A D53 repression motif induces oligomerization of TOPLESS corepressors and promotes assembly of a corepressor-nucleosome complex

TaSPL6B, a member of the Squamosa promoter binding protein-like family, regulates shoot branching and florescence in Arabidopsis thaliana

BACKGROUND

Squamosa promoter-binding protein-like (SPL) proteins are essential to plant growth and development as plant-specific transcription factors. However, the functions of SPL proteins in wheat need to be further explored.

RESULTS

We cloned and characterized TaSPL6B of wheat in this study. Analysis of physicochemical properties revealed that it contained 961 amino acids and had a molecular weight of 105 kDa. Full-length TaSPL6B transcription activity was not validated in yeast and subcellular localization analysis revealed that TaSPL6B was distributed in the nucleus. Ectopic expression of TaSPL6B in Arabidopsis led to increasing number of branches and early flowering. TaSPL6B was highly transcribed in internodes of transgenic Arabidopsis. The expression of AtSMXL6/AtSMXL7/AtSMXL8 (homologous genes of TaD53) was markedly increased, whereas the expression of AtSPL2 (homologous genes of TaSPL3) and AtBRC1 (homologous genes of TaTB1) was markedly reduced in the internodes of transgenic Arabidopsis. Besides, TaSPL6B, TaSPL3 and TaD53 interacted with one another, as demonstrated by yeast two-hybrid and bimolecular fluorescence complementation assays. Therefore, we speculated that TaSPL6B brought together TaD53 and TaSPL3 and enhanced the inhibition effect of TaD53 on TaSPL3 through integrating light and strigolactone signaling pathways, followed by suppression of TaTB1, a key repressor of tillering.

CONCLUSIONS

As a whole, our findings contribute to a better understanding of how SPL genes work in wheat and will be useful for further research into how TaSPL6B affects yield-related traits in wheat.

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.

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.

A conserved function of corepressors is to nucleate assembly of the transcriptional preinitiation complex

The plant corepressor TPL is recruited to diverse chromatin contexts, yet its mechanism of repression remains unclear. Previously, we have leveraged the fact that TPL retains its function in a synthetic transcriptional circuit in the yeast model Saccharomyces cerevisiae to localize repressive function to two distinct domains. Here, we employed two unbiased whole genome approaches to map the physical and genetic interactions of TPL at a repressed locus. We identified SPT4, SPT5 and SPT6 as necessary for repression with the SPT4 subunit acting as a bridge connecting TPL to SPT5 and SPT6. We also discovered the association of multiple additional constituents of the transcriptional preinitiation complex at TPL-repressed promoters, specifically those involved in early transcription initiation events. These findings were validated in yeast and plants through multiple assays, including a novel method to analyze conditional loss of function of essential genes in plants. Our findings support a model where TPL nucleates preassembly of the transcription activation machinery to facilitate rapid onset of transcription once repression is relieved.

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.

The fungal pathogen Ustilago maydis targets the maize corepressor RELK2 to modulate host transcription for tumorigenesis

Ustilago maydis is a biotrophic fungus that causes tumor formation on all aerial parts of maize. U. maydis secretes effector proteins during penetration and colonization to successfully overcome the plant immune response and reprogram host physiology to promote infection. In this study, we functionally characterized the U. maydis effector protein Topless (TPL) interacting protein 6 (Tip6). We found that Tip6 interacts with the N-terminus of RELK2 through its two Ethylene-responsive element binding factor-associated amphiphilic repression (EAR) motifs. We show that the EAR motifs are essential for the virulence function of Tip6 and critical for altering the nuclear distribution pattern of RELK2. We propose that Tip6 mimics the recruitment of RELK2 by plant repressor proteins, thus disrupting host transcriptional regulation. We show that a large group of AP2/ERF B1 subfamily transcription factors are misregulated in the presence of Tip6. Our study suggests a regulatory mechanism where the U. maydis effector Tip6 utilizes repressive domains to recruit the corepressor RELK2 to disrupt the transcriptional networks of the host plant.

The D14 and KAI2 Orthologs of Gymnosperms Sense Strigolactones and KL Mimics, Respectively, and the Signals Are Transduced to Control Downstream Genes

Strigolactones (SLs), lactone-containing carotenoid derivatives, function as signaling molecules in the rhizosphere, inducing symbiosis with arbuscular mycorrhizal. In addition, as a class of plant hormones, SLs control plant growth and development in flowering plants (angiosperms). Recent studies show that the ancestral function of SLs, which precede terrestrialization of plants, is as rhizosphere signaling molecules. SLs were then recruited as a class of plant hormones through the step-by-step acquisition of signaling components. The D14 gene encoding the SL receptor arose by gene duplication of KARRIKIN INSENSITIVE2 (KAI2), the receptor of karrikins and KAI2 ligand (KL), an unknown ligand, in the common ancestor of seed plants. KL signaling targets SMAX1, a repressor protein. On the other hand, the SL signaling targets SMXL78 subclade repressors, which arose by duplication of SMAX1 in angiosperms. Thus, gymnosperms contain the SL receptor D14 but not SMXL78, the SL signaling-specific repressor proteins. We studied two gymnosperm species, ginkgo (Ginkgo biloba) and Japanese umbrella pine (Sciadopitys verticillata), to clarify whether SLs are perceived and the signals are transduced in gymnosperms. We show that D14 and KAI2 of ginkgo and Japanese umbrella pine specifically perceive an SL analog and KL mimic, respectively. Furthermore, our results suggest that both SL signaling and KL signaling target SMAX1, and the specific localization of the receptor may result in the specificity of the signaling in gymnosperms.

