SMAX1-LIKE/D53 Family Members Enable Distinct MAX2-Dependent Responses to Strigolactones and Karrikins in Arabidopsis

Transcriptional regulation of development by SMAX1-LIKE proteins - targets of strigolactone and karrikin/KAI2 ligand signaling

SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE (SMXL) proteins comprise a family of plant growth regulators that includes downstream targets of the karrikin (KAR)/KAI2 ligand (KL) and strigolactone (SL) signaling pathways. Following the perception of KAR/KL or SL signals by α/β hydrolases, some types of SMXL proteins are polyubiquitinated by an E3 ubiquitin ligase complex containing the F-box protein MORE AXILLARY GROWTH2 (MAX2)/DWARF3 (D3), and proteolyzed. Because SMXL proteins interact with TOPLESS (TPL) and TPL-related (TPR) transcriptional co-repressors, SMXL degradation initiates changes in gene expression. This simplified model of SMXL regulation and function in plants must now be revised in light of recent discoveries. It has become apparent that SMXL abundance is not regulated by KAR/KL or SL alone, and that some SMXL proteins are not regulated by MAX2/D3 at all. Therefore, SMXL proteins should be considered as signaling hubs that integrate multiple cues. Here we review the current knowledge of how SMXL proteins impose transcriptional regulation of plant development and environmental responses. SMXL proteins can bind DNA directly and interact with transcriptional regulators from several protein families. Multiple mechanisms of downstream genetic control by SMXL proteins have been identified recently that do not involve the recruitment of TPL/TPR, expanding the paradigm of SMXL function.

Strigolactones optimise plant water usage by modulating vessel formation

Wood formation is crucial for plant growth, enabling water and nutrient transport through vessel elements, derived from cambium stem cells (CSCs). CSCs produce vascular cell types in a bidirectional manner, but their regulation and cell fate trajectories remain unclear. Here, using single-cell transcriptome analysis in Arabidopsis thaliana, we reveal that the strigolactone (SL) signalling pathway negatively regulates vessel element formation, impacting plant water usage. While SL signalling is generally active in differentiating vascular tissues, it is low in developing vessels and CSCs, where it modulates cell fate decisions and drought response. SL-dependent changes in vessel element formation directly affect transpiration rates via stomata, underscoring the importance of vascular tissue composition in water balance. Our findings demonstrate the role of structural alignment in water-transport tissues under unstable water conditions, offering insights for enhancing drought resistance in plants through long-term modulation of vascular development.

Strigolactone promotes cotton fiber cell elongation by de-repressing DWARF53 on linolenic acid biosynthesis

Strigolactone (SL) is a plant hormone required for plant development. DWARF53 (D53) functions as a transcription repressor in SL signaling. However, the role of D53 in cotton (Gossypium hirsutum, Gh) fiber development remains unclear. Here, we identify that GhD53 suppresses fiber elongation by repressing transcription of GhFAD3 genes, which control linolenic acid (C18:3) biosynthesis. Mechanistically, GhD53 interacts with SL-related transcriptional activate factor (GhSLRF) to prevent its binding on Omega-3 fatty acid desaturase gene (GhFAD3) promoters, thereby inhibiting GhFAD3 transcription. Upon SL exposure, GhD53 is degraded and leads to GhSLRF activation. This activation further promotes GhFAD3 transcription, C18:3 biosynthesis, and fiber elongation. Our findings identify the molecular mechanism of how SL controls cell elongation via D53 and offer potential strategies to improve cotton quality through SL application.

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.

Strigolactone insensitivity affects the hormonal homeostasis in barley

In response to environmental changes, plants continuously make architectural changes in order to optimize their growth and development. The regulation of plant branching, influenced by environmental conditions and affecting hormone balance and gene expression, is crucial for agronomic purposes due to its direct correlation with yield. Strigolactones (SL), the youngest class of phytohormones, function to shape the architecture of plants by inhibiting axillary outgrowth. Barley plants harboring the mutation in the HvDWARF14 (HvD14) gene, which encodes the SL-specific receptor, produce almost twice as many tillers as wild-type (WT) Sebastian plants. Here, through hormone profiling and comparison of transcriptomic and proteomic changes between 2- and 4-week-old plants of WT and hvd14 genotypes, we elucidate a regulatory mechanism that might affect the tillering of SL-insensitive plants. The analysis showed statistically significant increased cytokinin content and decreased auxin and abscisic acid content in 'bushy' hvd14 compared to WT, which aligns with the commonly known actions of these hormones regarding branching regulation. Further, transcriptomic and proteomic analysis revealed a set of differentially expressed genes (DEG) and abundant proteins (DAP), among which 11.6% and 14.6% were associated with phytohormone-related processes, respectively. Bioinformatics analyses then identified a series of potential SL-dependent transcription factors (TF), which may control the differences observed in the hvd14 transcriptome and proteome. Comparison to available Arabidopsis thaliana data implicates a sub-selection of these TF as being involved in the transduction of SL signal in both monocotyledonous and dicotyledonous plants.

Evolution of the JULGI-SMXL4/5 module for phloem development in angiosperms

Bifacial cambium, which produces xylem and phloem, and monopodial architecture, characterized by apical dominance and lateral branching from axillary buds, are key developmental features of seed plants, consisting of angiosperms and gymnosperms. These allow seed plants to adapt to diverse environments by optimizing resource allocation and structural integrity. In seed plants, SUPPRESSOR OF MAX2-LIKE (SMXL) family members function in phloem development and strigolactone-induced inhibition of axillary bud outgrowth. Although strigolactone signaling regulates most SMXL family members, the only known regulator of SMXL4 and SMXL5 is the RNA-binding protein JULGI. We demonstrate that in angiosperms, by directly regulating SMXL4/5 expression, JULGI uncouples SMXL4/5 activity from strigolactone signaling. JULGI and ancestral SMXLs from seedless vascular plants or SMXL4/5 from seed plants are coexpressed in the phloem tissues of vascular plants, from lycophytes to angiosperms. Core angiosperm SMXL4/5 mRNAs contain a G-rich element in the 5' untranslated region (UTR) that serves as a target sequence for JULGI to negatively regulate SMXL4/5 expression. Heterologous expression of JULGIs from various angiosperms rescued the Arabidopsis jul1 jul2 mutant. Expressing SMXL4/5s from seed plants and ancestral SMXLs rescued Arabidopsis smxl4 smxl5. Angiosperm SMXL4/5s lack an RGKT motif for proteasomal degradation. Indeed, treatment with the synthetic strigolactone analog rac-GR24 induced proteasomal degradation of SMXL from ferns and SMXL5a from gymnosperms, but not SMXL4/5 from angiosperms. These findings suggest that in ancestral angiosperms, the 5' UTR of SMXL4/5 gained G-rich elements, creating a regulatory module with JULGI that allows the phloem development pathway to act independently of strigolactone signaling.

