Plant hormones and signaling peptides
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Frontiers | Auxin-BR Interaction Regulates Plant Growth and Development | Plant Science

Frontiers | Auxin-BR Interaction Regulates Plant Growth and Development | Plant Science | Plant hormones and  signaling peptides | Scoop.it
Plants develop a high flexibility to alter growth, development, and metabolism to adapt to the ever-changing environments. Multiple signaling pathways are involved in these processes and the molecular pathways to transduce various developmental signals are not linear but are interconnected by a complex network and even feedback mutually to achieve the final outcome. This review will focus on two important plant hormones, auxin and brassinosteroid (BR), based on the most recent progresses about these two hormone regulated plant growth and development in Arabidopsis, and highlight the cross-talks between these two phytohormones.
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SUMO Suppresses the Activity of the Jasmonic Acid Receptor CORONATINE INSENSITIVE1

SUMO Suppresses the Activity of the Jasmonic Acid Receptor CORONATINE INSENSITIVE1 | Plant hormones and  signaling peptides | Scoop.it
Plants respond rapidly to sudden environmental cues, often responding prior to changes in the hormone levels that coordinate these responses. How this is achieved is not fully understood. The integrative role of the phytohormone jasmonic acid (JA) relies upon the plant’s ability to control the levels of JASMONATE ZIM (JAZ) domain-containing repressor proteins. Here, we demonstrate that regardless of intrinsic JA levels, Small Ubiquitin-like Modifier (SUMO)-conjugated JAZ proteins inhibit the JA receptor CORONATINE INSENSITIVE1 (COI1) from mediating non-SUMOylated JAZ degradation. The SUMO-deconjugating proteases OVERLY TOLERANT TO SALT1 (OTS1) and OTS2 regulate JAZ protein SUMOylation and stability. The ots1 ots2 double mutants accumulate SUMOylated and non-SUMOylated JAZ repressor proteins but show no change in endogenous JA levels compared with wild-type plants. SUMO1-conjugated JAZ proteins bind to COI1 independently of the JA mimic coronatine. SUMO inhibits JAZ binding to COI1. We identify the SUMO interacting motif in COI1 and demonstrate that this is vital to SUMO-dependent inhibition of COI1. Necrotroph infection of Arabidopsis thaliana promotes SUMO protease degradation, and this increases JAZ SUMOylation and abundance, which in turn inhibits JA signaling. This study reveals a mechanism for rapidly regulating JA responses, allowing plants to adapt to environmental changes.
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It starts with TIRs

It starts with TIRs | Plant hormones and  signaling peptides | Scoop.it
The canonical auxin receptor complex mediates gene expression, but it is also necessary for responses far too rapid to be mediated by transcription. An innovative setup that uses advanced microscopy and microfluidics can record auxin-induced changes within 30 seconds during root growth.
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Canonical and noncanonical ethylene signaling pathways that regulate Arabidopsis susceptibility to the cyst nematode Heterodera schachtii - Piya - - New Phytologist -

Canonical and noncanonical ethylene signaling pathways that regulate Arabidopsis susceptibility to the cyst nematode Heterodera schachtii - Piya - - New Phytologist - | Plant hormones and  signaling peptides | Scoop.it
Plant‐parasitic cyst nematodes successfully exploit various phytohormone signaling pathways to establish a new hormonal equilibrium that facilitates nematode parasitism. Although it is largely accepted that ethylene regulates plant responses to nematode infection, a mechanistic understanding of how ethylene shapes plant–nematode interactions remains largely unknown.
In this study, we examined the involvement of various components regulating ethylene perception and signaling in establishing Arabidopsis susceptibility to the cyst nematode Heterodera schachtii using a large set of well‐characterized single and higher order mutants.
Our analyses revealed the existence of two pathways that separately engage ethylene with salicylic acid (SA) and cytokinin signaling during plant response to nematode infection. One pathway involves the canonical ethylene signaling pathway in which activation of ethylene signaling results in suppression of SA‐based immunity. The second pathway involves the ethylene receptor ETR1, which signals independently of SA acid to affect immunity, instead altering cytokinin‐mediated regulation of downstream components.
Our results reveal important mechanisms through which cyst nematodes exploit components of ethylene perception and signaling to affect the balance of hormonal signaling through ethylene interaction with SA and cytokinin networks. This hormonal interaction overcomes plant defense and provokes a susceptible response.
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A novel tomato F‐box protein, SlEBF3, is involved in tuning ethylene signaling during plant development and climacteric fruit ripening - Deng - 2018 - The Plant Journal