Serine protease NAL1 exerts pleiotropic functions through degradation of TOPLESS-related corepressor in rice

NARROW LEAF 1 (NAL1) is a breeding-valuable pleiotropic gene that affects multiple agronomic traits in rice, although the molecular mechanism is largely unclear. Here, we report that NAL1 is a serine protease and displays a novel hexameric structure consisting of two ATP-mediated doughnut-shaped trimeric complexes. Moreover, we identified TOPLESS-related corepressor OsTPR2 involved in multiple growth and development processes as the substrate of NAL1. We found that NAL1 degraded OsTPR2, thus modulating the expression of downstream genes related to hormone signalling pathways, eventually achieving its pleiotropic physiological function. An elite allele, NAL1A, which may have originated from wild rice, could increase grain yield. Furthermore, the NAL1 homologues in different crops have a similar pleiotropic function to NAL1. Our study uncovers a NAL1-OsTPR2 regulatory module and provides gene resources for the design of high-yield crops.

A novel transcriptional repressor complex MYB22-TOPLESS-HDAC1 promotes rice resistance to brown planthopper by repressing F3'H expression

The brown planthopper (BPH) is the most destructive pest of rice. The MYB transcription factors are vital for rice immunity, but most are activators. Although MYB22 positively regulates rice resistance to BPH and has an EAR motif associated with active repression, it remains unclear whether it is a transcriptional repressor affecting rice-BPH interaction. Genetic analyses revealed that MYB22 regulates rice resistance to BPH via its EAR motif. Several biochemical experiments (e.g. transient transcription assay, Y2H, LCA, and BiFC) indicated that MYB22 is a transcriptional repressor that interacts with the corepressor TOPLESS via its EAR motif and recruits HDAC1 to form a tripartite complex. Flavonoid-3'-hydroxylase (F3'H) is a flavonoid biosynthesis pathway-related gene that negatively regulates rice resistance to BPH. Based on a bioinformatics analysis and the results of EMSA and transient transcription assays, MYB22 can bind directly to the F3'H promoter and repress gene expression along with TOPLESS and HDAC1. We revealed a transcriptional regulatory mechanism influencing the rice-BPH interaction that differs from previously reported mechanisms. Specifically, MYB22-TOPLESS-HDAC1 is a novel transcriptional repressor complex with components that synergistically and positively regulate rice resistance to BPH through the transcriptional repression of F3'H.

Strigolactones positively regulate Verticillium wilt resistance in cotton via crosstalk with other hormones

Verticillium wilt caused by Verticillium dahliae is a serious vascular disease in cotton (Gossypium spp.). V. dahliae induces the expression of the CAROTENOID CLEAVAGE DIOXYGENASE 7 (GauCCD7) gene involved in strigolactone (SL) biosynthesis in Gossypium australe, suggesting a role for SLs in Verticillium wilt resistance. We found that the SL analog rac-GR24 enhanced while the SL biosynthesis inhibitor TIS108 decreased cotton resistance to Verticillium wilt. Knock-down of GbCCD7 and GbCCD8b genes in island cotton (Gossypium barbadense) decreased resistance, whereas overexpression of GbCCD8b in upland cotton (Gossypium hirsutum) increased resistance to Verticillium wilt. Additionally, Arabidopsis (Arabidopsis thaliana) SL mutants defective in CCD7 and CCD8 putative orthologs were susceptible, whereas both Arabidopsis GbCCD7- and GbCCD8b-overexpressing plants were more resistant to Verticillium wilt than wild-type (WT) plants. Transcriptome analyses showed that several genes related to the jasmonic acid (JA)- and abscisic acid (ABA)-signaling pathways, such as MYELOCYTOMATOSIS 2 (GbMYC2) and ABA-INSENSITIVE 5, respectively, were upregulated in the roots of WT cotton plants in responses to rac-GR24 and V. dahliae infection but downregulated in the roots of both GbCCD7- and GbCCD8b-silenced cotton plants. Furthermore, GbMYC2 suppressed the expression of GbCCD7 and GbCCD8b by binding to their promoters, which might regulate the homeostasis of SLs in cotton through a negative feedback loop. We also found that GbCCD7- and GbCCD8b-silenced cotton plants were impaired in V. dahliae-induced reactive oxygen species (ROS) accumulation. Taken together, our results suggest that SLs positively regulate cotton resistance to Verticillium wilt through crosstalk with the JA- and ABA-signaling pathways and by inducing ROS accumulation.

Insights to Gossypium defense response against Verticillium dahliae: the Cotton Cancer

The soil-borne pathogen Verticillium dahliae, also referred as "The Cotton Cancer," is responsible for causing Verticillium wilt in cotton crops, a destructive disease with a global impact. To infect cotton plants, the pathogen employs multiple virulence mechanisms such as releasing enzymes that degrade cell walls, activating genes that contribute to virulence, and using protein effectors. Conversely, cotton plants have developed numerous defense mechanisms to combat the impact of V. dahliae. These include strengthening the cell wall by producing lignin and depositing callose, discharging reactive oxygen species, and amassing hormones related to defense. Despite the efforts to develop resistant cultivars, there is still no permanent solution to Verticillium wilt due to a limited understanding of the underlying molecular mechanisms that drive both resistance and pathogenesis is currently prevalent. To address this challenge, cutting-edge technologies such as clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), host-induced gene silencing (HIGS), and gene delivery via nano-carriers could be employed as effective alternatives to control the disease. This article intends to present an overview of V. dahliae virulence mechanisms and discuss the different cotton defense mechanisms against Verticillium wilt, including morphophysiological and biochemical responses and signaling pathways including jasmonic acid (JA), salicylic acid (SA), ethylene (ET), and strigolactones (SLs). Additionally, the article highlights the significance of microRNAs (miRNAs), circular RNAs (circRNAs), and long non-coding RNAs (lncRNAs) in gene expression regulation, as well as the different methods employed to identify and functionally validate genes to achieve resistance against this disease. Gaining a more profound understanding of these mechanisms could potentially result in the creation of more efficient strategies for combating Verticillium wilt in cotton crops.