Manipulation of a strigolactone transporter in tomato confers resistance to the parasitic weed broomrape

Parasitic weeds of the Orobanchaceae family cause substantial economic losses and pose significant threats to global agriculture. However, management of such parasitism is challenging, and very few resistance genes have been cloned and characterized in depth. Here, we performed a genome-wide association study using 152 tomato accessions and identified SlABCG45 as a key gene that mediates host resistance to Phelipanche aegyptiaca by affecting the level of strigolactones (SLs) in root exudates. SLs are synthesized and released by host plants and act as germination stimulants for parasitic weeds. We found that SlABCG45 and its close homolog SlABCG44 were membrane-localized SL transporters with essential roles in exudation of SLs to the rhizosphere, resistance to Phelipanche and Orobanche, and upward transport of SLs from roots to shoots. As a predominant environmental stimulant exacerbates parasitism, phosphorus deficiency dramatically induced SlABCG45 expression and weakly induced SlABCG44 expression via the transcription factors SlNSP1 and SlNSP2. Knockout of SlABCG45 in tomato had little effect on yield traits in a broomrape-free field, but conferred increased resistance to different Phelipanche and Orobanche species, resulting in an ∼30% yield increase in a Phelipanche-infested field. Our findings reveal that targeting a single gene by genome editing can confer broad-spectrum parasite resistance in tomato, providing an effective strategy for the sustainable control of parasitic plants in agriculture.

Identification and functional analysis of strigolactone pathway genes regulating tillering traits in sugarcane

Saccharum officinarum and Saccharum spontaneum are two fundamental species of modern sugarcane cultivars, exhibiting divergent tillering patterns crucial for sugarcane architecture and yield. Strigolactones (SLs), a class of plant hormones, are considered to play a central role in shaping plant form and regulating tillering. Our study highlights the distinct tillering patterns observed between S. officinarum and S. spontaneum and implicates significant differences in SL levels in root exudates between the two species. Treatment with rac-GR24 (an artificial SL analog) suppressed tillering in S. spontaneum. Based on transcriptome analysis, we focused on two genes, TRANSCRIPTION ELONGATION FACTOR 1 (TEF1) and CIRCADIAN CLOCK ASSOCIATED1 (CCA1), which show higher expression in S. spontaneum or S. officinarum, respectively. While the overexpression of SoCCA1 did not lead to significant phenotypic differences, overexpression of SsTEF1 in rice stimulated tillering and inhibited plant height, demonstrating its role in tillering regulation. However, the overexpression of suggests that SoCCA1 may not be the key regulator of sugarcane tillering. Yeast one-hybrid assays identified four transcription factors (TFs) regulating SsTEF1 and four and five TFs regulating SsCCA1 and SoCCA1. This study provides a theoretical foundation for deciphering the molecular mechanisms underlying the different tillering behaviors between S. officinarum and S. spontaneum, providing valuable insights for the molecular-based design of sugarcane breeding strategies.

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.

KAI2-dependent signaling controls vegetative reproduction in Marchantia polymorpha through activation of LOG-mediated cytokinin synthesis (14)

Marchantia polymorpha reproduces vegetatively (asexually) by producing propagules known as gemmae within gemma cups and sexually through spores. We previously reported that KARRIKIN INSENSITIVE2 (KAI2)-dependent signaling promotes gemma cup and gemma formation. KAI2A perceives unidentified endogenous ligand(s), tentatively referred to as KAI2 ligands (KL). Perception of KL by KAI2 triggers MORE AXILLARY GROWTH2 (MAX2)-dependent proteolysis of MpSUPPRESSOR of MORE AXILLALRY GROWTH2 1-LIKE (MpSMXL). In this study, we identify genes working downstream of KAI2-dependent signaling in M. polymorpha. We find that KAI2-dependent signaling positively controls the expression of MpLONELY GUY (MpLOG), encoding a cytokinin biosynthesis enzyme. Disruption of the MpLOG function decreases endogenous cytokinin levels and causes defects similar to KAI2-dependent signaling mutants. Moreover, supplying exogenous cytokinins rescues the defects of Mplog and KAI2-dependent signaling mutants, implying that cytokinins work downstream of KAI2-dependent signaling. Activation of MpLOG by KAI2-dependent signaling occurs in a highly cell-type-specific manner, leading to cell-specific induction of GEMMA CUP-ASSOCIATED MYB1 (GCAM1), the master regulator of vegetative reproduction of M. polymorpha. We propose a genetic cascade, starting from KAI2-dependent signaling, that promotes vegetative reproduction through the induction of MpLOG and GCAM1. The interaction between KAI2-dependent signaling and cytokinin in M. polymorpha provides insights into the function and evolution of KAI2-dependent signaling.

Cotton GhMAX2 promotes single-celled fiber elongation by releasing the GhS1FA-mediated inhibition of fatty acid biosynthesis

KEY MESSAGE

Cotton GhMAX2 positively regulates fiber elongation by mediating the degradation of GhS1FA, which transcriptionally represses GhKCS9 expression. Strigolactones (SLs) are known to promote cotton fiber development. However, the precise molecular relationship between SL signaling and fiber cell elongation remains unclear. In this study, we investigate the role of F-box E3 ligase MORE AXILLARY GROWTH2 (MAX2) in upland cotton in relation to the regulation of fiber development. GhMAX2b and GhMAX2f act as key components for SL signal transduction, with their loss-of-function leading to a notable reduction in fiber length. Biochemical analysis showed that GhMAX2b/f trigger the ubiquitination and subsequent degradation of the transcription repressor strigolactone-1-factor-At (GhS1FA), which function as a substrate for these E3 ligases. Furthermore, GhS1FA inhibits fatty acids biosynthesis by directly binding to the W-box element within the promoter of 3-ketoacyl-CoA synthases 9 (GhKCS9) and repressing its expression. In summary, we propose that GhMAX2b/f promote fiber elongation, potentially operating partially independently of GhD53 degradation.

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.

Smoke-water treatment of seeds, an ancient technique for increasing seed vigor

Germination is an essential phenomenon in the life cycle of plants, and a variety of external and internal factors influence it. Fire and the produced smoke have been vital environmental stimulants for the germination of seeds in many plant species, like Leucospermum cordifolium and Serruria florida. These plants do not germinate at all if fire and smoke are not present. This phenomenon of germination in plant species has existed in the ecosystem since ancient times. Various studies to study the response of seeds to smoke and its extracts have been undertaken for stimulation of germination by burning various plant materials and bubbling the smoke produced through water. The application of plant-derived smoke and smoke water is well known for promoting germination, breaking dormancy, and checking abiotic stress. This significantly indicates that plant-derived smoke contains some bioactive metabolites responsible for the physiological metabolism of seed germination and is involved in enhancing seed vigor. The present review deals with the ancient use of smoke and smoke extracts for seed priming, the cost-efficient method of its preparation, the mode of action of karrikins relating to its perception by plants, and its significant effects on various crops, including its ability to check biotic and abiotic stresses.

Regulatory mechanisms of strigolactone perception in rice

Strigolactones (SLs) are hormones essential for plant development and environmental responses. SL perception requires the formation of the complex composed of an SL receptor DWARF14 (D14), F-box protein D3, and transcriptional repressor D53, triggering ubiquitination and degradation of D53 to activate signal transduction. However, mechanisms of SL perception and their influence on plant architecture and environmental responses remain elusive and controversial. Here, we report that key residues at interfaces of the AtD14-D3-ASK1 complex are essential for the activation of SL perception, discover that overexpression of the D3-CTH motif negatively regulates SL perception to enhance tillering, and reveal the importance of phosphorylation and N-terminal disordered (NTD) domain in mediating ubiquitination and degradation of D14. Importantly, low nitrogen promotes phosphorylation and stabilization of D14 to repress rice tillering. These findings reveal a panorama of the activation, termination, and regulation of SL perception, which determines the plasticity of plant architecture in complex environments.

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.

