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

The Multifaceted Impact of Karrikin Signaling in Plants

Karrikins (KARs), produced during wildfires, are bioactive compounds that stimulate seed germination in fire-prone ecosystems and influence broader plant-environment interactions. These compounds act through the α/β hydrolase receptor KARRIKIN INSENSITIVE2 (KAI2), which perceives KARs as analogs of the hypothesized phytohormone KAI2 ligand (KL). KAR signaling shares molecular parallels with strigolactones (SLs), another class of butenolide plant hormones, and regulates diverse processes such as seedling development, root architecture, photomorphogenesis, and stress responses. Despite its multifaceted roles, the mechanistic basis of KAR-mediated regulation remains poorly understood. This review synthesizes insights into KAR signaling mechanisms, emphasizing recent advances in signal transduction pathways and functional studies. It also addresses key unresolved questions, including the identity of endogenous KL and the crosstalk between KARs and other hormonal networks. By elucidating these mechanisms, KAR-based strategies hold promises for enhancing crop resilience and sustainability, offering novel avenues for agricultural innovation in changing environments.

Structural requirements of KAI2 ligands for activation of signal transduction

Karrikin Insensitive 2 (KAI2), identified as the receptor protein for karrikins (KARs), which are smoke-derived seed germination stimulants, belongs to the same α/β-hydrolase family as D14, the receptor for strigolactones (SLs). KAI2 is believed to recognize an endogenous butenolide (KAI2 ligand; KL), but the identity of this compound remains unknown. Recent studies have suggested that ligand hydrolysis by KAI2 is a prerequisite for receptor activation to induce interaction with the target proteins, similar to the situation with D14. However, direct experimental evidence has been lacking. Here, we designed KAI2 ligands (carba-dMGers) whose butenolide rings were modified so that they cannot be hydrolyzed or dissociated from the original ligand molecule by KAI2, by structurally modifying dMGer, a potent and selective KAI2 agonist. Using these dMGer analogs, we found that the strongly bioactive ligand, (+)-dMGer, was hydrolyzed by KAI2 at a lower enzymatic rate compared with the weakly bioactive ligand, (+)-1'-carba-dMGer, and the hydrolyzed butenolide ring of (+)-dMGer was transiently trapped in the catalytic pocket of KAI2. Additionally, structural analysis revealed that (+)-6'-carba-dMGer bound to the catalytic pocket of KAI2 in the unhydrolyzed state. However, this binding did not induce the interaction between KAI2 and SMAX1, indicating that ligand binding to the receptor alone was not sufficient for KAI2 signaling. This study showed experimental data from a ligand structure-activity study that ligand hydrolysis and subsequent covalent adduct formation with the catalytic triad plays a key role in KAI2 activation, providing insight into the chemical structure of the Arabidopsis KL.

Identification and structure-guided development of triazole urea-based selective antagonists of Arabidopsis karrikin signaling

The smoke-derived butenolides, karrikins (KARs), regulate many aspects of plant growth and development. However, KARs and a plant hormone, strigolactones (SLs), have high resemblance in signal perception and transduction, making it hard to delineate KARs response due to the shortage of chemical-genetic tools. Here, we identify a triazole urea KK181N1 as an inhibitor of the KARs receptor KAI2. KK181N1 selectively depress the KAR-induced phenotypes in Arabidopsis. We further elucidate the antagonistic, KAI2 binding mechanism of KK181N1, showing that KK181N1 binds to the catalytic pockets of KAI2 in a non-covalent binding manner. Our experiments also demonstrate the binding affinity of triazole urea compounds are regulated by the structured water molecule networks. By fine-tuning this network, we successfully develop a more potent derivative of KK181N1. We anticipate that these chemicals will be applicable to the elucidation of KARs biology, especially for discriminating the molecular and physiological aspects of KARs and SL signaling.