OBERON3 and SUPPRESSOR OF MAX2 1-LIKE proteins form a regulatory module driving phloem development

Spatial specificity of cell fate decisions is central for organismal development. The phloem tissue mediates long-distance transport of energy metabolites along plant bodies and is characterized by an exceptional degree of cellular specialization. How a phloem-specific developmental program is implemented is, however, unknown. Here we reveal that the ubiquitously expressed PHD-finger protein OBE3 forms a central module with the phloem-specific SMXL5 protein for establishing the phloem developmental program in Arabidopsis thaliana. By protein interaction studies and phloem-specific ATAC-seq analyses, we show that OBE3 and SMXL5 proteins form a complex in nuclei of phloem stem cells where they promote a phloem-specific chromatin profile. This profile allows expression of OPS, BRX, BAM3, and CVP2 genes acting as mediators of phloem differentiation. Our findings demonstrate that OBE3/SMXL5 protein complexes establish nuclear features essential for determining phloem cell fate and highlight how a combination of ubiquitous and local regulators generate specificity of developmental decisions 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.

Rhizogenic Agrobacterium protein RolB interacts with the TOPLESS repressor proteins to reprogram plant immunity and development

Rhizogenic Agrobacterium strains comprise biotrophic pathogens that cause hairy root disease (HRD) on hydroponically grown Solanaceae and Cucurbitaceae crops, besides being widely explored agents for the creation of hairy root cultures for the sustainable production of plant-specialized metabolites. Hairy root formation is mediated through the expression of genes encoded on the T-DNA of the root-inducing (Ri) plasmid, of which several, including root oncogenic locus B (rolB), play a major role in hairy root development. Despite decades of research, the exact molecular function of the proteins encoded by the rol genes remains enigmatic. Here, by means of TurboID-mediated proximity labeling in tomato (Solanum lycopersicum) hairy roots, we identified the repressor proteins TOPLESS (TPL) and Novel Interactor of JAZ (NINJA) as direct interactors of RolB. Although these interactions allow RolB to act as a transcriptional repressor, our data hint at another in planta function of the RolB oncoprotein. Hence, by a series of plant bioassays, transcriptomic and DNA-binding site enrichment analyses, we conclude that RolB can mitigate the TPL functioning so that it leads to a specific and partial reprogramming of phytohormone signaling, immunity, growth, and developmental processes. Our data support a model in which RolB manipulates host transcription, at least in part, through interaction with TPL, to facilitate hairy root development. Thereby, we provide important mechanistic insights into this renowned oncoprotein in HRD.

Genome-Wide Identification of SMXL Gene Family in Soybean and Expression Analysis of GmSMXLs under Shade Stress

SMXL6,7,8 are important target proteins in strigolactone (SL) signal pathway, which negatively regulate the reception and response of SL signal, and play an important role in regulating plant branching. However, there is a relative lack of research on soybean SMXL gene family. In this study, 31 soybean SMXL genes were identified by phylogenetic analysis and divided into three groups. Based on the analysis of GmSMXL gene’s structure and motif composition, it was found that the GmSMXL members in the same group were similar. The results of cis-element analysis showed that GmSMXL genes may regulate the growth and development of soybean by responding to hormones and environment. Based on the tissue specificity analysis and GR24 treatment, the results showed that four GmSMXLs in G1 group were predominantly expressed in stems, axillary buds and leaves and involved in SL signal pathway. Finally, under shading stress, the expression of four genes in G1 group was slightly different in different varieties, which may be the reason for the difference in branching ability of different varieties under shading stress. We have systematically studied the SMXL gene family in soybean, which may lay a foundation for the study of the function of GmSMXL gene in the future.

Regulation of jasmonate signaling by reversible acetylation of TOPLESS in Arabidopsis

The plant hormone jasmonate (JA) regulates plant immunity and adaptive growth by orchestrating a genome-wide transcriptional program. Key regulators of JA-responsive gene expression include the master transcription factor MYC2, which is repressed by the conserved Groucho/Tup1-like corepressor TOPLESS (TPL) in the resting state. However, the mechanisms underlying TPL-mediated transcriptional repression of MYC2 activity and hormone-dependent switching between repression and de-repression remain enigmatic. Here, we report the regulation of TPL activity and JA signaling by reversible acetylation of TPL. We found that the histone acetyltransferase GCN5 could mediate TPL acetylation, which enhances its interaction with the NOVEL-INTERACTOR-OF-JAZ (NINJA) adaptor and promotes its recruitment to MYC2 target promoters, facilitating transcriptional repression. Conversely, TPL deacetylation by the histone deacetylase HDA6 weakens TPL-NINJA interaction and inhibits TPL recruitment to MYC2 target promoters, facilitating transcriptional activation. In the resting state, the opposing activities of GCN5 and HDA6 maintain TPL acetylation homeostasis, promoting transcriptional repression activity of TPL. In response to JA elicitation, HDA6 expression is transiently induced, resulted in decreased TPL acetylation and repressor activity, thereby transcriptional activation of MYC2 target genes. Thus, the GCN5-TPL-HDA6 module maintains the homeostasis of acetylated TPL, thereby determining the transcriptional state of JA-responsive genes. Our findings uncovered a mechanism by which the TPL corepressor activity in JA signaling is actively tuned in a rapid and reversible manner.