Novel mechanisms of strigolactone-induced DWARF14 degradation in Arabidopsis thaliana

In angiosperms, the strigolactone receptor is the α/β hydrolase DWARF14 (D14) that, upon strigolactone binding, undergoes conformational changes, triggers strigolactone-dependent responses, and hydrolyses strigolactones. Strigolactone signalling involves the formation of a complex between strigolactone-bound D14, the E3-ubiquitin ligase SCFMAX2, and the transcriptional corepressors SMXL6/7/8, which become ubiquitinated and degraded by the proteasome. Strigolactone also destabilizes the D14 receptor. The current model proposes that D14 degradation occurs after ubiquitination of the SMXLs via SCFMAX2 and proteasomal degradation. Using fluorescence and luminescence assays on transgenic lines expressing D14 fused to GREEN FLUORESCENT PROTEIN or LUCIFERASE, we showed that strigolactone-induced D14 degradation may also occur independently of SCFMAX2 and/or SMXL6/7/8 through a proteasome-independent mechanism. Furthermore, strigolactone hydrolysis was not essential for triggering either D14 or SMXL7 degradation. The activity of mutant D14 proteins predicted to be non-functional for strigolactone signalling was also examined, and their capability to bind strigolactones in vitro was studied using differential scanning fluorimetry. Finally, we found that under certain conditions, the efficiency of D14 degradation was not aligned with that of SMXL7 degradation. These findings indicate a more complex regulatory mechanism governing D14 degradation than previously anticipated and provide novel insights into the dynamics of strigolactone signalling in Arabidopsis.

The strigolactone receptor DWARF14 regulates flowering time in Arabidopsis

Multiple plant hormones, including strigolactone (SL), play key roles in regulating flowering time. The Arabidopsis (Arabidopsis thaliana) DWARF14 (AtD14) receptor perceives SL and recruits F-box protein MORE AXILLARY GROWTH2 (MAX2) and the SUPPRESSOR OF MAX2-LIKE (SMXL) family proteins. These interactions lead to the degradation of the SMXL repressor proteins, thereby regulating shoot branching, leaf shape, and other developmental processes. However, the molecular mechanism by which SL regulates plant flowering remains elusive. Here, we demonstrate that intact strigolactone biosynthesis and signaling pathways are essential for normal flowering in Arabidopsis. Loss-of-function mutants in both SL biosynthesis (max3) and signaling (Atd14 and max2) pathways display earlier flowering, whereas the repressor triple mutant smxl6/7/8 (s678) exhibits the opposite phenotype. Retention of AtD14 in the cytoplasm leads to its inability to repress flowering. Moreover, we show that nuclear-localized AtD14 employs dual strategies to enhance the function of the AP2 transcription factor TARGET OF EAT1 (TOE1). AtD14 directly binds to TOE1 in an SL-dependent manner and stabilizes it. In addition, AtD14-mediated degradation of SMXL7 releases TOE1 from the repressor protein, allowing it to bind to and inhibit the FLOWERING LOCUS T (FT) promoter. This results in reduced FT transcription and delayed flowering. In summary, AtD14 perception of SL enables the transcription factor TOE1 to repress flowering, providing insights into hormonal control of plant flowering.

Strigolactone signalling inhibits trehalose 6-phosphate signalling independently of BRC1 to suppress shoot branching

The phytohormone strigolactone (SL) inhibits shoot branching, whereas the signalling metabolite trehalose 6-phosphate (Tre6P) promotes branching. How Tre6P and SL signalling may interact and which molecular mechanisms might be involved remains largely unknown. Transcript profiling of Arabidopsis SL mutants revealed a cluster of differentially expressed genes highly enriched in the Tre6P pathway compared with wild-type (WT) plants or brc1 mutants. Tre6P-related genes were also differentially expressed in axillary buds of garden pea (Pisum sativum) SL mutants. Tre6P levels were elevated in the SL signalling mutant more axillary (max) growth 2 compared with other SL mutants or WT plants indicating a role of MAX2-dependent SL signalling in regulating Tre6P levels. A transgenic approach to increase Tre6P levels demonstrated that all SL mutant lines and brc1 flowered earlier, showing all of these mutants were responsive to Tre6P. Elevated Tre6P led to increased branching in WT plants but not in max2 and max4 mutants, indicating some dependency between the SL pathway and Tre6P regulation of shoot branching. By contrast, elevated Tre6P led to an enhanced branching phenotype in brc1 mutants indicating independence between BRC1 and Tre6P. A model is proposed whereby SL signalling represses branching via Tre6P and independently of the BRC1 pathway.

The SMXL8-AGL9 module mediates crosstalk between strigolactone and gibberellin to regulate strigolactone-induced anthocyanin biosynthesis in apple

Although the strigolactone (SL) signaling pathway and SL-mediated anthocyanin biosynthesis have been reported, the molecular association between SL signaling and anthocyanin biosynthesis remains unclear. In this study, we identified the SL signal transduction pathway associated with anthocyanin biosynthesis and the crosstalk between gibberellin (GA) and SL signaling in apple (Malus × domestica). ELONGATED HYPOCOTYL5 (HY5) acts as a key node integrating SL signaling and anthocyanin biosynthesis, and the SL-response factor AGAMOUS-LIKE MADS-BOX9 (AGL9) promotes anthocyanin biosynthesis by activating HY5 transcription. The SL signaling repressor SUPPRESSOR OF MAX2 1-LIKE8 (SMXL8) interacts with AGL9 to form a complex that inhibits anthocyanin biosynthesis by downregulating HY5 expression. Moreover, the E3 ubiquitin ligase PROTEOLYSIS1 (PRT1) mediates the ubiquitination-mediated degradation of SMXL8, which is a key part of the SL signal transduction pathway associated with anthocyanin biosynthesis. In addition, the GA signaling repressor REPRESSOR-of-ga1-3-LIKE2a (RGL2a) mediates the crosstalk between GA and SL by disrupting the SMXL8-AGL9 interaction that represses HY5 transcription. Taken together, our study reveals the regulatory mechanism of SL-mediated anthocyanin biosynthesis and uncovers the role of SL-GA crosstalk in regulating anthocyanin biosynthesis in apple.

At the crossroads: strigolactones mediate changes in cytokinin synthesis and signalling in response to nitrogen limitation

Strigolactones (SLs) are key regulators of shoot growth and responses to environmental stimuli. Numerous studies have indicated that nitrogen (N) limitation induces SL biosynthesis, suggesting that SLs may play a pivotal role in coordinating systemic responses to N availability, but this idea has not been clearly demonstrated. Here, we generated triple knockout mutants in the SL synthesis gene TaDWARF17 (TaD17) in bread wheat and investigated their phenotypic and transcriptional responses under N limitation, aiming to elucidate the role of SLs in the adaptation to N limitation. Tad17 mutants display typical SL mutant phenotypes, and fail to adapt their shoot growth appropriately to N. Despite exhibiting an increased tillering phenotype, Tad17 mutants continued to respond to N limitation by reducing tiller number, suggesting that SLs are not the sole regulators of tillering in response to N availability. RNA-seq analysis of basal nodes revealed that the loss of D17 significantly altered the transcriptional response of N-responsive genes, including changes in the expression profiles of key N response master regulators. Crucially, our findings suggest that SLs are required for the transcriptional downregulation of cytokinin (CK) synthesis and signalling in response to N limitation. Collectively, our results suggest that SLs are essential for the appropriate morphological and transcriptional adaptation to N limitation in wheat, and that the repressive effect of SLs on shoot growth is partly mediated by their repression of CK synthesis.

