, and reveals a role for ABA in the regulation of rice tillering.

Triflumizole as a Novel Lead Compound for Strigolactone Biosynthesis Inhibitor

Strigolactones (SLs) are carotenoid-derived plant hormones involved in the development of various plants. SLs also stimulate seed germination of the root parasitic plants, Striga spp. and Orobanche spp., which reduce crop yield. Therefore, regulating SL biosynthesis may lessen the damage of root parasitic plants. Biosynthetic inhibitors effectively control biological processes by targeted regulation of biologically active compounds. In addition, biosynthetic inhibitors regulate endogenous levels in developmental stage- and tissue-specific manners. To date, although some chemicals have been found as SL biosynthesis inhibitor, these are derived from only three lead chemicals. In this study, to find a novel lead chemical for SL biosynthesis inhibitor, 27 nitrogen-containing heterocyclic derivatives were screened for inhibition of SL biosynthesis. Triflumizole most effectively reduced the levels of rice SL, 4-deoxyorobanchol (4DO), in root exudates. In addition, triflumizole inhibited endogenous 4DO biosynthesis in rice roots by inhibiting the enzymatic activity of Os900, a rice enzyme that converts the SL intermediate carlactone to 4DO. A Striga germination assay revealed that triflumizole-treated rice displayed a reduced level of germination stimulation for Striga. These results identify triflumizole as a novel lead compound for inhibition of SL biosynthesis.

Diverse Roles of MAX1 Homologues in Rice

Cytochrome P450 enzymes encoded by MORE AXILLARY GROWTH1 (MAX1)-like genes produce most of the structural diversity of strigolactones during the final steps of strigolactone biosynthesis. The diverse copies of MAX1 in Oryza sativa provide a resource to investigate why plants produce such a wide range of strigolactones. Here we performed in silico analyses of transcription factors and microRNAs that may regulate each rice MAX1, and compared the results with available data about MAX1 expression profiles and genes co-expressed with MAX1 genes. Data suggest that distinct mechanisms regulate the expression of each MAX1. Moreover, there may be novel functions for MAX1 homologues, such as the regulation of flower development or responses to heavy metals. In addition, individual MAX1s could be involved in specific functions, such as the regulation of seed development or wax synthesis in rice. Our analysis reveals potential new avenues of strigolactone research that may otherwise not be obvious.

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.

Integrative Metabolomic and Transcriptomic Analyses Reveal Metabolic Changes and Its Molecular Basis in Rice Mutants of the Strigolactone Pathway

Plants have evolved many metabolites to meet the demands of growth and adaptation. Although strigolactones (SLs) play vital roles in controlling plant architecture, their function in regulating plant metabolism remains elusive. Here we report the integrative metabolomic and transcriptomic analyses of two rice SL mutants, d10 (a biosynthesis mutant) and d14 (a perception mutant). Both mutants displayed a series of metabolic and transcriptional alterations, especially in the lipid, flavonoid, and terpenoid pathways. Levels of several diterpenoid phytoalexins were substantially increased in d10 and d14, together with the induction of terpenoid gene cluster and the corresponding upstream transcription factor WRKY45, an established determinant of plant immunity. The fact that WRKY45 is a target of IPA1, which acted as a downstream transcription factor of SL signaling, suggests that SLs contribute to plant defense through WRKY45 and phytoalexins. Moreover, our data indicated that SLs may modulate rice metabolism through a vast number of clustered or tandemly duplicated genes. Our work revealed a central role of SLs in rice metabolism. Meanwhile, integrative analysis of the metabolome and transcriptome also suggested that SLs may contribute to metabolite-associated growth and defense.

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.

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.