Highly Sensitive Strigolactone Perception by a Divergent Clade KAI2 Receptor in a Facultative Root Parasitic Plant, Phtheirospermum japonicum

Phtheirospermum japonicum, a member of the Orobanchaceae family, is a facultative root parasitic plant that can survive without parasitizing the host. In contrast, obligate root parasitic plants, such as Striga and Orobanche, which are also members of the Orobanchaceae family, cannot survive in the absence of the host. The germination of obligate root parasitic plants is typically induced by host root-derived strigolactones (SLs) at very low concentrations. The KAI2/HTL family proteins have been found to be involved in the perception of karrikin (KAR), a smoke-derived germination inducer and unidentified endogenous ligand, in non-parasitic plants. Obligate root parasitic plants possess uniquely diverged KAI2 clade genes, which are collectively referred to as KAI2d. Many of those have been shown to function as SL receptors. Intriguingly, the KAI2d clade genes are also conserved in P. japonicum, even though this plant does not require SLs for germination. The biochemical and physiological functions of the KAI2d proteins in P. japonicum remain unclear. Here, we report that some of these proteins can function as SL receptors in P. japonicum. Moreover, we found that one of them, PjKAI2d4, is highly sensitive to SLs when expressed in Arabidopsis, and it is similar to the sensitive SL receptors found in Striga and Orobanche. These results suggest that the KAI2d clade SL receptors play a crucial role not only in obligate parasites but also in facultative parasitic plants.

Dominant-Negative KAI2d Paralogs Putatively Attenuate Strigolactone Responses in Root Parasitic Plants

Many root parasitic plants in the Orobanchaceae use host-derived strigolactones (SLs) as germination cues. This adaptation facilitates attachment to a host and is particularly important for the success of obligate parasitic weeds that cause substantial crop losses globally. Parasite seeds sense SLs through 'divergent' KARRIKIN INSENSITIVE2 (KAI2d)/HYPOSENSITIVE TO LIGHT α/β-hydrolases that have undergone substantial duplication and diversification in Orobanchaceae genomes. After germination, chemotropic growth of parasite roots toward a SL source also occurs in some species. We investigated which of the seven KAI2d genes found in a facultative hemiparasite, Phtheirospermum japonicum, may enable chemotropic responses to SLs. To do so, we developed a triple mutant Nbd14a,b kai2i line of Nicotiana benthamiana in which SL-induced degradation of SUPPRESSOR OF MORE AXILLARY GROWTH2 (MAX2) 1 (SMAX1), an immediate downstream target of KAI2 signaling, is disrupted. In combination with a transiently expressed, ratiometric reporter of SMAX1 protein abundance, this mutant forms a system for the functional analysis of parasite KAI2d proteins in a plant cellular context. Using this system, we unexpectedly found three PjKAI2d proteins that do not trigger SMAX1 degradation in the presence of SLs. Instead, these PjKAI2d proteins inhibit the perception of low SL concentrations by SL-responsive PjKAI2d in a dominant-negative manner that depends upon an active catalytic triad. Similar dominant-negative KAI2d paralogs were identified in an obligate hemiparasitic weed, Striga hermonthica. These proteins suggest a mechanism for attenuating SL signaling in parasites, which might be used to enhance the perception of shallow SL gradients during root growth toward a host or to restrict germination responses to specific SLs.

Structural insights into rice KAI2 receptor provide functional implications for perception and signal transduction

KAI2 receptors, classified as plant α/β hydrolase enzymes, are capable of perceiving smoke-derived butenolide signals and endogenous yet unidentified KAI2-ligands (KLs). While the number of functional KAI2 receptors varies among land plant species, rice has only one KAI2 gene. Rice, a significant crop and representative of grasses, relies on KAI2-mediated Arbuscular mycorrhiza (AM) symbioses to flourish in traditionally arid and nutrient-poor environments. This study presents the first crystal structure of an active rice (Oryza sativa, Os) KAI2 hydrolase receptor. Our structural and biochemical analyses uncover grass-unique pocket residues influencing ligand sensitivity and hydrolytic activity. Through structure-guided analysis, we identify a specific residue whose mutation enables the increase or decrease of ligand perception, catalytic activity, and signal transduction. Furthermore, we investigate OsKAI2-mediated signaling by examining its ability to form a complex with its binding partner, the F-box protein DWARF3 (D3) ubiquitin ligase and subsequent degradation of the target substrate OsSMAX1, demonstrating the significant role of hydrophobic interactions in the OsKAI2-D3 interface. This study provides new insights into the diverse and pivotal roles of the OsKAI2 signaling pathway in the plant kingdom, particularly in grasses.

