Regulation of auxin transcriptional responses

Identification of plant transcriptional activation domains

Gene expression in Arabidopsis is regulated by more than 1,900 transcription factors (TFs), which have been identified genome-wide by the presence of well-conserved DNA-binding domains. Activator TFs contain activation domains (ADs) that recruit coactivator complexes; however, for nearly all Arabidopsis TFs, we lack knowledge about the presence, location and transcriptional strength of their ADs1. To address this gap, here we use a yeast library approach to experimentally identify Arabidopsis ADs on a proteome-wide scale, and find that more than half of the Arabidopsis TFs contain an AD. We annotate 1,553 ADs, the vast majority of which are, to our knowledge, previously unknown. Using the dataset generated, we develop a neural network to accurately predict ADs and to identify sequence features that are necessary to recruit coactivator complexes. We uncover six distinct combinations of sequence features that result in activation activity, providing a framework to interrogate the subfunctionalization of ADs. Furthermore, we identify ADs in the ancient AUXIN RESPONSE FACTOR family of TFs, revealing that AD positioning is conserved in distinct clades. Our findings provide a deep resource for understanding transcriptional activation, a framework for examining function in intrinsically disordered regions and a predictive model of ADs.

Leaf Vein Patterning

Leaves form veins whose patterns vary from a single vein running the length of the leaf to networks of staggering complexity where huge numbers of veins connect to other veins at both ends. For the longest time, vein formation was thought to be controlled only by the polar, cell-to-cell transport of the plant hormone auxin; recent evidence suggests that is not so. Instead, it turns out that vein patterning features are best accounted for by a combination of polar auxin transport, facilitated auxin diffusion through plasmodesma intercellular channels, and auxin signal transduction-though the latter's precise contribution remains unclear. Equally unclear remain the sites of auxin production during leaf development, on which that vein patterning mechanism ought to depend. Finally, whether that vein patterning mechanism can account for the variety of vein arrangements found in nature remains unknown. Addressing those questions will be the exciting challenge of future research.

Enigmatic role of auxin response factors in plant growth and stress tolerance

Abiotic and biotic stresses globally constrain plant growth and impede the optimization of crop productivity. The phytohormone auxin is involved in nearly every aspect of plant development. Auxin acts as a chemical messenger that influences gene expression through a short nuclear pathway, mediated by a family of specific DNA-binding transcription factors known as Auxin Response Factors (ARFs). ARFs thus act as effectors of auxin response and translate chemical signals into the regulation of auxin responsive genes. Since the initial discovery of the first ARF in Arabidopsis, advancements in genetics, biochemistry, genomics, and structural biology have facilitated the development of models elucidating ARF action and their contributions to generating specific auxin responses. Yet, significant gaps persist in our understanding of ARF transcription factors despite these endeavors. Unraveling the functional roles of ARFs in regulating stress response, alongside elucidating their genetic and molecular mechanisms, is still in its nascent phase. Here, we review recent research outcomes on ARFs, detailing their involvement in regulating leaf, flower, and root organogenesis and development, as well as stress responses and their corresponding regulatory mechanisms: including gene expression patterns, functional characterization, transcriptional, post-transcriptional and post- translational regulation across diverse stress conditions. Furthermore, we delineate unresolved questions and forthcoming challenges in ARF research.

Synthetically derived BiAux modulates auxin co-receptor activity to stimulate lateral root formation

The root system plays an essential role in plant growth and adaptation to the surrounding environment. The root clock periodically specifies lateral root prebranch sites (PBS), where a group of pericycle founder cells (FC) is primed to become lateral root founder cells and eventually give rise to lateral root primordia or lateral roots (LRs). This clock-driven organ formation process is tightly controlled by modulation of auxin content and signaling. Auxin perception entails the physical interaction of TRANSPORT INHIBITOR RESPONSE 1 (TIR1) or AUXIN SIGNALING F-BOX (AFBs) proteins with AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) repressors to form a co-receptor system. Despite the apparent simplicity, the understanding of how specific auxin co-receptors are assembled remains unclear. We identified the compound bis-methyl auxin conjugated with N-glucoside, or BiAux, in Arabidopsis (Arabidopsis thaliana) that specifically induces the formation of PBS and the emergence of LR, with a slight effect on root elongation. Docking analyses indicated that BiAux binds to F-box proteins, and we showed that BiAux function depends on TIR1 and AFB2 F-box proteins and AUXIN RESPONSE FACTOR 7 activity, which is involved in FC specification and LR formation. Finally, using a yeast (Saccharomyces cerevisiae) heterologous expression system, we showed that BiAux favors the assemblage of specific co-receptors subunits involved in LR formation and enhances AUXIN/INDOLE-3-ACETIC ACID 28 protein degradation. These results indicate that BiAux acts as an allosteric modulator of specific auxin co-receptors. Therefore, BiAux exerts a fine-tune regulation of auxin signaling aimed to the specific formation of LR among the many development processes regulated by auxin.

Transcriptome-wide identification of ARF gene family in medicinal plant Polygonatum kingianum and expression analysis of PkARF members in different tissues

BACKGROUND

Polygonatum kingianum holds significant importance in Traditional Chinese Medicine due to its medicinal properties, characterized by its diverse chemical constituents including polysaccharides, terpenoids, flavonoids, phenols, and phenylpropanoids. The Auxin Response Factor (ARF) is a pivotal transcription factor known for its regulatory role in both primary and secondary metabolite synthesis. However, our understanding of the ARF gene family in P. kingianum remains limited.

METHODS AND RESULTS

We employed RNA-Seq to sequence three distinct tissues (leaf, root, and stem) of P. kingianum. The analysis revealed a total of 31,558 differentially expressed genes (DEGs), with 43 species of transcription factors annotated among them. Analyses via gene ontology and the Kyoto Encyclopedia of Genes and Genomes demonstrated that these DEGs were predominantly enriched in metabolic pathways and secondary metabolite biosynthesis. The proposed temporal expression analysis categorized the DEGs into nine clusters, suggesting the same expression trends that may be coordinated in multiple biological processes across the three tissues. Additionally, we conducted screening and expression pattern analysis of the ARF gene family, identifying 12 significantly expressed PkARF genes in P. kingianum roots. This discovery lays the groundwork for investigations into the role of PkARF genes in root growth, development, and secondary metabolism regulation.

