Rapid auxin signaling: Unknowns old and new

Anchorene, a carotenoid-derived growth regulator, modulates auxin homeostasis by suppressing GH3-mediated auxin conjugation

Anchorene, identified as an endogenous bioactive carotenoid-derived dialdehyde and diapocarotenoid, affects root development by modulating auxin homeostasis. However, the precise interaction between anchorene and auxin, as well as the mechanisms by which anchorene modulates auxin levels, remain largely elusive. In this study, we conducted a comparative analysis of anchorene's bioactivities alongside auxin and observed that anchorene induces multifaceted auxin-like effects. Through genetic and pharmacological examinations, we revealed that anchorene's auxin-like activities depend on the indole-3-pyruvate-dependent auxin biosynthesis pathway, as well as the auxin inactivation pathway mediated by Group II Gretchen Hagen 3 (GH3) proteins that mainly facilitate the conjugation of indole-3-acetic acid (IAA) to amino acids, leading to the formation of inactivated storage forms. Our measurements indicated that anchorene treatment elevates IAA levels while reducing the quantities of inactivated IAA-amino acid conjugates and oxIAA. RNA sequencing further revealed that anchorene triggers the expression of numerous auxin-responsive genes in a manner reliant on Group II GH3s. Additionally, our in vitro enzymatic assays and biolayer interferometry (BLI) assay demonstrated anchorene's robust suppression of GH3.17-mediated IAA conjugation with glutamate. Collectively, our findings highlight the significant role of carotenoid-derived metabolite anchorene in modulating auxin homeostasis, primarily through the repression of GH3-mediated IAA conjugation and inactivation pathways, offering novel insights into the regulatory mechanisms of plant bioactive apocarotenoids.

Crosstalk between Rho of Plants GTPase signalling and plant hormones

Rho of Plants (ROPs) constitute a plant-specific subset of small guanine nucleotide-binding proteins within the Cdc42/Rho/Rac family. These versatile proteins regulate diverse cellular processes, including cell growth, cell division, cell morphogenesis, organ development, and stress responses. In recent years, the dynamic cellular and subcellular behaviours orchestrated by ROPs have unveiled a notable connection to hormone-mediated organ development and physiological responses, thereby expanding our knowledge of the functions and regulatory mechanisms of this signalling pathway. This review delineates advancements in understanding the interplay between plant hormones and the ROP signalling cascade, focusing primarily on the connections with auxin and abscisic acid pathways, alongside preliminary discoveries in cytokinin, brassinosteroid, and salicylic acid responses. It endeavours to shed light on the intricate, coordinated mechanisms bridging cell- and tissue-level signals that underlie plant cell behaviour, organ development, and physiological processes, and highlights future research prospects and challenges in this rapidly developing field.

The interplay between extracellular and intracellular auxin signaling in plants

The phytohormone auxin exerts control over remarkable developmental processes in plants. It moves from cell to cell, resulting in the creation of both extracellular auxin and intracellular auxin, which are recognized by distinct auxin receptors. These two auxin signaling systems govern different auxin responses while working together to regulate plant development. In this review, we outline the latest research advancements in unraveling these auxin signaling pathways, encompassing auxin perception and signaling transductions. We emphasize the interaction between extracellular auxin and intracellular auxin, which contributes to the intricate role of auxin in plant development.

VIK-Mediated Auxin Signaling Regulates Lateral Root Development in Arabidopsis

The crucial role of TIR1-receptor-mediated gene transcription regulation in auxin signaling has long been established. In recent years, the significant role of protein phosphorylation modifications in auxin signal transduction has gradually emerged. To further elucidate the significant role of protein phosphorylation modifications in auxin signaling, a phosphoproteomic analysis in conjunction with auxin treatment has identified an auxin activated Mitogen-activated Protein Kinase Kinase Kinase (MAPKKK) VH1-INTERACTING Kinase (VIK), which plays an important role in auxin-induced lateral root (LR) development. In the vik mutant, auxin-induced LR development is significantly attenuated. Further investigations show that VIK interacts separately with the positive regulator of LR development, LATERAL ORGAN BOUNDARIES-DOMAIN18 (LBD18), and the negative regulator of LR emergence, Ethylene Responsive Factor 13 (ERF13). VIK directly phosphorylates and stabilizes the positive transcription factor LBD18 in LR formation. In the meantime, VIK directly phosphorylates the negative regulator ERF13 at Ser168 and Ser172 sites, causing its degradation and releasing the repression by ERF13 on LR emergence. In summary, VIK-mediated auxin signaling regulates LR development by enhancing the protein stability of LBD18 and inducing the degradation of ERF13, respectively.

Plant physiology: RAF kinases claim a conserved role in rapid auxin responses

A recent study spotlights B-RAF kinases as central mediators of rapid auxin responses across diverse plant species. Coupled with other current studies, this discovery illuminates the essential role of B-RAF kinases in orchestrating growth, stress responses, and various other biological processes in plants.

Plant Reactome Knowledgebase: empowering plant pathway exploration and OMICS data analysis

Plant Reactome (https://plantreactome.gramene.org) is a freely accessible, comprehensive plant pathway knowledgebase. It provides curated reference pathways from rice (Oryza sativa) and gene-orthology-based pathway projections to 129 additional species, spanning single-cell photoautotrophs, non-vascular plants, and higher plants, thus encompassing a wide-ranging taxonomic diversity. Currently, Plant Reactome houses a collection of 339 reference pathways, covering metabolic and transport pathways, hormone signaling, genetic regulations of developmental processes, and intricate transcriptional networks that orchestrate a plant's response to abiotic and biotic stimuli. Beyond being a mere repository, Plant Reactome serves as a dynamic data discovery platform. Users can analyze and visualize omics data, such as gene expression, gene-gene interaction, proteome, and metabolome data, all within the rich context of plant pathways. Plant Reactome is dedicated to fostering data interoperability, upholding global data standards, and embracing the tenets of the Findable, Accessible, Interoperable and Re-usable (FAIR) data policy.

The Roles of GRETCHEN HAGEN3 (GH3)-Dependent Auxin Conjugation in the Regulation of Plant Development and Stress Adaptation

The precise control of free auxin (indole-3-acetic acid, IAA) gradient, which is orchestrated by biosynthesis, conjugation, degradation, hydrolyzation, and transport, is critical for all aspects of plant growth and development. Of these, the GRETCHEN HAGEN 3 (GH3) acyl acid amido synthetase family, pivotal in conjugating IAA with amino acids, has garnered significant interest. Recent advances in understanding GH3-dependent IAA conjugation have positioned GH3 functional elucidation as a hot topic of research. This review aims to consolidate and discuss recent findings on (i) the enzymatic mechanisms driving GH3 activity, (ii) the influence of chemical inhibitor on GH3 function, and (iii) the transcriptional regulation of GH3 and its impact on plant development and stress response. Additionally, we explore the distinct biological functions attributed to IAA-amino acid conjugates.