A novel tomato F‐box protein, SlEBF3, is involved in tuning ethylene signaling during plant development and climacteric fruit ripening - Deng - 2018 - The Plant Journal | Plant hormones and  signaling peptides | Scoop.it
Ethylene is instrumental to climacteric fruit ripening and EIN3 BINDING F‐BOX (EBF) proteins have been assigned a central role in mediating ethylene responses by regulating EIN3/EIL degradation in Arabidopsis. However, the role and mode of action of tomato EBFs in ethylene‐dependent processes like fruit ripening remains unclear. Two novel EBF genes, SlEBF3 and SlEBF4, were identified in the tomato genome, and SlEBF3 displayed a ripening‐associated expression pattern suggesting its potential involvement in controlling ethylene response during fruit ripening. SlEBF3 downregulated tomato lines failed to show obvious ripening‐related phenotypes likely due to functional redundancy among SlEBF family members. By contrast, SlEBF3 overexpression lines exhibited pleiotropic ethylene‐related alterations, including inhibition of fruit ripening, attenuated triple‐response and delayed petal abscission. Yeast‐two‐hybrid system and bimolecular fluorescence complementation approaches indicated that SlEBF3 interacts with all known tomato SlEIL proteins and, consistently, total SlEIL protein levels were decreased in SlEBF3 overexpression fruits, supporting the idea that the reduced ethylene sensitivity and defects in fruit ripening are due to the SlEBF3‐mediated degradation of EIL proteins. Moreover, SlEBF3 expression is regulated by EIL1 via a feedback loop, which supposes its role in tuning ethylene signaling and responses. Overall, the study reveals the role of a novel EBF tomato gene in climacteric ripening, thus providing a new target for modulating fleshy fruit ripening.
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Abscisic acid negatively modulates plant defence against rice black‐streaked dwarf virus infection by suppressing the jasmonate pathway and regulating reactive oxygen species levels in rice - Xie -...

Abscisic acid negatively modulates plant defence against rice black‐streaked dwarf virus infection by suppressing the jasmonate pathway and regulating reactive oxygen species levels in rice - Xie -... | Plant hormones and  signaling peptides | Scoop.it
Abscisic acid (ABA) plays a multifaceted role in plant immunity and can either increase resistance or increase susceptibility to some bacterial and fungal pathogens depending on the pathosystem. ABA is also known to mediate plant defence to some viruses. In this study, the relationship between the ABA pathway and rice black‐streaked dwarf virus (RBSDV) was investigated in rice. The expression of ABA pathway genes was significantly reduced upon RBSDV infection. Application of exogenous hormones and various ABA pathway mutants revealed that the ABA pathway plays a negative role in rice defence against RBSDV. Exogenous hormone treatment and virus inoculation showed that ABA inhibits the jasmonate‐mediated resistance to RBSDV. ABA treatment also suppressed accumulation of reactive oxygen species by inducing the expression of superoxidase dismutases and catalases. Thus, ABA modulates the rice–RBSDV interaction by suppressing the jasmonate pathway and regulating reactive oxygen species levels. This is the first example of ABA increasing susceptibility to a plant virus.
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Broad spectrum developmental role of Brachypodium AUX1 - Schuren - 2018 - New Phytologist -