CONCLUSION

The obtained data and insights serve as a focal point for further research studies, centred on genetic manipulation of growth and secondary metabolism in P. kingianum. Furthermore, these findings contribute to the understanding of functional genomics in P. kingianum, offering valuable genetic resources.

Unlocking nature's (sub)cellular symphony: Phase separation in plant meristems

Plant development is based on the balance of stem cell maintenance and differentiation in the shoot and root meristems. The necessary cell fate decisions are regulated by intricate networks of proteins and biomolecules within plant cells and require robust and dynamic compartmentalization strategies, including liquid-liquid phase separation (LLPS), which allows the formation of membrane-less compartments. This review summarizes the current knowledge about the emerging field of LLPS in plant development, with a particular focus on the shoot and root meristems. LLPS regulates not only floral transition and flowering time while integrating environmental signals in the shoots but also influences auxin signalling and is putatively involved in maintaining the stem cell niche (SCN) in the roots. Therefore, LLPS has the potential to play a crucial role in the plasticity of plant development, necessitating further research for a comprehensive understanding.

Game of thrones among AUXIN RESPONSE FACTORs-over 30 years of MONOPTEROS research

For many years, research has been carried out with the aim of understanding the mechanism of auxin action, its biosynthesis, catabolism, perception, and transport. One central interest is the auxin-dependent gene expression regulation mechanism involving AUXIN RESPONSE FACTOR (ARF) transcription factors and their repressors, the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) proteins. Numerous studies have been focused on MONOPTEROS (MP)/ARF5, an activator of auxin-dependent gene expression with a crucial impact on plant development. This review summarizes over 30 years of research on MP/ARF5. We indicate the available analytical tools to study MP/ARF5 and point out the known mechanism of MP/ARF5-dependent regulation of gene expression during various developmental processes, namely embryogenesis, leaf formation, vascularization, and shoot and root meristem formation. However, many questions remain about the auxin dose-dependent regulation of gene transcription by MP/ARF5 and its isoforms in plant cells, the composition of the MP/ARF5 protein complex, and, finally, all the genes under its direct control. In addition, information on post-translational modifications of MP/ARF5 protein is marginal, and knowledge about their consequences on MP/ARF5 function is limited. Moreover, the epigenetic factors and other regulators that act upstream of MP/ARF5 are poorly understood. Their identification will be a challenge in the coming years.

E3 ubiquitin ligases SINA4 and SINA11 regulate anthocyanin biosynthesis by targeting the IAA29-ARF5-1-ERF3 module in apple

Auxin/indole-3-acetic acid (AUX/IAA) and auxin response factor (ARF) proteins are important components of the auxin signalling pathway, but their ubiquitination modification and the mechanism of auxin-mediated anthocyanin biosynthesis remain elusive. Here, the ARF MdARF5-1 was identified as a negative regulator of anthocyanin biosynthesis in apple, and it integrates auxin and ethylene signals by inhibiting the expression of the ethylene response factor MdERF3. The auxin repressor MdIAA29 decreased the inhibitory effect of MdARF5-1 on anthocyanin biosynthesis by attenuating the transcriptional inhibition of MdERF3 by MdARF5-1. In addition, the E3 ubiquitin ligases MdSINA4 and MdSINA11 played negative and positive regulatory roles in anthocyanin biosynthesis by targeting MdIAA29 and MdARF5-1 for ubiquitination degradation, respectively. MdSINA4 destabilized MdSINA11 to regulate anthocyanin accumulation in response to auxin signalling. In sum, our data revealed the crosstalk between auxin and ethylene signals mediated by the IAA29-ARF5-1-ERF3 module and provide new insights into the ubiquitination modification of the auxin signalling pathway.

Auxin and abiotic stress responses

Plants are exposed to a variety of abiotic stresses; these stresses have profound effects on plant growth, survival, and productivity. Tolerance and adaptation to stress require sophisticated stress sensing, signaling, and various regulatory mechanisms. The plant hormone auxin is a key regulator of plant growth and development, playing pivotal roles in the integration of abiotic stress signals and control of downstream stress responses. In this review, we summarize and discuss recent advances in understanding the intersection of auxin and abiotic stress in plants, with a focus on temperature, salt, and drought stresses. We also explore the roles of auxin in stress tolerance and opportunities arising for agricultural applications.

Comparative mutant analyses reveal a novel mechanism of ARF regulation in land plants

A major challenge in plant biology is to understand how the plant hormone auxin regulates diverse transcriptional responses throughout development, in different environments, and in different species. The answer may lie in the specific complement of auxin signaling components in each cell. The balance between activators (class-A AUXIN RESPONSE FACTORS) and repressors (class-B ARFs) is particularly important. It is unclear how this balance is achieved. Through comparative analysis of novel, dominant mutants in maize and the moss Physcomitrium patens , we have discovered a ∼500-million-year-old mechanism of class-B ARF protein level regulation, important in determining cell fate decisions across land plants. Thus, our results add a key piece to the puzzle of how auxin regulates plant development.

AUXIN RESPONSE FACTOR protein accumulation and function

Auxin is a key regulator of plant developmental processes. Its effects on transcription are mediated by the AUXIN RESPONSE FACTOR (ARF) family of transcription factors. ARFs tightly control specific auxin responses necessary for proper plant growth and development. Recent research has revealed that regulated ARF protein accumulation and ARF nucleo-cytoplasmic partitioning can determine auxin transcriptional outputs. In this review, we explore these recent findings and consider the potential for regulated ARF accumulation in driving auxin responses in plants.

