Proteome-wide cellular thermal shift assay reveals unexpected cross-talk between brassinosteroid and auxin signaling

Assessing Metabolite Interactions With Chloroplastic Proteins via the PISA Assay

Plants rely on metabolite regulation of proteins to control their metabolism and adapt to environmental changes, but studying these complex interaction networks remains challenging. The proteome integral solubility alteration (PISA) assay, a high-throughput chemoproteomic technique, was originally developed for mammalian systems to investigate drug targets. PISA detects changes in protein stability upon interaction with small molecules, quantified through LC-MS. Here, we present an adapted PISA protocol for Arabidopsis thaliana chloroplasts to identify potential protein interactions with ascorbate. Chloroplasts are extracted using a linear Percoll gradient, treated with multiple ascorbate concentrations, and subjected to heat-induced protein denaturation. Soluble proteins are extracted via ultracentrifugation, and proteome-wide stability changes are quantified using multiplexed LC-MS. We provide instructions for deconvolution of LC-MS spectra and statistical analysis using freely available software. This protocol enables unbiased screening of protein regulation by small molecules in plants without requiring prior knowledge of interaction partners, chemical probe design, or genetic modifications. Key features • Optimization of the PISA assay to study protein-ligand interactions in plant chloroplasts, including isolation of chloroplasts. • Study of regulation on a proteome level, without genetic manipulation or prior knowledge of interaction partners. • High proteome coverage, low sample requirement, 5-fold reduction of TMT-labeling cost, and short LC-MS analysis time. • Adaptable to other organisms, such as bacteria, with minor modifications.

Applications of the Cellular Thermal Shift Assay to Drug Discovery in Natural Products: A Review

Natural products play a crucial role in drug discovery because of their structural diversity and biological activity. However, identifying their molecular targets remains a challenge. Traditional target identification approaches such as affinity-based protein profiling and activity-based protein profiling are limited by the need for chemical modification or reactive groups in natural products. The emergence of label-free techniques offers a powerful alternative for studying drug-target engagement in a physiological context. In particular, the cellular thermal shift assay (CETSA) exploits ligand-induced protein stabilization-a phenomenon where ligand binding enhances a protein's thermal stability by reducing conformational flexibility-to assess drug binding without requiring chemical modifications. CETSA's integration with advanced mass spectrometry and high-throughput platforms has dramatically expanded proteome coverage and sensitivity, enabling the simultaneous quantification of thousands of proteins and the identification of low-abundance targets in native cellular environments. This review highlights the application of key CETSA-based methods to target identification in natural products including Western blot-based CETSA, isothermal dose-response CETSA, mass spectrometry-based CETSA, and high-throughput CETSA. Case studies are presented that demonstrate their effectiveness in uncovering the mechanisms of action of different drugs. The current limitations of CETSA-based strategies are also explored, and future improvements to optimize their potential for drug discovery are discussed. Integrating CETSA with complementary approaches can enhance the target identification accuracy and efficiency for natural products and ultimately advance development of therapeutic applications.

Plant steroids on the move: mechanisms of brassinosteroid export

Brassinosteroids (BRs) are essential plant steroidal hormones that regulate growth and development. The recent discoveries of ATP-binding cassette subfamily B (ABCB) members, ABCB19 and ABCB1, as BR transporters highlight the significance of active export to the apoplast in maintaining BR homeostasis and enabling effective signaling. This review focuses on the latest progress in understanding ABCB-mediated BR transport, with particular attention to the structural and functional characterization of arabidopsis ABCB19 and ABCB1. These findings reveal both conserved and distinct features in substrate recognition and transport mechanisms, providing valuable insights into their roles in hormonal regulation. Additionally, the evolutionary conservation of ABC transporters in mediating steroid-based signaling across biological kingdoms underscores their fundamental biological significance.