Hydroxyl carlactone derivatives are predominant strigolactones in Arabidopsis

Strigolactones (SLs) regulate important aspects of plant growth and stress responses. Many diverse types of SL occur in plants, but a complete picture of biosynthesis remains unclear. In Arabidopsis thaliana, we have demonstrated that MAX1, a cytochrome P450 monooxygenase, converts carlactone (CL) into carlactonoic acid (CLA) and that LBO, a 2-oxoglutarate-dependent dioxygenase, can convert methyl carlactonoate (MeCLA) into a metabolite called [MeCLA + 16 Da]. In the present study, feeding experiments with deuterated MeCLAs revealed that [MeCLA + 16 Da] is hydroxymethyl carlactonoate (1'-HO-MeCLA). Importantly, this LBO metabolite was detected in plants. Interestingly, other related compounds, methyl 4-hydroxycarlactonoate (4-HO-MeCLA) and methyl 16-hydroxycarlactonoate (16-HO-MeCLA), were also found to accumulate in lbo mutants. 3-HO-, 4-HO-, and 16-HO-CL were detected in plants, but their expected corresponding metabolites, HO-CLAs, were absent in max1 mutants. These results suggest that HO-CL derivatives may be predominant SLs in Arabidopsis, produced through MAX1 and LBO.

The negative regulator SMAX1 controls mycorrhizal symbiosis and strigolactone biosynthesis in rice

Most plants associate with beneficial arbuscular mycorrhizal (AM) fungi that facilitate soil nutrient acquisition. Prior to contact, partner recognition triggers reciprocal genetic remodelling to enable colonisation. The plant Dwarf14-Like (D14L) receptor conditions pre-symbiotic perception of AM fungi, and also detects the smoke constituent karrikin. D14L-dependent signalling mechanisms, underpinning AM symbiosis are unknown. Here, we present the identification of a negative regulator from rice, which operates downstream of the D14L receptor, corresponding to the homologue of the Arabidopsis thaliana Suppressor of MAX2-1 (AtSMAX1) that functions in karrikin signalling. We demonstrate that rice SMAX1 is a suppressor of AM symbiosis, negatively regulating fungal colonisation and transcription of crucial signalling components and conserved symbiosis genes. Similarly, rice SMAX1 negatively controls strigolactone biosynthesis, demonstrating an unexpected crosstalk between the strigolactone and karrikin signalling pathways. We conclude that removal of SMAX1, resulting from D14L signalling activation, de-represses essential symbiotic programmes and increases strigolactone hormone production.

Signaling via the D14L karrikin receptor conditions rice roots for association with arbuscular mycorrhizal fungi. Here, Choi et al. show that SMAX1, a rice homolog of an Arabidopsis repressor of karrikin signaling, acts downstream of D14L to suppress mycorrhizal symbiosis and strigolactone biosynthesis.

Evolution of Plant Hormone Response Pathways

This review focuses on the evolution of plant hormone signaling pathways. Like the chemical nature of the hormones themselves, the signaling pathways are diverse. Therefore, we focus on a group of hormones whose primary perception mechanism involves an Skp1/Cullin/F-box-type ubiquitin ligase: auxin, jasmonic acid, gibberellic acid, and strigolactone. We begin with a comparison of the core signaling pathways of these four hormones, which have been established through studies conducted in model organisms in the Angiosperms. With the advent of next-generation sequencing and advanced tools for genetic manipulation, the door to understanding the origins of hormone signaling mechanisms in plants beyond these few model systems has opened. For example, in-depth phylogenetic analyses of hormone signaling components are now being complemented by genetic studies in early diverging land plants. Here we discuss recent investigations of how basal land plants make and sense hormones. Finally, we propose connections between the emergence of hormone signaling complexity and major developmental transitions in plant evolution.

CYP722C from Gossypium arboreum catalyzes the conversion of carlactonoic acid to 5-deoxystrigol

CYP722C from cotton, a homolog of the enzyme involved in orobanchol synthesis in cowpea and tomato, catalyzes the conversion of carlactonoic acid to 5-deoxystrigol. Strigolactones (SLs) are important phytohormones with roles in the regulation of plant growth and development. These compounds also function as signaling molecules in the rhizosphere by interacting with beneficial arbuscular mycorrhizal fungi and harmful root parasitic plants. Canonical SLs, such as 5-deoxystrigol (5DS), consist of a tricyclic lactone ring (ABC-ring) connected to a methylbutenolide (D-ring). Although it is known that 5DS biosynthesis begins with carlactonoic acid (CLA) derived from β-carotene, the enzyme that catalyzes the conversion of CLA remains elusive. Recently, we identified cytochrome P450 (CYP) CYP722C as the enzyme that catalyzes direct conversion of CLA to orobanchol in cowpea and tomato (Wakabayashi et al., Sci Adv 5:eaax9067, 2019). Orobanchol has a different C-ring configuration from that of 5DS. The present study aimed to characterize the homologous gene, designated GaCYP722C, from cotton (Gossypium arboreum) to examine whether this gene is involved in 5DS biosynthesis. Expression of GaCYP722C was upregulated under phosphate starvation, which is an SL-producing condition. Recombinant GaCYP722C was expressed in a baculovirus-insect cell expression system and was found to catalyze the conversion of CLA to 5DS but not to 4-deoxyorobanchol. These results strongly suggest that GaCYP722C from cotton is a 5DS synthase and that CYP722C is the crucial CYP subfamily involved in the generation of canonical SLs, irrespective of the different C-ring configurations.

