Specialisation within the DWARF14 protein family confers distinct responses to karrikins and strigolactones in Arabidopsis

Molecular basis for high ligand sensitivity and selectivity of strigolactone receptors in Striga

Seeds of the root parasitic plant Striga hermonthica can sense very low concentrations of strigolactones (SLs) exuded from host roots. The S. hermonthica hyposensitive to light (ShHTL) proteins are putative SL receptors, among which ShHTL7 reportedly confers sensitivity to picomolar levels of SL when expressed in Arabidopsis thaliana. However, the molecular mechanism underlying ShHTL7 sensitivity is unknown. Here we determined the ShHTL7 crystal structure and quantified its interactions with various SLs and key interacting proteins. We established that ShHTL7 has an active-site pocket with broad-spectrum response to different SLs and moderate affinity. However, in contrast to other ShHTLs, we observed particularly high affinity of ShHTL7 for F-box protein AtMAX2. Furthermore, ShHTL7 interacted with AtMAX2 and with transcriptional regulator AtSMAX1 in response to nanomolar SL concentration. ShHTL7 mutagenesis analyses identified surface residues that contribute to its high-affinity binding to AtMAX2 and residues in the ligand binding pocket that confer broad-spectrum response to SLs with various structures. Crucially, yeast-three hybrid experiments showed that AtMAX2 confers responsiveness of the ShHTL7-AtSMAX1 interaction to picomolar levels of SL in line with the previously reported physiological sensitivity. These findings highlight the key role of SL-induced MAX2-ShHTL7-SMAX1 complex formation in determining the sensitivity to SL. Moreover, these data suggest a strategy to screen for compounds that could promote suicidal seed germination at physiologically relevant levels.

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.

Orobanchaceae parasite-host interactions

Parasitic plants in the family Orobanchaceae, such as Striga, Orobanche and Phelipanche, often cause significant damage to agricultural crops. The Orobanchaceae family comprises more than 2000 species in about 100 genera, providing an excellent system for studying the molecular basis of parasitism and its evolution. Notably, the establishment of model Orobanchaceae parasites, such as Triphysaria versicolor and Phtheirospermum japonicum, that can infect the model host Arabidopsis, has greatly facilitated transgenic analyses of genes important for parasitism. In addition, recent genomic and transcriptomic analyses of several Orobanchaceae parasites have revealed fascinating molecular insights into the evolution of parasitism and strategies for adaptation in this family. This review highlights recent progress in understanding how Orobanchaceae parasites attack their hosts and how the hosts mount a defense against the threats.

Unraveling the MAX2 Protein Network in Arabidopsis thaliana: Identification of the Protein Phosphatase PAPP5 as a Novel MAX2 Interactor

The F-box protein MORE AXILLARY GROWTH 2 (MAX2) is a central component in the signaling cascade of strigolactones (SLs) as well as of the smoke-derived karrikins (KARs) and the so far unknown endogenous KAI2 ligand (KL). The two groups of molecules are involved in overlapping and unique developmental processes, and signal-specific outcomes are attributed to perception by the paralogous α/β-hydrolases DWARF14 (D14) for SL and KARRIKIN INSENSITIVE 2/HYPOSENSITIVE TO LIGHT (KAI2/HTL) for KAR/KL. In addition, depending on which receptor is activated, specific members of the SUPPRESSOR OF MAX2 1 (SMAX1)-LIKE (SMXL) family control KAR/KL and SL responses. As proteins that function in the same signal transduction pathway often occur in large protein complexes, we aimed at discovering new players of the MAX2, D14, and KAI2 protein network by tandem affinity purification in Arabidopsis cell cultures. When using MAX2 as a bait, various proteins were copurified, among which were general components of the Skp1-Cullin-F-box complex and members of the CONSTITUTIVE PHOTOMORPHOGENIC 9 signalosome. Here, we report the identification of a novel interactor of MAX2, a type 5 serine/threonine protein phosphatase, designated PHYTOCHROME-ASSOCIATED PROTEIN PHOSPHATASE 5 (PAPP5). Quantitative affinity purification pointed at PAPP5 as being more present in KAI2 rather than in D14 protein complexes. In agreement, mutant analysis suggests that PAPP5 modulates KAR/KL-dependent seed germination under suboptimal conditions and seedling development. In addition, a phosphopeptide enrichment experiment revealed that PAPP5 might dephosphorylate MAX2 in vivo independently of the synthetic SL analog, rac-GR24. Together, by analyzing the protein complexes to which MAX2, D14, and KAI2 belong, we revealed a new MAX2 interactor, PAPP5, that might act through dephosphorylation of MAX2 to control mainly KAR/KL-related phenotypes and, hence, provide another link with the light pathway.

MAX2-independent transcriptional responses to rac-GR24 in Lotus japonicus roots

Strigolactones (SLs) and smoke-derived Karrikins (KARs) are structurally similar butenolide compounds that control distinct aspects of plant development. They are perceived by two closely related α/β hydrolases D14 and KAI2, respectively. Responses to both molecules involve the F-box protein MAX2 that participates in the Skp1-Cullin-F-box (SCF) complex, which ubiquitylates developmental regulators of the SMXL family to mark them for degradation by the 26S proteasome, enabling SL and KAR responses. Current research on SL and KAR signaling uses the synthetic molecules rac-GR24, KAR₁ and KAR₂ for pharmacological treatments. In a previous microarray analysis, we observed transcriptional activation in response to rac-GR24 in Lotus japonicus seedling roots. We retested transcript accumulation of selected genes by quantitative PCR in the wild type and the max2-4 mutant, and found that surprisingly, a number of them respond to rac-GR24 in a MAX2-independent manner, and also respond in roots of d14 and kai2a kai2b double mutants. Thus, the synthetic compounds induce transcriptional responses independent of their perception by the canonical receptor complex.

Bioassays for the Effects of Strigolactones and Other Small Molecules on Root and Root Hair Development

Growth and development of plant roots are highly dynamic and adaptable to environmental conditions. They are under the control of several plant hormone signaling pathways, and therefore root developmental responses can be used as bioassays to study the action of plant hormones and other small molecules. In this chapter, we present different procedures to measure root traits of the model plant Arabidopsis thaliana. We explain methods for phenotypic analysis of lateral root development, primary root length, root skewing and straightness, and root hair density and length. We describe optimal growth conditions for Arabidopsis seedlings for reproducible root and root hair developmental outputs; and how to acquire images and measure the different traits using image analysis with relatively low-tech equipment. We provide guidelines for a semiautomatic image analysis of primary root length, root skewing, and root straightness in Fiji and a script to automate the calculation of root angle deviation from the vertical and root straightness. By including mutants defective in strigolactone (SL) or KAI2 ligand (KL) synthesis and/or signaling, these methods can be used as bioassays for different SLs or SL-like molecules. In addition, the techniques described here can be used for studying seedling root system architecture, root skewing, and root hair development in any context.