Polarity-guided uneven mitotic divisions control brassinosteroid activity in proliferating plant root cells

Brassinosteroid hormones are positive regulators of plant organ growth, yet their function in proliferating tissues remains unclear. Here, through integrating single-cell RNA sequencing with long-term live-cell imaging of the Arabidopsis root, we reveal that brassinosteroid activity fluctuates throughout the cell cycle, decreasing during mitotic divisions and increasing during the G1 phase. The post-mitotic recovery of brassinosteroid activity is driven by the intrinsic polarity of the mother cell, resulting in one daughter cell with enhanced brassinosteroid signaling, while the other supports brassinosteroid biosynthesis. The coexistence of these distinct daughter cell states during the G1 phase circumvents a negative feedback loop to facilitate brassinosteroid production while signaling increases. Our findings uncover polarity-guided, uneven mitotic divisions in the meristem, which control brassinosteroid hormone activity to ensure optimal root growth.

Reduced RG-II pectin dimerization disrupts differential growth by attenuating hormonal regulation

Defects in cell wall integrity (CWI) profoundly affect plant growth, although, underlying mechanisms are not well understood. We show that in Arabidopsis mur1 mutant, CWI defects from compromising dimerization of RG-II pectin, a key component of cell wall, attenuate the expression of auxin response factors ARF7-ARF19. As a result, polar auxin transport components are misexpressed, disrupting auxin response asymmetry, leading to defective apical hook development. Accordingly, mur1 hook defects are suppressed by enhancing ARF7 expression. In addition, expression of brassinosteroid biosynthesis genes is down-regulated in mur1 mutant, and supplementing brassinosteroid or enhancing brassinosteroid signaling suppresses mur1 hook defects. Intriguingly, brassinosteroid enhances RG-II dimerization, showing hormonal feedback to the cell wall. Our results thus reveal a previously unrecognized link between cell wall defects from reduced RG-II dimerization and growth regulation mediated via modulation of auxin-brassinosteroid pathways in early seedling development.

Inhibition of CDH11 Activates cGAS-STING by Stimulating Branched Chain Amino Acid Catabolism and Mitigates Lung Metastasis of Adenoid Cystic Carcinoma

Salivary adenoid cystic carcinoma (SACC) is an intractable malignant tumor originates in the secretory glands and frequently metastasizes to the lungs. Hybrid epithelial-mesenchymal transition (EMT) cells within the tumors are correlated with augmented proliferative capacity and facilitation of lung metastasis. Single-cell RNA sequencing and spatial transcriptomic sequencing are employed to reveal the hybrid EMT subsets within the vascular fibroblast microenvironment. These hybrid EMT cells exhibit a pro-tumorigenic impact in vitro. Notably, cadherin 11 (CDH11), a specific marker for hybrid EMT cells, may exert its regulatory role in cellular function by interfering with branched-chain amino acids (BCAA) metabolism by inhibiting branched-chain ketoacid dehydrogenase to activate the mammalian target of the rapamycin pathway, thus making it a potential therapeutic target for SACC. Furthermore, celecoxib and its derivatives are specific CDH11 inhibitors that regulate BCAA metabolism, increase reactive oxygen species production, and subsequently activate the cyclic GMP-AMP synthase-stimulator of the interferongene pathway (cGAS-STING). They also inhibit lung metastasis in NOD-SCID mice in vivo. Overall, these findings suggest a promising treatment strategy that targets hybrid EMT cells to mitigate lung metastasis in SACC. Celecoxib may serve as a promising clinical intervention for the treatment of lung metastases in patients with SACC.

DDX50 cooperates with STAU1 to effect stabilization of pro-differentiation RNAs

Glucose binding can alter protein oligomerization to enable differentiation. Here, we demonstrate that glucose binding is a general capacity of DExD/H-box RNA helicases, including DDX50, which was found to be essential for the differentiation of diverse cell types. Glucose binding to conserved DDX50 ATP binding sequences altered protein conformation and dissociated DDX50 dimers. DDX50 monomers bound STAU1 to redirect STAU1 from an RNA-decay-promoting complex with UPF1 to a DDX50-STAU1 ribonuclear complex. DDX50 and STAU1 bound and stabilized a common set of essential pro-differentiation RNAs, including JUN, OVOL1, CEBPB, PRDM1, and TINCR, whose structures they also modified. These findings uncover a DDX50-mediated mechanism of reprograming STAU1 from its canonical role in Staufen-mediated mRNA decay to an opposite role stabilizing pro-differentiation RNAs and establish an activity for glucose in controlling RNA structure and stability.