Exploring silique number in Brassica napus L.: Genetic and molecular advances for improving yield

Silique number is a crucial yield-related trait for the genetic enhancement of rapeseed (Brassica napus L.). The intricate molecular process governing the regulation of silique number involves various factors. Despite advancements in understanding the mechanisms regulating silique number in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), the molecular processes involved in controlling silique number in rapeseed remain largely unexplored. In this review, we identify candidate genes and review the roles of genes and environmental factors in regulating rapeseed silique number. We use genetic regulatory networks for silique number in Arabidopsis and grain number in rice to uncover possible regulatory pathways and molecular mechanisms involved in regulating genes associated with rapeseed silique number. A better understanding of the genetic network regulating silique number in rapeseed will provide a theoretical basis for the genetic improvement of this trait and genetic resources for the molecular breeding of high-yielding rapeseed.

Volatile communication in plants relies on a KAI2-mediated signaling pathway

Plants are constantly exposed to volatile organic compounds (VOCs) that are released during plant-plant communication, within-plant self-signaling, and plant-microbe interactions. Therefore, understanding VOC perception and downstream signaling is vital for unraveling the mechanisms behind information exchange in plants, which remain largely unexplored. Using the hormone-like function of volatile terpenoids in reproductive organ development as a system with a visual marker for communication, we demonstrate that a petunia karrikin-insensitive receptor, PhKAI2ia, stereospecifically perceives the (-)-germacrene D signal, triggering a KAI2-mediated signaling cascade and affecting plant fitness. This study uncovers the role(s) of the intermediate clade of KAI2 receptors, illuminates the involvement of a KAI2ia-dependent signaling pathway in volatile communication, and provides new insights into plant olfaction and the long-standing question about the nature of potential endogenous KAI2 ligand(s).

Karrikin signalling: impacts on plant development and abiotic stress tolerance

Plants rely upon a diverse range of metabolites to control growth and development, and to overcome stress that results from suboptimal conditions. Karrikins (KARs) are a class of butenolide compounds found in smoke that stimulate seed germination and regulate various developmental processes in plants. KARs are perceived via a plant α/β-hydrolase called KARRIKIN INSENSITIVE2 (KAI2), which also functions as a receptor for a postulated phytohormone, provisionally termed KAI2 ligand (KL). Considered natural analogues of KL, KARs have been extensively studied for their effects on plant growth and their crosstalk with plant hormones. The perception and response pathway for KAR-KL signalling is closely related to that of strigolactones, another class of butenolides with numerous functions in regulating plant growth. KAR-KL signalling influences seed germination, seedling photomorphogenesis, root system architecture, abiotic stress responses, and arbuscular mycorrhizal symbiosis. Here, we summarize current knowledge of KAR-KL signalling, focusing on its role in plant development, its effects on stress tolerance, and its interaction with other signalling mechanisms.