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.

Ligand-receptor interactions in plant hormone signaling

Small-molecule plant hormones principally control plant growth, development, differentiation, and environmental responses. Nine types of plant hormones are ubiquitous in angiosperms, and the molecular mechanisms of their hormone actions have been elucidated during the last two decades by genomic decoding of model plants with genetic mutants. In particular, the discovery of hormone receptors has greatly contributed to the understanding of signal transduction systems. The three-dimensional structure of the ligand-receptor complex has been determined for eight of the nine hormones by X-ray crystal structure analysis, and ligand perception mechanisms have been revealed at the atomic level. Collective research has revealed the molecular function of plant hormones that act as either molecular glue or an allosteric regulator for activation of receptors. In this review, we present an overview of the respective hormone signal transduction and describe the structural bases of ligand-receptor interactions.

On the outside looking in: roles of endogenous and exogenous strigolactones

A collection of small molecules called strigolactones (SLs) act as both endogenous hormones to control plant development and as ecological communication cues between organisms. SL signalling overlaps with that of a class of smoke-derived compounds, karrikins (KARs), which have distinct yet overlapping developmental effects on plants. Although the roles of SLs in shoot and root development, in the promotion of arbuscular mycorrhizal (AM) fungal branching and in parasitic plant germination have been well characterized, recent data have illustrated broader roles for these compounds in the rhizosphere. Here, we review the known roles of SLs in development, growth of AM fungi and germination of parasitic plants to develop a framework for understanding the use of SLs as molecules of communication in the rhizosphere. It appears, for example, that there are many connections between SLs and phosphate utilization. Low phosphate levels regulate SL metabolism and, in turn, SLs sculpt root and shoot architecture to coordinate growth and optimize phosphate uptake from the environment. Plant-exuded SLs attract fungal symbionts to deliver inorganic phosphate (Pi) to the host. These and other examples suggest the boundary between exogenous and endogenous SL functions can be easily blurred and a more holistic view of these small molecules is likely to be required to fully understand SL biology. Related to this, we summarize and discuss evidence for a primitive role of SLs in moss as a quorum sensing-like molecule, providing a unifying concept of SLs as endogenous and exogenous signalling molecules.

Overexpression of Karrikins Receptor Gene Sapium sebiferum KAI2 Promotes the Cold Stress Tolerance via Regulating the Redox Homeostasis in Arabidopsis thaliana

KARRIKINS INSENSITIVE2 (KAI2) is the receptor gene for karrikins, recently found to be involved in seed germination, hypocotyl development, and the alleviation of salinity and osmotic stresses. Nevertheless, whether KAI2 could regulate cold tolerance remains elusive. In the present study, we identified that Arabidopsis mutants of KAI2 had a high mortality rate, while overexpression of, a bioenergy plant, Sapium sebiferum KAI2 (SsKAI2) significantly recovered the plants after cold stress. The results showed that the SsKAI2 overexpression lines (OEs) had significantly increased levels of proline, total soluble sugars, and total soluble protein. Meanwhile, SsKAI2 OEs had a much higher expression of cold-stress-acclimation-relate genes, such as Cold Shock Proteins and C-REPEAT BINDING FACTORS under cold stress. Moreover, the results showed that SsKAI2 OEs were hypersensitive to abscisic acid (ABA), and ABA signaling genes were w significantly affected in SsKAI2 OEs under cold stress, suggesting a potential interaction between SsKAI2 and ABA downstream signaling. In SsKAI2 OEs, the electrolyte leakage, hydrogen peroxide, and malondialdehyde contents were reduced under cold stress in Arabidopsis. SsKAI2 OEs enhanced the anti-oxidants like ascorbate peroxidase, catalase, peroxidase, superoxide dismutase, and total glutathione level under cold stress. Conclusively, these results provide novel insights into the understanding of karrikins role in the regulation of cold stress adaptation.

Sensors for the quantification, localization and analysis of the dynamics of plant hormones

Plant hormones play important roles in plant growth and development and physiology, and in acclimation to environmental changes. The hormone signaling networks are highly complex and interconnected. It is thus important to not only know where the hormones are produced, how they are transported and how and where they are perceived, but also to monitor their distribution quantitatively, ideally in a non-invasive manner. Here we summarize the diverse set of tools available for quantifying and visualizing hormone distribution and dynamics. We provide an overview over the tools that are currently available, including transcriptional reporters, degradation sensors, and luciferase and fluorescent sensors, and compare the tools and their suitability for different purposes.

DLK2 regulates arbuscule hyphal branching during arbuscular mycorrhizal symbiosis

D14 and KAI2 receptors enable plants to distinguish between strigolactones (SLs) and karrikins (KARs), respectively, in order to trigger appropriate environmental and developmental responses. Both receptors are related to the regulation of arbuscular mycorrhiza (AM) formation and are members of the RsbQ-like family of α,β-hydrolases. DLK2 proteins, whose function remains unknown, constitute a third clade from the RsbQ-like protein family. We investigated whether the tomato SlDLK2 is a new regulatory component in the AM symbiosis. Genetic approaches were conducted to analyze SlDLK2 expression and to understand SlDLK2 function in AM symbiosis. We show that SlDLK2 expression in roots is AM-dependent and is associated with cells containing arbuscules. SlDLK2 ectopic expression arrests arbuscule branching and downregulates AM-responsive genes, even in the absence of symbiosis; while the opposite effect was observed upon SlDLK2 silencing. Moreover, SlDLK2 overexpression in Medicago truncatula roots showed the same altered phenotype observed in tomato roots. Interestingly, SlDLK2 interacts with DELLA, a protein that regulates arbuscule formation/degradation in AM roots. We propose that SlDLK2 is a new component of the complex plant-mediated mechanism regulating the life cycle of arbuscules in AM symbiosis.