Ticarcillin degradation product thiophene acetic acid is a novel auxin analog that promotes organogenesis in tomato

Efficient regeneration of transgenic plants from explants after transformation is one of the crucial steps in developing genetically modified plants with desirable traits. Identification of novel plant growth regulators and developmental regulators will assist to enhance organogenesis in culture. In this study, we observed enhanced shoot regeneration from tomato cotyledon explants in culture media containing timentin, an antibiotic frequently used to prevent Agrobacterium overgrowth after transformation. Comparative transcriptome analysis of explants grown in the presence and absence of timentin revealed several genes previously reported to play important roles in plant growth and development, including Auxin Response Factors (ARFs), GRF Interacting Factors (GIFs), Flowering Locus T (SP5G), Small auxin up-regulated RNAs (SAUR) etc. Some of the differentially expressed genes were validated by quantitative real-time PCR. We showed that ticarcillin, the main component of timentin, degrades into thiophene acetic acid (TAA) over time. TAA was detected in plant tissue grown in media containing timentin. Our results showed that TAA is indeed a plant growth regulator that promotes root organogenesis from tomato cotyledons in a manner similar to the well-known auxins, indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA). In combination with the cytokinin 6-benzylaminopurine (BAP), TAA was shown to promote shoot organogenesis from tomato cotyledon in a concentration-dependent manner. To the best of our knowledge, the present study reports for the first time demonstrating the function of TAA as a growth regulator in a plant species. Our work will pave the way for future studies involving different combinations of TAA with other plant hormones which may play an important role in in vitro organogenesis of recalcitrant species. Moreover, the differentially expressed genes and long noncoding RNAs identified in our transcriptome studies may serve as contender genes for studying molecular mechanisms of shoot organogenesis.

Involvement of small molecules and metabolites in regulation of biomolecular condensate properties

Biomolecular condensate (BMCs) formation facilitates the grouping of molecules, including proteins, nucleic acids, and small molecules, creating specific microenvironments with particular functions. They are often assembled through liquid-liquid phase separation (LLPS), a phenomenon that arises when specific proteins, nucleic acids, and small molecules demix from the aqueous environment into another phase with different physiochemical properties. BMCs assemble and disassemble in response to external and internal stimuli such as temperature, molecule concentration, ionic strength, pH, and cellular redox state. Likewise, the nature of the regulatory stimuli may affect the lifespan, morphology, and content of BMCs. In humans, compelling evidence points to the critical role of BMCs in diseases. By contrast, the link between BMC formation, stress resistance, and cell survival has not been revealed in plants. Recent studies have pointed out the nascent roles of small molecules in the assembly and dynamics of BMCs; however, this is still an emerging field of study. This review briefly highlights the most significant efforts to identify the molecular mechanisms between small molecules and BMC formation and regulation in plants and other organisms. We then discuss (i) how small molecules exert control over the BMC assembly and dynamics in plants and (ii) how small molecules can influence the formation and material properties of plant BMCs. Finally, we propose novel alternatives that might help to understand the relationship between chemicals and condensation dynamics and their possible application to plant biotechnology.

Effect of Multi-walled Carbon Nanotubes Functionalized with Indol-3-butyric Acid on the Development of Avena sativa

In pioneering research, it has been documented that the CNT influences the development of plants through the balance of phytoregulators. Therefore, in this work the objective is to evaluate the effects of CNT functionalized by non-covalent method with indole-3-butyric acid that they have on Avena sativa. The CNT was characterized by FTIR and Raman to confirm functionalization. It was observed that in the germination stage the seeds treated with IBA inhibited germination, however, when functionalizing the CNT with IBA it was observed that the CNT is contributing to counteract this inhibition.

Nitric oxide-mediated S-nitrosylation of IAA17 protein in intrinsically disordered region represses auxin signaling

The phytohormone auxin plays crucial roles in nearly every aspect of plant growth and development. Auxin signaling is activated through the phytohormone-induced proteasomal degradation of the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) family of transcriptional repressors. Notably, many auxin-modulated physiological processes are also regulated by nitric oxide (NO) that executes its biological effects predominantly through protein S-nitrosylation at specific cysteine residues. However, little is known about the molecular mechanisms in regulating the interactive NO and auxin networks. Here, we show that NO represses auxin signaling by inhibiting IAA17 protein degradation. NO induces the S-nitrosylation of Cys-70 located in the intrinsically disordered region of IAA17, which inhibits the TIR1-IAA17 interaction and consequently the proteasomal degradation of IAA17. The accumulation of a higher level of IAA17 attenuates auxin response. Moreover, an IAA17C70W nitrosomimetic mutation renders the accumulation of a higher level of the mutated protein, thereby causing partial resistance to auxin and defective lateral root development. Taken together, these results suggest that S-nitrosylation of IAA17 at Cys-70 inhibits its interaction with TIR1, thereby negatively regulating auxin signaling. This study provides unique molecular insights into the redox-based auxin signaling in regulating plant growth and development.

Temperature changes in the root ecosystem affect plant functionality

Climate change is increasing the frequency of extreme heat events that aggravate its negative impact on plant development and agricultural yield. Most experiments designed to study plant adaption to heat stress apply homogeneous high temperatures to both shoot and root. However, this treatment does not mimic the conditions in natural fields, where roots grow in a dark environment with a descending temperature gradient. Excessively high temperatures severely decrease cell division in the root meristem, compromising root growth, while increasing the division of quiescent center cells, likely in an attempt to maintain the stem cell niche under such harsh conditions. Here, we engineered the TGRooZ, a device that generates a temperature gradient for in vitro or greenhouse growth assays. The root systems of plants exposed to high shoot temperatures but cultivated in the TGRooZ grow efficiently and maintain their functionality to sustain proper shoot growth and development. Furthermore, gene expression and rhizosphere or root microbiome composition are significantly less affected in TGRooZ-grown roots than in high-temperature-grown roots, correlating with higher root functionality. Our data indicate that use of the TGRooZ in heat-stress studies can improve our knowledge of plant response to high temperatures, demonstrating its applicability from laboratory studies to the field.

Gibberellins promote polar auxin transport to regulate stem cell fate decisions in cambium

Vascular cambium contains bifacial stem cells, which produce secondary xylem to one side and secondary phloem to the other. However, how these fate decisions are regulated is unknown. Here we show that the positioning of an auxin signalling maximum within the cambium determines the fate of stem cell daughters. The position is modulated by gibberellin-regulated, PIN1-dependent polar auxin transport. Gibberellin treatment broadens auxin maximum from the xylem side of the cambium towards the phloem. As a result, xylem-side stem cell daughter preferentially differentiates into xylem, while phloem-side daughter retains stem cell identity. Occasionally, this broadening leads to direct specification of both daughters as xylem, and consequently, adjacent phloem-identity cell reverts to being stem cell. Conversely, reduced gibberellin levels favour specification of phloem-side stem cell daughter as phloem. Together, our data provide a mechanism by which gibberellin regulates the ratio of xylem and phloem production.