Perception of butenolides by Bacillus subtilis via the α/β hydrolase RsbQ

The regulation of behavioral and developmental decisions by small molecules is common to all domains of life. In plants, strigolactones and karrikins are butenolide growth regulators that influence several aspects of plant growth and development, as well as interactions with symbiotic fungi.1,2,3 DWARF14 (D14) and KARRIKIN INSENSITIVE2 (KAI2) are homologous enzyme-receptors that perceive strigolactones and karrikins, respectively, and that require hydrolase activity to effect signal transduction.4,5,6,7 RsbQ, a homolog of D14 and KAI2 from the gram-positive bacterium Bacillus subtilis, regulates growth responses to nutritional stress via the alternative transcription factor SigmaB (σB).8,9 However, the molecular function of RsbQ is unknown. Here, we show that RsbQ perceives butenolide compounds that are bioactive in plants. RsbQ is thermally destabilized by the synthetic strigolactone GR24 and its desmethyl butenolide equivalent dGR24. We show that, like D14 and KAI2, RsbQ is a functional butenolide hydrolase that undergoes covalent modification of the catalytic histidine residue. Exogenous application of both GR24 and dGR24 inhibited the endogenous signaling function of RsbQ in vivo, with dGR24 being 10-fold more potent. Application of dGR24 to B. subtilis phenocopied loss-of-function rsbQ mutations and led to a significant downregulation of σB-regulated transcripts. We also discovered that exogenous butenolides promoted the transition from planktonic to biofilm growth. Our results suggest that butenolides may serve as inter-kingdom signaling compounds between plants and bacteria to help shape rhizosphere communities.

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

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

Karrikin perception and signalling

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

KARRIKIN UPREGULATED F-BOX 1 negatively regulates drought tolerance in Arabidopsis

The karrikin (KAR) receptor and several related signaling components have been identified by forward genetic screening, but only a few studies have reported on upstream and downstream KAR signaling components and their roles in drought tolerance. Here, we characterized the functions of KAR UPREGULATED F-BOX 1 (KUF1) in drought tolerance using a reverse genetics approach in Arabidopsis (Arabidopsis thaliana). We observed that kuf1 mutant plants were more tolerant to drought stress than wild-type (WT) plants. To clarify the mechanisms by which KUF1 negatively regulates drought tolerance, we performed physiological, transcriptome, and morphological analyses. We found that kuf1 plants limited leaf water loss by reducing stomatal aperture and cuticular permeability. In addition, kuf1 plants showed increased sensitivity of stomatal closure, seed germination, primary root growth, and leaf senescence to abscisic acid (ABA). Genome-wide transcriptome comparisons of kuf1 and WT rosette leaves before and after dehydration showed that the differences in various drought tolerance-related traits were accompanied by differences in the expression of genes associated with stomatal closure (e.g. OPEN STOMATA 1), lipid and fatty acid metabolism (e.g. WAX ESTER SYNTHASE), and ABA responsiveness (e.g. ABA-RESPONSIVE ELEMENT 3). The kuf1 mutant plants had higher root/shoot ratios and root hair densities than WT plants, suggesting that they could absorb more water than WT plants. Together, these results demonstrate that KUF1 negatively regulates drought tolerance by modulating various physiological traits, morphological adjustments, and ABA responses and that the genetic manipulation of KUF1 in crops is a potential means of enhancing their drought tolerance.

Rapid analysis of strigolactone receptor activity in a Nicotiana benthamiana dwarf14 mutant

DWARF14 (D14) is an ɑ/β-hydrolase and receptor for the plant hormone strigolactone (SL) in angiosperms. Upon SL perception, D14 works with MORE AXILLARY GROWTH2 (MAX2) to trigger polyubiquitination and degradation of DWARF53(D53)-type proteins in the SUPPRESSOR OF MAX2 1-LIKE (SMXL) family. We used CRISPR-Cas9 to generate knockout alleles of the two homoeologous D14 genes in the Nicotiana benthamiana genome. The Nbd14a,b double mutant had several phenotypes that are consistent with the loss of SL perception in other plants, including increased axillary bud outgrowth, reduced height, shortened petioles, and smaller leaves. A ratiometric fluorescent reporter system was used to monitor degradation of SMXL7 from Arabidopsis thaliana (AtSMXL7) after transient expression in N. benthamiana and treatment with the strigolactone analog GR24. AtSMXL7 was degraded after treatment with GR245DS, which has the stereochemical configuration of natural SLs, as well as its enantiomer GR24 ent-5DS. In Nbd14a,b leaves, AtSMXL7 abundance was unaffected by rac-GR24 or either GR24 stereoisomer. Transient coexpression of AtD14 with the AtSMXL7 reporter in Nbd14a,b restored the degradation response to rac-GR24, but required an active catalytic triad. We used this platform to evaluate the ability of several AtD14 mutants that had not been characterized in plants to target AtSMXL7 for degradation.

