Spatiotemporal brassinosteroid signaling and antagonism with auxin pattern stem cell dynamics in Arabidopsis roots

From trade-off to synergy: microbial insights into enhancing plant growth and immunity

The reduction in crop yield caused by pathogens and pests presents a significant challenge to global food security. Genetic engineering, which aims to bolster plant defence mechanisms, emerges as a cost-effective solution for disease control. However, this approach often incurs a growth penalty, known as the growth-defence trade-off. The precise molecular mechanisms governing this phenomenon are still not completely understood, but they generally fall under two main hypotheses: a "passive" redistribution of metabolic resources, or an "active" regulatory choice to optimize plant fitness. Despite the knowledge gaps, considerable practical endeavours are in the process of disentangling growth from defence. The plant microbiome, encompassing both above- and below-ground components, plays a pivotal role in fostering plant growth and resilience to stresses. There is increasing evidence which indicates that plants maintain intimate associations with diverse, specifically selected microbial communities. Meta-analyses have unveiled well-coordinated, two-way communications between plant shoots and roots, showcasing the capacity of plants to actively manage their microbiota for balancing growth with immunity, especially in response to pathogen incursions. This review centers on successes in making use of specific root-associated microbes to mitigate the growth-defence trade-off, emphasizing pivotal advancements in unravelling the mechanisms behind plant growth and defence. These findings illuminate promising avenues for future research and practical applications.

Transcriptome Reveals the Regulation of Exogenous Auxin Inducing Rooting of Non-Rooting Callus of Tea Cuttings

Cuttage is the main propagation method of tea plant cultivars in China. However, some tea softwood cuttings just form an expanded and loose callus at the base, without adventitious root (AR) formation during the propagation period. Meanwhile, exogenous auxin could promote the AR formation of tea plant cuttings, but the regulation mechanism has not yet explained clearly. We conducted this study to elucidate the regulatory mechanism of exogenous auxin-induced adventitious root (AR) formation of such cuttings. The transcriptional expression profile of non-rooting tea calluses in response to exogenous IBA and NAA was analyzed using ONT RNA Seq technology. In total, 56,178 differentially expressed genes (DEGs) were detected, and most of genes were significantly differentially expressed after 12 h of exogenous auxin treatment. Among these DEGs, we further identified 80 DEGs involved in the auxin induction pathway and AR formation. Specifically, 14 auxin respective genes (ARFs, GH3s, and AUX/IAAs), 3 auxin transporters (AUX22), 19 auxin synthesis- and homeostasis-related genes (cytochrome P450 (CYP450) and calmodulin-like protein (CML) genes), and 44 transcription factors (LOB domain-containing protein (LBDs), SCARECROW-LIKE (SCL), zinc finger protein, WRKY, MYB, and NAC) were identified from these DEGs. Moreover, we found most of these DEGs were highly up-regulated at some stage before AR formation, suggesting that they may play a potential role in the AR formation of tea plant cuttings. In summary, this study will provide a theoretical foundation to deepen our understanding of the molecular mechanism of AR formation in tea cuttings induced by auxin during propagation time.

Computational Reconstruction of the Transcription Factor Regulatory Network Induced by Auxin in Arabidopsis thaliana L

In plant hormone signaling, transcription factor regulatory networks (TFRNs), which link the master transcription factors to the biological processes under their control, remain insufficiently characterized despite their crucial function. Here, we identify a TFRN involved in the response to the key plant hormone auxin and define its impact on auxin-driven biological processes. To reconstruct the TFRN, we developed a three-step procedure, which is based on the integrated analysis of differentially expressed gene lists and a representative collection of transcription factor binding profiles. Its implementation is available as a part of the CisCross web server. With the new method, we distinguished two transcription factor subnetworks. The first operates before auxin treatment and is switched off upon hormone application, the second is switched on by the hormone. Moreover, we characterized the functioning of the auxin-regulated TFRN in control of chlorophyll and lignin biosynthesis, abscisic acid signaling, and ribosome biogenesis.

Interaction of the Transcription Factors BES1/BZR1 in Plant Growth and Stress Response

Bri1-EMS Suppressor 1 (BES1) and Brassinazole Resistant 1 (BZR1) are two key transcription factors in the brassinosteroid (BR) signaling pathway, serving as crucial integrators that connect various signaling pathways in plants. Extensive genetic and biochemical studies have revealed that BES1 and BZR1, along with other protein factors, form a complex interaction network that governs plant growth, development, and stress tolerance. Among the interactome of BES1 and BZR1, several proteins involved in posttranslational modifications play a key role in modifying the stability, abundance, and transcriptional activity of BES1 and BZR1. This review specifically focuses on the functions and regulatory mechanisms of BES1 and BZR1 protein interactors that are not involved in the posttranslational modifications but are crucial in specific growth and development stages and stress responses. By highlighting the significance of the BZR1 and BES1 interactome, this review sheds light on how it optimizes plant growth, development, and stress responses.

BZR1 and BES1 transcription factors mediate brassinosteroid control over root system architecture in response to nitrogen availability

Plants modify their root system architecture (RSA) in response to nitrogen (N) deficiency. The plant steroidal hormone, brassinosteroid (BR), plays important roles in root growth and development. This study demonstrates that optimal levels of exogenous BR impact significant increases in lateral root length and numbers in Arabidopsis seedlings under mild N-deficient conditions as compared to untreated seedlings. The impact of BR on RSA was stronger under mild N deficiency than under N-sufficient conditions. The BR effects on RSA were mimicked in dominant mutants of BZR1 and BES1 (bzr1-1D and bes1-D) transcription factors, while the RSA was highly reduced in the BR-insensitive mutant bri1-6, confirming that BR signaling is essential for the development of RSA under both N-sufficient and N-deficient conditions. Exogenous BR and constitutive activity of BZR1 and BES1 in dominant mutants led to enhanced root meristem, meristematic cell number, and cortical cell length. Under mild N deficiency, bzr1-1D displayed higher fresh and dry shoot weights, chlorophyll content, and N levels in the shoot, as compared to the wild type. These results indicate that BR modulates RSA under both N-sufficient and N-deficient conditions via the transcription factors BES1/BZR1 module and confers tolerance to N deficiency.

A century of studying plant secondary metabolism-From "what?" to "where, how, and why?"

Over the past century, early advances in understanding the identity of the chemicals that collectively form a living plant have led scientists to deeper investigations exploring where these molecules localize, how they are made, and why they are synthesized in the first place. Many small molecules are specific to the plant kingdom and have been termed plant secondary metabolites, despite the fact that they can play primary and essential roles in plant structure, development, and response to the environment. The past 100 yr have witnessed elucidation of the structure, function, localization, and biosynthesis of selected plant secondary metabolites. Nevertheless, many mysteries remain about the vast diversity of chemicals produced by plants and their roles in plant biology. From early work characterizing unpurified plant extracts, to modern integration of 'omics technology to discover genes in metabolite biosynthesis and perception, research in plant (bio)chemistry has produced knowledge with substantial benefits for society, including human medicine and agricultural biotechnology. Here, we review the history of this work and offer suggestions for future areas of exploration. We also highlight some of the recently developed technologies that are leading to ongoing research advances.