Stomatal opening under high temperatures is controlled by the OST1-regulated TOT3-AHA1 module

Plants continuously respond to changing environmental conditions to prevent damage and maintain optimal performance. To regulate gas exchange with the environment and to control abiotic stress relief, plants have pores in their leaf epidermis, called stomata. Multiple environmental signals affect the opening and closing of these stomata. High temperatures promote stomatal opening (to cool down), and drought induces stomatal closing (to prevent water loss). Coinciding stress conditions may evoke conflicting stomatal responses, but the cellular mechanisms to resolve these conflicts are unknown. Here we demonstrate that the high-temperature-associated kinase TARGET OF TEMPERATURE 3 directly controls the activity of plasma membrane H+-ATPases to induce stomatal opening. OPEN STOMATA 1, which regulates stomatal closure to prevent water loss during drought stress, directly inactivates TARGET OF TEMPERATURE 3 through phosphorylation. Taken together, this signalling axis harmonizes stomatal opening and closing under high temperatures and/or drought. In the context of global climate change, understanding how different stress signals converge on stomatal regulation allows the development of climate-change-ready crops.

Glucose binds and activates NSUN2 to promote translation and epidermal differentiation

Elevations in intracellular glucose concentrations are essential for epithelial cell differentiation by mechanisms that are not fully understood. Glucose has recently been found to directly bind several proteins to alter their functions to enhance differentiation. Among the newly identified glucose-binding proteins is NSUN2, an RNA-binding protein that we identified as indispensable for epidermal differentiation. Glucose was found to bind conserved sequences within NSUN2, enhancing its binding to S-adenosyl-L-methionine and boosting its enzymatic activity. Additionally, glucose enhanced NSUN2's proximity to proteins involved in mRNA translation, with NSUN2 modulating global messenger RNA (mRNA) translation, particularly that of key pro-differentiation mRNAs containing m5C modifications, such as GRHL3. Glucose thus engages diverse molecular mechanisms beyond its energetic roles to facilitate cellular differentiation processes.

Cellular thermal shift assay: an approach to identify and assess protein target engagement

INTRODUCTION

A comprehensive and global knowledge of protein target engagement is of vital importance for mechanistic studies and in drug development. Since its initial introduction, the cellular thermal shift assay (CETSA) has proven to be a reliable and flexible technique that can be widely applied to multiple contexts and has profound applications in facilitating the identification and assessment of protein target engagement.

AREAS COVERED

This review introduces the principle of CETSA, elaborates on western blot-based CETSA and MS-based thermal proteome profiling (TPP) as well as the major applications and prospects of these approaches.

EXPERT OPINION

CETSA primarily evaluates a given ligand binding to a particular target protein in cells and tissues with the protein thermal stabilities analyzed by western blot. When coupling mass spectrometry with CETSA, thermal proteome profiling allows simultaneous proteome-wide experiment that greatly increased the efficiency of target engagement evaluation, and serves as a promising strategy to identify protein targets and off-targets as well as protein-protein interactions to uncover the biological effects. The CETSA approaches have broad applications and potentials in drug development and clinical research.

Engineering cell-free systems by chemoproteomic-assisted phenotypic screening

Phenotypic screening is a valuable tool to both understand and engineer complex biological systems. We demonstrate the functionality of this approach in the development of cell-free protein synthesis (CFPS) technology. Phenotypic screening identified numerous compounds that enhanced protein production in yeast lysate CFPS reactions. Notably, many of these were competitive ATP kinase inhibitors, with the exploitation of their inherent substrate promiscuity redirecting ATP flux towards heterologous protein expression. Chemoproteomic-guided strain engineering partially phenocopied drug effects, with a 30% increase in protein yield observed upon deletion of the ATP-consuming SSA1 component of the HSP70 chaperone. Moreover, drug-mediated metabolic rewiring coupled with template optimization generated the highest protein yields in yeast CFPS to date using a hitherto less efficient, but more cost-effective glucose energy regeneration system. Our approach highlights the utility of target-agnostic phenotypic screening and target identification to deconvolute cell-lysate complexity, adding to the expanding repertoire of strategies for improving CFPS.