KARRIKIN INSENSITIVE2 regulates leaf development, root system architecture and arbuscular-mycorrhizal symbiosis in Brachypodium distachyon

KARRIKIN INSENSITIVE2 (KAI2) is an α/β-hydrolase required for plant responses to karrikins, which are abiotic butenolides that can influence seed germination and seedling growth. Although represented by four angiosperm species, loss-of-function kai2 mutants are phenotypically inconsistent and incompletely characterised, resulting in uncertainties about the core functions of KAI2 in plant development. Here we characterised the developmental functions of KAI2 in the grass Brachypodium distachyon using molecular, physiological and biochemical approaches. Bdkai2 mutants exhibit increased internode elongation and reduced leaf chlorophyll levels, but only a modest increase in water loss from detached leaves. Bdkai2 shows increased numbers of lateral roots and reduced root hair growth, and fails to support normal root colonisation by arbuscular-mycorrhizal (AM) fungi. The karrikins KAR₁ and KAR₂ , and the strigolactone (SL) analogue rac-GR24, each elicit overlapping but distinct changes to the shoot transcriptome via BdKAI2. Finally, we show that BdKAI2 exhibits a clear ligand preference for desmethyl butenolides and weak responses to methyl-substituted SL analogues such as GR24. Our findings suggest that KAI2 has multiple roles in shoot development, root system development and transcriptional regulation in grasses. Although KAI2-dependent AM symbiosis is likely conserved within monocots, the magnitude of the effect of KAI2 on water relations may vary across angiosperms.

Structural and functional analyses explain Pea KAI2 receptor diversity and reveal stereoselective catalysis during signal perception

KAI2 proteins are plant α/β hydrolase receptors which perceive smoke-derived butenolide signals and endogenous, yet unidentified KAI2-ligands (KLs). The number of functional KAI2 receptors varies among species and KAI2 gene duplication and sub-functionalization likely plays an adaptative role by altering specificity towards different KLs. Legumes represent one of the largest families of flowering plants and contain many agronomic crops. Prior to their diversification, KAI2 underwent duplication resulting in KAI2A and KAI2B. Here we demonstrate that Pisum sativum KAI2A and KAI2B are active receptors and enzymes with divergent ligand stereoselectivity. KAI2B has a higher affinity for and hydrolyses a broader range of substrates including strigolactone-like stereoisomers. We determine the crystal structures of PsKAI2B in apo and butenolide-bound states. The biochemical, structural, and mass spectra analyses of KAI2s reveal a transient intermediate on the catalytic serine and a stable adduct on the catalytic histidine, confirming its role as a bona fide enzyme. Our work uncovers the stereoselectivity of ligand perception and catalysis by diverged KAI2 receptors and proposes adaptive sensitivity to KAR/KL and strigolactones by KAI2B.

Masks Start to Drop: Suppressor of MAX2 1-Like Proteins Reveal Their Many Faces

Although the main players of the strigolactone (SL) signaling pathway have been characterized genetically, how they regulate plant development is still poorly understood. Of central importance are the SUPPRESSOR OF MAX2 1-LIKE (SMXL) proteins that belong to a family of eight members in Arabidopsis thaliana, of which one subclade is involved in SL signaling and another one in the pathway of the chemically related karrikins. Through proteasomal degradation of these SMXLs, triggered by either DWARF14 (D14) or KARRIKIN INSENSITIVE2 (KAI2), several physiological processes are controlled, such as, among others, shoot and root architecture, seed germination, and seedling photomorphogenesis. Yet another clade has been shown to be involved in vascular development, independently of the D14 and KAI2 actions and not relying on proteasomal degradation. Despite their role in several aspects of plant development, the exact molecular mechanisms by which SMXLs regulate them are not completely unraveled. To fill the major knowledge gap in understanding D14 and KAI2 signaling, SMXLs are intensively studied, making it challenging to combine all the insights into a coherent characterization of these important proteins. To this end, this review provides an in-depth exploration of the recent data regarding their physiological function, evolution, structure, and molecular mechanism. In addition, we propose a selection of future perspectives, focusing on the apparent localization of SMXLs in subnuclear speckles, as observed in transient expression assays, which we couple to recent advances in the field of biomolecular condensates and liquid-liquid phase separation.