The BAS chromatin remodeler determines brassinosteroid-induced transcriptional activation and plant growth in Arabidopsis

Brassinosteroid (BR) signaling leads to the nuclear accumulation of the BRASSINAZOLE-RESISTANT 1 (BZR1) transcription factor, which plays dual roles in activating or repressing the expression of thousands of genes. BZR1 represses gene expression by recruiting histone deacetylases, but how it activates transcription of BR-induced genes remains unclear. Here, we show that BR reshapes the genome-wide chromatin accessibility landscape, increasing the accessibility of BR-induced genes and reducing the accessibility of BR-repressed genes in Arabidopsis. BZR1 physically interacts with the BRAHMA-associated SWI/SNF (BAS)-chromatin-remodeling complex on the genome and selectively recruits the BAS complex to BR-activated genes. Depletion of BAS abrogates the capacities of BZR1 to increase chromatin accessibility, activate gene expression, and promote cell elongation without affecting BZR1's ability to reduce chromatin accessibility and expression of BR-repressed genes. Together, these data identify that BZR1 recruits the BAS complex to open chromatin and to mediate BR-induced transcriptional activation of growth-promoting genes.

DNA double-strand breaks enhance brassinosteroid signaling to activate quiescent center cell division in Arabidopsis

In Arabidopsis roots, the quiescent center (QC), a group of slowly dividing cells located at the center of the stem cell niche, functions as an organizing center to maintain the stemness of neighboring cells. Recent studies have shown that they also act as a reservoir for backup cells, which replenish DNA-damaged stem cells by activating cell division. The latter function is essential for maintaining stem cells under stressful conditions, thereby guaranteeing post-embryonic root development in fluctuating environments. In this study, we show that one of the brassinosteroid receptors in Arabidopsis, BRASSINOSTEROID INSENSITIVE1-LIKE3 (BRL3), plays a major role in activating QC division in response to DNA double-strand breaks. SUPPRESSOR OF GAMMA RESPONSE 1, a master transcription factor governing DNA damage response, directly induces BRL3. DNA damage-induced QC division was completely suppressed in brl3 mutants, whereas QC-specific overexpression of BRL3 activated QC division. Our data also showed that BRL3 is required to induce the AP2-type transcription factor ETHYLENE RESPONSE FACTOR 115, which triggers regenerative cell division. We propose that BRL3-dependent brassinosteroid signaling plays a unique role in activating QC division and replenishing dead stem cells, thereby enabling roots to restart growing after recovery from genotoxic stress.

Identification and Characterization of the BZR Transcription Factor Genes Family in Potato (Solanum tuberosum L.) and Their Expression Profiles in Response to Abiotic Stresses

Brassinazole resistant (BZR) genes act downstream of the brassinosteroid signaling pathway regulating plant growth and development and participating in plant stress responses. However, the BZR gene family has not systematically been characterized in potato. We identified eight BZR genes in Solanum tuberosum, which were distributed among seven chromosomes unequally and were classified into three subgroups. Potato and tomato BZR proteins were shown to be closely related with high levels of similarity. The BZR gene family members in each subgroup contained similar conserved motifs. StBZR genes exhibited tissue-specific expression patterns, suggesting their functional differentiation during evolution. StBZR4, StBZR7, and StBZR8 were highly expressed under white light in microtubers. StBZR1 showed a progressive up-regulation from 0 to 6 h and a progressive down-regulation from 6 to 24 h after drought and salt stress. StBZR1, StBZR2, StBZR4, StBZR5, StBZR6, StBZR7 and StBZR8 were significantly induced from 0 to 3 h under BR treatment. This implied StBZR genes are involved in phytohormone and stress response signaling pathways. Our results provide a theoretical basis for understanding the functional mechanisms of BZR genes in potato.

The hidden harmony: Exploring ROS-phytohormone nexus for shaping plant root architecture in response to environmental cues

Root system architecture, encompassing lateral roots and root hairs, plays a vital in overall plant growth and stress tolerance. Reactive oxygen species (ROS) and plant hormones intricately regulate root growth and development, serving as signaling molecules that govern processes such as cell proliferation and differentiation. Manipulating the interplay between ROS and hormones has the potential to enhance nutrient absorption, stress tolerance, and agricultural productivity. In this review, we delve into how studying these processes provides insights into how plants respond to environmental changes and optimize growth patterns to better control cellular processes and stress responses in crops. We discuss various factors and complex signaling networks that may exist among ROS and phytohormones during root development. Additionally, the review highlights possible role of reactive nitrogen species (RNS) in ROS-phytohormone interactions and in shaping root system architecture according to environmental cues.

Transcriptome analysis reveals the effect of acidic environment on adventitious root differentiation in Camellia sinensis

The generation of adventitious roots (ARs) is the key to the success of cuttings. The appropriate environment for AR differentiation in tea plants is acidic. However, the mechanism is unclear. In this study, pH 4.5 was suitable condition for the differentiation of AR in tea plants. At the base of cuttings, the root primordia differentiated ARs more rapidly at pH 4.5 than pH 7.0, and nine AR differentiation-related genes were found to be differentially expressed in 30 days, the result was also validated by qRT-PCR. The promoter regions of these genes contained auxin and brassinosteroid response elements. The expression levels of several genes which were involved in auxin and brassinosteroid synthesis as well as signaling at pH 4.5 compared to pH 7.0 occurred differential expression. Brassinolide (BL) and indole-3-acetic acid (IAA) could affect the differentiation of ARs under pH 4.5 and pH 7.0. By qRT-PCR analysis of genes during ARs generation, BL and IAA inhibited and promoted the expression of CsIAA14 gene, respectively, to regulate auxin signal transduction. Meanwhile, the expression levels of CsKNAT4, CsNAC2, CsNAC100, CsWRKY30 and CsLBD18 genes were up-regulated upon auxin treatment and were positively correlated with ARs differentiation.This study showed that pH 4.5 was the most suitable environment for the root primordia differentiation of AR in tea plant. Proper acidic pH conditions promoted auxin synthesis and signal transduction. The auxin initiated the expression of AR differentiation-related genes, and promoted its differentiated. BL was involved in ARs formation and elongation by regulating auxin signal transduction.

Single cell RNA-seq in phytohormone signaling: a promising future

Phytohormone signaling regulates plant growth and development. Single cell RNA sequencing (scRNA-seq) provides unprecedented opportunities to decipher hormone-mediated spatiotemporal gene regulatory networks. In a recent study, Nolan et al. used time-series scRNA-seq to identify the cortex as a key site for brassinosteroid (BR)-mediated gene expression and revealed a signaling network during cell phase transition.

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.

Brassinosteroid recruits FERONIA to safeguard cell expansion in Arabidopsis

Plant cell expansion is driven by turgor pressure and regulated by hormones. How plant cells avoid cell wall rupture during hormone-induced cell expansion remains a mystery. Here we show that brassinosteroid (BR), while stimulating cell elongation, promotes the plasma membrane (PM) accumulation of the receptor kinase FERONIA (FER), which monitors cell wall damage and in turn attenuates BR-induced cell elongation to prevent cell rupture. The GSK3-like kinase BIN2 phosphorylates FER, resulting in reduced FER accumulation and translocation from endoplasmic reticulum to PM. By inactivating BIN2, BR signaling promotes dephosphorylation and increases PM accumulation of FER, thereby enhancing the surveillance of cell wall integrity. Our study reveals a vital signaling circuit that coordinates hormone signaling with mechanical sensing to prevent cell bursting during hormone-induced cell expansion.