Sinapine targeting PLCβ3 EF hands disrupts Gαq-PLCβ3 interaction and ameliorates cardiovascular diseases

BACKGROUND

The renin-angiotensin-aldosterone system (RAAS) over-activation is highly involved in cardiovascular diseases (CVDs), with the Gαq-PLCβ3 axis acting as a core node of RAAS. PLCβ3 is a potential target of CVDs, and the lack of inhibitors has limited its drug development.

PURPOSE

Sinapine (SP) is a potential leading compound for treating CVDs. Thus, we aimed to elucidate the regulation of SP towards the Gαq-PLCβ3 axis and its molecular mechanism.

STUDY DESIGN

Aldosteronism and hypertension animal models were employed to investigate SP's inhibitory effect on the abnormal activation of the RAAS through the Gαq-PLCβ3 axis. We used chemical biology methods to identify potential targets and elucidate the underlying molecular mechanisms.

METHODS

The effects of SP on aldosteronism and hypertension were evaluated using an established animal model in our laboratory. Target identification and underlying molecular mechanism research were performed using activity-based protein profiling with a bio-orthogonal click chemistry reaction and other biochemical methods.

RESULTS

SP alleviated aldosteronism and hypertension in animal models by targeting PLCβ3. The underlying mechanism for blocking the Gαq-PLCβ3 interaction involves targeting the EF hands through the Asn-260 amino acid residue. SP regulated the Gαq-PLCβ3 axis more precisely than the Gαq-GEFT or Gαq-PKCζ axis in the cardiovascular system.

CONCLUSION

SP alleviated RAAS over-activation via Gαq-PLCβ3 interaction blockade by targeting the PLCβ3 EF hands domain, which provided a novel PLC inhibitor for treating CVDs. Unlike selective Gαq inhibitors, SP reduced the risk of side effects compared to Gαq inhibitors in treating CVDs.

Influence of Exogenous 24-Epicasterone on the Hormonal Status of Soybean Plants

Brassinosteroids (BRs) are key phytohormones involved in the regulation of major processes of cell metabolism that guide plant growth. In the past decades, new evidence has made it clear that BRs also play a key role in the orchestration of plant responses to many abiotic and biotic stresses. In the present work, we analyzed the impact of foliar treatment with 24-epicastasterone (ECS) on the endogenous content of major phytohormones (auxins, salicylic acid, jasmonic acid, and abscisic acid) and their intermediates in soybean leaves 7 days following the treatment. Changes in the endogenous content of phytohormones have been identified and quantified by LC/MS. The obtained results point to a clear role of ECS in the upregulation of auxin content (indole-3-acetic acid, IAA) and downregulation of salicylic, jasmonic, and abscisic acid levels. These data confirm that under optimal conditions, ECS in tested concentrations of 0.25 µM and 1 µM might promote growth in soybeans by inducing auxin contents. Benzoic acid (a precursor of salicylic acid (SA)), but not SA itself, has also been highly accumulated under ECS treatment, which indicates an activation of the adaptation strategies of cell metabolism to possible environmental challenges.

Orcinol glucoside targeted p38 as an agonist to promote osteogenesis and protect glucocorticoid-induced osteoporosis

BACKGROUND

Glucocorticoids (GC)-induced osteoporosis (GIOP) is the most common cause of secondary osteoporosis, which leads to an increased risk of fracture in patients. The inhibition of the osteoblast effect is one of the main pathological characteristics of GIOP, but without effective drugs on treatment.