Fluorescent biosensors illuminating plant hormone research

Phytohormones act as key regulators of plant growth that coordinate developmental and physiological processes across cells, tissues and organs. As such, their levels and distribution are highly dynamic owing to changes in their biosynthesis, transport, modification and degradation that occur over space and time. Fluorescent biosensors represent ideal tools to track these dynamics with high spatiotemporal resolution in a minimally invasive manner. Substantial progress has been made in generating a diverse set of hormone sensors with recent FRET biosensors for visualising hormone concentrations complementing information provided by transcriptional, translational and degron-based reporters. In this review, we provide an update on fluorescent biosensor designs, examine the key properties that constitute an ideal hormone biosensor, discuss the use of these sensors in conjunction with in vivo hormone perturbations and highlight the latest discoveries made using these tools.

Discovery of a Broad-Spectrum Fluorogenic Agonist for Strigolactone Receptors through a Computational Approach

Strigolactones (SLs) are plant hormones that play various roles in plant physiology, including provoking the germination of parasitic weeds Orobanche and Striga. A family of α/β-hydrolases have been proposed to be the SL receptor proteins. Effective assays for measuring the activity of SL receptors could promote the development of SL-related biology and chemistry. In this study, we developed a new approach called pharmacophore-linked probe virtual screening (PPVS). Its application yielded an effective "off-on" probe named Xilatone Red (XLR). This probe showed a broad spectrum and excellent sensitivity toward SL receptors, including ShD14 (Striga D14), for which the detection limit was determined to be in the micromolar range, outperforming that of the commercial fluorogenic agonist Yoshimulactone Green (YLG). Upon hydrolysis by SL receptors, XLR provided fluorogenic and colorimetric signaling responses. Furthermore, XLR could induce germination of Phelipanche aegyptiaca seeds and prevent Arabidopsis max4-1 branching defects at micromolar concentrations. Our molecular simulations revealed the essential factors in the molecular perception of XLR. We anticipate that this study can prompt the discovery of high-performance SL agonists/antagonists to combat parasitic weeds.

A Phelipanche ramosa KAI2 protein perceives strigolactones and isothiocyanates enzymatically

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

Karrikins control seedling photomorphogenesis and anthocyanin biosynthesis through a HY5-BBX transcriptional module

The butenolide molecule, karrikin (KAR), emerging in smoke of burned plant material, enhances light responses such as germination, inhibition of hypocotyl elongation, and anthocyanin accumulation in Arabidopsis. The KAR signaling pathway consists of KARRIKIN INSENSITIVE 2 (KAI2) and MORE AXILLARY GROWTH 2 (MAX2), which, upon activation, act in an SCF E3 ubiquitin ligase complex to target the downstream signaling components SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE 2 (SMXL2) for degradation. How degradation of SMAX1 and SMXL2 is translated into growth responses remains unknown. Although light clearly influences the activity of KAR, the molecular connection between the two pathways is still poorly understood. Here, we demonstrate that the KAR signaling pathway promotes the activity of a transcriptional module consisting of ELONGATED HYPOCOTYL 5 (HY5), B-BOX DOMAIN PROTEIN 20 (BBX20), and BBX21. The bbx20 bbx21 mutant is largely insensitive to treatment with KAR₂ , similar to a hy5 mutant, with regards to inhibition of hypocotyl elongation and anthocyanin accumulation. Detailed analysis of higher order mutants in combination with RNA-sequencing analysis revealed that anthocyanin accumulation downstream of SMAX1 and SMXL2 is fully dependent on the HY5-BBX module. However, the promotion of hypocotyl elongation by SMAX1 and SMXL2 is, in contrast to KAR₂ treatment, only partially dependent on BBX20, BBX21, and HY5. Taken together, these results suggest that light- and KAR-dependent signaling intersect at the HY5-BBX transcriptional module.