HTL/KAI2 signaling substitutes for light to control plant germination

Plants monitor multiple environmental cues, such as light and temperature, to ensure they germinate at the right time and place. Some specialist plants, like ephemeral fire-following weeds and root parasitic plants, germinate primarily in response to small molecules found in specific environments. Although these species come from distinct clades, they use the same HYPOSENSITIVE TO LIGHT/KARRIKIN INSENSITIVE 2 (HTL/KAI2) signaling pathway, to perceive different small molecules suggesting convergent evolution on this pathway. Here, we show that HTL/KAI2 signaling in Arabidopsis thaliana bypasses the light requirement for germination. The HTL/KAI2 downstream component, SUPPRESSOR OF MAX2 1 (SMAX1) accumulates in the dark and is necessary for PHYTOCHROME INTERACTING FACTOR 1/PHYTOCHROME INTERACTING FACTOR 3-LIKE 5 (PIF1/PIL5) to regulate hormone response pathways conducive to germination. The interaction of HTL/KAI2 and light signaling may help to explain how specialist plants like ephemeral and parasitic weeds evolved their germination behaviour in response to specific environments.

Exploring novel SNPs and candidate genes associated with seed allometry in Pisum sativum L

Seed size is an important agronomic trait that indicates seed quality. In legumes, pods with equal and larger seeds remain the first preference of farmers and consumers. Genetic understanding related to seed size including seed allometric traits has been limited in the case of peas. To fill this void the findings presented here used the genome-wide association studies (GWAS) to identify novel candidate gene(s) putatively linked with seed size in Pisum sativum L. The study was conducted on 240 Pea Single Plant Plus Collection (PSPPC) panels of pea germplasm. Allometric traits measured included seed_length, seed_width, seed_thickness, seed_volume, seed_biomass, and seed_biomass by volume (SB_V). GWAS was performed using the Genome Association and Prediction Integrated Tool (GAPIT) on R-studio. The Bayesian information and Linkage-disequilibrium Iteratively Nested Keyway (BLINK) model provided significant single nucleotide polymorphisms (SNPs) linked with all the seed allometric traits. When analyzed the genomic regions of these SNPs provided a list of candidate genes that may be related to seed size. The present study thus provides a list of significant SNPs and relevant genes viz. Psat2g072000 for seed_length, Psat4g104320 for seed_width, Psat6g125800 and Psat6g125840 for seed_thickness, Psat6g228320 for seed_volume, Psat2g143920 for seed_biomass, and Psat2g120400 for SB_V which may prove useful in the improvement of pea seed size using breeding programs or CRISPR intervention. Understanding the genetic basis of seed size could lead to crop development with desirable seed characteristics, such as equal and larger-sized seeds with maximum yield and higher nutritional content.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01499-6.

Non-transcriptional regulatory activity of SMAX1 and SMXL2 mediates karrikin-regulated seedling response to red light in Arabidopsis

Karrikins and strigolactones govern plant development and environmental responses through closely related signaling pathways. The transcriptional repressor proteins SUPPRESSOR OF MAX2 1 (SMAX1), SMAX1-like2 (SMXL2), and D53-like SMXLs mediate karrikin and strigolactone signaling by directly binding downstream genes or by inhibiting the activities of transcription factors. In this study, we characterized the non-transcriptional regulatory activities of SMXL proteins in Arabidopsis. We discovered that SMAX1 and SMXL2 with mutations in their ethylene-response factor-associated amphiphilic repression (EAR) motif had undetectable or weak transcriptional repression activities but still partially rescued the hypocotyl elongation defects and fully reversed the cotyledon epinasty defects of the smax1 smxl2 mutant. SMAX1 and SMXL2 directly interact with PHYTOCHROME INTERACTION FACTOR 4 (PIF4) and PIF5 to enhance their protein stability by interacting with phytochrome B (phyB) and suppressing the association of phyB with PIF4 and PIF5. The karrikin-responsive genes were then identified by treatment with GR24ent-5DS, a GR24 analog showing karrikin activity. Interestingly, INDOLE-3-ACETIC ACID INDUCIBLE 29 (IAA29) expression was repressed by GR24ent-5DS treatment in a PIF4- and PIF5-dependent and EAR-independent manner, whereas KARRIKIN UPREGULATED F-BOX 1 (KUF1) expression was induced in a PIF4- and PIF5-independent and EAR-dependent manner. Furthermore, the non-transcriptional regulatory activity of SMAX1, which is independent of the EAR motif, had a global effect on gene expression. Taken together, these results indicate that non-transcriptional regulatory activities of SMAX1 and SMXL2 mediate karrikin-regulated seedling response to red light.

Strigolactone induces D14-dependent large-scale changes in gene expression requiring SWI/SNF chromatin remodellers

Strigolactones (SL) function as plant hormones in control of multiple aspects of plant development, mostly via the regulation of gene expression. Immediate early-gene regulation by SL remains unexplored due to difficulty in dissecting early from late gene expression responses to SL. We used synthetic SL, rac-GR24 treatment of protoplasts and RNA-seq to explore early SL-induced changes in gene expression over time (5-180 minutes) and discovered rapid, dynamic and SL receptor D14-dependent regulation of gene expression in response to rac-GR24. Importantly, we discovered a significant dependence of SL signalling on chromatin remodelling processes, as the induction of a key SL-induced transcription factor BRANCHED1 requires the SWI/SNF chromatin remodelling ATPase SPLAYED (SYD) and leads to upregulation of a homologue SWI/SNF ATPase BRAHMA. ATAC-seq profiling of genome-wide changes in chromatin accessibility in response to rac-GR24 identified large-scale changes, with over 1400 differentially accessible regions. These changes in chromatin accessibility often precede transcriptional changes and are likely to harbour SL cis-regulatory elements. Importantly, we discovered that this early and extensive modification of the chromatin landscape also requires SYD. This study, therefore, provides evidence that SL signalling requires regulation of chromatin accessibility, and it identifies genomic locations harbouring likely SL cis-regulatory sequences.

Mucoromycotina 'fine root endophytes': a new molecular model for plant-fungal mutualisms?

The most studied plant-fungal symbioses to date are the interactions between plants and arbuscular mycorrhizal (AM) fungi of the Glomeromycotina clade. Advancements in phylogenetics and microbial community profiling have distinguished a group of symbiosis-forming fungi that resemble AM fungi as belonging instead to the Mucoromycotina. These enigmatic fungi are now known as Mucoromycotina 'fine root endophytes' and could provide a means to understand the origins of plant-fungal symbioses. Most of our knowledge of the mechanisms of fungal symbiosis comes from investigations using AM fungi. Here, we argue that inclusion of Mucoromycotina fine root endophytes in future studies will expand our understanding of the mechanisms, evolution, and ecology of plant-fungal symbioses.

Sequential activation of strigolactone and salicylate biosynthesis promotes leaf senescence

Leaf senescence is a complex process strictly regulated by various external and endogenous factors. However, the key signaling pathway mediating leaf senescence remains unknown. Here, we show that Arabidopsis SPX1/2 negatively regulate leaf senescence genetically downstream of the strigolactone (SL) pathway. We demonstrate that the SL receptor AtD14 and MAX2 mediate the age-dependent degradation of SPX1/2. Intriguingly, we uncover an age-dependent accumulation of SLs in leaves via transcriptional activation of SL biosynthetic genes by the transcription factors (TFs) SPL9/15. Furthermore, we reveal that SPX1/2 interact with the WRKY75 subclade TFs to inhibit their DNA-binding ability and thus repress transcriptional activation of salicylic acid (SA) biosynthetic gene SA Induction-Deficient 2, gating the age-dependent SA accumulation in leaves at the leaf senescence onset stage. Collectively, our new findings reveal a signaling pathway mediating sequential activation of SL and salicylate biosynthesis for the onset of leaf senescence in Arabidopsis.