Broad spectrum developmental role of Brachypodium AUX1 - Schuren - 2018 - New Phytologist - | Plant hormones and  signaling peptides | Scoop.it
Targeted cellular auxin distribution is required for morphogenesis and adaptive responses of plant organs. In Arabidopsis thaliana (Arabidopsis), this involves the prototypical auxin influx facilitator AUX1 and its LIKE‐AUX1 (LAX) homologs, which act partially redundantly in various developmental processes. Interestingly, AUX1 and its homologs are not strictly essential for the Arabidopsis life cycle. Indeed, aux1 lax1 lax2 lax3 quadruple knock‐outs are mostly viable and fertile, and strong phenotypes are only observed at low penetrance.
Here we investigated the Brachypodium distachyon (Brachypodium) AUX1 homolog BdAUX1 by genetic, cell biological and physiological analyses.
We report that BdAUX1 is essential for Brachypodium development. Bdaux1 loss‐of‐function mutants are dwarfs with aberrant flower development, and consequently infertile. Moreover, they display a counter‐intuitive root phenotype. Although Bdaux1 roots are agravitropic as expected, in contrast to Arabidopsis aux1 mutants they are dramatically longer than wild type roots because of exaggerated cell elongation. Interestingly, this correlates with higher free auxin content in Bdaux1 roots. Consistently, their cell wall characteristics and transcriptome signature largely phenocopy other Brachypodium mutants with increased root auxin content.
Our results imply fundamentally different wiring of auxin transport in Brachypodium roots and reveal an essential role of BdAUX1 in a broad spectrum of developmental processes, suggesting a central role for AUX1 in pooideae.
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Plant Hormonomics: Multiple Phytohormone Profiling by Targeted Metabolomics

Plant Hormonomics: Multiple Phytohormone Profiling by Targeted Metabolomics | Plant hormones and  signaling peptides | Scoop.it
Phytohormones are physiologically important small molecules that play essential roles in intricate signaling networks that regulate diverse processes in plants. We present a method for the simultaneous targeted profiling of 101 phytohormone-related analytes from minute amounts of fresh plant material (less than 20 mg). Rapid and nonselective extraction, fast one-step sample purification, and extremely sensitive ultra-high-performance liquid chromatography-tandem mass spectrometry enable concurrent quantification of the main phytohormone classes: cytokinins, auxins, brassinosteroids, gibberellins, jasmonates, salicylates, and abscisates. We validated this hormonomic approach in salt-stressed and control Arabidopsis (Arabidopsis thaliana) seedlings, quantifying a total of 43 endogenous compounds in both root and shoot samples. Subsequent multivariate statistical data processing and cross-validation with transcriptomic data highlighted the main hormone metabolites involved in plant adaptation to salt stress.
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The SERK3 elongated allele defines a role for BIR ectodomains in brassinosteroid signalling

The SERK3 elongated allele defines a role for BIR ectodomains in brassinosteroid signalling | Plant hormones and  signaling peptides | Scoop.it
The leucine-rich repeat receptor kinase (LRR-RK) BRASSINOSTEROID INSENSITIVE 1 (BRI1) requires a shape-complementary SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) co-receptor for brassinosteroid sensing and receptor activation1. Interface mutations that weaken the interaction between receptor and co-receptor in vitro reduce brassinosteroid signalling responses2. The SERK3 elongated (elg) allele3,4,5 maps to the complex interface and shows enhanced brassinosteroid signalling, but surprisingly no tighter binding to the BRI1 ectodomain in vitro. Here, we report that rather than promoting the interaction with BRI1, the elg mutation disrupts the ability of the co-receptor to interact with the ectodomains of BRI1-ASSOCIATED-KINASE1 INTERACTING KINASE (BIR) receptor pseudokinases, negative regulators of LRR-RK signalling6. A conserved lateral surface patch in BIR LRR domains is required for targeting SERK co-receptors and the elg allele maps to the core of the complex interface in a 1.25 Å BIR3–SERK1 structure. Collectively, our structural, quantitative biochemical and genetic analyses suggest that brassinosteroid signalling complex formation is negatively regulated by BIR receptor ectodomains.
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Suppression of DELLA signaling induces procambial cell formation in culture - Yamazaki - 2018 - The Plant Journal -

Suppression of DELLA signaling induces procambial cell formation in culture - Yamazaki - 2018 - The Plant Journal - | Plant hormones and  signaling peptides | Scoop.it
The post‐embryonic growth of plants requires the activities of apical meristems and lateral meristems. In the meristems, self‐proliferation and differentiation of stem cells is tightly modulated by plant hormone signaling networks and specific transcription factors. Despite extensive studies on stem cell maintenance in plants, the mechanism by which stem cells are initially established is largely unknown. Vascular stem cells consisting of procambial/cambial cells give rise to xylem and phloem cells. In this study, we analyzed the establishment of procambial cells using the in vitro culture system VISUAL, in which mesophyll cells rapidly differentiate into xylem tracheary elements and phloem sieve elements via procambial cells. We found that procambial cell formation in VISUAL is initiated by light, which can be replaced by application of gibberellin (GA). Gibberellin was able to promote procambial cell formation through degradation of DELLA, whereas light did not elevate the endogenous GA content. Indeed, light in combination with bikinin reduced the accumulation of DELLA protein in VISUAL. Consistently, overexpression of a constitutively active DELLA protein repressed vascular cell differentiation even under light. These combined results suggest that DELLA signaling suppresses procambial cell formation during vascular development in VISUAL.
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A Bypass in Jasmonate Biosynthesis – the OPR3-independent Formation