TOPLESS in the regulation of plant immunity

This review presents the multiple ways how topless and topless-related proteins regulate defense activation in plants and help in optimizing the defense-growth tradeoff. Eukaryotic gene expression is tightly regulated at various levels by hormones, transcription regulators, post-translational modifications, and transcriptional coregulators. TOPLESS (TPL)/TOPLESS-related (TPR) corepressors regulate gene expression by interacting with other transcription factors. TPRs regulate auxin, gibberellins, jasmonic acid, strigolactone, and brassinosteroid signaling in plants. In general, except for GA, TPLs suppress these signaling pathways to prevent unwanted activation of hormone signaling. The association of TPL/TPRs in these hormonal signaling reflects a wide role of this class of corepressors in plants' normal and stress physiology. The involvement of TPL in immune responses was first demonstrated a decade ago as a repressor of DND1 and DND2 that are negative regulators of plant immune response. Over the last decade, several research groups have established a larger role of TPL/TPRs in plant immunity during both pattern- and effector-triggered immunity. Very recent research unraveled the significant involvement of TPRs in balancing the growth and defense trade-off. TPRs, along with proteasomal degradation complex, miRNA, and phasiRNA, suppress the activation of autoimmunity in plants under normal conditions and promote defense under pathogen attack.

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.

The Multifaceted Roles of MYC2 in Plants: Toward Transcriptional Reprogramming and Stress Tolerance by Jasmonate Signaling

Environmental stress is one of the major restrictions on plant development and foodstuff production. The adaptive response in plants largely occurs through an intricate signaling system, which is crucial for regulating the stress-responsive genes. Myelocytomatosis (MYC) transcription factors are the fundamental regulators of the jasmonate (JA) signaling branch that participates in plant development and multiple stresses. By binding to the cis-acting elements of a large number of stress-responsive genes, JA-responsive transcription factors activate the stress-resistant defense genes. The mechanism of stress responses concerns myriad regulatory processes at the physiological and molecular levels. Discovering stress-related regulatory factors is of great value in disclosing the response mechanisms of plants to biotic or abiotic stress, which could guide the genetic improvement of plant resistance. This review summarizes recent researches in various aspects of MYC2-mediated JA signaling and emphasizes MYC2 involvement in plant growth and stress response.

The molecular and genetic regulation of shoot branching

The architecture of flowering plants exhibits both phenotypic diversity and plasticity, determined, in part, by the number and activity of axillary meristems and, in part, by the growth characteristics of the branches that develop from the axillary buds. The plasticity of shoot branching results from a combination of various intrinsic and genetic elements, such as number and position of nodes and type of growth phase, as well as environmental signals such as nutrient availability, light characteristics, and temperature (Napoli et al., 1998; Bennett and Leyser, 2006; Janssen et al., 2014; Teichmann and Muhr, 2015; Ueda and Yanagisawa, 2019). Axillary meristem initiation and axillary bud outgrowth are controlled by a complex and interconnected regulatory network. Although many of the genes and hormones that modulate branching patterns have been discovered and characterized through genetic and biochemical studies, there are still many gaps in our understanding of the control mechanisms at play. In this review, we will summarize our current knowledge of the control of axillary meristem initiation and outgrowth into a branch.

DWARF53 interacts with transcription factors UB2/UB3/TSH4 to regulate maize tillering and tassel branching

Strigolactones (SLs) are a recently identified class of phytohormones that regulate diverse developmental processes in land plants. However, the signaling mechanism of SLs in maize (Zea mays) remains largely unexplored. Here, we identified the maize gene DWARF 53 (ZmD53) and demonstrated that ZmD53 interacts with the SL receptors DWARF 14A/B (ZmD14A/B) in a rac-GR24-dependent manner. Transgenic maize plants expressing a gain-of-function mutant version of Zmd53 exhibited insensitivity to exogenous rac-GR24 treatment and a highly pleiotropic phenotype, including excess tillering and reduced tassel branching, indicating that ZmD53 functions as an authentic SL signaling repressor in maize. In addition, we showed that ZmD53 interacts with two homologous maize SPL transcription factors, UB3 and TSH4, and suppresses their transcriptional activation activity on TB1 to promote tillering. We also showed that UB2, UB3, and TSH4 can physically interact with each other and themselves, and that they can directly regulate the expression of TSH4, thus forming a positive feedback loop. Furthermore, we demonstrated that ZmD53 can repress the transcriptional activation activity of UB3 and TSH4 on their own promoters, thus decreasing tassel branch number. Our results reveal new insights into the integration of SL signaling and the miR156/SPL molecular module to coordinately regulate maize development.

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

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

The GW2-WG1-OsbZIP47 pathway controls grain size and weight in rice

Regulation of seed size is a key strategy for improving crop yield and is also a basic biological question. However, the molecular mechanisms by which plants determine their seed size remain elusive. Here, we report that the GW2-WG1-OsbZIP47 regulatory module controls grain width and weight in rice. WG1, which encodes a glutaredoxin protein, promotes grain growth by increasing cell proliferation. Interestingly, WG1 interacts with the transcription factor OsbZIP47 and represses its transcriptional activity by associating with the transcriptional co-repressor ASP1, indicating that WG1 may act as an adaptor protein to recruit the transcriptional co-repressor. In contrary, OsbZIP47 restricts grain growth by decreasing cell proliferation. Further studies reveal that the E3 ubiquitin ligase GW2 ubiquitinates WG1 and targets it for degradation. Genetic analyses confirm that GW2, WG1, and OsbZIP47 function in a common pathway to control grain growth. Taken together, our findings reveal a genetic and molecular framework for the control of grain size and weight by the GW2-WG1-OsbZIP47 regulatory module, providing new targets for improving seed size and weight in crops.