Lotus japonicus karrikin receptors display divergent ligand-binding specificities and organ-dependent redundancy

Karrikins (KARs), smoke-derived butenolides, are perceived by the α/β-fold hydrolase KARRIKIN INSENSITIVE2 (KAI2) and thought to mimic endogenous, yet elusive plant hormones tentatively called KAI2-ligands (KLs). The sensitivity to different karrikin types as well as the number of KAI2 paralogs varies among plant species, suggesting diversification and co-evolution of ligand-receptor relationships. We found that the genomes of legumes, comprising a number of important crops with protein-rich, nutritious seed, contain two or more KAI2 copies. We uncover sub-functionalization of the two KAI2 versions in the model legume Lotus japonicus and demonstrate differences in their ability to bind the synthetic ligand GR24ent-5DS in vitro and in genetic assays with Lotus japonicus and the heterologous Arabidopsis thaliana background. These differences can be explained by the exchange of a widely conserved phenylalanine in the binding pocket of KAI2a with a tryptophan in KAI2b, which arose independently in KAI2 proteins of several unrelated angiosperms. Furthermore, two polymorphic residues in the binding pocket are conserved across a number of legumes and may contribute to ligand binding preferences. The diversification of KAI2 binding pockets suggests the occurrence of several different KLs acting in non-fire following plants, or an escape from possible antagonistic exogenous molecules. Unexpectedly, L. japonicus responds to diverse synthetic KAI2-ligands in an organ-specific manner. Hypocotyl growth responds to KAR1, KAR2 and rac-GR24, while root system development responds only to KAR1. This differential responsiveness cannot be explained by receptor-ligand preferences alone, because LjKAI2a is sufficient for karrikin responses in the hypocotyl, while LjKAI2a and LjKAI2b operate redundantly in roots. Instead, it likely reflects differences between plant organs in their ability to transport or metabolise the synthetic KLs. Our findings provide new insights into the evolution and diversity of butenolide ligand-receptor relationships, and open novel research avenues into their ecological significance and the mechanisms controlling developmental responses to divergent KLs.

Interaction Mechanism of the Germination Stimulants Karrikins and Their Receptor ShKAI2iB

The significance of karrikins (KARs) in plant physiology opens a door for their application in the agricultural production. As the first event of the whole signaling pathway, the binding of smoke-derived signal molecules KARs to the receptor protein KAI2 triggers the germination of the primary dormant seeds of all angiosperms, not just the "fire-prone" taxa. In the present study, all-atom molecular dynamics simulations, along with the accurate estimation of the ligand-receptor binding free energy, were used to investigate the atomic level interaction of all the members of the KARs family (from KAR₁ to KAR₆) with the receptor ShKAI2iB, an intermediate-evolving KAI2 from Striga hermonthica. The calculated binding energy value of KAR₁ to ShKAI2iB, -5.64 kcal/mol, is in good agreement with the available experimental data, -5.67 kcal/mol. The further analysis of the detailed interaction between each KAR and the protein reveals the primary reasons for the difference of the affinity of the diverse ligands with the receptor and displays the regional characteristics of the protein's active site. Our research will not only provide clues for the study of equivalent endogenous phytohormone, but also contribute to the development of synthetic germinating chemicals.

Plant-Derived Smoke Affects Biochemical Mechanism on Plant Growth and Seed Germination

The role of plant-derived smoke, which is changed in mineral-nutrient status, in enhancing germination and post-germination was effectively established. The majority of plant species positively respond to plant-derived smoke in the enhancement of seed germination and plant growth. The stimulatory effect of plant-derived smoke on normally growing and stressed plants may help to reduce economic and human resources, which validates its candidature as a biostimulant. Plant-derived smoke potentially facilitates the early harvest and increases crop productivity. Karrikins and cyanohydrin are the active compound in plant-derived smoke. In this review, data from the latest research explaining the effect of plant-derived smoke on morphological, physiological, biochemical, and molecular responses of plants are presented. The pathway for reception and interaction of compounds of plant-derived smoke at the cellular and molecular level of plant is described and discussed.

Multi-Omics Strategies for Decoding Smoke-Assisted Germination Pathways and Seed Vigour

The success of seed germination and the successful establishment of seedlings across diverse environmental conditions depends on seed vigour, which is of both economic and ecologic importance. The smoke-derived exogenous compound karrikins (KARs) and the endogenous plant hormone strigolactone (SL) are two classes of butanolide-containing molecules that follow highly similar signalling pathways to control diverse biological activities in plants. Unravelling the precise mode-of-action of these two classes of molecules in model species has been a key research objective. However, the specific and dynamic expression of biomolecules upon stimulation by these signalling molecules remains largely unknown. Genomic and post-genomic profiling approaches have enabled mining and association studies across the vast genetic diversity and phenotypic plasticity. Here, we review the background of smoke-assisted germination and vigour and the current knowledge of how plants perceive KAR and SL signalling and initiate the crosstalk with the germination-associated hormone pathways. The recent advancement of 'multi-omics' applications are discussed in the context of KAR signalling and with relevance to their adoption for superior agronomic trait development. The remaining challenges and future opportunities for integrating multi-omics datasets associated with their application in KAR-dependent seed germination and abiotic stress tolerance are also discussed.

Synthetic strigolactone (rac-GR24) alleviates the adverse effects of heat stress on seed germination and photosystem II function in lupine seedlings

There is increasing experimental evidence that strigolactones, a class of carotenoid-derived sesquiterpenoid hormones, and their downstream signal components play a role in plant resilience to abiotic stress. Strigolactones positively influence plant coping mechanisms in response to abiotic stressors like drought and high salinity. In this study, we examined the effects of rac-GR24 (a synthetic strigolactone analog) and strigolactone inhibitors on the physiological and molecular responses associated with thermotolerance during seed germination and seedling development in Lupinus angustifolius under heat stress. Photosystem I & II functions were also evaluated via Chl a fluorescence transient analysis in heat stressed lupine seedlings. Our results suggest a putative role for GR24 in mediating tolerance to heat stress during seed germination and seedling development albeit these responses appeared independent of D14-mediated signalling. Seeds primed with GR24 had the highest of all germination indices, enhanced proline content and reduced peroxidation of lipids. GR24 also enhanced the activities of enzymes of the antioxidant and glyoxalase systems in lupine seedlings. The JIP-test indicated that GR24 conferred resistance to heat stress-induced damage to the oxygen evolution complex while also preventing the inactivation of PSII reaction centres thus ensuring PSII thermotolerance.

Barley strigolactone signalling mutant hvd14.d reveals the role of strigolactones in abscisic acid-dependent response to drought

Strigolactones (SLs) are a group of plant hormones involved in many aspects of plant development and stress adaptation. Here, we investigated the drought response of a barley (Hordeum vulgare L.) mutant carrying a missense mutation in the gene encoding the SL-specific receptor HvD14. Our results clearly showed that hvd14.d mutant is hyper-sensitive to drought stress. This was illustrated by a lower leaf relative water content (RWC), impaired photosynthesis, disorganization of chloroplast structure, altered stomatal density and slower closure of stomata in response to drought in the mutant compared to the wild type parent cultivar Sebastian. Although the content of abscisic acid (ABA) and its derivatives remained unchanged in the mutant, significant differences in expression of genes related to ABA biosynthesis were observed. Moreover, hvd14.d was insensitive to ABA during seed germination. Analysis of Arabidopsis thaliana mutant atd14-1 also demonstrated that mutation in the SL receptor resulted in increased sensitivity to drought. Our results indicate that the drought-sensitive phenotype of barley SL mutant might be caused by a disturbed ABA metabolism and/or signalling pathways. These results together uncovered a link between SL signalling and ABA-dependent drought stress response in barley.