A Bypass in Jasmonate Biosynthesis – the OPR3-independent Formation | Plant hormones and  signaling peptides | Scoop.it
For the first time in 25 years, a new pathway for biosynthesis of jasmonic acid (JA) has been identified. JA production takes place via 12-oxo-phytodienoic acid (OPDA) including reduction by OPDA reductases (OPRs). A loss-of-function allele, opr3-3, revealed an OPR3-independent pathway converting OPDA to JA.
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Frontiers | Strigolactone Levels in Dicot Roots Are Determined by an Ancestral Symbiosis-Regulated Clade of the PHYTOENE SYNTHASE Gene Family | Plant Science

Frontiers | Strigolactone Levels in Dicot Roots Are Determined by an Ancestral Symbiosis-Regulated Clade of the PHYTOENE SYNTHASE Gene Family | Plant Science | Plant hormones and  signaling peptides | Scoop.it
Strigolactones (SLs) are apocarotenoid phytohormones synthesized from carotenoid precursors. They are produced most abundantly in roots for exudation into the rhizosphere to cope with mineral nutrient starvation through support of root symbionts. Abscisic acid (ABA) is another apocarotenoid phytohormone synthesized in roots, which is involved in responses to abiotic stress. Typically low carotenoid levels in roots raise the issue of precursor supply for the biosynthesis of these two apocarotenoids in this organ. Increased ABA levels upon abiotic stress in Poaceae roots are known to be supported by a particular isoform of phytoene synthase (PSY), catalyzing the rate-limiting step in carotenogenesis. Here we report on novel PSY3 isogenes from Medicago truncatula (MtPSY3) and Solanum lycopersicum (SlPSY3) strongly expressed exclusively upon root interaction with symbiotic arbuscular mycorrhizal (AM) fungi and moderately in response to phosphate starvation. They belong to a widespread clade of conserved PSYs restricted to dicots (dPSY3) distinct from the Poaceae-PSY3s involved in ABA formation. An ancient origin of dPSY3s and a potential co-evolution with the AM symbiosis is discussed in the context of PSY evolution. Knockdown of MtPSY3 in hairy roots of M. truncatula strongly reduced SL and AM-induced C13 α-ionol/C14 mycorradicin apocarotenoids. Inhibition of the reaction subsequent to phytoene synthesis revealed strongly elevated levels of phytoene indicating induced flux through the carotenoid pathway in roots upon mycorrhization. dPSY3 isogenes are coregulated with upstream isogenes and downstream carotenoid cleavage steps toward SLs (D27, CCD7, CCD8) suggesting a combined carotenoid/apocarotenoid pathway, which provides “just in time”-delivery of precursors for apocarotenoid formation.
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Frontiers | Ethylene and 1-Aminocyclopropane-1-carboxylate (ACC) in Plant–Bacterial Interactions | Plant Science

Frontiers | Ethylene and 1-Aminocyclopropane-1-carboxylate (ACC) in Plant–Bacterial Interactions | Plant Science | Plant hormones and  signaling peptides | Scoop.it
Ethylene and its precursor 1-aminocyclopropane-1-carboxylate (ACC) actively participate in plant developmental, defense and symbiotic programs. In this sense, ethylene and ACC play a central role in the regulation of bacterial colonization (rhizospheric, endophytic, and phyllospheric) by the modulation of plant immune responses and symbiotic programs, as well as by modulating several developmental processes, such as root elongation. Plant-associated bacterial communities impact plant growth and development, both negatively (pathogens) and positively (plant-growth promoting and symbiotic bacteria). Some members of the plant-associated bacterial community possess the ability to modulate plant ACC and ethylene levels and, subsequently, modify plant defense responses, symbiotic programs and overall plant development. In this work, we review and discuss the role of ethylene and ACC in several aspects of plant-bacterial interactions. Understanding the impact of ethylene and ACC in both the plant host and its associated bacterial community is key to the development of new strategies aimed at increased plant growth and protection.
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AUXIN RESPONSE FACTOR3 Regulates Floral Meristem Determinacy by Repressing Cytokinin Biosynthesis and Signaling