Three mutations repurpose a plant karrikin receptor to a strigolactone receptor

Uncovering the basis of small-molecule hormone receptors' evolution is paramount to a complete understanding of how protein structure drives function. In plants, hormone receptors for strigolactones are well suited to evolutionary inquiries because closely related homologs have different ligand preferences. More importantly, because of facile plant transgenic systems, receptors can be swapped and quickly assessed functionally in vivo. Here, we show that only three mutations are required to turn the nonstrigolactone receptor, KAI2, into a receptor that recognizes the plant hormone strigolactone. This modified receptor still retains its native function to perceive KAI2 ligands. Our directed evolution studies indicate that only a few keystone mutations are required to increase receptor promiscuity of KAI2, which may have implications for strigolactone receptor evolution in parasitic plants.

Desmethyl butenolides are optimal ligands for karrikin receptor proteins

Strigolactones and karrikins are butenolide molecules that regulate plant growth. They are perceived by the α/β-hydrolase DWARF14 (D14) and its homologue KARRIKIN INSENSITIVE2 (KAI2), respectively. Plant-derived strigolactones have a butenolide ring with a methyl group that is essential for bioactivity. By contrast, karrikins are abiotic in origin, and the butenolide methyl group is nonessential. KAI2 is probably a receptor for an endogenous butenolide, but the identity of this compound remains unknown. Here we characterise the specificity of KAI2 towards differing butenolide ligands using genetic and biochemical approaches. We find that KAI2 proteins from multiple species are most sensitive to desmethyl butenolides that lack a methyl group. Desmethyl-GR24 and desmethyl-CN-debranone are active by KAI2 but not D14. They are more potent KAI2 agonists compared with their methyl-substituted reference compounds both in vitro and in plants. The preference of KAI2 for desmethyl butenolides is conserved in Selaginella moellendorffii and Marchantia polymorpha, suggesting that it is an ancient trait in land plant evolution. Our findings provide insight into the mechanistic basis for differential ligand perception by KAI2 and D14, and support the view that the endogenous substrates for KAI2 and D14 have distinct chemical structures and biosynthetic origins.

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.

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.

The Use of Differential Scanning Fluorimetry to Assess Strigolactone Receptor Function

Differential scanning fluorimetry (DSF) is a method used for assessing the interaction of ligands with proteins. In most cases binding of a ligand to proteins tends to increase the melting temperature (T_m) of the protein involved. However, in the case of strigolactone receptors (e.g., D14, AtD14, DAD2, RMS3) from plants, the T_m tends to be reduced in the presence of strigolactones. This is likely due to increased flexibility of the receptors in the presence of hormone ligands.DSF experiments are simple, fast, amenable to high-throughput formats, and cost effective. They have therefore gained in popularity, including within the field of SL signaling. Typically in DSF the receptor protein is purified and incubated with the ligand (strigolactone, agonist, or antagonist) and a (fluorescent) reporter dye. The mixture is then placed in a quantitative PCR instrument and subjected to an increasing temperature gradient. Changes in fluorescence are recorded along the gradient, as the dye interacts with unfolded portions of the protein becoming accessible when the protein "melts". Differences in the temperature at which the protein unfolds in the absence and in the presence of the ligand are interpreted as indicating interactions between the ligand and the receptor.

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.

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.

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

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

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.