SUPPRESSOR OF MAX2 1-LIKE (SMXL) homologs are MAX2-dependent repressors of Physcomitrium patens growth

SUPPRESSOR OF MAX2 (SMAX)1-LIKE (SMXL) proteins are a plant-specific clade of type I HSP100/Clp-ATPases. SMXL genes are present in virtually all land plant genomes. However, they have mainly been studied in angiosperms. In Arabidopsis (Arabidopsis thaliana), 3 functional SMXL subclades have been identified: SMAX1/SMXL2, SMXL345, and SMXL678. Of these, 2 subclades ensure endogenous phytohormone signal transduction. SMAX1/SMXL2 proteins are involved in KAI2 ligand (KL) signaling, while SMXL678 proteins are involved in strigolactone (SL) signaling. Many questions remain regarding the mode of action of these proteins, as well as their ancestral roles. We addressed these questions by investigating the functions of the 4 SMXL genes in the moss Physcomitrium patens. We demonstrate that PpSMXL proteins are involved in the conserved ancestral MAX2-dependent KL signaling pathway and negatively regulate growth. However, PpSMXL proteins expressed in Arabidopsis cannot replace SMAX1 or SMXL2 function in KL signaling, whereas they can functionally replace SMXL4 and SMXL5 and restore root growth. Therefore, the molecular functions of SMXL proteins are conserved, but their interaction networks are not. Moreover, the PpSMXLC/D clade positively regulates SL signal transduction in P. patens. Overall, our data reveal that SMXL proteins in moss mediate crosstalk between the SL and KL signaling pathways.

Environmentally adaptive reshaping of plant photomorphogenesis by karrikin and strigolactone signaling

Coordinated morphogenic adaptation of growing plants is critical for their survival and propagation under fluctuating environments. Plant morphogenic responses to light and warm temperatures, termed photomorphogenesis and thermomorphogenesis, respectively, have been extensively studied in recent decades. During photomorphogenesis, plants actively reshape their growth and developmental patterns to cope with changes in light regimes. Accordingly, photomorphogenesis is closely associated with diverse growth hormonal cues. Notably, accumulating evidence indicates that light-directed morphogenesis is profoundly affected by two recently identified phytochemicals, karrikins (KARs) and strigolactones (SLs). KARs and SLs are structurally related butenolides acting as signaling molecules during a variety of developmental steps, including seed germination. Their receptors and signaling mediators have been identified, and associated working mechanisms have been explored using gene-deficient mutants in various plant species. Of particular interest is that the KAR and SL signaling pathways play important roles in environmental responses, among which their linkages with photomorphogenesis are most comprehensively studied during seedling establishment. In this review, we focus on how the phytochemical and light signals converge on the optimization of morphogenic fitness. We also discuss molecular mechanisms underlying the signaling crosstalks with an aim of developing potential ways to improve crop productivity under climate changes.

Detailing Early Shoot Growth Arrest in Kro-0 x BG-5 Hybrids of Arabidopsis thaliana

Shoot growth directly impacts plant productivity. Plants adjust their shoot growth in response to varying environments to maximize resource capture and stress resilience. While several factors controlling shoot growth are known, the complexity of the regulation and the input of the environment are not fully understood. We have investigated shoot growth repression induced by low ambient temperatures in hybrids of Arabidopsis thaliana Kro-0 and BG-5 accessions. To continue our previous studies, we confirmed that the Kro-0 allele of DYNAMIN-RELATED PROTEIN 3B causes stunted shoot growth in the BG-5 background. We also found that shoot growth repression was most pronounced near the apex at a lower temperature and that the cells in the hybrid stem failed to elongate correctly. Furthermore, we observed that shoot growth repression in hybrids depended on light availability. Global gene expression analysis indicated the involvement of hormones, especially strigolactone, associated with the dwarf phenotype. Altogether, this study enhances our knowledge on the genetic, physiological and environmental factors associated with shoot growth regulation.

SMXL5 attenuates strigolactone signaling in Arabidopsis thaliana by inhibiting SMXL7 degradation

Hormone-activated proteolysis is a recurring theme of plant hormone signaling mechanisms. In strigolactone signaling, the enzyme receptor DWARF14 (D14) and an F-box protein, MORE AXILLARY GROWTH2 (MAX2), mark SUPPRESSOR OF MAX2 1-LIKE (SMXL) family proteins SMXL6, SMXL7, and SMXL8 for rapid degradation. Removal of these transcriptional corepressors initiates downstream growth responses. The homologous proteins SMXL3, SMXL4, and SMXL5, however, are resistant to MAX2-mediated degradation. We discovered that the smxl4 smxl5 mutant has enhanced responses to strigolactone. SMXL5 attenuates strigolactone signaling by interfering with AtD14-SMXL7 interactions. SMXL5 interacts with AtD14 and SMXL7, providing two possible ways to inhibit SMXL7 degradation. SMXL5 function is partially dependent on an ethylene-responsive-element binding-factor-associated amphiphilic repression (EAR) motif, which typically mediates interactions with the TOPLESS family of transcriptional corepressors. However, we found that loss of the EAR motif reduces SMXL5-SMXL7 interactions and the attenuation of strigolactone signaling by SMXL5. We hypothesize that integration of SMXL5 into heteromeric SMXL complexes reduces the susceptibility of SMXL6/7/8 proteins to strigolactone-activated degradation and that the EAR motif promotes the formation or stability of these complexes. This mechanism may provide a way to spatially or temporally fine-tune strigolactone signaling through the regulation of SMXL5 expression or translation.

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).

Comprehensive Evolutionary Analysis of the SMXL Gene Family in Rosaceae: Further Insights into Its Origin, Expansion, Diversification, and Role in Regulating Pear Branching

SMXL genes constitute a conserved gene family that is ubiquitous in angiosperms and involved in regulating various plant processes, including branching, leaf elongation, and anthocyanin biosynthesis, but little is known about their molecular functions in pear branching. Here, we performed genome-wide identification and investigation of the SMXL genes in 16 angiosperms and analyzed their phylogenetics, structural features, conserved motifs, and expression patterns. In total, 121 SMXLs genes were identified and were classified into four groups. The number of non-redundant SMXL genes in each species varied from 3 (Amborella trichopoda Baill.) to 18 (Glycine max Merr.) and revealed clear gene expansion events over evolutionary history. All the SMXL genes showed conserved structures, containing no more than two introns. Three-dimensional protein structure prediction revealed distinct structures between but similar structures within groups. A quantitative real-time PCR analysis revealed different expressions of 10 SMXL genes from pear branching induced by fruit-thinning treatment. Overall, our study provides a comprehensive investigation of SMXL genes in the Rosaceae family, especially pear. The results offer a reference for understanding the evolutionary history of SMXL genes and provide excellent candidates for studying fruit tree branching regulation, and in facilitating pear pruning and planting strategies.