PURPOSE

The aim of this study was to investigate the potential effects of orcinol glucoside (OG) on osteoblast cells and GIOP mice, as well as the mechanism of the underlying molecular target protein of OG both in vitro osteoblast cell and in vivo GIOP mice model.

METHODS

GIOP mice were used to determine the effect of OG on bone density and bone formation. Then, a cellular thermal shift assay coupled with mass spectrometry (CETSA-MS) method was used to identify the target of OG. Surface plasmon resonance (SPR), enzyme activity assay, molecular docking, and molecular dynamics were used to detect the affinity, activity, and binding site between OG and its target, respectively. Finally, the anti-osteoporosis effect of OG through the target signal pathway was investigated in vitro osteoblast cell and in vivo GIOP mice model.

RESULTS

OG treatment increased bone mineral density (BMD) in GIOP mice and effectively promoted osteoblast proliferation, osteogenic differentiation, and mineralization in vitro. The CETSA-MS result showed that the target of OG acting on the osteoblast is the p38 protein. SPR, molecular docking assay and enzyme activity assay showed that OG could direct bind to the p38 protein and is a p38 agonist. The cellular study found that OG could promote p38 phosphorylation and upregulate the proteins expression of its downstream osteogenic (Runx2, Osx, Collagen Ⅰ, Dlx5). Meanwhile, it could also inhibit the nuclear transport of GR by increasing the phosphorylation site at GR226 in osteoblast cell. In vivo GIOP mice experiment further confirmed that OG could prevent bone loss in the GIOP mice model through promoting p38 activity as well as its downstream proteins expression and activity.

CONCLUSIONS

This study has established that OG could promote osteoblast activity and revise the bone loss in GIOP mice by direct binding to the p38 protein and is a p38 agonist to improve its downstream signaling, which has great potential in GIOP treatment for targeting p38. This is the first report to identify OG anti-osteoporosis targets using a label-free strategy (CETSA-MS).

Brassinosteroid signaling regulator BIM1 integrates brassinolide and jasmonic acid signaling during cold tolerance in apple

Although brassinolide (BR) and jasmonic acid (JA) play essential roles in the regulation of cold stress responses, the molecular basis of their crosstalk remains elusive. Here, we show a key component of BR signaling in apple (Malus × domestica), BR INSENSITIVE1 (BRI1)-EMS-SUPPRESSOR1 (BES1)-INTERACTING MYC-LIKE PROTEIN1 (MdBIM1), increases cold tolerance by directly activating expression of C-REPEAT BINDING FACTOR1 (MdCBF1) and forming a complex with C-REPEAT BINDING FACTOR2 (MdCBF2) to enhance MdCBF2-activated transcription of cold-responsive genes. Two repressors of JA signaling, JAZMONATE ZIM-DOMAIN1 (MdJAZ1) and JAZMONATE ZIM-DOMAIN2 (MdJAZ2), interact with MdBIM1 to integrate BR and JA signaling under cold stress. MdJAZ1 and MdJAZ2 reduce MdBIM1-promoted cold stress tolerance by attenuating transcriptional activation of MdCBF1 expression by MdBIM1 and interfering with the formation of the MdBIM1-MdCBF2 complex. Furthermore, the E3 ubiquitin ligase ARABIDOPSIS TÓXICOS en LEVADURA73 (MdATL73) decreases MdBIM1-promoted cold tolerance by targeting MdBIM1 for ubiquitination and degradation. Our results not only reveal crosstalk between BR and JA signaling mediated by a JAZ-BIM1-CBF module but also provide insights into the posttranslational regulatory mechanism of BR signaling.