Repression by the Arabidopsis TOPLESS corepressor requires association with the core mediator complex

The plant corepressor TOPLESS (TPL) is recruited to a large number of loci that are selectively induced in response to developmental or environmental cues, yet the mechanisms by which it inhibits expression in the absence of these stimuli are poorly understood. Previously, we had used the N-terminus of Arabidopsis thaliana TPL to enable repression of a synthetic auxin response circuit in Saccharomyces cerevisiae (yeast). Here, we leveraged the yeast system to interrogate the relationship between TPL structure and function, specifically scanning for repression domains. We identified a potent repression domain in Helix 8 located within the CRA domain, which directly interacted with the Mediator middle module subunits Med21 and Med10. Interactions between TPL and Mediator were required to fully repress transcription in both yeast and plants. In contrast, we found that multimer formation, a conserved feature of many corepressors, had minimal influence on the repression strength of TPL.

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.

The PEAPOD Pathway and Its Potential To Improve Crop Yield

A key strategy to increase plant productivity is to improve intrinsic organ growth. Some of the regulatory networks underlying organ growth and development, as well as the interconnections between these networks, are highly conserved. An example of such a growth-regulatory module with a highly conserved role in final organ size and shape determination in eudicot species is the PEAPOD (PPD)/KINASE-INDUCIBLE DOMAIN INTERACTING (KIX)/STERILE APETALA (SAP) module. We review the proteins constituting the PPD pathway and their roles in different plant developmental processes, and explore options for future research. We also speculate on strategies to exploit knowledge about the PPD pathway for targeted yield improvement to engineer crop traits of agronomic interest, such as leaf, fruit, and seed size.

Structure of nucleosome-bound DNA methyltransferases DNMT3A and DNMT3B

CpG methylation by de novo DNA methyltransferases (DNMTs) 3A and 3B is essential for mammalian development and differentiation and is frequently dysregulated in cancer1. These two DNMTs preferentially bind to nucleosomes, yet cannot methylate the DNA wrapped around the nucleosome core2, and they favour the methylation of linker DNA at positioned nucleosomes3,4. Here we present the cryo-electron microscopy structure of a ternary complex of catalytically competent DNMT3A2, the catalytically inactive accessory subunit DNMT3B3 and a nucleosome core particle flanked by linker DNA. The catalytic-like domain of the accessory DNMT3B3 binds to the acidic patch of the nucleosome core, which orients the binding of DNMT3A2 to the linker DNA. The steric constraints of this arrangement suggest that nucleosomal DNA must be moved relative to the nucleosome core for de novo methylation to occur.

Karrikin Signaling Acts Parallel to and Additively with Strigolactone Signaling to Regulate Rice Mesocotyl Elongation in Darkness

Seedling emergence in monocots depends mainly on mesocotyl elongation, requiring coordination between developmental signals and environmental stimuli. Strigolactones (SLs) and karrikins are butenolide compounds that regulate various developmental processes; both are able to negatively regulate rice (Oryza sativa) mesocotyl elongation in the dark. Here, we report that a karrikin signaling complex, DWARF14-LIKE (D14L)-DWARF3 (D3)-O. sativa SUPPRESSOR OF MAX2 1 (OsSMAX1) mediates the regulation of rice mesocotyl elongation in the dark. We demonstrate that D14L recognizes the karrikin signal and recruits the SCFD3 ubiquitin ligase for the ubiquitination and degradation of OsSMAX1, mirroring the SL-induced and D14- and D3-dependent ubiquitination and degradation of D53. Overexpression of OsSMAX1 promoted mesocotyl elongation in the dark, whereas knockout of OsSMAX1 suppressed the elongated-mesocotyl phenotypes of d14l and d3 OsSMAX1 localizes to the nucleus and interacts with TOPLESS-RELATED PROTEINs, regulating downstream gene expression. Moreover, we showed that the GR24 enantiomers GR245DS and GR24 ent-5DS specifically inhibit mesocotyl elongation and regulate downstream gene expression in a D14- and D14L-dependent manner, respectively. Our work revealed that karrikin and SL signaling play parallel and additive roles in modulating downstream gene expression and negatively regulating mesocotyl elongation in the dark.

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

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

Structure-Function Analysis of SMAX1 Reveals Domains That Mediate Its Karrikin-Induced Proteolysis and Interaction with the Receptor KAI2

Karrikins (KARs) are butenolides found in smoke that can influence germination and seedling development of many plants. The KAR signaling mechanism is hypothesized to be very similar to that of the plant hormone strigolactone (SL). Both pathways require the F-box protein MORE AXILLARY GROWTH2 (MAX2), and other core signaling components have shared ancestry. Putatively, KAR activates the receptor KARRIKIN INSENSITIVE2 (KAI2), triggering its association with the E3 ubiquitin ligase complex SCFMAX2 and downstream targets SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2). Polyubiquitination and proteolysis of SMAX1 and SMXL2 then enable growth responses to KAR. However, many of the assumptions of this model have not been demonstrated. Therefore, we investigated the posttranslational regulation of SMAX1 from the model plant Arabidopsis (Arabidopsis thaliana). We find evidence that SMAX1 is degraded by KAI2-SCFMAX2 but is also subject to MAX2-independent turnover. We identify SMAX1 domains that are responsible for its nuclear localization, KAR-induced degradation, association with KAI2, and ability to interact with other SMXL proteins. KAI2 undergoes MAX2-independent degradation after KAR treatment, which we propose results from its association with SMAX1 and SMXL2. Finally, we discover an SMXL domain that mediates receptor-target interaction preferences in KAR and SL signaling, laying the foundation for understanding how these highly similar pathways evolved to fulfill different roles.