Nanotechnology, a frontier in agricultural science, a novel approach in abiotic stress management and convergence with new age medicine-A review

Climate change imposes various environmental stresses which substantially impact plant growth and productivity. Salinity, drought, temperature extremes, heavy metals, and nutritional imbalances are among several abiotic stresses contributing to high yield losses of crops in various parts of the world, resulting in food insecurity. Many interesting strategies are being researched in the attempt to improve plants' environmental stress tolerance. These include the application of nanoparticles, which have been found to improve plant function under stress situations. Nanotechnology will be a key driver in the upcoming agri-tech and pharmaceutical revolution, which promises a more sustainable, efficient, and resilient agricultural and medical system Nano-fertilizers can help plants utilise nutrients more efficiently by releasing nutrients slowly and sustainably. Plant physiology and nanomaterial features (such as size, shape, and charge) are important aspects influencing the impact on plant growth. Here, we discussed the most promising new opportunities and methodologies for using nanotechnology to increase the efficiency of critical inputs for crop agriculture, as well as to better manage biotic and abiotic stress. Potential development and implementation challenges are highlighted, emphasising the importance of designing suggested nanotechnologies using a systems approach. Finally, the strengths, flaws, possibilities, and risks of nanotechnology are assessed and analysed in order to present a comprehensive and clear picture of the nanotechnology potentials, as well as future paths for nano-based agri-food applications towards sustainability. Future research directions have been established in order to support research towards the long-term development of nano-enabled agriculture and evolution of pharmaceutical industry.

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

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

Genome-Wide Identification and Expression Analysis of DWARF53 Gene in Response to GA and SL Related to Plant Height in Banana

Dwarfing is one of the common phenotypic variations in asexually reproduced progeny of banana, and dwarfed banana is not only windproof and anti-fallout but also effective in increasing acreage yield. As a key gene in the strigolactone signalling pathway, DWARF53 (D53) plays an important role in the regulation of the height of plants. In order to gain insight into the function of the banana D53 gene, this study conducted genome-wide identification of banana D53 gene based on the M. acuminata, M. balbisiana and M. itinerans genome database. Analysis of MaD53 gene expression under high temperature, low temperature and osmotic stress based on transcriptome data and RT-qPCR was used to analyse MaD53 gene expression in different tissues as well as in different concentrations of GA and SL treatments. In this study, we identified three MaD53, three MbD53 and two MiD53 genes in banana. Phylogenetic tree analysis showed that D53 Musa are equally related to D53 Asparagales and Poales. Both high and low-temperature stresses substantially reduced the expression of the MaD53 gene, but osmotic stress treatments had less effect on the expression of the MaD53 gene. GR24 treatment did not significantly promote the height of the banana, but the expression of the MaD53 gene was significantly reduced in roots and leaves. GA treatment at 100 mg/L significantly promoted the expression of the MaD53 gene in roots, but the expression of this gene was significantly reduced in leaves. In this study, we concluded that MaD53 responds to GA and SL treatments, but "Yinniaijiao" dwarf banana may not be sensitive to GA and SL.

Genetic Model Identification and Major QTL Mapping for Petiole Thickness in Non-Heading Chinese Cabbage

Petioles of non-heading Chinese cabbage are not only an important edible part but also a conduit for nutrient transport, holding significant agricultural and research value. In this study, we conducted a comprehensive genetic analysis of petiole-related traits using a segregating population. Modern quantitative genetic approaches were applied to investigate the genetic regulation of petiole thickness. The results indicated that petiole thickness is a quantitative trait, and the identified genetic model was consistent with two pairs of additive-dominant main genes and additive-dominant polygenes (2MG-AD). BSA-seq analysis identified a major effect of QTL controlling petiole thickness on chromosome A09: 42.08-45.09 Mb, spanning 3.01 Mb, designated as QTL-BrLH9. Utilizing InDel markers, the interval was narrowed down to 51 kb, encompassing 14 genes with annotations for 10 of them. Within the interval, four mutated genes were detected. Combined with gene annotation, protein sequence analysis, and homology alignment, it was found that BraA09g063520.3C's homologous gene SMXL6 in Arabidopsis (Arabidopsis thaliana (L.) Heynh) is an inhibitor of the coding and synthesis of the strigolactone pathway. Strigolactone (SLs) plays an important role in plant growth and development. The cloning results showed that multiple frameshift mutations and non-synonymous mutations occurred on the exon. The qPCR results showed that the expression of the gene was significantly different between the two parents at the adult stage, so it was speculated that it would lead to changes in petiole thickness. BraA09g063520.3C was predicted as the final candidate gene.

Effect of strigolactone on growth, photosynthetic efficiency, antioxidant activity, and osmolytes accumulation in different maize (Zea mays L.) hybrids grown under drought stress

Drought alters plant physiology, morphology, and biochemical pathways, necessitating effective mitigation strategies. Strigolactones (SLs) are phytohormones known to enhance plant growth under abiotic stress. However, their specific impact on drought stress in maize remains unclear. This study aimed to determine the optimal SL concentration for mitigating drought stress in two maize hybrids (HY-1898, FH-1046). Maize plants were subjected to 60% field capacity drought stress in a pot experiment. After 40 d, different concentrations (0, 0.001, 0.01, and 0.1 mg L-1) of the synthetic SL analogue GR24 were applied to evaluate their effects on growth features, photosynthesis attributes, and osmolyte accumulation in the maize hybrids. Results showed that exogenous SL application significantly increased photosynthetic pigments in maize hybrids under drought stress. Chlorophyll content, gas exchange characteristics, net CO2 assimilation rate, stomatal conductance, water use efficiency, and antioxidant activities were enhanced by GR24. Leaf ascorbic acid and total phenolics also increased with SL application. Organic osmolytes, such as glycine betaine and free proline, were elevated in both maize hybrids under drought stress. Yield-related parameters, including cob diameter, cob weight, number of seeds per cob, and number of seeds per plant, were significantly increased by GR24 under drought stress. Our findings highlight the potential of GR24 foliar application to mitigate drought stress and promote maize growth and grain yield in a concentration-dependent manner. The minimum effective SL concentration against drought stress was determined to be 0.01 mg L-1. Overall, foliar application of GR24 could serve as a sustainable approach for drought tolerance in agriculture.

The role of strigolactone analog (GR24) in endogenous hormone metabolism and hormone-related gene expression in tobacco axillary buds

KEY MESSAGE

Strigolactone has the potential to influence hormone metabolism, in addition to having a role in inhibiting axillary bud elongation, which could be regulated by the expression of phytohormones-related genes. The elongation of axillary buds affects the economic benefits of tobacco. In this study, it was investigated the effect of strigolactone (SL) on the elongation of tobacco axillary buds and its endogenous hormone metabolism and related gene expression by applying the artificial analog of SL, GR24, and an inhibitor of SL synthesis, TIS-108, to the axillary buds. The results showed that the elongation of axillary buds was significantly inhibited by GR24 on day 2 and day 9. Ultra-high-performance liquid-chromatography-mass spectrometry results further showed that SL significantly affected the metabolism of endogenous plant hormones, altering both their levels and the ratios between each endogenous hormone. Particularly, the levels of auxin (IAA), trans-zeatin-riboside (tZR), N6-(∆2-isopentenyl) adenine (iP), gibberellin A4 (GA4), jasmonic acid (JA), and jasmonoyl isoleucine (JA-Ile) were decreased after GR24 treatment on day 9, but the levels of 1-aminocyclopropane-1-carboxylic acid (ACC) and gibberellin A1 (GA1) were significantly increased. Further analysis of endogenous hormonal balance revealed that after the treatment with GR24 on day 9, the ratio of IAA to cytokinin (CTK) was markedly increased, but the ratios of IAA to abscisic acid (ABA), salicylic acid (SA), ACC, JAs, and, GAs were notably decreased. In addition, according to RNA-seq analysis, multiple differentially expressed genes were found, such as GH3.1, AUX/IAA, SUAR20, IPT, CKX1, GA2ox1, ACO3, ERF1, PR1, and HCT, which may play critical roles in the biosynthesis, deactivation, signaling pathway of phytohormones, and the biosynthesis of flavonoids to regulate the elongation of axillary buds in tobacco. This work lays the certain theoretical foundation for the application of SL in regulating the elongation of axillary buds of tobacco.