Recent advances in understanding of the epigenetic regulation of plant regeneration

Ever since the concept of "plant cell totipotency" was first proposed in the early twentieth century, plant regeneration has been a major focus of study. Regeneration-mediated organogenesis and genetic transformation are important topics in both basic research and modern agriculture. Recent studies in the model plant Arabidopsis thaliana and other species have expanded our understanding of the molecular regulation of plant regeneration. The hierarchy of transcriptional regulation driven by phytohormone signaling during regeneration is associated with changes in chromatin dynamics and DNA methylation. Here, we summarize how various aspects of epigenetic regulation, including histone modifications and variants, chromatin accessibility dynamics, DNA methylation, and microRNAs, modulate plant regeneration. As the mechanisms of epigenetic regulation are conserved in many plants, research in this field has potential applications in boosting crop breeding, especially if coupled with emerging single-cell omics technologies.

An In vitro Assay to Recapitulate Hormone-Triggered and SCF-Mediated Protein Ubiquitylation

Signaling proteins trigger a sequence of molecular switches in the cell, which permit development, growth, and rapid adaptation to changing environmental conditions. SCF-type E3 ubiquitin ligases recognize signaling proteins prompting changes in their fate, one of these being ubiquitylation followed by degradation by the proteasome. SCFs together with their ubiquitylation targets (substrates) often serve as phytohormone receptors, responding and/or assembling in response to fluctuating intracellular hormone concentrations. Tracing and understanding phytohormone perception and SCF-mediated ubiquitylation of proteins could provide powerful clues on the molecular mechanisms utilized for plant adaptation. Here, we describe an adaptable in vitro system that uses recombinant proteins and enables the study of hormone-triggered SCF-substrate interaction and the dynamics of protein ubiquitylation. This system can serve to predict the requirements for protein recognition and to understand how phytohormone levels have the power to control protein fate.

Characteristics and potential functional effects of long insertions in Asian butternuts

Background

Structural variants (SVs) play important roles in adaptation evolution and species diversification. Especially, in plants, many phenotypes of response to the environment were found to be associated with SVs. Despite the prevalence and significance of SVs, long insertions remain poorly detected and studied in all but model species.

Results

We used whole-genome resequencing of paired reads from 80 Asian butternuts to detect long insertions and further analyse their characteristics and potential functional effects. By combining of mapping-based and de novo assembly-based methods, we obtained a multiple related species pangenome representing higher taxonomic groups. We obtained 89,312 distinct contigs totaling 147,773,999 base pair (bp) of new sequences, of which 347 were putative long insertions placed in the reference genome. Most of the putative long insertions appeared in multiple species; in contrast, only 62 putative long insertions appeared in one species, which may be involved in the response to the environment. 65 putative long insertions fell into 61 distinct protein-coding genes involved in plant development, and 105 putative long insertions fell into upstream of 106 distinct protein-coding genes involved in cellular respiration. 3,367 genes were annotated in 2,606 contigs. We propose PLAINS (https://github.com/CMB-BNU/PLAINS.git), a streamlined, comprehensive pipeline for the prediction and analysis of long insertions using whole-genome resequencing.

Conclusions

Our study lays down an important foundation for further whole-genome long insertion studies, allowing the investigation of their effects by experiments.

Molecular Mechanisms of Diverse Auxin Responses during Plant Growth and Development

The plant hormone auxin acts as a signaling molecule to regulate numerous developmental processes throughout all stages of plant growth. Understanding how auxin regulates various physiological and developmental processes has been a hot topic and an intriguing field. Recent studies have unveiled more molecular details into how diverse auxin responses function in every aspect of plant growth and development. In this review, we systematically summarized and classified the molecular mechanisms of diverse auxin responses, and comprehensively elaborated the characteristics and multilevel regulation mechanisms of the canonical transcriptional auxin response. On this basis, we described the characteristics and differences between different auxin responses. We also presented some auxin response genes that have been genetically modified in plant species and how their changes impact various traits of interest. Finally, we summarized some important aspects and unsolved questions of auxin responses that need to be focused on or addressed in future research. This review will help to gain an overall understanding of and some insights into the diverse molecular mechanisms of auxin responses in plant growth and development that are instrumental in harnessing genetic resources in molecular breeding of extant plant species.

Regulation of AUXIN RESPONSE FACTOR condensation and nucleo-cytoplasmic partitioning

Auxin critically regulates plant growth and development. Auxin-driven transcriptional responses are mediated through the AUXIN RESPONSE FACTOR (ARF) family of transcription factors. ARF protein condensation attenuates ARF activity, resulting in dramatic shifts in the auxin transcriptional landscape. Here, we perform a forward genetics screen for ARF hypercondensation, identifying an F-box protein, which we named AUXIN RESPONSE FACTOR F-BOX1 (AFF1). Functional characterization of SCFAFF1 revealed that this E3 ubiquitin ligase directly interacts with ARF19 and ARF7 to regulate their accumulation, condensation, and nucleo-cytoplasmic partitioning. Mutants defective in AFF1 display attenuated auxin responsiveness, and developmental defects, suggesting that SCFAFF1 -mediated regulation of ARF protein drives aspects of auxin response and plant development.

Pleiotropic and nonredundant effects of an auxin importer in Setaria and maize

Directional transport of auxin is critical for inflorescence and floral development in flowering plants, but the role of auxin influx carriers (AUX1 proteins) has been largely overlooked. Taking advantage of available AUX1 mutants in green millet (Setaria viridis) and maize (Zea mays), we uncover previously unreported aspects of plant development that are affected by auxin influx, including higher order branches in the inflorescence, stigma branch number, glume (floral bract) development, and plant fertility. However, disruption of auxin flux does not affect all parts of the plant, with little obvious effect on inflorescence meristem size, time to flowering, and anther morphology. In double mutant studies in maize, disruptions of ZmAUX1 also affect vegetative development. A green fluorescent protein (GFP)-tagged construct of the Setaria AUX1 protein Sparse Panicle1 (SPP1) under its native promoter showed that SPP1 localizes to the plasma membrane of outer tissue layers in both roots and inflorescences, and accumulates specifically in inflorescence branch meristems, consistent with the mutant phenotype and expected auxin maxima. RNA-seq analysis indicated that most gene expression modules are conserved between mutant and wild-type plants, with only a few hundred genes differentially expressed in spp1 inflorescences. Using clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology, we disrupted SPP1 and the other four AUX1 homologs in S. viridis. SPP1 has a larger effect on inflorescence development than the others, although all contribute to plant height, tiller formation, and leaf and root development. The AUX1 importers are thus not fully redundant in S. viridis. Our detailed phenotypic characterization plus a stable GFP-tagged line offer tools for future dissection of the function of auxin influx proteins.