Brassinosteroid catabolic enzyme CYP734A129 regulates the morphologies of leaves and floral organs in woodland strawberry

Brassinosteroids (BRs) play critical roles in plant growth and development and regulate many important agronomic traits. However, the functions of BRs in strawberry are unclear. This study identified two mutants, named P6 and R87, in woodland strawberry (Fragaria vesca) from EMS mutagenesis populations that exhibit narrow leaves, petals and sepals. Mapping by sequencing and genetic studies revealed that the F. vesca CYP734A129, encoding a putative BR catabolic enzyme, is the causative gene for both P6 and R87. Overexpression of CYP734A129 in both F. vesca and Arabidopsis causes a severe dwarf phenotype, and the BRI1-EMS-SUPPRESSOR 1 (BES1) protein is less abundant in the CYP734A129-overexpressing Arabidopsis seedlings. This suggests that CYP734A129 is functionally conserved with CYP734A1, as a BR-inactivating enzyme. Transcriptome analysis of young leaves revealed that four BR biosynthetic genes were significantly downregulated in P6 (cyp734a129), and photosynthesis-related genes were highly enriched among the up-regulated genes in P6 compared to the wild type. This further supports that CYP734A129 inactivates BRs in F. vesca. Furthermore, we showed that mutations in CYP734A129 do not affect fruit shape and color during ripening in strawberry. Overall, our results suggest that F. vesca CYP734A129 is a BR catabolic enzyme, and provide insights into the roles of CYP734A129 in strawberry.

Jasmonates regulate apical hook development by repressing brassinosteroid biosynthesis and signaling

An apical hook is a special structure formed during skotomorphogenesis in dicotyledonous plant species. It is critical for protecting the shoot apical meristem from mechanical damage during seed germination and hypocotyl elongation in soil. Brassinosteroid (BR) and jasmonate (JA) phytohormones antagonistically regulate apical hook formation. However, the interrelationship between BRs and JAs in this process has not been well elucidated. Here, we reveal that JAs repress BRs to regulate apical hook development in Arabidopsis (Arabidopsis thaliana). Exogenous application of methyl jasmonate (MeJA) repressed the expression of the rate-limiting BR biosynthetic gene DWARF4 (DWF4) in a process relying on 3 key JA-dependent transcription factors, MYC2, MYC3, and MYC4. We demonstrated that MYC2 interacts with the critical BR-activated transcription factor BRASSINAZOLE RESISTANT 1 (BZR1), disrupting the association of BZR1 with its partner transcription factors, such as those of the PHYTOCHROME INTERACTING FACTOR (PIF) family and downregulating the expression of their target genes, such as WAVY ROOT GROWTH 2 (WAG2), encoding a protein kinase essential for apical hook development. Our results indicate that JAs not only repress the expression of BR biosynthetic gene DWF4 but, more importantly, attenuate BR signaling by inhibiting the transcriptional activation of BZR1 by MYC2 during apical hook development.

Wheat type one protein phosphatase promotes salt and osmotic stress tolerance in arabidopsis via auxin-mediated remodelling of the root system

The control of optimal root growth and plant stress responses depends largely on a variety of phytohormones among which auxin and brassinosteroids (BRs) are the most influential. We have previously reported that the durum wheat type 1 protein phosphatase TdPP1 participates in the control of root growth by modulating BR signaling. In this study, we pursue our understanding of how TdPP1 fulfills this regulatory function on root growth by evaluating the physiological and molecular responses of Arabidopsis TdPP1 over-expressing lines to abiotic stresses. Our results showed that when exposed to 300 mM Mannitol or 100 mM NaCl, the seedlings of TdPP1 over-expressors exhibit modified root architecture with higher lateral root density, and longer root hairs concomitant with a lower inhibition of the primary root growth. These lines also exhibit faster gravitropic response and a decrease in primary root growth inhibition when exposed to high concentrations of exogenous IAA. On another hand, a cross between TdPP1 overexpressors and DR5:GUS marker line was performed to monitor auxin accumulation in roots. Remarkably, the TdPP1 overexpression resulted in an enhanced auxin gradient under salt stress with a higher accumulation in primary and lateral root tips. Moreover, TdPP1 transgenics exhibit a significant induction of a subset of auxin-responsive genes under salt stress conditions. Therefore, our results reveal a role of PP1 in enhancing auxin signaling to help shape greater root plasticity thus improving plant stress resilience.

Brassinosteroid signaling and molecular crosstalk with nutrients in plants

As sessile organisms, plants have evolved sophisticated mechanisms to optimize their growth and development in response to fluctuating nutrient levels. Brassinosteroids (BRs) are a group of plant steroid hormones that play critical roles in plant growth and developmental processes as well as plant responses to environmental stimuli. Recently, multiple molecular mechanisms have been proposed to explain the integration of BRs with different nutrient signaling processes to coordinate gene expression, metabolism, growth, and survival. Here, we review recent advances in understanding the molecular regulatory mechanisms of the BR signaling pathway and the multifaceted roles of BR in the intertwined sensing, signaling, and metabolic processes of sugar, nitrogen, phosphorus, and iron. Further understanding and exploring these BR-related processes and mechanisms will facilitate advances in crop breeding for higher resource efficiency.

The BR signaling pathway regulates primary root development and drought stress response by suppressing the expression of PLT1 and PLT2 in Arabidopsis thaliana

Introduction

With the warming global climate, drought stress has become an important abiotic stress factor limiting plant growth and crop yield. As the most rapidly drought-sensing organs of plants, roots undergo a series of changes to enhance their ability to absorb water, but the molecular mechanism is unclear.

Results and methods

In this study, we found that PLT1 and PLT2, two important transcription factors of root development in Arabidopsis thaliana, are involved in the plant response to drought and are inhibited by BR signaling. PLT1- and PLT2-overexpressing plants showed greater drought tolerance than wild-type plants. Furthermore, we found that BZR1 could bind to the promoter of PLT1 and inhibit its transcriptional activity in vitro and in vivo. PLT1 and PLT2 were regulated by BR signaling in root development and PLT2 could partially rescue the drought sensitivity of bes1-D. In addition, RNA-seq data analysis showed that BR-regulated root genes and PLT1/2 target genes were also regulated by drought; for example, CIPK3, RCI2A, PCaP1, PIP1;5, ERF61 were downregulated by drought and PLT1/2 but upregulated by BR treatment; AAP4, WRKY60, and AT5G19970 were downregulated by PLT1/2 but upregulated by drought and BR treatment; and RGL2 was upregulated by drought and PLT1/2 but downregulated by BR treatment.

Discussion

Our findings not only reveal the mechanism by which BR signaling coordinates root growth and drought tolerance by suppressing the expression of PLT1 and PLT2 but also elucidates the relationship between drought and root development. The current study thus provides an important theoretical basis for the improvement of crop yield under drought conditions.

Brassinosteroid transcription factor BES1 modulates nitrate deficiency by promoting NRT2.1 and NRT2.2 transcription in Arabidopsis

Nitrogen (N) is one of the most essential mineral elements for plants. Brassinosteroids (BRs) play key roles in plant growth and development. Emerging evidence indicates that BRs participate in the responses to nitrate deficiency. However, the precise molecular mechanism underlying the BR signaling pathway in regulating nitrate deficiency remains largely unknown. The transcription factor BES1 regulates the expression of many genes in response to BRs. Root length, nitrate uptake and N concentration of bes1-D mutants were higher than those of wild-type under nitrate deficiency. BES1 levels strongly increased under low nitrate conditions, especially in the non-phosphorylated (active) form. Furthermore, BES1 directly bound to the promoters of NRT2.1 and NRT2.2 to promote their expression under nitrate deficiency. Taken together, BES1 is a key mediator that links BR signaling under nitrate deficiency by modulating high affinity nitrate transporters in plants.