Latest knowledge on strigolactone biosynthesis and perception

Strigolactones (SLs) are a class of terpenoid lactones initially identified as seed germination stimulants for root parasitic plants more than 50 years ago. Long after this initial discovery, SLs were re-characterized as the symbiotic signals for arbuscular mycorrhizal fungi that supply inorganic nutrients, such as phosphate, to their host plants. In 2008, SLs were found to be endogenous plant hormones that regulate shoot branching in plants. The discovery of SLs as a new class of plant hormones has significantly advanced research in this field. Studies over the past 15 years have elucidated almost the entire pathway of SL biosynthesis and the overall mechanism of its signaling. This review summarizes research on the SL biosynthetic pathway, and the current state of knowledge of the SL perception mechanism.

Genome-wide analysis of the SMXL gene family in common bean and identification of karrikin-responsive PvSMXL2 as a negative regulator of PEG-induced drought stress

Common bean (Phaseolus vulgaris L.) is a major legume crop worldwide, but its growth and development frequently face challenges due to abiotic stresses, particularly drought. Proper supplement of copper could mitigate the adverse effects of drought, but excessive accumulation of this metal in plants can be harmful. The suppressor of MAX2 1-like (SMXL) gene family, which plays important roles in various plant processes, including stress responses, remains poorly understood in common bean. In this study, we identified nine orthologues of SMXL genes in common bean, which are located on six chromosomes and classified into four subgroups. Basic molecular properties, including theoretical isoelectric point (PI), molecular weight (MW), grand average of hydropathicity (GVIO), gene structure, and conserved motifs were characterized, and numerous cis-elements in promoters were predicted. The expression patterns of PvSMXL genes were found to be distinct under 10% polyethylene glycol (PEG)-induced drought stress and 200 μM Cu treatments. Most PvSMXLs showed reduced expression in response to Cu treatment, whereas nearly half PvSMXLs exhibited inducible expression under drought stress. PvSMXL2, which exhibited a rapid response to karrikin 1 (KAR1), an active form of the plant growth regulators newly found in the smoke of burning plant material, was down-regulated by both PEG-induced drought and Cu stresses. Transient silencing of PvSMXL2 resulted in enhanced drought stress tolerance without conferring Cu tolerance. These findings provide valuable insights into the functions of SMXL genes in common bean under abiotic stress conditions.

Identification and characterization of the karrikins signaling gene SsSMAX1 in Sapium sebiferum

SUPPRESSOR OF MAX2 LIKE 1 (SMAX1) is a member of the SUPPRESSOR of MAX2 1‑LIKE family of genes and is known as a target protein of KARRIKIN INSENSITIVE2 (KAI2)-MORE AXILLARY BRANCHES2 (MAX2), which mediates karrikin signaling in Arabidopsis. SMAX1 plays a significant role in seed germination, hypocotyl elongation, and root hair development in Arabidopsis. SMAX1 has not yet been identified and characterized in woody plants. This study identified and characterized SsSMAX1 in Sapium sebiferum and found that SsSMAX1 was highly expressed in the seed, hypocotyl, and root tips of S. sebiferum. SsSMAX1 was functionally characterized by ectopic expression in Arabidopsis. SsSMAX1 overexpression lines of Arabidopsis showed significantly delayed seed germination and produced seedlings with longer hypocotyl and roots than wild-type and Atsmax1 functional mutants. SsSMAX1 overexpression lines of Arabidopsis also had broader and longer leaves and petioles than wild-type and Atsmax1, suggesting that SsSMAX1 is functionally conserved. This study characterizes the SMAX1 gene in a woody and commercially valuable bioenergy plant, Sapium sebiferum. The results of this study are beneficial to future research on the molecular biology of woody plants.

Identification and expression profile of the SMAX/SMXL family genes in chickpea and lentil provide important players of biotechnological interest involved in plant branching

MAIN CONCLUSION

SMAX/SMXL family genes were successfully identified and characterized in the chickpea and lentil and gene expression data revealed several genes associated with the modulation of plant branching and powerful targets for use in transgenesis and genome editing. Strigolactones (SL) play essential roles in plant growth, rooting, development, and branching, and are associated with plant resilience to abiotic and biotic stress conditions. Likewise, karrikins (KAR) are "plant smoke-derived molecules" that act in a hormonal signaling pathway similar to SL playing an important role in seed germination and hairy root elongation. The SMAX/SMXL family genes are part of these two signaling pathways, in addition to some of these members acting in a still little known SL- and KAR-independent signaling pathway. To date, the identification and functional characterization of the SMAX/SMXL family genes has not been performed in the chickpea and lentil. In this study, nine SMAX/SMXL genes were systematically identified and characterized in the chickpea and lentil, and their expression profiles were explored under different unstressless or different stress conditions. After a comprehensive in silico characterization of the genes, promoters, proteins, and protein-protein interaction network, the expression profile for each gene was determined using a meta-analysis from the RNAseq datasets and complemented with real-time PCR analysis. The expression profiles of the SMAX/SMXL family genes were very dynamic in different chickpea and lentil organs, with some genes assuming a tissue-specific expression pattern. In addition, these genes were significantly modulated by different stress conditions, indicating that SMAX/SMXL genes, although working in three distinct signaling pathways, can act to modulate plant resilience. Most CaSMAX/SMXL and partner genes such as CaTiE1 and CaLAP1, have a positive correlation with the plant branching level, while most LcSMAX/SMXL genes were less correlated with the plant branching level. The SMXL6, SMXL7, SMXL8, TiE1, LAP1, BES1, and BRC1 genes were highlighted as powerful targets for use in transgenesis and genome editing aiming to develop chickpea and lentil cultivars with improved architecture. Therefore, this study presented a detailed characterization of the SMAX/SMXL genes in the chickpea and lentil, and provided new insights for further studies focused on each SMAX/SMXL gene.

Genomic region and origin for selected traits during differentiation of small-fruit cultivars in Japanese apricot (Prunus mume)

The Japanese apricot (Prunus mume) is a popular fruit tree in Japan. However, the genetic factors associated with fruit trait variations are poorly understood. In this study, we investigated nine fruit-associated traits, including harvesting time, fruit diameter, fruit shape, fruit weight, stone (endocarp) weight, ratio of stone weight to fruit weight, and rate of fruit gumming, using 110 Japanese apricot accessions over four years. A genome-wide association study (GWAS) was performed for these traits and strong signals were detected on chromosome 6 for harvesting time and fruit diameters. These peaks were shown to undergo strong artificial selection during the differentiation of small-fruit cultivars. The genomic region defined by the GWAS and XP-nSL analyses harbored several candidate genes associated with plant hormone regulation. Furthermore, the alleles of small-fruit cultivars in this region were shown to have genetic proximity to some Chinese cultivars of P. mume. These results indicate that the small-fruit trait originated in China; after being introduced into Japan, it was preferred and selected by the Japanese people, resulting in the differentiation of small-fruit cultivars.