SUPPRESSOR OF MAX2 1-LIKE 5 promotes secondary phloem formation during radial stem growth

As a pre-requisite for constant growth, plants produce vascular tissues at different sites within their post-embryonic body. Interestingly, the formation of vascular tissues during longitudinal and radial expansion of shoot and root axes differs fundamentally with respect to its anatomical configuration. This raises the question to which level regulatory mechanisms of vascular tissue formation are shared throughout plant development. Here, we show that, similar to primary phloem formation during longitudinal growth, the cambium-based formation of secondary phloem depends on the function of SUPPRESSOR OF MAX2 1-LIKE (SMXL) genes. In particular, local SMXL5 deficiency results in the absence of secondary phloem. Moreover, the additional disruption of SMXL4 activity increases tissue production in the cambium region without secondary phloem being formed. Using promoter-reporter lines, we observed that SMXL4 and SMXL5 activities are associated with different stages of secondary phloem formation in the Arabidopsis stem. Based on genome-wide transcriptional profiling and expression analyses of phloem-related markers, we concluded that early steps of phloem formation are impaired in smxl4;smxl5 double mutants and that the additional cambium-derived cells fail to establish phloem-related features. Our results showed that molecular mechanisms determining primary and secondary phloem formation share important properties, but differ slightly with SMXL5 playing a more dominant role in the formation of secondary phloem.

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

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

Strigolactones and Brassinosteroids Antagonistically Regulate the Stability of the D53-OsBZR1 Complex to Determine FC1 Expression in Rice Tillering

Rice tillering, a key architecture trait determining grain yield, is highly regulated by a class of newly identified phytohormones, strigolactones (SLs). However, the whole SL signaling pathway from the receptor to downstream transcription factors to finally inhibit tillering remains unrevealed. In this study, we first found that brassinosteroids (BRs) strongly enhance tillering by promoting bud outgrowth in rice, which is largely different from the function of BRs in Arabidopsis. Genetic and biochemical analyses indicated that both the SL and BR signaling pathways control rice tillering by regulating the stability of D53 and/or the OsBZR1-RLA1-DLT module, a transcriptional complex in the rice BR signaling pathway. We further found that D53 interacts with OsBZR1 to inhibit the expression of FC1, a local inhibitor of tillering, and that this inhibition depends on direct DNA binding by OsBZR1, which recruits D53 to the FC1 promoter in rice buds. Taken together, these findings uncover a mechanism illustrating how SLs and BRs coordinately regulate rice tillering via the early responsive gene FC1.

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

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

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.

Strigolactones and their crosstalk with other phytohormones

BACKGROUND

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

SCOPE

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

CONCLUSION

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

A single residue change in the product of the chrysanthemum gene TPL1-2 leads to a failure in its repression of flowering

The TPL/TPR co-repressor is involved in many plant signaling pathways, including those regulating the switch from vegetative to reproductive growth. Here, a TPL homolog (TPL 1-2) was isolated from chrysanthemum. Its product was found to be deposited in the nucleus. The abundance of TPL1-2 transcript varied across the plant, with its highest level being recorded in the stem apex, and its lowest in the root and stem. In the leaf, the abundance of TPL1-2 transcript was highest at dusk in plants exposed to long days, and at dawn in those exposed to short days. Site-directed mutagenesis was used to induce an N176H mutation in TPL1-2. The constitutive expression in Arabidopsis thaliana of the wild type and the mutated alleles of TPL1-2 had a contrasting effect on flowering time, with the mutant transgene expressors flowering later than the wild type transgene expressors. The flowering-related genes FT, TSF, FUL and AP1 were all more strongly transcribed in the mutant transgene expressors than in the wild type transgene expressors.

Accelerating structure-function mapping using the ViVa webtool to mine natural variation

Thousands of sequenced genomes are now publicly available capturing a significant amount of natural variation within plant species; yet, much of these data remain inaccessible to researchers without significant bioinformatics experience. Here, we present a webtool called ViVa (Visualizing Variation) which aims to empower any researcher to take advantage of the amazing genetic resource collected in the Arabidopsis thaliana 1001 Genomes Project (http://1001genomes.org). ViVa facilitates data mining on the gene, gene family, or gene network level. To test the utility and accessibility of ViVa, we assembled a team with a range of expertise within biology and bioinformatics to analyze the natural variation within the well-studied nuclear auxin signaling pathway. Our analysis has provided further confirmation of existing knowledge and has also helped generate new hypotheses regarding this well-studied pathway. These results highlight how natural variation could be used to generate and test hypotheses about less-studied gene families and networks, especially when paired with biochemical and genetic characterization. ViVa is also readily extensible to databases of interspecific genetic variation in plants as well as other organisms, such as the 3,000 Rice Genomes Project ( http://snp-seek.irri.org/) and human genetic variation ( https://www.ncbi.nlm.nih.gov/clinvar/).

Strigolactone promotes cytokinin degradation through transcriptional activation of CYTOKININ OXIDASE/DEHYDROGENASE 9 in rice

Strigolactone plays a vital role in plant growth and development, but its response genes remain to be identified. In this study, we found that cytokinin content is markedly increased in the strigolactone signaling mutant d53, and that OsCKX9, which encodes a cytokinin oxidase to catalyze the degradation of cytokinin, functions as a primary strigolactone-responsive gene to regulate rice tillering, plant height, and panicle size, likely via a secondary response gene, OsRR5, which encodes a cytokinin-inducible rice type-A response regulator, demonstrating that strigolactone regulates rice shoot architecture through enhanced cytokinin catabolism by modulating OsCKX9 expression.