Genome-wide identification and expression analysis of BZR gene family and associated responses to abiotic stresses in cucumber (Cucumis sativus L.)

BACKGROUND

BRASSINAZOLE-RESISTANT (BZR) is a class of specific transcription factor (TFs) involved in brassinosteroid (BR) signal transduction. The regulatory mechanism of target genes mediated by BZR has become one of the key research areas in plant BR signaling networks. However, the functions of the BZR gene family in cucumber have not been well characterized.

RESULTS

In this study, six CsBZR gene family members were identified by analyzing the conserved domain of BES1 N in the cucumber genome. The size of CsBZR proteins ranges from 311 to 698 amino acids and are mostly located in the nucleus. Phylogenetic analysis divided CsBZR genes into three subgroups. The gene structure and conserved domain showed that the BZR genes domain in the same group was conserved. Cis-acting element analysis showed that cucumber BZR genes were mainly involved in hormone response, stress response and growth regulation. The qRT-PCR results also confirmed CsBZR response to hormones and abiotic stress.

CONCLUSION

Collectively, the CsBZR gene is involved in regulating cucumber growth and development, particularly in hormone response and response to abiotic stress. These findings provide valuable information for understanding the structure and expression patterns of BZR genes.

Brassinolide promotes interaction between chloroplasts and mitochondria during the optimization of photosynthesis by the mitochondrial electron transport chain in mesophyll cell protoplasts of Arabidopsis thaliana

The current experimental data unveils the role of brassinolide (BL), a phytohormone of class brassinosteroids (BRs), in augmenting the cross-talk between the mitochondrial electron transport chain (mETC) and chloroplasts to strengthen the efficiency of the Calvin-Benson cycle (CBC) for higher assimilation of carbon dioxide in the mesophyll cell protoplasts (MCP) of Arabidopsis thaliana. The outcome of total respiration (TR) and photosynthetic carbon assimilation (PCA) was monitored as O₂ uptake under dark and NaHCO₃-dependent O₂ evolution under light, respectively, after pre-incubation of MCP at a broad spectrum of BL concentration from 0.05 pM to 5 pM at 25 °C and optimum light intensity of 1000 μmol m^-2 s^-1. The addition of optimal concentration (0.5 pM) of BL to MCP stimulated the (i) TR, (ii) PCA, and (iii) para-benzoquinone-dependent O₂ evolution (PSII activity). Further, in response to BL, the enzyme activity or transcript levels of redox-regulated CBC enzymes and glucose-6-phosphate raised considerably. Also, the addition of BL to MCP remarkably accelerated the capacity of the cytochrome oxidase (COX) and alternative oxidase (AOX) pathways concurrently with an increase in total cellular pyruvate and reactive oxygen species (ROS) levels. Besides, malate valve components (Malate, Chl-MDH, M-MDH) increased in response to BL. At the same time, the cellular redox ratios of pyridine nucleotides (NADPH and NADH) were kept low in the presence of BL. However, BL could not keep up the CBC activity of photosynthesis along with its associated light-activated enzymes/transcripts when mETC through COX or AOX pathway is restricted by antimycin A (AA) or salicylhydroxamic acid (SHAM), respectively. In contrast, adding BL to MCP under restricted mETC showed aggravation in total cellular ROS, pyruvate, malate, and redox ratio of pyridine nucleotides with a concomitant increase in transcripts associated with malate valve and antioxidant systems. These results suggest that BL enhances the PCA by coordinating in cross-talk of chloroplasts and mitochondria to regulate the cellular redox ratio or ROS through the involvement of COX and AOX pathways along with the malate valve and antioxidant systems.

HD-ZIP III-dependent local promotion of brassinosteroid synthesis suppresses vascular cell division in Arabidopsis root apical meristem

Spatiotemporal control of cell division in the meristem is vital for plant growth. In the stele of the root apical meristem (RAM), procambial cells divide periclinally to increase the number of vascular cell files. Class III homeodomain leucine zipper (HD-ZIP III) proteins are key transcriptional regulators of RAM development and suppress the periclinal division of vascular cells in the stele; however, the mechanism underlying the regulation of vascular cell division by HD-ZIP III transcription factors (TFs) remains largely unknown. Here, we performed transcriptome analysis to identify downstream genes of HD-ZIP III and found that HD-ZIP III TFs positively regulate brassinosteroid biosynthesis-related genes, such as CONSTITUTIVE PHOTOMORPHOGENIC DWARF (CPD), in vascular cells. Introduction of pREVOLUTA::CPD in a quadruple loss-of-function mutant of HD-ZIP III genes partly rescued the phenotype in terms of the vascular defect in the RAM. Treatment of a quadruple loss-of-function mutant, a gain-of-function mutant of HD-ZIP III, and the wild type with brassinosteroid and a brassinosteroid synthesis inhibitor also indicated that HD-ZIP III TFs act together to suppress vascular cell division by increasing brassinosteroid levels. Furthermore, brassinosteroid application suppressed the cytokinin response in vascular cells. Together, our findings suggest that the suppression of vascular cell division by HD-ZIP III TFs is caused, at least in part, by the increase in brassinosteroid levels through the transcriptional activation of brassinosteroid biosynthesis genes in the vascular cells of the RAM. This elevated brassinosteroid level suppresses cytokinin response in vascular cells, inhibiting vascular cell division in the RAM.

Specificity in root domain accumulation of Phytoglobin1 and nitric oxide (NO) determines meristematic viability in water-stressed Brassica napus roots

BACKGROUND AND AIMS

Drought reduces plant productivity, especially in the susceptible species Brassica napus. Water stress, mimicked by applications of 10 % polyethylene glycol (PEG), elevates nitric oxide (NO) in root cells after a few hours, contributing to degradation of the root apical meristems (RAMs), the function of which relies on auxin and brassinosteroids (BRs). Phytoglobins (Pgbs) are effective NO scavengers induced by this stress. This study examines the effects of BnPgb1 dysregulation in dehydrating B. napus roots, and the spatiotemporal relationship between Pgb1 and activities of auxin and BRs in the regulation of the RAM.

METHODS

Brassica napus lines over-expressing [BnPgb1(S)] or down-regulating [BnPgb1(RNAi)] BnPgb1 were exposed to PEG-induced water stress. The localization of BnPgb1, NO, auxin and PIN1 were analysed during the first 48 h, while the expression level of biosynthetic auxin and BR genes was measured during the first 24 h. Pharmacological treatments were conducted to assess the requirement of auxin and BR in dehydrating roots.

KEY RESULTS

During PEG stress, BnPgb1 protein accumulated preferentially in the peripheral domains of the root elongation zone, exposing the meristem to NO, which inhibits polar auxin transport (PAT), probably by interfering with PIN1 localization and the synthesis of auxin. Diminished auxin at the root tip depressed the synthesis of BR and caused the degradation of the RAMs. The strength of BnPgb1 signal in the elongation zone was increased in BnPgb1(S) roots, where NO was confined to the most apical cells. Consequently, PAT and auxin synthesis were retained, and the definition of RAMs was maintained. Auxin preservation of the RAM required BRs, although BRs alone was not sufficient to fully rescue drought-damaged RAMs in auxin-depleted environments.

CONCLUSIONS

The tissue-specific localization of BnPgb1 and NO determine B. napus root responses to water stress. A model is proposed in which auxin and BRs act as downstream components of BnPgb1 signalling in the preservation of RAMs in dehydrating roots.