Different strategies of strigolactone and karrikin signals in regulating the resistance of Arabidopsis thaliana to water-deficit stress

Strigolactone and karrikin receptors, DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2), respectively, have been shown to positively regulate drought resistance in Arabidopsis thaliana by modulating abscisic acid responsiveness, anthocyanin accumulation, stomatal closure, cell membrane integrity and cuticle formation. Here, we aim to identify genes specifically or commonly regulated by D14 and KAI2 under water scarcity, using comparative analysis of the transcriptome data of the A. thaliana d14-1 and kai2-2 mutants under dehydration conditions. In comparison with wild-type, under dehydration conditions, the expression levels of genes related to photosynthesis and the metabolism of glucosinolates and trehalose were significantly changed in both d14-1 and kai2-2 mutant plants, whereas the transcript levels of genes related to the metabolism of cytokinins and brassinosteroids were significantly altered in the d14-1 mutant plants only. These results suggest that cytokinin and brassinosteroid metabolism might be specifically regulated by the D14 pathway, whereas photosynthesis and metabolism of glucosinolates and trehalose are potentially regulated by both D14 and KAI2 pathways in plant response to water scarcity.

The effects of gibberellin on the expression of symbiosis-related genes in Paris-type arbuscular mycorrhizal symbiosis in Eustoma grandiflorum

Arbuscular mycorrhiza (AM) is a symbiotic interaction in terrestrial plants that is colonized by fungi in the Glomeromycotina. The morphological types of AM, including the Arum-type and Paris-type, are distinct, depending on the host plant species. A part of the regulatory pathways in Arum-type AM symbiosis has been revealed because most model plants form the Arum-type AM with a model AM fungus, Rhizophagus irregularis. Moreover, gibberellin (GA) is known to severely inhibit AM fungal colonization in Arum-type AM symbiosis. Recently, we showed that exogenous GA treatment significantly promoted AM fungal colonization in Paris-type AM symbiosis in Eustoma grandiflorum. In this study, we focused on the transcriptional changes in AM symbiosis-related genes in GA-treated E. grandiflorum. The expression levels of all examined E. grandiflorum genes were maintained or increased by GA treatment compared with those of the control treatment. Our new results suggest that signaling pathway(s) required for establishing AM symbiosis in E. grandiflorum may be distinct from the well-characterized pathway for that in model plants.

The karrikin signaling regulator SMAX1 controls Lotus japonicus root and root hair development by suppressing ethylene biosynthesis

An evolutionarily ancient plant hormone receptor complex comprising the α/β-fold hydrolase receptor KARRIKIN INSENSITIVE 2 (KAI2) and the F-box protein MORE AXILLARY GROWTH 2 (MAX2) mediates a range of developmental responses to smoke-derived butenolides called karrikins (KARs) and to yet elusive endogenous KAI2 ligands (KLs). Degradation of SUPPRESSOR OF MAX2 1 (SMAX1) after ligand perception is considered to be a key step in KAR/KL signaling. However, molecular events which regulate plant development downstream of SMAX1 removal have not been identified. Here we show that Lotus japonicus SMAX1 is specifically degraded in the presence of KAI2 and MAX2 and plays an important role in regulating root and root hair development. smax1 mutants display very short primary roots and elongated root hairs. Their root transcriptome reveals elevated ethylene responses and expression of ACC Synthase 7 (ACS7), which encodes a rate-limiting enzyme in ethylene biosynthesis. smax1 mutants release increased amounts of ethylene and their root phenotype is rescued by treatment with ethylene biosynthesis and signaling inhibitors. KAR treatment induces ACS7 expression in a KAI2-dependent manner and root developmental responses to KAR treatment depend on ethylene signaling. Furthermore, in Arabidopsis, KAR-induced root hair elongation depends on ACS7 Thus, we reveal a connection between KAR/KL and ethylene signaling in which the KAR/KL signaling module (KAI2-MAX2-SMAX1) regulates the biosynthesis of ethylene to fine-tune root and root hair development, which are important for seedling establishment at the beginning of the plant life cycle.

Strigolactone signaling regulates specialized metabolism in tobacco stems and interactions with stem-feeding herbivores

Plants are attacked by herbivores, which often specialize on different tissues, and in response, have evolved sophisticated resistance strategies that involve different types of chemical defenses frequently targeted to different tissues. Most known phytohormones have been implicated in regulating these defenses, with jasmonates (JAs) playing a pivotal role in complex regulatory networks of signaling interactions, often generically referred to as "cross talk." The newly identified class of phytohormones, strigolactones (SLs), known to regulate the shoot architecture, remain unstudied with regard to plant-herbivore interactions. We explored the role of SL signaling in resistance to a specialist weevil (Trichobaris mucorea) herbivore of the native tobacco, Nicotiana attenuata, that attacks the root-shoot junction (RSJ), the part of the plant most strongly influenced by alterations in SL signaling (increased branching). As SL signaling shares molecular components, such as the core F-box protein MORE AXILLARY GROWTH 2 (MAX2), with another new class of phytohormones, the karrikins (KARs), which promote seed germination and seedling growth, we generated transformed lines, individually silenced in the expression of NaMAX2, DWARF 14 (NaD14: the receptor for SL) and CAROTENOID CLEAVAGE DIOXYGENASE 7 (NaCCD7: a key enzyme in SL biosynthesis), and KARRIKIN INSENSITIVE 2 (NaKAI2: the KAR receptor). The mature stems of all transgenic lines impaired in the SL, but not the KAR signaling pathway, overaccumulated anthocyanins, as did the stems of plants attacked by the larvae of weevil, which burrow into the RSJs to feed on the pith of N. attenuata stems. T. mucorea larvae grew larger in the plants silenced in the SL pathway, but again, not in the KAI2-silenced plants. These phenotypes were associated with elevated JA and auxin (indole-3-acetic acid [IAA]) levels and significant changes in the accumulation of defensive compounds, including phenolamides and nicotine. The overaccumulation of phenolamides and anthocyanins in the SL pathway-silenced plants likely resulted from antagonism between the SL and JA pathway in N. attenuata. We show that the repressors of SL signaling, suppressor of max2-like (NaSMXL6/7), and JA signaling, jasmonate zim-domain (NaJAZs), physically interact, promoting NaJAZb degradation and releasing JASMONATE INSENSITIVE 1 (JIN1/MYC2) (NaMYC2), a critical transcription factor promoting JA responses. However, the increased performance of T. mucorea larvae resulted from lower pith nicotine levels, which were inhibited by increased IAA levels in SL pathway-silenced plants. This inference was confirmed by decapitation and auxin transport inhibitor treatments that decreased pith IAA and increased nicotine levels. In summary, SL signaling tunes specific sectors of specialized metabolism in stems, such as phenylpropanoid and nicotine biosynthesis, by tailoring the cross talk among phytohormones, including JA and IAA, to mediate herbivore resistance of stems. The metabolic consequences of the interplay of SL, JA, and IAA signaling revealed here could provide a mechanism for the commonly observed pattern of herbivore tolerance/resistance trade-offs.