AUXIN RESPONSE FACTOR3 Regulates Floral Meristem Determinacy by Repressing Cytokinin Biosynthesis and Signaling | Plant hormones and  signaling peptides | Scoop.it
Successful floral meristem (FM) determinacy is critical for subsequent reproductive development and the plant life cycle. Although the phytohormones cytokinin and auxin interact to coregulate many aspects of plant development, whether and how cytokinin and auxin function in FM determinacy remain unclear. Here, we show that in Arabidopsis thaliana, cytokinin homeostasis is critical for FM determinacy. In this developmental context, auxin promotes the expression of AUXIN RESPONSE FACTOR3 (ARF3) to repress cytokinin activity. ARF3 directly represses the expression of ISOPENTENYLTRANSFERASE (IPT) family genes and indirectly represses LONELY GUY (LOG) family genes, both of which encode enzymes required for cytokinin biosynthesis. ARF3 also directly inhibits the expression of ARABIDOPSIS HISTIDINE KINASE4, a cytokinin receptor gene, resulting in reduced cytokinin activity. Consequently, ARF3 controls cell division by regulating cell cycle gene expression through cytokinin. In flowers, we show that AGAMOUS (AG) dynamically regulates the expression of ARF3 and IPTs, resulting in coordinated regulation of FM maintenance and termination through cell division. Moreover, genome-wide transcriptional profiling revealed both repressive and active roles for ARF3 in early flower development. Our findings establish a molecular link between AG and auxin/cytokinin and shed light on the mechanisms of stem cell maintenance and termination in the FM.
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Integrated Regulation of Apical Hook Development by Transcriptional Coupling of EIN3/EIL1 and PIFs in Arabidopsis

Integrated Regulation of Apical Hook Development by Transcriptional Coupling of EIN3/EIL1 and PIFs in Arabidopsis | Plant hormones and  signaling peptides | Scoop.it
The apical hook protects the meristems of dicot seedlings as they protrude through the soil; multiple factors, including phytohormones and light, mediate apical hook development. HOOKLESS1 (HLS1) plays an indispensable role, as HLS1 mutations cause a hookless phenotype. The ETHYLENE INSENSITIVE3 (EIN3) and EIN3-LIKE1 (EIL1) transcription factors integrate multiple signals (ethylene, gibberellins, and jasmonate) and activate HLS1 expression to enhance hook development. Here, we found that Arabidopsis thaliana PHYTOCHROME INTERACTING FACTOR (PIF) transcription factors act in parallel with EIN3/EIL1 and promote hook curvature by activating HLS1 transcription at a distinct binding motif. EIN3/EIL1 and PIFs can promote hook formation in the absence of the other. Jasmonate represses PIF function to inhibit hook development. Like EIN3 and EIL1, MYC2 interacts with PIF4 and hampers its activity. Acting together, EIN3/EIL1 and PIFs alleviate the negative effects of jasmonate/light and facilitate the positive effects of ethylene/gibberellins. Mutating EIN3/EIL1 and PIFs causes a complete hookless phenotype, marginal HLS1 expression, and insensitivity to upstream signals. Transcriptome profiling revealed that EIN3/EIL1 and PIFs additively and distinctly regulate a wide array of processes, including apical hook development. Together, our findings identify an integrated framework underlying the regulation of apical hook development and show that EIN3/EIL1 and PIFs fine-tune adaptive growth in response to hormone and light signals.
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Frontiers | Salicylic Acid: A Double-Edged Sword for Programed Cell Death in Plants | Plant Science

Frontiers | Salicylic Acid: A Double-Edged Sword for Programed Cell Death in Plants | Plant Science | Plant hormones and  signaling peptides | Scoop.it
In plants, salicylic acid (SA) plays important roles in regulating immunity and programed cell death. Early studies revealed that increased SA accumulation is associated with the onset of hypersensitive reaction during resistance gene-mediated defense responses. SA was also found to accumulate to high levels in lesion-mimic mutants and in some cases the accumulation of SA is required for the spontaneous cell death phenotype. Meanwhile, high levels of SA have been shown to negatively regulate plant cell death during effector-triggered immunity, suggesting that SA has dual functions in cell death control. The molecular mechanisms of how SA regulates cell death in plants are discussed.
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Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future | Molecular Plant-Microbe Interactions

Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future | Molecular Plant-Microbe Interactions | Plant hormones and  signaling peptides | Scoop.it
Salicylic acid (SA) is a critical plant hormone that regulates numerous aspects of plant growth and development as well as the activation of defenses against biotic and abiotic stress. Here, we present a historical overview of the progress that has been made to date in elucidating the role of SA in signaling plant immune responses. The ability of plants to develop acquired immunity after pathogen infection was first proposed in 1933. However, most of our knowledge about plant immune signaling was generated over the last three decades, following the discovery that SA is an endogenous defense signal. During this timeframe, researchers have identified i) two pathways through which SA can be synthesized, ii) numerous proteins that regulate SA synthesis and metabolism, and iii) some of the signaling components that function downstream of SA, including a large number of SA targets or receptors. In addition, it has become increasingly evident that SA does not signal immune responses by itself but, rather, as part of an intricate network that involves many other plant hormones. Future efforts to develop a comprehensive understanding of SA-mediated immune signaling will therefore need to close knowledge gaps that exist within the SA pathway itself as well as clarify how crosstalk among the different hormone signaling pathways leads to an immune response that is both robust and optimized for maximal efficacy, depending on the identity of the attacking pathogen.
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Ethylene signaling is critical for synergid cell functional specification and pollen tube attraction - Zhang - 2018 - The Plant Journal -

Ethylene signaling is critical for synergid cell functional specification and pollen tube attraction - Zhang - 2018 - The Plant Journal - | Plant hormones and  signaling peptides | Scoop.it
ETHYLENE INSENSITIVE 3 (EIN3) is a key regulator of ethylene signaling, and EIN3‐BINDING F‐BOX1 (EBF1) and EBF2 are responsible for EIN3 degradation. Previous reports have shown that the ebf1 ebf2 double homozygous mutant cannot be identified. In this study, the genetic analysis revealed that the ebf1 ebf2 female gametophyte is defective. The pollination experiment showed that ebf1 ebf2 ovules failed to attract pollen tubes. In female gametophyte/ovule, the synergid cell is responsible for pollen tube attraction. Observation of the pEIN3::EIN3‐GFP transgenic lines showed that EIN3 signal was over‐accumulated at the micropylar end of ebf1 ebf2 female gametophyte. The overexpression of stabilized EIN3 in synergid cell led to the defect of pollen tube guidance. These results suggested that the over‐accumulated EIN3 in ebf1 ebf2 synergid cell blocks its pollen tube attraction which leads to the failure of ebf1 ebf2 homozygous plant. We identified that EIN3 directly activated the expression of a sugar transporter, SENESCENCE‐ASSOCIATED GENE29 (SAG29/SWEET15). Overexpression of SAG29 in synergid cells blocked pollen tube attraction, suggesting that SAG29 might play a role in ethylene signaling to repel pollen tube entry. Taken together, our study reveals that strict control of ethylene signaling is critical for the synergid cell function during plant reproduction.
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A Molecular Framework for Auxin-Controlled Homeostasis of Shoot Stem Cells in Arabidopsis - ScienceDirect

A Molecular Framework for Auxin-Controlled Homeostasis of Shoot Stem Cells in Arabidopsis - ScienceDirect | Plant hormones and  signaling peptides | Scoop.it
The classic phytohormone auxin plays an essential role in priming meristematic cell differentiation in the shoot apical meristem to promote lateral organ initiation. Recently, several lines of evidence have suggested that auxin is not only transported to new primordia but also descends to the stem cells in the central zone. However, the function of auxin in stem cell regulation has remained elusive. Here, we show that auxin signaling in stem cells is mediated, at least in part, by AUXIN RESPONSE FACTOR 5/MONOPTEROS (ARF5/MP), which directly represses the transcription of DORNROSCHEN/ENHANCER OF SHOOT REGENERATION 1 (DRN/ESR1). DRN expressed in stem cells positively regulates CLAVATA3 (CLV3) expression and has important meristematic functions. Our results provide a mechanistic framework for auxin control of shoot stem cell homeostasis and demonstrate how auxin differentially controls plant stem cell maintenance and differentiation.
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Cytokinin Targets Auxin Transport to Promote Shoot Branching