Comparative functional genomics analysis of cytochrome P450 gene superfamily in wheat and maize

BACKGROUND

The cytochrome P450s (CYP450s) as the largest enzyme family of plant metabolism participate in various physiological processes, whereas no study has demonstrated interest in comprehensive comparison of the genes in wheat and maize. Genome-wide survey, characterization and comparison of wheat and maize CYP450 gene superfamily are useful for genetic manipulation of the Gramineae crops.

RESULTS

In total, 1285 and 263 full-length CYP450s were identified in wheat and maize, respectively. According to standard nomenclature, wheat CYP450s (TaCYP450s) were categorized into 45 families, while maize CYP450s (ZmCYP450s) into 43 families. A comprehensive analysis of wheat and maize CYP450s, involved in functional domains, conserved motifs, phylogeny, gene structures, chromosome locations and duplicated events was performed. The result showed that each family/subfamily in both species exhibited characteristic features, suggesting their phylogenetic relationship and the potential divergence in their functions. Functional divergence analysis at the amino acid level of representative clans CYP51, CYP74 and CYP97 in wheat, maize and rice identified some critical amino acid sites that are responsible for functional divergence of a gene family. Expression profiles of Ta-, ZmCYP450s were investigated using RNA-seq data, which contribute to infer the potential functions of the genes during development and stress responses. We found in both species CYP450s had preferential expression in specific tissues, and many tissue-specific genes were identified. Under water-deficit condition, 82 and 39 significantly differentially expressed CYP450s were respectively detected in wheat and maize. These genes may have some roles in protecting plants against drought damage. Thereinto, fourteen CYP450s were selected to validate their expression level through qRT-PCR. To further elucidating molecular mechanisms of CYP450 action, gene co-expression network was constructed. In total, 477 TaCYP450s were distributed in 22 co-expression modules, and some co-expressed genes that likely take part in the same biochemical pathway were identified. For instance, the expression of TaCYP74A98_4D was highly correlated with TaLOX9, TaLOX36, TaLOX39, TaLOX44 and TaOPR8, and all of them may be involved in jasmonate (JA) biosynthesis. TaCYP73A201_3A showed coexpression with TaPAL1.25, TaCCoAOMT1.2, TaCOMT.1, TaCCR1.6 and TaLAC5, which probably act in the wheat stem and/or root lignin synthesis pathway.

CONCLUSION

Our study first established systematic information about evolutionary relationship, expression pattern and function characterization of CYP450s in wheat and maize.

Strigolactones Decrease Leaf Angle in Response to Nutrient Deficiencies in Rice

Strigolactones (SLs) are a class of plant hormones that are synthesized from β-carotene through sequential reactions catalyzed by DWARF (D) 27, D17, D10, and OsMORE AXILLARY GROWTH (MAX) 1 in rice (Oryza sativa L.). In rice, endogenous SL levels increase in response to deficiency of nitrogen, phosphate, or sulfate (-N, -P, or -S). Rice SL mutants show increased lamina joint (LJ) angle as well as dwarfism, delayed leaf senescence, and enhanced shoot branching. The LJ angle is an important trait that determines plant architecture. To evaluate the effect of endogenous SLs on LJ angle in rice, we measured LJ angle and analyzed the expression of SL-biosynthesis genes under macronutrient deficiencies. In the "Shiokari" background, LJ angle was significantly larger in SL mutants than in the wild-type (WT). In WT and SL-biosynthesis mutants, direct treatment with the SL synthetic analog GR24 decreased the LJ angle. In WT, deficiency of N, P, or S, but not of K, Ca, Mg, or Fe decreased LJ angle. In SL mutants, deficiency of N, P, or S had no such effect. We analyzed the time course of SL-related gene expression in the LJ of WT deficient in N, P, or S, and found that expression of SL-biosynthesis genes increased 2 or 3 days after the onset of deficiency. Expression levels of both the SL-biosynthesis and signaling genes was particularly strongly increased under -P. Rice cultivars "Nipponbare", "Norin 8", and "Kasalath" had larger LJ angle than "Shiokari", interestingly with no significant differences between WT and SL mutants. In "Nipponbare", endogenous SL levels increased and the LJ angle was decreased under -N and -P. These results indicate that SL levels increased in response to nutrient deficiencies, and that elevated endogenous SLs might negatively regulate leaf angle in rice.