On the biosynthesis and evolution of apocarotenoid plant growth regulators

Carotenoids are an important source of metabolites with regulatory function, which include the plant hormones abscisic acid (ABA) and strigolactones (SLs), and several recently identified growth regulators and signaling molecules. These carotenoid-derivatives originate from oxidative breakdown of double bonds in the carotenoid polyene, a common metabolic process that gives rise to diverse carbonyl cleavage-products known as apocarotenoids. Apocarotenoids exert biologically important functions in all taxa. In plants, they are a major regulator of plant growth, development and response to biotic and abiotic environmental stimuli, and mediate plant's communication with surrounding organisms. In this article, we provide a general overview on the biology of plant apocarotenoids, focusing on ABA, SLs, and recently identified apocarotenoid growth regulators. Following an introduction on carotenoids, we describe plant apocarotenoid biosynthesis, signal transduction, and evolution and summarize their biological functions. Moreover, we discuss the evolution of these intriguing metabolites, which has not been adequately addressed in the literature.

The SUPPRESSOR of MAX2 1 (SMAX1)-Like SMXL6, SMXL7 and SMXL8 Act as Negative Regulators in Response to Drought Stress in Arabidopsis

Drought represents a major threat to crop growth and yields. Strigolactones (SLs) contribute to regulating shoot branching by targeting the SUPPRESSOR OF MORE AXILLARY GROWTH2 (MAX2)-LIKE6 (SMXL6), SMXL7 and SMXL8 for degradation in a MAX2-dependent manner in Arabidopsis. Although SLs are implicated in plant drought response, the functions of the SMXL6, 7 and 8 in the SL-regulated plant response to drought stress have remained unclear. Here, we performed transcriptomic, physiological and biochemical analyses of smxl6, 7, 8 and max2 plants to understand the basis for SMXL6/7/8-regulated drought response. We found that three D53 (DWARF53)-Like SMXL members, SMXL6, 7 and 8, are involved in drought response as the smxl6smxl7smxl8 triple mutants showed markedly enhanced drought tolerance compared to wild type (WT). The smxl6smxl7smxl8 plants exhibited decreased leaf stomatal index, cuticular permeability and water loss, and increased anthocyanin biosynthesis during dehydration. Moreover, smxl6smxl7smxl8 were hypersensitive to ABA-induced stomatal closure and ABA responsiveness during and after germination. In addition, RNA-sequencing analysis of the leaves of the D53-like smxl mutants, SL-response max2 mutant and WT plants under normal and dehydration conditions revealed an SMXL6/7/8-mediated network controlling plant adaptation to drought stress via many stress- and/or ABA-responsive and SL-related genes. These data further provide evidence for crosstalk between ABA- and SL-dependent signaling pathways in regulating plant responses to drought. Our results demonstrate that SMXL6, 7 and 8 are vital components of SL signaling and are negatively involved in drought responses, suggesting that genetic manipulation of SMXL6/7/8-dependent SL signaling may provide novel ways to improve drought resistance.

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.

Vernalization shapes shoot architecture and ensures the maintenance of dormant buds in the perennial Arabis alpina

Perennials have a complex shoot architecture with axillary meristems organized in zones of differential bud activity and fate. This includes zones of buds maintained dormant for multiple seasons and used as reservoirs for potential growth in case of damage. The shoot of Arabis alpina, a perennial relative of Arabidopsis thaliana, consists of a zone of dormant buds placed between subapical vegetative and basal flowering branches. This shoot architecture is shaped after exposure to prolonged cold, required for flowering. To understand how vernalization ensures the maintenance of dormant buds, we performed physiological and transcriptome studies, followed the spatiotemporal changes of auxin, and generated transgenic plants. Our results demonstrate that the complex shoot architecture in A. alpina is shaped by its flowering behavior, specifically the initiation of inflorescences during cold treatment and rapid flowering after subsequent exposure to growth-promoting conditions. Dormant buds are already formed before cold treatment. However, dormancy in these buds is enhanced during, and stably maintained after, vernalization by a BRC1-dependent mechanism. Post-vernalization, stable maintenance of dormant buds is correlated with increased auxin response, transport, and endogenous indole-3-acetic acid levels in the stem. Here, we provide a functional link between flowering and the maintenance of dormant buds in perennials.

An Updated Overview on the Regulation of Seed Germination

The ability of a seed to germinate and establish a plant at the right time of year is of vital importance from an ecological and economical point of view. Due to the fragility of these early growth stages, their swiftness and robustness will impact later developmental stages and crop yield. These traits are modulated by a continuous interaction between the genetic makeup of the plant and the environment from seed production to germination stages. In this review, we have summarized the established knowledge on the control of seed germination from a molecular and a genetic perspective. This serves as a “backbone” to integrate the latest developments in the field. These include the link of germination to events occurring in the mother plant influenced by the environment, the impact of changes in the chromatin landscape, the discovery of new players and new insights related to well-known master regulators. Finally, results from recent studies on hormone transport, signaling, and biophysical and mechanical tissue properties are underscoring the relevance of tissue-specific regulation and the interplay of signals in this crucial developmental process.