Physiological and iTRAQ-based quantitative proteomics analyses reveal the similarities and differences in stress responses between short-term boron deficiency and toxicity in wheat roots

BACKGROUND

Boron (B) is a trace element that is essential for normal wheat development, such as root growth. In wheat, roots are important organs that absorb nutrients and water. However, at present, there is insufficient research on the molecular mechanism underlying how short-term B stress affects wheat root growth.

METHODS AND RESULTS

Here, the optimal concentration of B for wheat root growth was determined, and the proteomic profiles of roots under short-term B deficiency and toxicity were analyzed and compared by the isobaric tag for relative and absolute quantitation (iTRAQ) technique. A total of 270 differentially abundant proteins (DAPs) that accumulated in response to B deficiency and 263 DAPs that accumulated in response to B toxicity were identified. Global expression analysis revealed that ethylene, auxin, abscisic acid (ABA), and Ca2+ signals were involved in the responses to these two stresses. Under B deficiency, DAPs related to auxin synthesis or signaling and DAPs involved in calcium signaling increased in abundance. In striking contrast, auxin and calcium signals were repressed under B toxicity. Twenty-one DAPs were detected under both conditions, including RAN1 that played a core role in the auxin and calcium signals. Overexpression of RAN1 was shown to confer plant resistance to B toxicity by activating auxin response genes, including TIR and those identified by iTRAQ in this research. Moreover, growth of the primary roots of tir mutant was significantly inhibited under B toxicity.

CONCLUSION

Taken together, these results indicate that some connections were present between RAN1 and the auxin signaling pathway under B toxicity. Therefore, this research provides data for improving the understanding of the molecular mechanism underlying the response to B stress.

Brassinosteroid gene regulatory networks at cellular resolution in the Arabidopsis root

Brassinosteroids are plant steroid hormones that regulate diverse processes, such as cell division and cell elongation, through gene regulatory networks that vary in space and time. By using time series single-cell RNA sequencing to profile brassinosteroid-responsive gene expression specific to different cell types and developmental stages of the Arabidopsis root, we identified the elongating cortex as a site where brassinosteroids trigger a shift from proliferation to elongation associated with increased expression of cell wall-related genes. Our analysis revealed HOMEOBOX FROM ARABIDOPSIS THALIANA 7 (HAT7) and GT-2-LIKE 1 (GTL1) as brassinosteroid-responsive transcription factors that regulate cortex cell elongation. These results establish the cortex as a site of brassinosteroid-mediated growth and unveil a brassinosteroid signaling network regulating the transition from proliferation to elongation, which illuminates aspects of spatiotemporal hormone responses.

Asymmetric cell division in plant development

Asymmetric cell division (ACD) is a fundamental process that generates new cell types during development in eukaryotic species. In plant development, post-embryonic organogenesis driven by ACD is universal and more important than in animals, in which organ pattern is preset during embryogenesis. Thus, plant development provides a powerful system to study molecular mechanisms underlying ACD. During the past decade, tremendous progress has been made in our understanding of the key components and mechanisms involved in this important process in plants. Here, we present an overview of how ACD is determined and regulated in multiple biological processes in plant development and compare their conservation and specificity among different model cell systems. We also summarize the molecular roles and mechanisms of the phytohormones in the regulation of plant ACD. Finally, we conclude with the overarching paradigms and principles that govern plant ACD and consider how new technologies can be exploited to fill the knowledge gaps and make new advances in the field.

New Paradigms in Brassinosteroids, Strigolactones, Sphingolipids, and Nitric Oxide Interaction in the Control of Lateral and Adventitious Root Formation

The root system is formed by the primary root (PR), which forms lateral roots (LRs) and, in some cases, adventitious roots (ARs), which in turn may produce their own LRs. The formation of ARs is also essential for vegetative propagation in planta and in vitro and for breeding programs. Root formation and branching is coordinated by a complex developmental network, which maximizes the plant's ability to cope with abiotic stress. Rooting is also a response caused in a cutting by wounding and disconnection from the donor plant. Brassinosteroids (BRs) are steroid molecules perceived at the cell surface. They act as plant-growth-regulators (PGRs) and modulate plant development to provide stress tolerance. BRs and auxins control the formation of LRs and ARs. The auxin/BR interaction involves other PGRs and compounds, such as nitric oxide (NO), strigolactones (SLs), and sphingolipids (SPLs). The roles of these interactions in root formation and plasticity are still to be discovered. SLs are carotenoid derived PGRs. SLs enhance/reduce LR/AR formation depending on species and culture conditions. These PGRs possibly crosstalk with BRs. SPLs form domains with sterols within cellular membranes. Both SLs and SPLs participate in plant development and stress responses. SPLs are determinant for auxin cell-trafficking, which is essential for the formation of LRs/ARs in planta and in in vitro systems. Although little is known about the transport, trafficking, and signaling of SPLs, they seem to interact with BRs and SLs in regulating root-system growth. Here, we review the literature on BRs as modulators of LR and AR formation, as well as their crosstalk with SLs and SPLs through NO signaling. Knowledge on the control of rooting by these non-classical PGRs can help in improving crop productivity and enhancing AR-response from cuttings.

Auxin promotes hypocotyl elongation by enhancing BZR1 nuclear accumulation in Arabidopsis

Auxin and brassinosteroids (BRs) are two major growth-promoting phytohormones that shape hypocotyl elongation; however, the cross-talk between auxin and BR in this process is not fully understood. In this study, we found that auxin-induced hypocotyl elongation is dependent on brassinazole-resistant 1 (BZR1), a core BR signaling component. Auxin promotes BZR1 nuclear accumulation in hypocotyl cells, a process dependent on mitogen-activated protein kinase 3 (MPK3) and MPK6, which are both activated by auxin and whose encoding genes are highly expressed in hypocotyls. We determined that MPK3/MPK6 phosphorylate and reduce the protein stability of general regulatory factor 4 (GRF4), a member of the 14-3-3 family of proteins that retain BZR1 in the cytoplasm. In summary, this study reveals the molecular mechanism by which auxin promotes hypocotyl elongation by enhancing BZR1 nuclear accumulation via MPK3/MPK6-regulated GRF4 protein stability.

Brassinosteroid signaling positively regulates abscisic acid biosynthesis in response to chilling stress in tomato

Brassinosteroids (BRs) and abscisic acid (ABA) are essential regulators of plant growth and stress tolerance. Although the antagonistic interaction of BRs and ABA is proposed to ensure the balance between growth and defense in model plants, the crosstalk between BRs and ABA in response to chilling in tomato (Solanum lycopersicum), a warm-climate horticultural crop, is unclear. Here, we determined that overexpression of the BR biosynthesis gene DWARF (DWF) or the key BR signaling gene BRASSINAZOLE-RESISTANT1 (BZR1) increases ABA levels in response to chilling stress via positively regulating the expression of the ABA biosynthesis gene 9-CIS-EPOXYCAROTENOID DIOXYGENASE1 (NCED1). BR-induced chilling tolerance was mostly dependent on ABA biosynthesis. Chilling stress or high BR levels decreased the abundance of BRASSINOSTEROID-INSENSITIVE2 (BIN2), a negative regulator of BR signaling. Moreover, we observed that chilling stress increases BR levels and results in the accumulation of BZR1. BIN2 negatively regulated both the accumulation of BZR1 protein and chilling tolerance by suppressing ABA biosynthesis. Our results demonstrate that BR signaling positively regulates chilling tolerance via ABA biosynthesis in tomato. The study has implications in production of warm-climate crops in horticulture.