Pleiotropic and nonredundant effects of an auxin importer in Setaria and maize

Directional transport of auxin is critical for inflorescence and floral development in flowering plants, but the role of auxin influx carriers (AUX1 proteins) has been largely overlooked. Taking advantage of available AUX1 mutants in green millet (Setaria viridis) and maize (Zea mays), we uncover previously unreported aspects of plant development that are affected by auxin influx, including higher order branches in the inflorescence, stigma branch number, glume (floral bract) development, and plant fertility. However, disruption of auxin flux does not affect all parts of the plant, with little obvious effect on inflorescence meristem size, time to flowering, and anther morphology. In double mutant studies in maize, disruptions of ZmAUX1 also affect vegetative development. A green fluorescent protein (GFP)-tagged construct of the Setaria AUX1 protein Sparse Panicle1 (SPP1) under its native promoter showed that SPP1 localizes to the plasma membrane of outer tissue layers in both roots and inflorescences, and accumulates specifically in inflorescence branch meristems, consistent with the mutant phenotype and expected auxin maxima. RNA-seq analysis indicated that most gene expression modules are conserved between mutant and wild-type plants, with only a few hundred genes differentially expressed in spp1 inflorescences. Using clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology, we disrupted SPP1 and the other four AUX1 homologs in S. viridis. SPP1 has a larger effect on inflorescence development than the others, although all contribute to plant height, tiller formation, and leaf and root development. The AUX1 importers are thus not fully redundant in S. viridis. Our detailed phenotypic characterization plus a stable GFP-tagged line offer tools for future dissection of the function of auxin influx proteins.

The mycorrhizal-induced growth promotion and insect resistance reduction in Populus alba × P. berolinensis seedlings: a multi-omics study

Arbuscular mycorrhizal (AM) fungi are an alternative to chemical insecticides or fertilizers, and there is an urgent need to extend the application of AM fungi to woody plants. This study aims to investigate the growth and resistance against the gypsy moth larvae (Lymantria dispar) in Glomus intraradices-colonized Populus alba × P. berolinensis seedlings, and to unravel the transcriptome and metabolome phenotypes recruited by AM fungus colonization that affect plant growth and insect resistance. Our results showed a positive mycorrhizal growth response, i.e., growth and biomass of mycorrhizal seedlings were enhanced. However, AM fungus inoculation reduced the resistance of poplar to gypsy moth larvae, as evidenced by the decreased carbon/nitrogen ratio in leaves, as well as the increased larval growth and shortened larval developmental duration. Transcriptome analysis revealed that in both auxin and gibberellin signaling transductions, all nodes were responsive to AM symbiosis and most differentially expressed genes belonging to effectors were up-regulated in mycorrhizal seedlings. Furthermore, the two key enzymes (4-coumarate-CoA ligase and trans-cinnamate 4-monooxygenase) involved in the synthesis of p-Coumaroyl-CoA, an initial metabolite in flavonoid biosynthesis and the first rate-limiting enzyme (chalcone synthase) in flavonoid biosynthesis, were down-regulated at the transcriptional level. Consistent with the transcriptome results, metabolome analysis found that the amounts of all differentially accumulated flavonoid compounds (e.g., catechin and quercetin) identified in mycorrhizal seedlings were decreased. Taken together, these findings highlight the diverse outcomes of AM fungi-host plant-insect interaction and reveal the regulatory network of the positive mycorrhizal growth response and mycorrhizal-induced reduction of insect resistance in poplar.

Bending to auxin: fast acid growth for tropisms

The phytohormone auxin is the major growth regulator governing tropic responses including gravitropism. Auxin build-up at the lower side of stimulated shoots promotes cell expansion, whereas in roots it inhibits growth, leading to upward shoot bending and downward root bending, respectively. Yet it remains an enigma how the same signal can trigger such opposite cellular responses. In this review, we discuss several recent unexpected insights into the mechanisms underlying auxin regulation of growth, challenging several existing models. We focus on the divergent mechanisms of apoplastic pH regulation in shoots and roots revisiting the classical Acid Growth Theory and discuss coordinated involvement of multiple auxin signaling pathways. From this emerges a more comprehensive, updated picture how auxin regulates growth.

Characterization of Transcription Regulatory Domains of OsMADS29: Identification of Proximal Auxin-Responsive Domains and a Strong Distal Negative Element

OsMADS29 (M29) is a seed-specific MADS-box transcription factor involved in programmed cell death of nucellar tissue and maintaining auxin:cytokinin homeostasis. It affects embryo and endosperm development and starch filling during seed development in rice. Its expression seems to be tightly regulated by developmental, spatial, and temporal cues; however, cis- and trans-regulatory factors that affect its expression are largely unknown. In silico analysis of the 1.7 kb upstream regulatory region (URR) consisting of 1,290 bp promoter and 425 bp 5'-UTR regions revealed several auxin-responsive and seed-specific cis-regulatory elements distributed across the URR. In this study, the analysis of four URR deletions fused to a downstream β-glucuronidase (GUS) reporter in transgenic rice has revealed the presence of several proximal positive elements and a strong distal negative element (NE). The promoter regions containing auxin-responsive elements responded positively to the exogenous application of auxins to transgenic seedlings. The proximal positive elements are capable of driving reporter expression in both vegetative and reproductive tissues. In contrast, the NE strongly suppresses reporter gene expression in both vegetative and reproductive tissues. In a transient onion peel assay system, the NE could reduce the efficacy of a 2x CaMV 35S promoter by ∼90%. Our results indicate the existence of a complex array of positive and negative regulatory regions along with auxin-responsive elements guiding the development-dependent and spatial expression of M29.