A series of dual-reporter vectors for ratiometric analysis of protein abundance in plants

Ratiometric reporter systems enable comparisons of the abundance of a protein of interest, or "target," relative to a reference protein. Both proteins are encoded on a single transcript but are separated during translation. This arrangement bypasses the potential for discordant expression that can arise when the target and reference proteins are encoded by separate genes. We generated a set of 18 Gateway-compatible vectors termed pRATIO that combine a variety of promoters, fluorescent, and bioluminescent reporters, and 2A "self-cleaving" peptides. These constructs are easily modified to produce additional combinations or introduce new reporter proteins. We found that mScarlet-I provides the best signal-to-noise ratio among several fluorescent reporter proteins during transient expression experiments in Nicotiana benthamiana. Firefly and Gaussia luciferase also produce high signal-to-noise in N. benthamiana. As proof of concept, we used this system to investigate whether degradation of the receptor KAI2 after karrikin treatment is influenced by its subcellular localization. KAI2 is normally found in the cytoplasm and the nucleus of plant cells. In N. benthamiana, karrikin-induced degradation of KAI2 was only observed when it was retained in the nucleus. These vectors are tools to easily monitor in vivo the abundance of a protein that is transiently expressed in plants, and will be particularly useful for investigating protein turnover in response to different stimuli.

SMAX1-dependent seed germination bypasses GA signalling in Arabidopsis and Striga

Parasitic plant infestations dramatically reduce the yield of many major food crops of sub-Saharan Africa and pose a serious threat to food security on that continent¹. The first committed step of a successful infestation is the germination of parasite seeds primarily in response to a group of related small-molecule hormones called strigolactones (SLs), which are emitted by host roots². Despite the important role of SLs, it is not clear how host-derived SLs germinate parasitic plants. In contrast, gibberellins (GA) acts as the dominant hormone for stimulation of germination in non-parasitic plant species by inhibiting a set of DELLA repressors³. Here, we show that expression of SL receptors from the parasitic plant Striga hermonthica in the presence of SLs circumvents the GA requirement for germination of Arabidopsis thaliana seed. Striga receptors co-opt and enhance signalling through the HYPOSENSITIVE TO LIGHT/KARRIKIN INSENSITIVE 2 (AtHTL/KAI2) pathway, which normally plays a rudimentary role in Arabidopsis seed germination^4,5. AtHTL/KAI2 negatively controls the SUPPRESSOR OF MAX2 1 (SMAX1) protein⁵, and loss of SMAX1 function allows germination in the presence of DELLA repressors. Our data suggest that ligand-dependent inactivation of SMAX1 in Striga and Arabidopsis can bypass GA-dependent germination in these species.

BES1 Functions as the Co-regulator of D53-like SMXLs to Inhibit BRC1 Expression in Strigolactone-Regulated Shoot Branching in Arabidopsis

Shoot branching, determining plant architecture and crop yield, is critically controlled by strigolactones (SLs). However, how SLs inhibit shoot branching after its perception by the receptor complex remains largely obscure. In this study, using the transcriptomic and genetic analyss as well as biochemical studies, we reveal the key role of BES1 in the SL-regulated shoot branching. We demonstrate that BES1 and D53-like SMXLs, the substrates of SL receptor complex D14-MAX2, interact with each other to inhibit BRC1 expression, which specifically triggers the SL-regulated transcriptional network in shoot branching. BES1 directly binds the BRC1 promoter and recruits SMXLs to inhibit BRC1 expression. Interestingly, despite being the shared component by SL and brassinosteroid (BR) signaling, BES1 gains signal specificity through different mechanisms in response to BR and SL signals.

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.

Negative Roles of Strigolactone-Related SMXL6, 7 and 8 Proteins in Drought Resistance in Arabidopsis

Previous investigations have shown that the SUPPRESSORS OF MAX2 1-LIKE6, 7 and 8 (SMXL6, 7 and 8) proteins redundantly repress strigolactone (SL) signaling in plant growth and development. Recently, a growing body of evidence indicated that SLs positively regulate plant drought resistance through functional analyses of genes involved in SL biosynthesis and positive regulation of SL signaling. However, the functions of the SL-signaling negative regulators SMXL6, 7 and 8 in drought resistance and the associated mechanisms remain elusive. To reveal the functions of these SMXL proteins, we analyzed the drought-resistant phenotype of the triple smxl6,7,8 mutant plants and studied several drought resistance-related traits. Our results showed that the smxl6,7,8 mutant plants were more resistant to drought than wild-type plants. Physiological investigations indicated that the smxl6,7,8 mutant plants exhibited higher leaf surface temperature, reduced cuticle permeability, as well as decreases in drought-induced water loss and cell membrane damage in comparison with wild-type plants. Additionally, smxl6,7,8 mutant plants displayed an increase in anthocyanin biosynthesis during drought, enhanced detoxification capacity and increased sensitivity to abscisic acid in cotyledon opening and growth inhibition assays. A good correlation between the expression levels of some relevant genes and the examined physiological and biochemical traits was observed. Our findings together indicate that the SMXL6, 7 and 8 act as negative regulators of drought resistance, and that disruption of these SMXL genes in crops may provide a novel way to improve their drought resistance.

Role of Strigolactones: Signalling and Crosstalk with Other Phytohormones

Plant hormones play important roles in controlling how plants grow and develop. While metabolism provides the energy needed for plant survival, hormones regulate the pace of plant growth. Strigolactones (SLs) were recently defined as new phytohormones that regulate plant metabolism and, in turn, plant growth and development. This group of phytohormones is derived from carotenoids and has been implicated in a wide range of physiological functions including regulation of plant architecture (inhibition of bud outgrowth and shoot branching), photomorphogenesis, seed germination, nodulation, and physiological reactions to abiotic factors. SLs also induce hyphal branching in germinating spores of arbuscular mycorrhizal fungi (AMF), a process that is important for initiating the connection between host plant roots and AMF. This review outlines the physiological roles of SLs and discusses the significance of interactions between SLs and other phytohormones to plant metabolic responses.

The Many Models of Strigolactone Signaling

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

Perception of karrikins by plants: a continuing enigma

Karrikins are small butenolide molecules with the capacity to promote germination and enhance seedling establishment. Generated abiotically from partial combustion of vegetation, karrikins are comparatively rare in the environment, but studying their mode of action has been most informative in revealing a new regulatory pathway for plant development that uses the karrikin perception machinery. Recent studies suggest that the karrikin receptor protein KAI2 and downstream transcriptional co-repressors in the SMXL family influence seed germination, seedling photomorphogenesis, root morphology, and responses to abiotic stress such as drought. Based on taxonomic distribution, this pathway is ubiquitous and likely to be evolutionarily ancient, originating prior to land plants. However, we still do not have a good grasp on how karrikins actually activate the receptor protein, and we have yet to discover the assumed endogenous ligand for KAI2 that karrikins are thought to mimic. This review covers recent progress in this field, as well as current gaps in our knowledge.