24-epibrassinolide improves cadmium tolerance and lateral root growth associated with regulating endogenous auxin and ethylene in Kentucky bluegrass

The application of phytohormones is a viable technique to increase the efficiency of phytoremediation in heavy metal-contaminated soils. The objective of this study was to determine how the application of 24-epibrassinolide (EBR), a brassinosteroid analog, could regulate root growth and tolerance to cadmium (Cd) stress in Kentucky bluegrass. As a result, the number of lateral root primordia and total root length in the Cd-treated seedlings decreased by 33.1 % and 56.5 %, respectively. After the application of EBR, Cd accumulation in roots and leaves, and the negative effect of Cd on root growth were reduced under Cd stress. Additionally, the expression of the brassinosteroid signaling gene PpBRI1 was significantly upregulated by exogenous EBR. Moreover, exogenous EBR upregulated the expression of genes encoding antioxidant enzymes and improved the activity of antioxidant enzymes, thereby reduced oxidative stress in roots. Finally, targeted hormonomics analysis highlighted the utility of the application of EBR to alleviate the effect of Cd on the reduction in auxin (IAA) content and the increase in ethylene (ACC) content. These were known to be associated with the upregulation in the expression of auxin biosynthesis gene PpYUCCA1 and downregulation in the expression of ethylene biosynthesis gene PpACO1 in the roots treated with Cd stress. Overall, the application of EBR alleviated Cd-induced oxidative stress in addition to improving root elongation and lateral root growth crosstalk with auxin and ethylene in Kentucky bluegrass subjected to Cd stress. This study further highlights the potential role of brassinosteroids in improving the efficiency of phytoremediation for Cd-contaminated soils.

A role for brassinosteroid signalling in decision-making processes in the Arabidopsis seedling

Plants often adapt to adverse conditions via differential growth, whereby limited resources are discriminately allocated to optimize the growth of one organ at the expense of another. Little is known about the decision-making processes that underly differential growth. In this study, we developed a screen to identify decision making mutants by deploying two tools that have been used in decision theory: a well-defined yet limited budget, as well as conflict-of-interest scenarios. A forward genetic screen that combined light and water withdrawal was carried out. This identified BRASSINOSTEROID INSENSITIVE 2 (BIN2) alleles as decision mutants with "confused" phenotypes. An assessment of organ and cell length suggested that hypocotyl elongation occurred predominantly via cellular elongation. In contrast, root growth appeared to be regulated by a combination of cell division and cell elongation or exit from the meristem. Gain- or loss- of function bin2 mutants were most severely impaired in their ability to adjust cell geometry in the hypocotyl or cell elongation as a function of distance from the quiescent centre in the root tips. This study describes a novel paradigm for root growth under limiting conditions, which depends not only on hypocotyl-versus-root trade-offs in the allocation of limited resources, but also on an ability to deploy different strategies for root growth in response to multiple stress conditions.

Signaling events for photomorphogenic root development

A germinating seedling incorporates environmental signals such as light into developmental outputs. Light is not only a source of energy, but also a central coordinative signal in plants. Traditionally, most research focuses on aboveground organs' response to light; therefore, our understanding of photomorphogenesis in roots is relatively scarce. However, root development underground is highly responsive to light signals from the shoot and understanding these signaling mechanisms will give a better insight into early seedling development. Here, we review the central light signaling hubs and their role in root growth promotion of Arabidopsis thaliana seedlings.

Combined Approach of GWAS and Phylogenetic Analyses to Identify New Candidate Genes That Participate in Arabidopsis thaliana Primary Root Development Using Cellular Measurements and Primary Root Length

Genome-wide association studies (GWAS) have allowed the identification of different loci associated with primary root (PR) growth, and Arabidopsis is an excellent model for these studies. The PR length is controlled by cell proliferation, elongation, and differentiation; however, the specific contribution of proliferation and differentiation in the control of PR growth is still poorly studied. To this end, we analyzed 124 accessions and used a GWAS approach to identify potential causal genomic regions related to four traits: PR length, growth rate, cell proliferation and cell differentiation. Twenty-three genes and five statistically significant SNPs were identified. The SNP with the highest score mapped to the fifth exon of NAC048 and this change makes a missense variant in only 33.3% of the accessions with a large PR, compared with the accessions with a short PR length. Moreover, we detected five more SNPs in this gene and in NAC3 that allow us to discover closely related accessions according to the phylogenetic tree analysis. We also found that the association between genetic variants among the 18 genes with the highest scores in our GWAS and the phenotypic classes into which we divided our accessions are not straightforward and likely follow historical patterns.

Biological and Molecular Components for Genetically Engineering Biosensors in Plants

Plants adapt to their changing environments by sensing and responding to physical, biological, and chemical stimuli. Due to their sessile lifestyles, plants experience a vast array of external stimuli and selectively perceive and respond to specific signals. By repurposing the logic circuitry and biological and molecular components used by plants in nature, genetically encoded plant-based biosensors (GEPBs) have been developed by directing signal recognition mechanisms into carefully assembled outcomes that are easily detected. GEPBs allow for in vivo monitoring of biological processes in plants to facilitate basic studies of plant growth and development. GEPBs are also useful for environmental monitoring, plant abiotic and biotic stress management, and accelerating design-build-test-learn cycles of plant bioengineering. With the advent of synthetic biology, biological and molecular components derived from alternate natural organisms (e.g., microbes) and/or de novo parts have been used to build GEPBs. In this review, we summarize the framework for engineering different types of GEPBs. We then highlight representative validated biological components for building plant-based biosensors, along with various applications of plant-based biosensors in basic and applied plant science research. Finally, we discuss challenges and strategies for the identification and design of biological components for plant-based biosensors.

The Arabidopsis TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE (TTL) family members are involved in root system formation via their interaction with cytoskeleton and cell wall remodeling

Lateral roots (LR) are essential components of the plant edaphic interface; contributing to water and nutrient uptake, biotic and abiotic interactions, stress survival, and plant anchorage. We have identified the TETRATRICOPEPTIDE-REPEAT THIOREDOXIN-LIKE 3 (TTL3) gene as being related to LR emergence and later development. Loss of function of TTL3 leads to a reduced number of emerged LR due to delayed development of lateral root primordia (LRP). This trait is further enhanced in the triple mutant ttl1ttl3ttl4. TTL3 interacts with microtubules and endomembranes, and is known to participate in the brassinosteroid (BR) signaling pathway. Both ttl3 and ttl1ttl3ttl4 mutants are less sensitive to BR treatment in terms of LR formation and primary root growth. The ability of TTL3 to modulate biophysical properties of the cell wall was established under restrictive conditions of hyperosmotic stress and loss of root growth recovery, which was enhanced in ttl1ttl3ttl4. Timing and spatial distribution of TTL3 expression is consistent with its role in development of LRP before their emergence and subsequent growth of LR. TTL3 emerged as a component of the root system morphogenesis regulatory network.