Protein interactome of 3',5'-cAMP reveals its role in regulating the actin cytoskeleton

Identification of protein interactors is ideally suited for the functional characterization of small molecules. 3',5'-cAMP is an evolutionary ancient signaling metabolite largely uncharacterized in plants. To tap into the physiological roles of 3',5'-cAMP, we used a chemo-proteomics approach, thermal proteome profiling (TPP), for the unbiased identification of 3',5'-cAMP protein targets. TPP measures shifts in the protein thermal stability upon ligand binding. Comprehensive proteomics analysis yielded a list of 51 proteins significantly altered in their thermal stability upon incubation with 3',5'-cAMP. The list contained metabolic enzymes, ribosomal subunits, translation initiation factors, and proteins associated with the regulation of plant growth such as CELL DIVISION CYCLE 48. To functionally validate obtained results, we focused on the role of 3',5'-cAMP in regulating the actin cytoskeleton suggested by the presence of actin among the 51 identified proteins. 3',5'-cAMP supplementation affected actin organization by inducing actin-bundling. Consistent with these results, the increase in 3',5'-cAMP levels, obtained either by feeding or by chemical modulation of 3',5'-cAMP metabolism, was sufficient to partially rescue the short hypocotyl phenotype of the actin2 actin7 mutant, severely compromised in actin level. The observed rescue was specific to 3',5'-cAMP, as demonstrated using a positional isomer 2',3'-cAMP, and true for the nanomolar 3',5'-cAMP concentrations reported for plant cells. In vitro characterization of the 3',5'-cAMP-actin pairing argues against a direct interaction between actin and 3',5'-cAMP. Alternative mechanisms by which 3',5'-cAMP would affect actin dynamics, such as by interfering with calcium signaling, are discussed. In summary, our work provides a specific resource, 3',5'-cAMP interactome, as well as functional insight into 3',5'-cAMP-mediated regulation in plants.

A Shift in Thinking: Cellular Thermal Shift Assay-Enabled Drug Discovery

A decade has passed since the cellular thermal shift assay (CETSA) was introduced to the drug discovery community. Over the years, the method has guided numerous projects by providing insights about, for example, target engagement, lead generation, target identification, lead optimization, and preclinical profiling. With this Microperspective, we intend to highlight recently published applications of CETSA and how the data generated can enable efficient decision-making and prioritization throughout the drug discovery and development value chain.

Mass spectrometric exploration of phytohormone profiles and signaling networks

Phytohormones have crucial roles in plant growth, development, and acclimation to environmental stress; however, measuring phytohormone levels and unraveling their complex signaling networks and interactions remains challenging. Mass spectrometry (MS) has revolutionized the study of complex biological systems, enabling the comprehensive identification and quantification of phytohormones and their related targets. Here, we review recent advances in MS technologies and highlight studies that have used MS to discover and analyze phytohormone-mediated molecular events. In particular, we focus on the application of MS for profiling phytohormones, elucidating phosphorylation signaling, and mapping protein interactions in plants.

Mapping the signaling network of BIN2 kinase using TurboID-mediated biotin labeling and phosphoproteomics

Elucidating enzyme-substrate relationships in posttranslational modification (PTM) networks is crucial for understanding signal transduction pathways but is technically difficult because enzyme-substrate interactions tend to be transient. Here, we demonstrate that TurboID-based proximity labeling (TbPL) effectively and specifically captures the substrates of kinases and phosphatases. TbPL-mass spectrometry (TbPL-MS) identified over 400 proximal proteins of Arabidopsis thaliana BRASSINOSTEROID-INSENSITIVE2 (BIN2), a member of the GLYCOGEN SYNTHASE KINASE 3 (GSK3) family that integrates signaling pathways controlling diverse developmental and acclimation processes. A large portion of the BIN2-proximal proteins showed BIN2-dependent phosphorylation in vivo or in vitro, suggesting that these are BIN2 substrates. Protein-protein interaction network analysis showed that the BIN2-proximal proteins include interactors of BIN2 substrates, revealing a high level of interactions among the BIN2-proximal proteins. Our proteomic analysis establishes the BIN2 signaling network and uncovers BIN2 functions in regulating key cellular processes such as transcription, RNA processing, translation initiation, vesicle trafficking, and cytoskeleton organization. We further discovered significant overlap between the GSK3 phosphorylome and the O-GlcNAcylome, suggesting an evolutionarily ancient relationship between GSK3 and the nutrient-sensing O-glycosylation pathway. Our work presents a powerful method for mapping PTM networks, a large dataset of GSK3 kinase substrates, and important insights into the signaling network that controls key cellular functions underlying plant growth and acclimation.