Strigolactones (SLs), a group of terpenoid lactones derived from carotenoids, are plant hormones that control numerous aspects of plant development. Although the framework of SL signaling that the repressor DWARF 53 (D53) could be SL-dependently degraded via the SL receptor D14 and F-box protein D3 has been established, the downstream response genes to SLs remain to be elucidated. Here we show that the cytokinin (CK) content is dramatically increased in shoot bases of the rice SL signaling mutant d53. By examining transcript levels of all the CK metabolism-related genes after treatment with SL analog GR24, we identified CYTOKININ OXIDASE/DEHYDROGENASE 9 (OsCKX9) as a primary response gene significantly up-regulated within 1 h of treatment in the wild type but not in d53. We also found that OsCKX9 functions as a cytosolic and nuclear dual-localized CK catabolic enzyme, and that the overexpression of OsCKX9 suppresses the browning of d53 calli. Both the CRISPR/Cas9-generated OsCKX9 mutants and OsCKX9-overexpressing transgenic plants showed significant increases in tiller number and decreases in plant height and panicle size, suggesting that the homeostasis of OsCKX9 plays a critical role in regulating rice shoot architecture. Moreover, we identified the CK-inducible rice type-A response regulator OsRR5 as the secondary SL-responsive gene, whose expression is significantly repressed after 4 h of GR24 treatment in the wild type but not in osckx9. These findings reveal a comprehensive plant hormone cross-talk in which SL can induce the expression of OsCKX9 to down-regulate CK content, which in turn triggers the response of downstream genes.

The C-terminal WD40 repeats on the TOPLESS co-repressor function as a protein-protein interaction surface

The two predicted WD40 propellers on TOPLESS function as protein-protein interaction domains. The 1st WD40 propeller mediates interaction with RAV1, and the 2nd WD40 propeller mediates interaction with VRN5. The TOPLESS/TOPLESS-RELATED (TPL/TPR) co-repressor family proteins are known to interact with a wide variety of proteins including transcription factors, Mediator subunits, histone deacetylases, and histone tails. Through these interactions, TPL/TPR act to repress transcription in an increasingly diverse array of plant pathways. Proteins that bind TPL/TPR typically contain one or more Repression Domains (RDs) that mediate the interaction. For example, the well-characterized Ethylene response factor-associated Amphiphilic Repression (EAR) motif is known to facilitate interaction by binding the TOPLESS Domain (TPD) located in the N-terminus. Here we show that in yeast two-hybrid assays, the non-EAR protein, Related to ABI3/VP1-1 (RAV1), binds a novel region located within the first nine WD40-repeats of TPL. Protein modeling and in silico analysis suggest that these nine WD40 repeats may form the first of two WD40 propellers located on C-terminus of TPL. The interaction between RAV1 and the 1st WD40 propeller is conserved with another RAV family member, TEMPRANILLO1 (TEM1) and is mediated by the B3 Repression Domain (BRD) located on both RAV1 and TEM1. Also, the predicted 2nd WD40 propeller was shown in yeast cells to bind Vernalization 5 (VRN5), which contains several unconfirmed partial RDs. Furthermore, we demonstrate that the 1st WD40 propeller of TPL can form a complex with RAV1 both in yeast and in Arabidopsis protoplasts.

Auxin EvoDevo: Conservation and Diversification of Genes Regulating Auxin Biosynthesis, Transport, and Signaling

The phytohormone auxin has been shown to be of pivotal importance in growth and development of land plants. The underlying molecular players involved in auxin biosynthesis, transport, and signaling are quite well understood in Arabidopsis. However, functional characterizations of auxin-related genes in economically important crops, specifically maize and rice, are still limited. In this article, we comprehensively review recent functional studies on auxin-related genes in both maize and rice, compared with what is known in Arabidopsis, and highlight conservation and diversification of their functions. Our analysis is illustrated by phylogenetic analysis and publicly available gene expression data for each gene family, which will aid in the identification of auxin-related genes for future research. Current challenges and future directions for auxin research in maize and rice are discussed. Developments in gene editing techniques provide powerful tools for overcoming the issue of redundancy in these gene families and will undoubtedly advance auxin research in crops.

Triazole Ureas Covalently Bind to Strigolactone Receptor and Antagonize Strigolactone Responses

Strigolactones, a class of plant hormones with multiple functions, mediate plant-plant and plant-microorganism communications in the rhizosphere. In this study, we developed potent strigolactone antagonists, which covalently bind to the strigolactone receptor D14, by preparing an array of triazole urea compounds. Using yeast two-hybrid and rice-tillering assays, we identified a triazole urea compound KK094 as a potent inhibitor of strigolactone receptors. Liquid chromatography-tandem mass spectrometry analysis and X-ray crystallography revealed that KK094 was hydrolyzed by D14, and that a reaction product of this degradation covalently binds to the Ser residue of the catalytic triad of D14. Furthermore, we identified two triazole urea compounds KK052 and KK073, whose effects on D14-D53/D14-SLR1 complex formation were opposite due to the absence (KK052) or presence (KK073) of a trifluoromethyl group on their phenyl ring. These results demonstrate that triazole urea compounds are potentially powerful tools for agricultural application and may be useful for the elucidation of the complicated mechanism underlying strigolactone perception.

RAMOSA1 ENHANCER LOCUS2-Mediated Transcriptional Repression Regulates Vegetative and Reproductive Architecture

Transcriptional repression in multicellular organisms orchestrates dynamic and precise gene expression changes that enable complex developmental patterns. Here, we present phenotypic and molecular characterization of the maize (Zea mays) transcriptional corepressor RAMOSA1 ENHANCER LOCUS2 (REL2), a unique member of the highly conserved TOPLESS (TPL) family. Analysis of single recessive mutations in rel2 revealed an array of vegetative and reproductive phenotypes, many related to defects in meristem initiation and maintenance. To better understand how REL2-mediated transcriptional complexes relate to rel2 phenotypes, we performed protein interaction assays and transcriptional profiling of mutant inflorescences, leading to the identification of different maize transcription factors and regulatory pathways that employ REL2 repression to control traits directly impacting maize yields. In addition, we used our REL2 interaction data to catalog conserved repression motifs present on REL2 interactors and showed that two of these, RLFGV- and DLN-type motifs, interact with the C-terminal WD40 domain of REL2 rather than the N terminus, which is known to bind LxLxL EAR motifs. These findings establish that the WD40 domain of TPL family proteins is an independent protein interaction surface that may work together with the N-terminal domain to allow the formation of large macromolecular complexes of functionally related transcription factors.