Divergent receptor proteins confer responses to different karrikins in two ephemeral weeds

Wildfires can encourage the establishment of invasive plants by releasing potent germination stimulants, such as karrikins. Seed germination of Brassica tournefortii, a noxious weed of Mediterranean climates, is strongly stimulated by KAR₁, the archetypal karrikin produced from burning vegetation. In contrast, the closely-related yet non-fire-associated ephemeral Arabidopsisthaliana is unusual because it responds preferentially to KAR₂. The α/β-hydrolase KARRIKIN INSENSITIVE 2 (KAI2) is the putative karrikin receptor identified in Arabidopsis. Here we show that B. tournefortii expresses three KAI2 homologues, and the most highly-expressed homologue is sufficient to confer enhanced responses to KAR₁ relative to KAR₂ when expressed in Arabidopsis. We identify two amino acid residues near the KAI2 active site that explain the ligand selectivity, and show that this combination has arisen independently multiple times within dicots. Our results suggest that duplication and diversification of KAI2 proteins could confer differential responses to chemical cues produced by environmental disturbance, including fire.

Karrikins are germination stimulants perceived by KAI2 in Arabidopsis. Here the authors show that Brassica tournefortii, a close relative to Arabidopsis, has multiple copies of KAI2 with amino acid substitutions that confer responsiveness to the specific karrikin compounds found in wildfire smoke.

Molecular basis of strigolactone perception in root-parasitic plants: aiming to control its germination with strigolactone agonists/antagonists

The genus Striga, also called "witchweed", is a member of the family Orobanchaceae, which is a major family of root-parasitic plants. Striga can lead to the formation of seed stocks in the soil and to explosive expansion with enormous seed production and stability once the crops they parasitize are cultivated. Understanding the molecular mechanism underlying the communication between Striga and their host plants through natural seed germination stimulants, "strigolactones (SLs)", is required to develop the technology for Striga control. This review outlines recent findings on the SL perception mechanism, which have been accumulated in Striga hermonthica by the similarity of the protein components that regulate SL signaling in nonparasitic model plants, including Arabidopsis and rice. HTL/KAI2 homologs were identified as SL receptors in the process of Striga seed germination. Recently, this molecular basis has further promoted the development of various types of SL agonists/antagonists as seed germination stimulants or inhibitors. Such chemical compounds are also useful to elucidate the dynamic behavior of SL receptors and the regulation of SL signaling.

Differential role of MAX2 and strigolactones in pathogen, ozone, and stomatal responses

Strigolactones are a group of phytohormones that control developmental processes including shoot branching and various plant-environment interactions in plants. We previously showed that the strigolactone perception mutant more axillary branches 2 (max2) has increased susceptibility to plant pathogenic bacteria. Here we show that both strigolactone biosynthesis (max3 and max4) and perception mutants (max2 and dwarf14) are significantly more sensitive to Pseudomonas syringae DC3000. Moreover, in response to P. syringae infection, high levels of SA accumulated in max2 and this mutant was ozone sensitive. Further analysis of gene expression revealed no major role for strigolactone in regulation of defense gene expression. In contrast, guard cell function was clearly impaired in max2 and depending on the assay used, also in max3, max4, and d14 mutants. We analyzed stomatal responses to stimuli that cause stomatal closure. While the response to abscisic acid (ABA) was not impaired in any of the mutants, the response to darkness and high CO2 was impaired in max2 and d14-1 mutants, and to CO2 also in strigolactone synthesis (max3, max4) mutants. To position the role of MAX2 in the guard cell signaling network, max2 was crossed with mutants defective in ABA biosynthesis or signaling. This revealed that MAX2 acts in a signaling pathway that functions in parallel to the guard cell ABA signaling pathway. We propose that the impaired defense responses of max2 are related to higher stomatal conductance that allows increased entry of bacteria or air pollutants like ozone. Furthermore, as MAX2 appears to act in a specific branch of guard cell signaling (related to CO2 signaling), this protein could be one of the components that allow guard cells to distinguish between different environmental conditions.

Knockout of VvCCD8 gene in grapevine affects shoot branching

BACKGROUND

Shoot branching is an important trait of plants that allows them to adapt to environment changes. Strigolactones (SLs) are newly identified plant hormones that inhibit shoot branching in plants. The SL biosynthesis genes CCD7 (carotenoid cleavage dioxygenase 7) and CCD8 have been found to regulate branching in several herbaceous plants by taking advantage of their loss-of-function mutants. However, the role for CCD7 and CCD8 in shoot branching control in grapevine is still unknown due to the lack of corresponding mutants.

RESULTS

Here we employed the CRISPR/Cas9 system to edit the VvCCD7 and VvCCD8 genes in the grape hybrid 41B. The 41B embryogenic cells can easily be transformed and used for regeneration of the corresponding transformed plants. Sequencing analysis revealed that gene editing has been used successfully to target both VvCCD genes in 41B embryogenic cells. After regeneration, six 41B plantlets were identified as transgenic plants carrying the CCD8-sgRNA expression cassette. Among these, four plants showed mutation in the target region and were selected as ccd8 mutants. These ccd8 mutants showed increased shoot branching compared to the corresponding wild-type plants. In addition, no off-target mutation was detected in the tested mutants at predicted off-target sites.

CONCLUSIONS

Our results underline the key role of VvCCD8 in the control of grapevine shoot branching.

Fellowship of the rings: a saga of strigolactones and other small signals

Strigolactones are an important class of plant signalling molecule with both external rhizospheric and internal hormonal functions in flowering plants. The past decade has seen staggering progress in strigolactone biology, permitting highly detailed understanding of their signalling, synthesis and biological roles - or so it seems. However, phylogenetic analyses show that strigolactone signalling mediated by the D14-SCF^MAX2 -SMXL7 complex is only one of a number of closely related signalling pathways, and is much less ubiquitous in land plants than might be expected. The existence of closely related pathways, such as the KAI2-SMAX1 module, challenges many of our assumptions about strigolactones, and in particular emphasises how little we understand about the specificity of strigolactone signalling with respect to related signalling pathways. In this review, we examine recent advances in strigolactone signalling, taking a holistic evolutionary view to identify the ambiguities and uncertainties in our understanding. We highlight that while we now have highly detailed molecular models for the core mechanism of D14-SMXL7 signalling, we still do not understand the ligand specificity of D14, the specificity of its interaction with SMXL7, nor the specificity of SMXL7 function. Our analysis therefore identifies key areas requiring further study.