Don't let go: co-fractionation mass spectrometry for untargeted mapping of protein-metabolite interactomes

The chemical complexity of metabolomes goes hand in hand with their functional diversity. Small molecules have many essential roles, many of which are executed by binding and modulating the function of a protein partner. The complex and dynamic protein-metabolite interaction (PMI) network underlies most if not all biological processes, but remains under-characterized. Herein, we highlight how co-fractionation mass spectrometry (CF-MS), a well-established approach to map protein assemblies, can be used for proteome and metabolome identification of the PMIs. We will review recent CF-MS studies, discuss the main advantages and limitations, summarize the available CF-MS guidelines, and outline future challenges and opportunities.

Cellular Thermal Shift Assay for the Detection of Small Molecule-Target Interactions in Arabidopsis Cells

Chemical genetics takes advantage of small molecule-protein interactions to explore various biological processes. Although an attractive alternative to classical genetics in plants, the identification of small-molecule targets remains a challenge and limits the broad use of the compounds. The cellular thermal shift assay (CETSA), based on the principle that binding of small molecules could affect the thermal stability of proteins, has been applied for target validation in plant cells. Combined with high-resolution mass spectrometry, CETSA can detect small-molecule targets by monitoring the changes in the protein thermal stability caused by the interactions with small molecules at the proteome level. Here we describe the small-molecule target validation as well as the target identification with mass spectrometry by means of CETSA.

Adenosine monophosphate deaminase modulates BIN2 activity through hydrogen peroxide-induced oligomerization

The Arabidopsis thaliana GSK3-like kinase, BRASSINOSTEROID-INSENSITIVE2 (BIN2) is a key negative regulator of brassinosteroid (BR) signaling and a hub for crosstalk with other signaling pathways. However, the mechanisms controlling BIN2 activity are not well understood. Here we performed a forward genetic screen for resistance to the plant-specific GSK3 inhibitor bikinin and discovered that a mutation in the ADENOSINE MONOPHOSPHATE DEAMINASE (AMPD)/EMBRYONIC FACTOR1 (FAC1) gene reduces the sensitivity of Arabidopsis seedlings to both bikinin and BRs. Further analyses revealed that AMPD modulates BIN2 activity by regulating its oligomerization in a hydrogen peroxide (H2O2)-dependent manner. Exogenous H2O2 induced the formation of BIN2 oligomers with a decreased kinase activity and an increased sensitivity to bikinin. By contrast, AMPD activity inhibition reduced the cytosolic reactive oxygen species (ROS) levels and the amount of BIN2 oligomers, correlating with the decreased sensitivity of Arabidopsis plants to bikinin and BRs. Furthermore, we showed that BIN2 phosphorylates AMPD to possibly alter its function. Our results uncover the existence of an H2O2 homeostasis-mediated regulation loop between AMPD and BIN2 that fine-tunes the BIN2 kinase activity to control plant growth and development.

New Wine in an Old Bottle: Utilizing Chemical Genetics to Dissect Apical Hook Development

The apical hook is formed by dicot seedlings to protect the tender shoot apical meristem during soil emergence. Regulated by many phytohormones, the apical hook has been taken as a model to study the crosstalk between individual signaling pathways. Over recent decades, the roles of different phytohormones and environmental signals in apical hook development have been illustrated. However, key regulators downstream of canonical hormone signaling have rarely been identified via classical genetics screening, possibly due to genetic redundancy and/or lethal mutation. Chemical genetics that utilize small molecules to perturb and elucidate biological processes could provide a complementary strategy to overcome the limitations in classical genetics. In this review, we summarize current progress in hormonal regulation of the apical hook, and previously reported chemical tools that could assist the understanding of this complex developmental process. We also provide insight into novel strategies for chemical screening and target identification, which could possibly lead to discoveries of new regulatory components in apical hook development, or unidentified signaling crosstalk that is overlooked by classical genetics screening.