PlantEAR: Functional Analysis Platform for Plant EAR Motif-Containing Proteins

The Ethylene-responsive element binding factor-associated Amphiphilic Repression (EAR) motifs, which were initially identified in members of the Arabidopsis ethylene response factor (ERF) family, are transcriptional repression motifs in plants and are defined by the consensus sequence patterns of either LxLxL or DLNxxP. EAR motif-containing proteins can function as transcription repressors, thus interacting with co-repressors, such as TOPLESS and AtSAP18, affecting the structure of chromatin by histone modifications and thereby repressing gene transcription. EAR motif-containing proteins are highly conserved across diverse plant species and play important roles in hormone signal transduction, stress responses and development, but they have not been identified in most plants. In this study, we identified 20,542 EAR motif-containing proteins from 71 plant species based on a Hidden Markov Model and orthologous gene search, and then we constructed a functional analysis platform for plant EAR motif-containing proteins (PlantEAR, http://structuralbiology.cau.edu.cn/plantEAR) by integrating a variety of functional annotations and processed data. Several tools were provided as functional support for EAR motif-containing proteins, such as browse, search, co-expression and protein-protein interaction (PPI) network analysis as well as cis-element analysis and gene set enrichment analysis (GSEA). In addition, basing on the identified EAR motif-containing proteins, we also explored their distribution in various species and found that the numbers of EAR motif-containing proteins showed an increasing trend in evolution from algae to angiosperms.

Structural plasticity of D3-D14 ubiquitin ligase in strigolactone signalling

The plant hormone strigolactones (SLs) regulate many aspects of plant physiology. In shoot branching inhibition, the SL-metabolizing α/β hydrolase D14 interacts with the F-box protein D3 to ubiquitinate and degrade the transcription repressor D53. Despite multiple modes of D14-SL interactions determined recently, how the hydrolase functions with D3 to mediate hormone-dependent D53 ubiquitination remains elusive. Here we show that D3 features a C-terminal α-helix (CTH), which can switch between two conformational states. Distinct from its engaged form, which facilitate the binding of D3 and D14 with a hydrolyzed SL intermediate, the dislodged D3 CTH can recognize unmodified D14 in an open conformation and inhibits its enzymatic activity. In an SL-dependent manner, the D3 CTH enables D14 to recruit D53, which in turn activates the hydrolase. By unraveling an unexpected structural plasticity in SCF^D3-D14 ubiquitin ligase, our results suggest an intricate mechanism by which the E3 coordinates SL signaling and metabolism.

Structural plasticity of D3-D14 ubiquitin ligase in strigolactone signalling

The strigolactones, a class of plant hormones, regulate many aspects of plant physiology. In the inhibition of shoot branching, the α/β hydrolase D14-which metabolizes strigolactone-interacts with the F-box protein D3 to ubiquitinate and degrade the transcription repressor D53. Despite the fact that multiple modes of interaction between D14 and strigolactone have recently been determined, how the hydrolase functions with D3 to mediate hormone-dependent D53 ubiquitination remains unknown. Here we show that D3 has a C-terminal α-helix that can switch between two conformational states. The engaged form of this α-helix facilitates the binding of D3 and D14 with a hydrolysed strigolactone intermediate, whereas the dislodged form can recognize unmodified D14 in an open conformation and inhibits its enzymatic activity. The D3 C-terminal α-helix enables D14 to recruit D53 in a strigolactone-dependent manner, which in turn activates the hydrolase. By revealing the structural plasticity of the SCF^D3-D14 ubiquitin ligase, our results suggest a mechanism by which the E3 coordinates strigolactone signalling and metabolism.

Spatial regulation of strigolactone function

Strigolactones are plant hormones that control many aspects of plant development and environmental responses. Despite recent and rapid progress in the biochemical and molecular understanding of strigolactone biosynthesis, transport, and signaling, our knowledge about where strigolactones are produced and where they act is fragmented. In this review, we summarize current knowledge about these aspects of strigolactones, obtained from mutant phenotypes, grafting experiments, gene expression patterns, and protein localization studies. We also discuss the potential of new imaging technologies to reveal the spatial regulation of strigolactone function.

Molecular evolution and diversification of the SMXL gene family

Strigolactones (SLs) are a relatively recent addition to the list of plant hormones that control different aspects of plant development. SL signalling is perceived by an α/β hydrolase, DWARF 14 (D14). A close homolog of D14, KARRIKIN INSENSTIVE2 (KAI2), is involved in perception of an uncharacterized molecule called karrikin (KAR). Recent studies in Arabidopsis identified the SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE 7 (SMXL7) to be potential SCF-MAX2 complex-mediated proteasome targets of KAI2 and D14, respectively. Genetic studies on SMXL7 and SMAX1 demonstrated distinct developmental roles for each, but very little is known about these repressors in terms of their sequence features. In this study, we performed an extensive comparative analysis of SMXLs and determined their phylogenetic and evolutionary history in the plant lineage. Our results show that SMXL family members can be sub-divided into four distinct phylogenetic clades/classes, with an ancient SMAX1. Further, we identified the clade-specific motifs that have evolved and that might act as determinants of SL-KAR signalling specificity. These specificities resulted from functional diversities among the clades. Our results suggest that a gradual co-evolution of SMXL members with their upstream receptors D14/KAI2 provided an increased specificity to both the SL perception and response in land plants.