Identification of two oxygenase genes involved in the respective biosynthetic pathways of canonical and non-canonical strigolactones in Lotus japonicus

A cytochrome P450 and a 2-oxoglutarate-dependent dioxygenase genes responsible, respectively, for the biosyntheses of canonical and non-canonical strigolactones in Lotus japonicus were identified by transcriptome profiling and mutant screening. Strigolactones (SLs) are a group of apocarotenoids with diverse structures that act as phytohormones and rhizosphere signals. The model legume Lotus japonicus produces both canonical and non-canonical SLs, 5-deoxystrigol (5DS) and lotuslactone (LL), respectively, through oxidation of a common intermediate carlactone by the cytochrome P450 (CYP) enzyme MAX1. However, the pathways downstream of MAX1 and the branching point in the biosyntheses of 5DS and LL have not been elucidated. Here, we identified a CYP and a 2-oxoglutarate-dependent dioxygenase (2OGD) genes responsible, respectively, for the formation of Lotus SLs by transcriptome profiling using RNA-seq and screening of SL-deficient mutants from the Lotus retrotransposon 1 (LORE1) insertion mutant resource. The CYP and 2OGD genes were named DSD and LLD, respectively, after 5DS or LL defective phenotype of the mutants. The involvements of the genes in Lotus SL biosyntheses were confirmed by restoration of the mutant phenotype using Agrobacterium rhizogenes-mediated transformation to generate transgenic roots expressing the coding sequence. The transcript levels of DSD and LLD in roots as well as the levels of 5DS and LL in root exudates were reduced by phosphate fertilization and gibberellin treatment. This study can provide the opportunity to investigate how and why plants produce the two classes of SLs.

Do Phosphate and Cytokinin Interact to Regulate Strigolactone Biosynthesis or Act Independently?

Strigolactones (SLs) are essential host recognition signals for both root-parasitic plants and arbuscular mycorrhizal (AM) fungi in the rhizosphere, and in planta SLs or their metabolites function as a novel class of plant hormones that regulate various aspects of plant growth through crosstalk with other hormones. Although nutrient availability is one of the important factors influencing SL production and exudation, and phosphate (Pi) deficiency significantly promotes SL production and exudation in host plants of AM fungi, how nutrient availability modulates SL production and exudation remains elusive. Cytokinin (CK), a canonical plant hormone, has extensively been studied as a shoot branching promoter and its biosynthesis is also influenced by mineral nutrients, especially nitrate, indicating that CK might be another key factor that affect SL production and exudation. In the present study, we show that CKs (t-zeatin, benzyladenine, kinetin, and CPPU) applied to hydroponic culture media significantly suppressed the SL levels in both the root exudates and the root tissues of rice plants grown under Pi deficiency. In a split-root system, CK suppressed SL production locally, while Pi affected SL production systemically, suggesting that Pi and CK act on SL production independently in rice plants.

Direct conversion of carlactonoic acid to orobanchol by cytochrome P450 CYP722C in strigolactone biosynthesis

Strigolactones (SLs) are carotenoid-derived phytohormones and rhizosphere signaling molecules for arbuscular mycorrhizal fungi and root parasitic weeds. Why and how plants produce diverse SLs are unknown. Here, cytochrome P450 CYP722C is identified as a key enzyme that catalyzes the reaction of BC-ring closure leading to orobanchol, the most prevalent canonical SL. The direct conversion of carlactonoic acid to orobanchol without passing through 4-deoxyorobanchol is catalyzed by the recombinant enzyme. By knocking out the gene in tomato plants, orobanchol was undetectable in the root exudates, whereas the architecture of the knockout and wild-type plants was comparable. These findings add to our understanding of the function of the diverse SLs in plants and suggest the potential of these compounds to generate crops with greater resistance to infection by noxious root parasitic weeds.