Comparing and Contrasting the Multiple Roles of Butenolide Plant Growth Regulators: Strigolactones and Karrikins in Plant Development and Adaptation to Abiotic Stresses

Strigolactones (SLs) and karrikins (KARs) are both butenolide molecules that play essential roles in plant growth and development. SLs are phytohormones, with SLs having known functions within the plant they are produced in, while KARs are found in smoke emitted from burning plant matter and affect seeds and seedlings in areas of wildfire. It has been suggested that SL and KAR signaling may share similar mechanisms. The α/β hydrolases DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2), which act as receptors of SL and KAR, respectively, both interact with the F-box protein MORE AXILLARY GROWTH 2 (MAX2) in order to target SUPPRESSOR OF MAX2 1 (SMAX1)-LIKE/D53 family members for degradation via the 26S proteasome. Recent reports suggest that SLs and/or KARs are also involved in regulating plant responses and adaptation to various abiotic stresses, particularly nutrient deficiency, drought, salinity, and chilling. There is also crosstalk with other hormone signaling pathways, including auxin, gibberellic acid (GA), abscisic acid (ABA), cytokinin (CK), and ethylene (ET), under normal and abiotic stress conditions. This review briefly covers the biosynthetic and signaling pathways of SLs and KARs, compares their functions in plant growth and development, and reviews the effects of any crosstalk between SLs or KARs and other plant hormones at various stages of plant development. We also focus on the distinct responses, adaptations, and regulatory mechanisms related to SLs and/or KARs in response to various abiotic stresses. The review closes with discussion on ways to gain additional insights into the SL and KAR pathways and the crosstalk between these related phytohormones.

Contalactone, a contaminant formed during chemical synthesis of the strigolactone reference GR24 is also a strigolactone mimic

Strigolactone (SL) plant hormones control plant architecture and are key players in both symbiotic and parasitic interactions. GR24, a synthetic SL analog, is the worldwide reference compound used in all bioassays for investigating the role of SLs in plant development and in rhizospheric interactions. In 2012, the first characterization of the SL receptor reported the detection of an unknown compound after incubation of GR24 samples with the SL receptor. We reveal here the origin of this compound (P270), which comes from a by-product formed during GR24 chemical synthesis. We present the identification of this by-product, named contalactone. A proposed chemical pathway for its formation is provided as well as an evaluation of its bioactivity on pea, Arabidopsis, root parasitic plant seeds and AM fungi, characterizing it as a SL mimic. Quality of GR24 samples can be easily checked by carrying out microscale hydrolysis in a basic aqueous medium to easily detect P270 as indicator of the presence of the contalactone impurity. In all cases, before being used for bioassays, GR24 must be careful purified by preparative HPLC.

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.

Network trade-offs and homeostasis in Arabidopsis shoot architectures

Understanding the optimization objectives that shape shoot architectures remains a critical problem in plant biology. Here, we performed 3D scanning of 152 Arabidopsis shoot architectures, including wildtype and 10 mutant strains, and we uncovered a design principle that describes how architectures make trade-offs between competing objectives. First, we used graph-theoretic analysis to show that Arabidopsis shoot architectures strike a Pareto optimal that can be captured as maximizing performance in transporting nutrients and minimizing costs in building the architecture. Second, we identify small sets of genes that can be mutated to shift the weight prioritizing one objective over the other. Third, we show that this prioritization weight feature is significantly less variable across replicates of the same genotype compared to other common plant traits (e.g., number of rosette leaves, total volume occupied). This suggests that this feature is a robust descriptor of a genotype, and that local variability in structure may be compensated for globally in a homeostatic manner. Overall, our work provides a framework to understand optimization trade-offs made by shoot architectures and provides evidence that these trade-offs can be modified genetically, which may aid plant breeding and selection efforts.

In both engineered and biological systems, there is often no single structure that performs optimally on all tasks. For example, a transport system that can very quickly shuttle people to and from work will often not be very cheap to build, and vice-versa. Thus, trade-offs are born, and it is natural to ask how well evolution has resolved trade-offs between competing tasks. Here, we use 3D laser scanning and network analysis to show that Arabidopsis plant architectures make Pareto optimal trade-offs, which means that improving upon one task requires a sacrifice in the other task. In other words, an architecture that performs better on both tasks cannot be built. We also identify a small set of genes that can change how the architecture prioritizes one task versus the other, which may allow for better crop design in the future. Finally, we show that two replicate architectures that look visually diverse (e.g., variation in size, number of leaves, number of branches, etc.) often prioritize each task similarly. This suggests that despite local variability in the architecture, there may be a homeostatic drive to maintain globally balanced trade-offs.

Genome Sequence of Striga asiatica Provides Insight into the Evolution of Plant Parasitism

Parasitic plants in the genus Striga, commonly known as witchweeds, cause major crop losses in sub-Saharan Africa and pose a threat to agriculture worldwide. An understanding of Striga parasite biology, which could lead to agricultural solutions, has been hampered by the lack of genome information. Here, we report the draft genome sequence of Striga asiatica with 34,577 predicted protein-coding genes, which reflects gene family contractions and expansions that are consistent with a three-phase model of parasitic plant genome evolution. Striga seeds germinate in response to host-derived strigolactones (SLs) and then develop a specialized penetration structure, the haustorium, to invade the host root. A family of SL receptors has undergone a striking expansion, suggesting a molecular basis for the evolution of broad host range among Striga spp. We found that genes involved in lateral root development in non-parasitic model species are coordinately induced during haustorium development in Striga, suggesting a pathway that was partly co-opted during the evolution of the haustorium. In addition, we found evidence for horizontal transfer of host genes as well as retrotransposons, indicating gene flow to S. asiatica from hosts. Our results provide valuable insights into the evolution of parasitism and a key resource for the future development of Striga control strategies.

Synthetic agonist of HTL/KAI2 shows potent stimulating activity for Arabidopsis seed germination

HTL/KAI2, a member of the α/β-fold hydrolase superfamily, is known to be a receptor-like protein of lactone compounds and that triggers seed germination of Arabidopsis. However, the endogenous ligand and physiological roles of HTL/KAI2 have remained unclear. To understand the mechanism underlying seed germination involved in HTL/KAI2 signaling, it is necessary to identify the endogenous ligand of HTL/KAI2. To date, even a biosynthetic mutant of the ligand has not yet been isolated. Because exogenous agonistic chemicals can only be purchased in small amounts at high prices, the limited supply of those chemicals has hampered any large-scale experiments, such as mutant screening. Therefore, easily synthesized and scalable artificial agonist would remove the limitation of the chemical supply and contribute to the identification of the endogenous ligand of HTL/KAI2 and/or the biosynthetic mutants. In this study, we demonstrated that designed chemicals with a phenoxyfuranone scaffold potently stimulated seed germination via HTL/KAI2 in Arabidopsis. As a result of screening of these chemicals, we selected a representative compound with convincing selectivity. Here in, we provide a new promising synthetic agonist of HTL/KAI2.