Diminished Auxin Signaling Triggers Cellular Reprogramming by Inducing a Regeneration Factor in the Liverwort Marchantia polymorpha

Regeneration in land plants is accompanied by the establishment of new stem cells, which often involves reactivation of the cell division potential in differentiated cells. The phytohormone auxin plays pivotal roles in this process. In bryophytes, regeneration is enhanced by the removal of the apex and repressed by exogenously applied auxin, which has long been proposed as a form of apical dominance. However, the molecular basis behind these observations remains unexplored. Here, we demonstrate that in the liverwort Marchantia polymorpha, the level of endogenous auxin is transiently decreased in the cut surface of decapitated explants, and identify by transcriptome analysis a key transcription factor gene, LOW-AUXIN RESPONSIVE (MpLAXR), which is induced upon auxin reduction. Loss of MpLAXR function resulted in delayed cell cycle reactivation, and transient expression of MpLAXR was sufficient to overcome the inhibition of regeneration by exogenously applied auxin. Furthermore, ectopic expression of MpLAXR caused cell proliferation in normally quiescent tissues. Together, these data indicate that decapitation causes a reduction of auxin level at the cut surface, where, in response, MpLAXR is up-regulated to trigger cellular reprogramming. MpLAXR is an ortholog of Arabidopsis ENHANCER OF SHOOT REGENERATION 1/DORNRÖSCHEN, which has dual functions as a shoot regeneration factor and a regulator of axillary meristem initiation, the latter of which requires a low auxin level. Thus, our findings provide insights into stem cell regulation as well as apical dominance establishment in land plants.

Signaling Mechanisms by Arabidopsis Cryptochromes

Cryptochromes (CRYs) are blue light photoreceptors that regulate growth, development, and metabolism in plants. In Arabidopsis thaliana (Arabidopsis), CRY1 and CRY2 possess partially redundant and overlapping functions. Upon exposure to blue light, the monomeric inactive CRYs undergo phosphorylation and oligomerization, which are crucial to CRY function. Both the N- and C-terminal domains of CRYs participate in light-induced interaction with multiple signaling proteins. These include the COP1/SPA E3 ubiquitin ligase, several transcription factors, hormone signaling intermediates and proteins involved in chromatin-remodeling and RNA N6 adenosine methylation. In this review, we discuss the mechanisms of Arabidopsis CRY signaling in photomorphogenesis and the recent breakthroughs in Arabidopsis CRY research.

Interaction between fructan metabolism and plant growth regulators

The relationship of fructan to plant growth regulators is clearly more complicated than it looks and is likely related to differences between fructan molecules in size and structure as well as localization. Fructans are a complex group of carbohydrates composed mainly of fructose units linked to a sucrose molecule. Fructans are present in plants as heterogeneous mixtures with diverse molecular structures and mass, different polymerization degrees, and linkage types between fructosyl residues. Like sucrose, they are frequently stored in leaves and other organs, acting as carbohydrate reserves. Fructans are synthesized in the cell vacuole by fructosyltransferase enzymes and catabolized by fructan exohydrolase enzymes. Several publications have shown that fructan metabolism varies with the stage of plant development and in response to the environment. Recent studies have shown a correlation between plant growth regulators (PGR), fructan metabolism, and tolerance to drought and cold. PGR are compounds that profoundly influence the growth and differentiation of plant cells, tissues, and organs. They play a fundamental role in regulating plant responses to developmental and environmental signals. In this review, we summarize the most up-to-date knowledge on the metabolism of fructans and their crosstalk with PGR signaling pathways. We identify areas that require more research to complete our understanding of the role of fructans in plants.

Structural Aspects of Auxin Signaling

Auxin signaling regulates growth and developmental processes in plants. The core of nuclear auxin signaling relies on just three components: TIR1/AFBs, Aux/IAAs, and ARFs. Each component is itself made up of several domains, all of which contribute to the regulation of auxin signaling. Studies of the structural aspects of these three core signaling components have deepened our understanding of auxin signaling dynamics and regulation. In addition to the structured domains of these components, intrinsically disordered regions within the proteins also impact auxin signaling outcomes. New research is beginning to uncover the role intrinsic disorder plays in auxin-regulated degradation and subcellular localization. Structured and intrinsically disordered domains affect auxin perception, protein degradation dynamics, and DNA binding. Taken together, subtle differences within the domains and motifs of each class of auxin signaling component affect signaling outcomes and specificity.

Temporal and spatial auxin responsive networks in maize primary roots

Auxin is a key regulator of root morphogenesis across angiosperms. To better understand auxin-regulated networks underlying maize root development, we have characterized auxin-responsive transcription across two time points (30 and 120 min) and four regions of the primary root: the meristematic zone, elongation zone, cortex and stele. Hundreds of auxin-regulated genes involved in diverse biological processes were quantified in these different root regions. In general, most auxin-regulated genes are region unique and are predominantly observed in differentiated tissues compared with the root meristem. Auxin gene regulatory networks were reconstructed with these data to identify key transcription factors that may underlie auxin responses in maize roots. Additionally, Auxin-Response Factor subnetworks were generated to identify target genes that exhibit tissue or temporal specificity in response to auxin. These networks describe novel molecular connections underlying maize root development and provide a foundation for functional genomic studies in a key crop.

Genome-Wide Comprehensive Analysis of the GASA Gene Family in Populus

Gibberellic acid-stimulated Arabidopsis (GASA) proteins, as cysteine-rich peptides (CRPs), play roles in development and reproduction and biotic and abiotic stresses. Although the GASA gene family has been identified in plants, the knowledge about GASAs in Populus euphratica, the woody model plant for studying abiotic stress, remains limited. Here, we referenced the well-sequenced Populus trichocarpa genome, and identified the GASAs in the whole genome of P. euphratica and P. trichocarpa. 21 candidate genes in P. trichocarpa and 19 candidate genes in P. euphratica were identified and categorized into three subfamilies by phylogenetic analysis. Most GASAs with signal peptides were located extracellularly. The GASA genes in Populus have experienced multiple gene duplication events, especially in the subfamily A. The evolution of the subfamily A, with the largest number of members, can be attributed to whole-genome duplication (WGD) and tandem duplication (TD). Collinearity analysis showed that WGD genes played a leading role in the evolution of GASA genes subfamily B. The expression patterns of P. trichocarpa and P. euphratica were investigated using the PlantGenIE database and the real-time quantitative PCR (qRT-PCR), respectively. GASA genes in P. trichocarpa and P. euphratica were mainly expressed in young tissues and organs, and almost rarely expressed in mature leaves. GASA genes in P. euphratica leaves were also widely involved in hormone responses and drought stress responses. GUS activity assay showed that PeuGASA15 was widely present in various organs of the plant, especially in vascular bundles, and was induced by auxin and inhibited by mannitol dramatically. In summary, this present study provides a theoretical foundation for further research on the function of GASA genes in P. euphratica.