Strigolactones and their crosstalk with other phytohormones

BACKGROUND

Strigolactones (SLs) are a diverse class of butenolide-bearing phytohormones derived from the catabolism of carotenoids. They are associated with an increasing number of emerging regulatory roles in plant growth and development, including seed germination, root and shoot architecture patterning, nutrient acquisition, symbiotic and parasitic interactions, as well as mediation of plant responses to abiotic and biotic cues.

SCOPE

Here, we provide a concise overview of SL biosynthesis, signal transduction pathways and SL-mediated plant responses with a detailed discourse on the crosstalk(s) that exist between SLs/components of SL signalling and other phytohormones such as auxins, cytokinins, gibberellins, abscisic acid, ethylene, jasmonates and salicylic acid.

CONCLUSION

SLs elicit their control on physiological and morphological processes via a direct or indirect influence on the activities of other hormones and/or integrants of signalling cascades of other growth regulators. These, among many others, include modulation of hormone content, transport and distribution within plant tissues, interference with or complete dependence on downstream signal components of other phytohormones, as well as acting synergistically or antagonistically with other hormones to elicit plant responses. Although much has been done to evince the effects of SL interactions with other hormones at the cell and whole plant levels, research attention must be channelled towards elucidating the precise molecular events that underlie these processes. More especially in the case of abscisic acid, cytokinins, gibberellin, jasmonates and salicylic acid for which very little has been reported about their hormonal crosstalk with SLs.

Apocarotenoids Involved in Plant Development and Stress Response

Carotenoids are isoprenoid pigments synthesized by all photosynthetic organisms and many heterotrophic microorganisms. They are equipped with a conjugated double-bond system that builds the basis for their role in harvesting light energy and in protecting the cell from photo-oxidation. In addition, the carotenoids polyene makes them susceptible to oxidative cleavage, yielding carbonyl products called apocarotenoids. This oxidation can be catalyzed by carotenoid cleavage dioxygenases or triggered nonenzymatically by reactive oxygen species. The group of plant apocarotenoids includes important phytohormones, such as abscisic acid and strigolactones, and signaling molecules, such as β-cyclocitral. Abscisic acid is a key regulator of plant's response to abiotic stress and is involved in different developmental processes, such as seed dormancy. Strigolactone is a main regulator of plant architecture and an important signaling molecule in the plant-rhizosphere communication. β-Cyclocitral, a volatile derived from β-carotene oxidation, mediates the response of cells to singlet oxygen stress. Besides these well-known examples, recent research unraveled novel apocarotenoid growth regulators and suggests the presence of yet unidentified ones. In this review, we describe the biosynthesis and biological functions of established regulatory apocarotenoids and touch on the recently identified anchorene and zaxinone, with emphasis on their role in plant growth, development, and stress response.

Regulation of biosynthesis, perception, and functions of strigolactones for promoting arbuscular mycorrhizal symbiosis and managing root parasitic weeds

Strigolactones (SLs) are carotenoid-derived plant secondary metabolites that play important roles in various aspects of plant growth and development as plant hormones, and in rhizosphere communications with symbiotic microbes and also root parasitic weeds. Therefore, sophisticated regulation of the biosynthesis, perception and functions of SLs is expected to promote symbiosis of beneficial microbes including arbuscular mycorrhizal (AM) fungi and also to retard parasitism by devastating root parasitic weeds. We have developed SL mimics with different skeletons, SL biosynthesis inhibitors acting at different biosynthetic steps, SL perception inhibitors that covalently bind to the SL receptor D14, and SL function inhibitors that bind to the serine residue at the catalytic site. In greenhouse pot tests, TIS108, an azole-type SL biosynthesis inhibitor effectively reduced numbers of attached root parasites Orobanche minor and Striga hermonthica without affecting their host plants; tomato and rice, respectively. AM colonization resulted in weak but distinctly enhanced plant resistance to pathogens. SL mimics can be used to promote AM symbiosis and to reduce the application rate of systemic-acquired resistance inducers which are generally phytotoxic to horticultural crops. © 2019 Society of Chemical Industry.

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.