Crystal structure and biochemical characterization of Striga hermonthica HYPO-SENSITIVE TO LIGHT 8 (ShHTL8) in strigolactone signaling pathway

Striga is a parasitic weed that disperses easily, and its seeds can persist in the soil for many years, presenting long-term threats to food security. If SLs stimulate the seed germination of root parasitic weeds before planting, weeds will wither due to no host. Therefore, it is necessary to determine the mechanism of strigolactone (SL) signaling in Striga to reduce the impacts of this parasitic weed. Striga has eleven different kinds of HYPO-SENSITIVE to LIGHT (ShHTL) hydrolases. Different ShHTL hydrolases exhibit distinct responses to SLs, despite these ShHTLs exhibiting more than 60% sequence identity. Currently, structural information is available for only five ShHTL proteins, and more structural information is needed to design Striga germination stimulants or inhibitors. In this paper, we report the crystal structure of ShHTL8, which is determined at a resolution of 1.4 Å. Scanning fluorimetry and HPLC assays indicate that L125, M147, M154 and I194 are important binding sites, and of which L125 may act as a key holder involved in the catalytic reaction. Additionally, the corresponding residue, Y124 of ShHTL1 and F135 of ShHTL2 also play a significant role in the substrate recognition.

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.

GmMAX2-D14 and -KAI interaction-mediated SL and KAR signaling play essential roles in soybean root nodulation

Despite of important functions of strigolactones (SLs) and karrikins (KARs) in plant development, plant-parasite and plant-fungi interactions, their roles in soybean-rhizobia interaction remain elusive. SL/KAR signaling genes GmMAX2a, GmD14s, and GmKAIs are activated by rhizobia infection. GmMAX2a restored atmax2 root hair defects and soybean root hairs were changed in GmMAX2a overexpression (GmMAX2a-OE) or knockdown (GmMAX2a-KD) mutants. GmMAX2a-KD gave fewer, whereas GmMAX2a-OE produced more nodules than GUS hairy roots. Mutation of GmMAX2a in its KD or OE transgenic hairy roots affected the rhizobia infection-induced increases in early nodulation gene expression. Both mutant hairy roots also displayed the altered auxin, jasmonate and abscisic acid levels, as further verified by transcriptomic analyses of their synthetic genes. Overexpression of an auxin synthetic gene GmYUC2a also affected SL and KAR signaling genes. GmMAX2a physically interacted with SL/KAR receptors GmD14s, GmKAIs, and GmD14Ls with different binding affinities, depending on variations in the critical amino acids, forming active D14/KAI-SCFMAX2 complexes. The knockdown mutant roots of the nodule-specifically expressing GmKAIs and GmD14Ls gave fewer nodules, with altered expression of several early nodulation genes. The expression levels of GmKAIs, and GmD14Ls were markedly changed in GmMAX2a mutant roots, so did their target repressor genes GmD53s and GmSMAX1s. Thus, SL and KAR signaling were involved in soybean-rhizobia interaction and nodulation partly through interactions with hormones, and this may explain the different effects of MXA2 orthologs on legume determinate and indeterminate nodulation. The study provides fresh insights into the roles of GmMAX2-mediated SL/KAR signaling in soybean root hair and nodule formation.

Structural analysis of HTL and D14 proteins reveals the basis for ligand selectivity in Striga

HYPOSENSITIVE TO LIGHT (HTL) and DWARF14 (D14) mediate the perception of karrikin and strigolactone, which stimulates germination of the parasitic weed Striga. However, their role in parasitic seeds is poorly understood, and the basis for their differing responsiveness remains unclear. Here, we show that Striga hermonthica HTL proteins (ShHTLs) in ‘conserved’ and ‘intermediate’ clades are able to bind karrikin. The ‘divergent’ clade is able to hydrolyze strigolactone. Unexpectedly, we find that ShD14 is also capable of hydrolyzing strigolactone. Through comparative analysis of ShHTLs and ShD14 crystal structures, we provide insights into the basis for their selectivity. Moreover, we show that both ShD14 and divergent clade ShHTLs, but not conserved and intermediate clade ShHTLs, can interact with the putative downstream signaling component ShMAX2 in the presence of the synthetic strigolactone, rac-GR24. These findings provide insight into how strigolactone is perceived and how ligand specificity is determined.

HTL and D14 receptors perceive the structurally similar signaling compounds karrikin and strigolactone. Here, the authors show that ShD14 and a divergent clade of ShHTLs from Strigae capable of recognizing strigolact are capable of recognizing strigolactone and provide structural insights into the evolution of ligand specificity.