Phase separation in plants: New insights into cellular compartmentalization

A fundamental challenge for cells is how to coordinate various biochemical reactions in space and time. To achieve spatiotemporal control, cells have developed organelles that are surrounded by lipid bilayer membranes. Further, membraneless compartmentalization, a process induced by dynamic physical association of biomolecules through phase transition offers another efficient mechanism for intracellular organization. While our understanding of phase separation was predominantly dependent on yeast and animal models, recent findings have provided compelling evidence for emerging roles of phase separation in plants. In this review, we first provide an overview of the current knowledge of phase separation, including its definition, biophysical principles, molecular features and regulatory mechanisms. Then we summarize plant-specific phase separation phenomena and describe their functions in plant biological processes in great detail. Moreover, we propose that phase separation is an evolutionarily conserved and efficient mechanism for cellular compartmentalization which allows for distinct metabolic processes and signaling pathways, and is especially beneficial for the sessile lifestyle of plants to quickly and efficiently respond to the changing environment.

A Gain-of-Function Mutant of IAA7 Inhibits Stem Elongation by Transcriptional Repression of EXPA5 Genes in Brassica napus

Plant height is one of the most important agronomic traits of rapeseeds. In this study, we characterized a dwarf Brassica napus mutant, named ndf-2, obtained from fast neutrons and DES mutagenesis. Based on BSA-Seq and genetic properties, we identified causal mutations with a time-saving approach. The ndf-2 mutation was identified on chromosome A03 and can result in an amino acid substitution in the conserved degron motif (GWPPV to EWPPV) of the Auxin/indole-3-acetic acid protein 7 (BnaA03.IAA7) encoded by the causative gene. Aux/IAA protein is one of the core components of the auxin signaling pathway, which regulates many growth and development processes. However, the molecular mechanism of auxin signal regulating plant height is still not well understood. In the following work, we identified that BnaARF6 and BnaARF8 as interactors of BnaA03.IAA7 and BnaEXPA5 as a target of BnaARF6 and BnaARF8. The three genes BnaA03.IAA7, BnaARF6/8 and BnaEXPA5 were highly expressed in stem, suggesting that these genes were involved in stem development. The overexpression of BnaEXPA5 results in larger rosettes leaves and longer inflorescence stems in Arabidopsis thaliana. Our results indicate that BnaA03.IAA7- and BnaARF6/8-dependent auxin signal control stem elongation and plant height by regulating the transcription of BnaEXPA5 gene, which is one of the targets of this signal.

Sequence determinants of in cell condensate morphology, dynamics, and oligomerization as measured by number and brightness analysis

BACKGROUND

Biomolecular condensates are non-stoichiometric assemblies that are characterized by their capacity to spatially concentrate biomolecules and play a key role in cellular organization. Proteins that drive the formation of biomolecular condensates frequently contain oligomerization domains and intrinsically disordered regions (IDRs), both of which can contribute multivalent interactions that drive higher-order assembly. Our understanding of the relative and temporal contribution of oligomerization domains and IDRs to the material properties of in vivo biomolecular condensates is limited. Similarly, the spatial and temporal dependence of protein oligomeric state inside condensates has been largely unexplored in vivo.

METHODS

In this study, we combined quantitative microscopy with number and brightness analysis to investigate the aging, material properties, and protein oligomeric state of biomolecular condensates in vivo. Our work is focused on condensates formed by AUXIN RESPONSE FACTOR 19 (ARF19), a transcription factor integral to the auxin signaling pathway in plants. ARF19 contains a large central glutamine-rich IDR and a C-terminal Phox Bem1 (PB1) oligomerization domain and forms cytoplasmic condensates.

RESULTS

Our results reveal that the IDR amino acid composition can influence the morphology and material properties of ARF19 condensates. In contrast the distribution of oligomeric species within condensates appears insensitive to the IDR composition. In addition, we identified a relationship between the abundance of higher- and lower-order oligomers within individual condensates and their apparent fluidity.

CONCLUSIONS

IDR amino acid composition affects condensate morphology and material properties. In ARF condensates, altering the amino acid composition of the IDR did not greatly affect the oligomeric state of proteins within the condensate. Video Abstract.

OsARF11 Promotes Growth, Meristem, Seed, and Vein Formation during Rice Plant Development

The plant hormone auxin acts as a mediator providing positional instructions in a range of developmental processes. Studies in Arabidopsis thaliana L. show that auxin acts in large part via activation of Auxin Response Factors (ARFs) that in turn regulate the expression of downstream genes. The rice (Oryza sativa L.) gene OsARF11 is of interest because of its expression in developing rice organs and its high sequence similarity with MONOPTEROS/ARF5, a gene with prominent roles in A. thaliana development. We have assessed the phenotype of homozygous insertion mutants in the OsARF11 gene and found that in relation to wildtype, osarf11 seedlings produced fewer and shorter roots as well as shorter and less wide leaves. Leaves developed fewer veins and larger areoles. Mature osarf11 plants had a reduced root system, fewer branches per panicle, fewer grains per panicle and fewer filled seeds. Mutants had a reduced sensitivity to auxin-mediated callus formation and inhibition of root elongation, and phenylboronic acid (PBA)-mediated inhibition of vein formation. Taken together, our results implicate OsARF11 in auxin-mediated growth of multiple organs and leaf veins. OsARF11 also appears to play a central role in the formation of lateral root, panicle branch, and grain meristems.