The regeneration factors ERF114 and ERF115 regulate auxin-mediated lateral root development in response to mechanical cues

Plants show an unparalleled regenerative capacity, allowing them to survive severe stress conditions, such as injury, herbivory attack, and harsh weather conditions. This potential not only replenishes tissues and restores damaged organs but can also give rise to whole plant bodies. Despite the intertwined nature of development and regeneration, common upstream cues and signaling mechanisms are largely unknown. Here, we demonstrate that in addition to being activators of regeneration, ETHYLENE RESPONSE FACTOR 114 (ERF114) and ERF115 govern developmental growth in the absence of wounding or injury. Increased ERF114 and ERF115 activity enhances auxin sensitivity, which is correlated with enhanced xylem maturation and lateral root formation, whereas their knockout results in a decrease in lateral roots. Moreover, we provide evidence that mechanical cues contribute to ERF114 and ERF115 expression in correlation with BZR1-mediated brassinosteroid signaling under both regenerative and developmental conditions. Antagonistically, cell wall integrity surveillance via mechanosensory FERONIA signaling suppresses their expression under both conditions. Taken together, our data suggest a molecular framework in which cell wall signals and mechanical strains regulate organ development and regenerative responses via ERF114- and ERF115-mediated auxin signaling.

Inhibition of the Glycogen Synthase Kinase 3 Family by the Bikinin Alleviates the Long-Term Effects of Salinity in Barley

Crops grown under stress conditions show restricted growth and, eventually, reduced yield. Among others, brassinosteroids (BRs) mitigate the effects of stress and improve plant growth. We used two barley cultivars with differing sensitivities to BRs, as determined by the lamina joint inclination test. Barley plants with the 2nd unfolded leaf were sprayed with a diluted series of bikinin, an inhibitor of the Glycogen Synthase Kinase 3 (GSK3) family, which controls the BR signaling pathway. Barley was grown under salt stress conditions up to the start of the 5th leaf growth stage. The phenotypical, molecular, and physiological changes were determined. Our results indicate that the salt tolerance of barley depends on its sensitivity to BRs. We confirmed that barley treatment with bikinin reduced the level of the phosphorylated form of HvBZR1, the activity of which is regulated by GSK3. The use of two barley varieties with different responses to salinity led to the identification of the role of BR signaling in photosynthesis activity. These results suggest that salinity reduces the expression of the genes controlling the BR signaling pathway. Moreover, the results also suggest that the functional analysis of the GSK3 family in stress responses can be a tool for plant breeding in order to improve crops' resistance to salinity or to other stresses.

Brassinosteroid signaling restricts root lignification by antagonizing SHORT-ROOT function in Arabidopsis

Cell wall lignification is a key step in forming functional endodermis and protoxylem (PX) in plant roots. Lignified casparian strips (CS) in endodermis and tracheary elements of PX are essential for selective absorption and transport of water and nutrients. Although multiple key regulators of CS and PX have been identified, the spatial information that drives the developmental shift to root lignification remains unknown. Here, we found that brassinosteroid (BR) signaling plays a key role in inhibiting root lignification in the root elongation zone. The inhibitory activity of BR signaling occurs partially through the direct binding of BRASSINAZOLE-RESISTANT 1 (BZR1) to SHORT-ROOT (SHR), repressing the SHR-mediated activation of downstream genes that are involved in root lignification. Upon entering the mature root zone, BR signaling declines rapidly, which releases SHR activity and initiates root lignification. Our results provide a mechanistic view of the developmental transition to cell wall lignification in Arabidopsis thaliana roots.

Computational modeling and quantitative physiology reveal central parameters for brassinosteroid-regulated early cell physiological processes linked to elongation growth of the Arabidopsis root

Brassinosteroids (BR) are key hormonal regulators of plant development. However, whereas the individual components of BR perception and signaling are well characterized experimentally, the question of how they can act and whether they are sufficient to carry out the critical function of cellular elongation remains open. Here, we combined computational modeling with quantitative cell physiology to understand the dynamics of the plasma membrane (PM)-localized BR response pathway during the initiation of cellular responses in the epidermis of the Arabidopsis root tip that are be linked to cell elongation. The model, consisting of ordinary differential equations, comprises the BR-induced hyperpolarization of the PM, the acidification of the apoplast and subsequent cell wall swelling. We demonstrate that the competence of the root epidermal cells for the BR response predominantly depends on the amount and activity of H+-ATPases in the PM. The model further predicts that an influx of cations is required to compensate for the shift of positive charges caused by the apoplastic acidification. A potassium channel was subsequently identified and experimentally characterized, fulfilling this function. Thus, we established the landscape of components and parameters for physiological processes potentially linked to cell elongation, a central process in plant development.

Application of 2,4-Epibrassinolide Improves Drought Tolerance in Tobacco through Physiological and Biochemical Mechanisms

Drought stress is a major abiotic stress that hinders plant growth and development. Brassinosteroids (BR), including 2,4-epibrassinolide (EBR), play important roles in plant growth, development, and responses to abiotic stresses, including drought stress. This work investigates exogenous EBR application roles in improving drought tolerance in tobacco. Tobacco plants were divided into three groups: WW (well-watered), DS (drought stress), and DSB (drought stress + 0.05 mM EBR). The results revealed that DS decreased the leaf thickness (LT), whereas EBR application upregulated genes related to cell expansion, which were induced by the BR (DWF4, HERK2, and BZR1) and IAA (ARF9, ARF6, PIN1, SAUR19, and ABP1) signaling pathway. This promoted LT by 28%, increasing plant adaptation. Furthermore, EBR application improved SOD (22%), POD (11%), and CAT (5%) enzyme activities and their related genes expression (FeSOD, POD, and CAT) along with a higher accumulation of osmoregulatory substances such as proline (29%) and soluble sugars (14%) under DS and conferred drought tolerance. Finally, EBR application augmented the auxin (IAA) (21%) and brassinolide (131%) contents and upregulated genes related to drought tolerance induced by the BR (BRL3 and BZR2) and IAA (YUCCA6, SAUR32, and IAA26) signaling pathways. These results suggest that it could play an important role in improving mechanisms of drought tolerance in tobacco.

The interplay of auxin and brassinosteroid signaling tunes root growth under low and different nitrogen forms

The coordinated signaling activity of auxin and brassinosteroids (BRs) is critical for optimal plant growth and development. Nutrient-derived signals regulate root growth by modulating the levels and spatial distribution of growth hormones to optimize nutrient uptake and assimilation. However, the effect of the interaction of these two hormones and their signaling on root plasticity during low and differential availability of nitrogen (N) forms (NH4+/NO3-) remains elusive. We demonstrate that root elongation under low N (LN) is an outcome of the interdependent activity of auxin and BR signaling pathways in Arabidopsis (Arabidopsis thaliana). LN promotes root elongation by increasing BR-induced auxin transport activity in the roots. Increased nuclear auxin signaling and its transport efficiency have a distinct impact on root elongation under LN conditions. High auxin levels reversibly inhibit BR signaling via BRI1 KINASE INHIBITOR1. Using the tissue-specific approach, we show that BR signaling from root vasculature (stele) tissues is sufficient to promote cell elongation and, hence, root growth under LN condition. Further, we show that N form-defined root growth attenuation or enhancement depends on the fine balance of BR and auxin signaling activity. NH4+ as a sole N source represses BR signaling and response, which in turn inhibits auxin response and transport, whereas NO3- promotes root elongation in a BR signaling-dependent manner. In this study, we demonstrate the interplay of auxin and BR-derived signals, which are critical for root growth in a heterogeneous N environment and appear essential for root N foraging response and adaptation.