Unlocking the Multifaceted Mechanisms of Bud Outgrowth: Advances in Understanding Shoot Branching

Shoot branching is a complex and tightly regulated developmental process that is essential for determining plant architecture and crop yields. The outgrowth of tiller buds is a crucial step in shoot branching, and it is influenced by a variety of internal and external cues. This review provides an extensive overview of the genetic, plant hormonal, and environmental factors that regulate shoot branching in several plant species, including rice, Arabidopsis, tomato, and wheat. We especially highlight the central role of TEOSINTE BRANCHED 1 (TB1), a key gene in orchestrating bud outgrowth. In addition, we discuss how the phytohormones cytokinins, strigolactones, and auxin interact to regulate tillering/branching. We also shed light on the involvement of sugar, an integral component of plant development, which can impact bud outgrowth in both trophic and signaling ways. Finally, we emphasize the substantial influence of environmental factors, such as light, temperature, water availability, biotic stresses, and nutrients, on shoot branching. In summary, this review offers a comprehensive evaluation of the multifaced regulatory mechanisms that underpin shoot branching and highlights the adaptable nature of plants to survive and persist in fluctuating environmental conditions.

Strigolactones promote plant freezing tolerance by releasing the WRKY41-mediated inhibition of CBF/DREB1 expression

Cold stress is a major abiotic stress that adversely affects plant growth and crop productivity. The C-REPEAT BINDING FACTOR/DRE BINDING FACTOR 1 (CBF/DREB1) transcriptional regulatory cascade plays a key role in regulating cold acclimation and freezing tolerance in Arabidopsis (Arabidopsis thaliana). Here, we show that max (more axillary growth) mutants deficient in strigolactone biosynthesis and signaling display hypersensitivity to freezing stress. Exogenous application of GR245DS , a strigolactone analog, enhances freezing tolerance in wild-type plants and strigolactone-deficient mutants and promotes the cold-induced expression of CBF genes. Biochemical analysis showed that the transcription factor WRKY41 serves as a substrate for the F-box E3 ligase MAX2. WRKY41 directly binds to the W-box in the promoters of CBF genes and represses their expression, negatively regulating cold acclimation and freezing tolerance. MAX2 ubiquitinates WRKY41, thus marking it for cold-induced degradation and thereby alleviating the repression of CBF expression. In addition, SL-mediated degradation of SMXLs also contributes to enhanced plant freezing tolerance by promoting anthocyanin biosynthesis. Taken together, our study reveals the molecular mechanism underlying strigolactones promote the cold stress response in Arabidopsis.

Multi-omics insights into the positive role of strigolactone perception in barley drought response

BACKGROUND

Drought is a major environmental stress that affects crop productivity worldwide. Although previous research demonstrated links between strigolactones (SLs) and drought, here we used barley (Hordeum vulgare) SL-insensitive mutant hvd14 (dwarf14) to scrutinize the SL-dependent mechanisms associated with water deficit response.

RESULTS

We have employed a combination of transcriptomics, proteomics, phytohormonomics analyses, and physiological data to unravel differences between wild-type and hvd14 plants under drought. Our research revealed that drought sensitivity of hvd14 is related to weaker induction of abscisic acid-responsive genes/proteins, lower jasmonic acid content, higher reactive oxygen species content, and lower wax biosynthetic and deposition mechanisms than wild-type plants. In addition, we identified a set of transcription factors (TFs) that are exclusively drought-induced in the wild-type barley.

CONCLUSIONS

Critically, we resolved a comprehensive series of interactions between the drought-induced barley transcriptome and proteome responses, allowing us to understand the profound effects of SLs in alleviating water-limiting conditions. Several new avenues have opened for developing barley more resilient to drought through the information provided. Moreover, our study contributes to a better understanding of the complex interplay between genes, proteins, and hormones in response to drought, and underscores the importance of a multidisciplinary approach to studying plant stress response mechanisms.

SUPPRESSOR OF MAX2 LIKE 6, 7, and 8 Interact with DDB1 BINDING WD REPEAT DOMAIN HYPERSENSITIVE TO ABA DEFICIENT 1 to Regulate the Drought Tolerance and Target SUCROSE NONFERMENTING 1 RELATED PROTEIN KINASE 2.3 to Abscisic Acid Response in Arabidopsis

SUPPRESSOR OF MAX2-LIKE 6, 7, and 8 (SMXL6,7,8) function as repressors and transcription factors of the strigolactone (SL) signaling pathway, playing an important role in the development and stress tolerance in Arabidopsis thaliana. However, the molecular mechanism by which SMXL6,7,8 negatively regulate drought tolerance and ABA response remains largely unexplored. In the present study, the interacting protein and downstream target genes of SMXL6,7,8 were investigated. Our results showed that the substrate receptor for the CUL4-based E3 ligase DDB1-BINDING WD-REPEAT DOMAIN (DWD) HYPERSENSITIVE TO ABA DEFICIENT 1 (ABA1) (DWA1) physically interacted with SMXL6,7,8. The degradation of SMXL6,7,8 proteins were partially dependent on DWA1. Disruption of SMXL6,7,8 resulted in increased drought tolerance and could restore the drought-sensitive phenotype of the dwa1 mutant. In addition, SMXL6,7,8 could directly bind to the promoter of SUCROSE NONFERMENTING 1 (SNF1)-RELATED PROTEIN KINASE 2.3 (SnRK2.3) to repress its transcription. The mutations in SnRK2.2/2.3 significantly suppressed the hypersensitivity of smxl6/7/8 to ABA-mediated inhibition of seed germination. Conclusively, SMXL6,7,8 interact with DWA1 to negatively regulate drought tolerance and target ABA-response genes. These data provide insights into drought tolerance and ABA response in Arabidopsis via the SMXL6,7,8-mediated SL signaling pathway.

Histone Deacetylases Regulate MORE AXILLARY BRANCHED 2-Dependent Germination of Arabidopsis thaliana

Under specific conditions, the germination of Arabidopsis thaliana is dependent on the activation of the KARRIKIN INSENSITIVE 2 (KAI2) signaling pathway by the KAI2-dependent perception of karrikin or the artificial strigolactone analogue, rac-GR24. To regulate the induction of germination, the KAI2 signaling pathway relies on MORE AXILLARY BRANCHED 2- (MAX2-)dependent ubiquitination and proteasomal degradation of the repressor protein SUPPRESSOR OF MAX2 1 (SMAX1). It is not yet known how the degradation of SMAX1 proteins eventually results in the regulation of seed germination, but it has been hypothesized that SMAX1-LIKE generally functions as transcriptional repressors through the recruitment of co-repressors TOPLESS (TPL) and TPL-related, which in turn interact with histone deacetylases. In this article, we show the involvement of histone deacetylases HDA6, HDA9, HDA19 and HDT1 in MAX2-dependent germination of Arabidopsis, and more specifically, that HDA6 is required for the induction of DWARF14-LIKE2 expression in response to rac-GR24 treatment.

KAI2 Can Do: Karrikin Receptor Function in Plant Development and Response to Abiotic and Biotic Factors

The α/β hydrolase KARRIKIN INSENSITIVE 2 (KAI2) functions as a receptor for a yet undiscovered phytohormone, provisionally termed KAI2 ligand (KL). In addition, it perceives karrikin, a butenolide compound found in the smoke of burnt plant material. KAI2-mediated signaling is involved in regulating seed germination and in shaping seedling and adult plant morphology, both above and below ground. It also governs responses to various abiotic stimuli and stresses and shapes biotic interactions. KAI2-mediated signaling is being linked to an elaborate cross-talk with other phytohormone pathways such as auxin, gibberellin, abscisic acid, ethylene and salicylic acid signaling, in addition to light and nutrient starvation signaling. Further connections will likely be revealed in the future. This article summarizes recent advances in unraveling the function of KAI2-mediated signaling and its interaction with other signaling pathways.