Overexpression of MusaSNAC1 improves shoot proliferation in transgenic banana lines

Augmenting shoot multiplication through genetic engineering is an emerging biotechnological application desirable in optimizing regeneration of genetically modified plants on selection medium and rapid clonal propagation of elite cultivars. Here, we report the improved shoot multiplication in transgenic banana lines with overexpression of MusaSNAC1, a drought-associated NAC transcription factor in banana. Overexpression of MusaSNAC1 induces hypersensitivity of transgenic banana lines toward 6-benzylaminopurine ensuing higher shoot number on different concentrations of 6-benzylaminopurine. Altered transcript levels of multiple genes involved in auxin signaling (Aux/IAA and ARFs) and cytokinin signaling pathways (ARRs) in banana plants overexpressing MusaSNAC1 corroborate the hypersensitivity of transgenic banana plants toward 6-benzylaminopurine. Modulation in expression of ARRs reported to be involved in ABA-hypersensitivity and closure of stomatal aperture correlates with the function of MusaSNAC1 as a drought-responsive NAC transcription factor. Present study suggests a prospective cross talk between shoot multiplication and drought responses coordinated by MusaSNAC1 in banana plants.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02744-5.

Emerging Roles for Phase Separation in Plants

The plant cell internal environment is a dynamic, intricate landscape composed of many intracellular compartments. Cells organize some cellular components through formation of biomolecular condensates-non-stoichiometric assemblies of protein and/or nucleic acids. In many cases, phase separation appears to either underly or contribute to the formation of biomolecular condensates. Many canonical membraneless compartments within animal cells form in a manner that is at least consistent with phase separation, including nucleoli, stress granules, Cajal bodies, and numerous additional bodies, regulated by developmental and environmental stimuli. In this Review, we examine the emerging roles for phase separation in plants. Further, drawing on studies carried out in other organisms, we identify cellular phenomenon in plants that might also arise via phase separation. We propose that plants make use of phase separation to a much greater extent than has been previously appreciated, implicating phase separation as an evolutionarily ancient mechanism for cellular organization.

A Gain-of-Function Mutant of IAA15 Inhibits Lateral Root Development by Transcriptional Repression of LBD Genes in Arabidopsis

Lateral root development is known to be regulated by Aux/IAA-ARF modules in Arabidopsis thaliana. As components, several Aux/IAAs have participated in these Aux/IAA-ARF modules. In this study, to identify the biological function of IAA15 in plant developments, transgenic plant overexpressing the gain-of-function mutant of IAA15 (IAA15^P75S OX) under the control of dexamethasone (DEX) inducible promoter, in which IAA15 protein was mutated by changing Pro-75 residue to Ser at the degron motif in conserved domain II, was constructed. As a result, we found that IAA15^P75S OX plants show a decreased number of lateral roots. Coincidently, IAA15 promoter-GUS reporter analysis revealed that IAA15 transcripts were highly detected in all stages of developing lateral root tissues. It was also verified that the IAA15^P75S protein is strongly stabilized against proteasome-mediated protein degradation by inhibiting its poly-ubiquitination, resulting in the transcriptional repression of auxin-responsive genes. In particular, transcript levels of LBD16 and LBD29, which are positive regulators of lateral root formation, dramatically repressed in IAA15^P75S OX plants. Furthermore, it was elucidated that IAA15 interacts with ARF7 and ARF19 and binds to the promoters of LBD16 and LBD29, strongly suggesting that IAA15 represses lateral root formation through the transcriptional suppression of LBD16 and LBD29 by inhibiting ARF7 and ARF19 activity. Taken together, this study suggests that IAA15 also plays a key negative role in lateral root formation as a component of Aux/IAA-ARF modules.

Molecular Evidences for the Interactions of Auxin, Gibberellin, and Cytokinin in Bent Peduncle Phenomenon in Rose (Rosa sp.)

In roses (Rosa sp.), peduncle morphology is an important ornamental feature. The common physiological abnormality known as the bent peduncle phenomenon (BPP) seriously decreases the quality of rose flowers and thus the commercial value. Because the molecular mechanisms underlying this condition are poorly understood, we analysed the transcriptional profiles and cellular structures of bent rose peduncles. Numerous differentially expressed genes involved in the auxin, cytokinin, and gibberellin signaling pathways were shown to be associated with bent peduncle. Paraffin sections showed that the cell number on the upper sides of bent peduncles was increased, while the cells on the lower sides were larger than those in normal peduncles. We also investigated the large, deformed sepals that usually accompany BPP and found increased expression level of some auxin-responsive genes and decreased expression level of genes that are involved in cytokinin and gibberellin synthesis in these sepals. Furthermore, removal of the deformed sepals partially relieved BPP. In summary, our findings suggest that auxin, cytokinin, and gibberellin all influence the development of BPP by regulating cell division and expansion. To effectively reduce BPP in roses, more efforts need to be devoted to the molecular regulation of gibberellins and cytokinins in addition to that of auxin.

Auxin-Abscisic Acid Interactions in Plant Growth and Development

Plant hormones regulate many aspects of plant growth, development, and response to biotic and abiotic stress. Much research has gone into our understanding of individual plant hormones, focusing primarily on their mechanisms of action and the processes that they regulate. However, recent research has begun to focus on a more complex problem; how various plant hormones work together to regulate growth and developmental processes. In this review, we focus on two phytohormones, abscisic acid (ABA) and auxin. We begin with brief overviews of the hormones individually, followed by in depth analyses of interactions between auxin and ABA, focusing on interactions in individual tissues and how these interactions are occurring where possible. Finally, we end with a brief discussion and future prospects for the field.

Regulation of auxin transcriptional responses

The plant hormone auxin acts as a signaling molecule to regulate a vast number of developmental responses throughout all stages of plant growth. Tight control and coordination of auxin signaling is required for the generation of specific auxin‐response outputs. The nuclear auxin signaling pathway controls auxin‐responsive gene transcription through the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F‐BOX pathway. Recent work has uncovered important details into how regulation of auxin signaling components can generate unique and specific responses to determine auxin outputs. In this review, we discuss what is known about the core auxin signaling components and explore mechanisms important for regulating auxin response specificity.

A review of recent updates to our understanding of auxin signaling.