Tripartite hormonal regulation of plasma membrane H+-ATPase activity

The enzyme activity of the plasma membrane (PM) proton pump, well known as arabidopsis PM H⁺-ATPase (AHA) in the model plant arabidopsis (Arabidopsis thaliana), is controlled by phosphorylation. Three different classes of phytohormones, brassinosteroids (BRs), abscisic acid (ABA), and auxin regulate plant growth and responses to environmental stimuli, at least in part by modulating the activity of the pump through phosphorylation of the penultimate Thr residue in its carboxyl terminus. Here, we review the current knowledge regarding this tripartite hormonal AHA regulation and highlight mechanisms of activation and deactivation, as well as the significance of hormonal crosstalk. Understanding the complexity of PM H⁺-ATPase regulation in plants might provide new strategies for sustainable agriculture.

Regulation of tomato fruit elongation by transcription factor BZR1.7 through promotion of SUN gene expression

Fruit shape is an important biological trait that is also of special commercial value in tomato. The SUN gene has been known as a key regulator of tomato fruit elongation for years, but the molecular mechanisms underlying its transcriptional regulation remain little understood. Here, a unique BZR1-like transcription factor, BZR1.7, was identified as a trans-acting factor of the SUN gene promoter that bound to the conserved E-box of the promoter to promote SUN gene expression. Overexpression of BZR1.7 in tomato led to elevated SUN gene expression and formation of elongated fruits. Plants of the BZR1.7 knockout mutant created by gene editing did not exhibit an observable fruit shape phenotype, suggesting possible functional redundancy of BZR1-like genes in tomato. There were seven BZR1-like genes in the tomato genome and overexpression of BZR1.5 and BZR1.6 led to elongated fruit phenotypes similar to those observed in the BZR1.7 overexpression lines, further supporting the notion of functional redundancy of BZR1-like genes in tomato fruit shape specification. Microscopic analysis revealed that there was a decreased number of cell layers in the fruit pericarp in the BZR1.7 overexpression lines. These findings offer new insights into the regulatory mechanism by which BZR1.7 promotes SUN gene expression and regulates fruit elongation in tomato.

Deubiquitinating enzymes UBP12 and UBP13 stabilize the brassinosteroid receptor BRI1

Protein ubiquitination is a dynamic and reversible post-translational modification that controls diverse cellular processes in eukaryotes. Ubiquitin-dependent internalization, recycling, and degradation are important mechanisms that regulate the activity and the abundance of plasma membrane (PM)-localized proteins. In plants, although several ubiquitin ligases are implicated in these processes, no deubiquitinating enzymes (DUBs), have been identified that directly remove ubiquitin from membrane proteins and limit their vacuolar degradation. Here, we discover two DUB proteins, UBP12 and UBP13, that directly target the PM-localized brassinosteroid (BR) receptor BR INSENSITIVE1 (BRI1) in Arabidopsis. BRI1 protein abundance is decreased in the ubp12i/ubp13 double mutant that displayed severe growth defects and reduced sensitivity to BRs. UBP13 directly interacts with and effectively removes K63-linked polyubiquitin chains from BRI1, thereby negatively modulating its vacuolar targeting and degradation. Our study reveals that UBP12 and UBP13 play crucial roles in governing BRI1 abundance and BR signaling activity to regulate plant growth.

Brassinosteroids are required for efficient root tip regeneration in Arabidopsis

Compared with other organisms, plants have an extraordinary capacity for self-repair. Even if the entire tissues, including the stem cells, are resected, most plant species are able to completely regenerate whole tissues. However, the mechanism by which plants efficiently regenerate the stem cell niche during tissue reorganization is still largely unknown. Here, we found that the signaling mediated by plant steroid hormones brassinosteroids is activated during stem cell formation after root tip excision in Arabidopsis. Treatment with brassinazole, an inhibitor of brassinosteroid biosynthesis, delayed the recovery of stem cell niche after root tip excision. Regeneration of root tip after resection was also delayed in a brassinosteroid receptor mutant. Therefore, we propose that brassinosteroids participate in efficient root tip regeneration, thereby enabling efficient tissue regeneration to ensure continuous root growth after resection.

ROPGAP-dependent interaction between brassinosteroid and ROP2-GTPase signaling controls pavement cell shape in Arabidopsis

The epidermal pavement cell shape in Arabidopsis is driven by chemical and mechanical cues that direct partitioning mechanisms required for the establishment of the lobe- and indentation-defining polar sites. Brassinosteroid (BR) hormones regulate pavement cell morphogenesis, but the underlying mechanism remains unclear. Here, we identified two PLECKSTRIN HOMOLOGY GTPase-ACTIVATING proteins (PHGAPs) as substrates of the GSK3-like kinase BR-INSENSITIVE2 (BIN2). The phgap1phgap2 mutant displayed severe epidermal cell shape phenotypes, and the PHGAPs were markedly enriched in the anticlinal face of the pavement cell indenting regions. BIN2 phosphorylation of PHGAPs was required for their stability and polarization. BIN2 inhibition activated ROP2-GTPase signaling specifically in the lobes because of PHGAP degradation, while the PHGAPs restrained ROP2 activity in the indentations. Hence, we connect BR and ROP2-GTPase signaling pathways via the regulation of PHGAPs and put forward the importance of spatiotemporal control of BR signaling for pavement cell interdigitation.

Spatial differences in stoichiometry of EGR phosphatase and Microtubule-associated Stress Protein 1 control root meristem activity during drought stress

During moderate severity drought and low water potential (ψw) stress, poorly understood signaling mechanisms restrict both meristem cell division and subsequent cell expansion. We found that the Arabidopsis thaliana Clade E Growth-Regulating 2 (EGR2) protein phosphatase and Microtubule-Associated Stress Protein 1 (MASP1) differed in their stoichiometry of protein accumulation across the root meristem and had opposing effects on root meristem activity at low ψw. Ectopic MASP1 or EGR expression increased or decreased, respectively, root meristem size and root elongation during low ψw stress. This, along with the ability of phosphomimic MASP1 to overcome the EGR-mediated suppression of root meristem size and the observation that ectopic EGR expression had no effect on unstressed plants, indicated that during low ψw EGR activation and attenuation of MASP1 phosphorylation in their overlapping zone of expression determines root meristem size and activity. Ectopic EGR expression also decreased root cell size at low ψw. Conversely, both the egr1-1 egr2-1 and egr1-1 egr2-1 masp1-1 mutants had similarly increased root cell size but only egr1-1egr2-1 had increased cell division. These observations demonstrated that EGRs affect meristem activity via MASP1 but affect cell expansion via other mechanisms. Interestingly, EGR2 was highly expressed in the root cortex, a cell type important for growth regulation and environmental response.

Transition Zone1 Negatively Regulates Arabidopsis Aluminum Resistance Through Interaction With Aconitases

The soluble form of aluminum (Al) is a major constraint to crop production in acidic soils. The Al exclusion correlated with the Al-induced organic acid is considered as an important mechanism of Al resistance. The regulation of organic acid exudation in response to Al stress mediated by the root organic acid transporters has been extensively studied. However, how plants respond to Al stress through the regulation of organic acid homeostasis is not well understood. In this study, we identified the functionally unknown Transition zone1 (TZ1) as an Al-inducible gene in the root transition zone, the most sensitive region to Al stress, in Arabidopsis. tz1 mutants showed enhanced Al resistance and displayed greatly reduced root growth inhibition. Furthermore, TZ1 was found to interact with the aconitases (ACOs) which can catalyze the conversion from citrate, one of the most important organic acids, into isocitrate. Consistently, in tz1 mutants, the citric acid content was highly increased. Collectively, this study provides evidence to show that TZ1 negatively regulates root growth response to Al stress through interacting with ACOs and regulating citric acid homeostasis.