Cytokinin Targets Auxin Transport to Promote Shoot Branching | Plant hormones and  signaling peptides | Scoop.it
Cytokinin promotes shoot branching by activating axillary buds, but its mechanism of action in Arabidopsis (Arabidopsis thaliana) in this process is unclear. We have shown previously that a hextuple mutant lacking a clade of type-A Arabidopsis Response Regulators (ARRs) known to act in cytokinin signaling has reduced shoot branching compared with the wild type. Since these proteins typically act as negative regulators of cytokinin signaling, this is an unexpected result. To explore this paradox more deeply, we characterized the effects of loss of function of the type-B ARR, ARR1, which positively regulates cytokinin-induced gene expression. The arr1 mutant has increased branching, consistent with a role antagonistic to the type-A ARRs but in apparent conflict with the known positive role for cytokinin in bud activation. We show that the arr branching phenotypes correlate with increases in stem auxin transport and steady-state levels of the auxin export proteins PIN3 and PIN7 on the plasma membrane of xylem-associated cells in the main stem. Cytokinin treatment results in an increased accumulation of PIN3, PIN7, and the closely related PIN4 within several hours, and loss of PIN3, PIN4, and PIN7 can partially rescue the arr1 branching phenotype. This suggests that there are multiple signaling pathways for cytokinin in bud outgrowth; one of these pathways regulates PIN proteins in shoots, independently of the canonical signaling function of the ARR genes tested here. A hypothesis consistent with the arr shoot phenotypes is that feedback control of biosynthesis leads to altered cytokinin accumulation, driving cytokinin signaling via this pathway.
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The tomato MAX1 homolog, SlMAX1, is involved in the biosynthesis of tomato strigolactones from carlactone - Zhang - 2018 - New Phytologist -

The tomato MAX1 homolog, SlMAX1, is involved in the biosynthesis of tomato strigolactones from carlactone - Zhang - 2018 - New Phytologist - | Plant hormones and  signaling peptides | Scoop.it
Strigolactones (SLs) are rhizosphere signalling molecules exuded by plants that induce seed germination of root parasitic weeds and hyphal branching of arbuscular mycorrhiza. They are also phytohormones regulating plant architecture. MORE AXILLARY GROWTH 1 (MAX1) and its homologs encode cytochrome P450 (CYP) enzymes that catalyse the conversion of the strigolactone precursor carlactone to canonical strigolactones in rice (Oryza sativa), and to an SL‐like compound in Arabidopsis. Here, we characterized the tomato (Solanum lycopersicum) MAX1 homolog, SlMAX1.
The targeting induced local lesions in genomes method was used to obtain Slmax1 mutants that exhibit strongly reduced production of orobanchol, solanacol and didehydro‐orobanchol (DDH) isomers. This results in a severe strigolactone mutant phenotype in vegetative and reproductive development.
Transient expression of SlMAX1 – together with SlD27, SlCCD7 and SlCCD8 – in Nicotiana benthamiana showed that SlMAX1 catalyses the formation of carlactonoic acid from carlactone.
Plant feeding assays showed that carlactone, but not 4‐deoxy‐orobanchol, is the precursor of orobanchol, which in turn is the precursor of solanacol and two of the three DDH isomers. Inhibitor studies suggest that a 2‐oxoglutarate‐dependent dioxygenase is involved in orobanchol biosynthesis from carlactone and that the formation of solanacol and DDH isomers from orobanchol is catalysed by CYPs.
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Frontiers | 14-3-3 Proteins in Plant Hormone Signaling: Doing Several Things at Once | Plant Science

Frontiers | 14-3-3 Proteins in Plant Hormone Signaling: Doing Several Things at Once | Plant Science | Plant hormones and  signaling peptides | Scoop.it
In this review we highlight the advances achieved in the investigation of the role of 14-3-3 proteins in hormone signaling, biosynthesis, and transport. 14-3-3 proteins are a family of conserved molecules that target a number of protein clients through their ability to recognize well-defined phosphorylated motifs. As a result, they regulate several cellular processes, ranging from metabolism to transport, growth, development, and stress response. High-throughput proteomic data and two-hybrid screen demonstrate that 14-3-3 proteins physically interact with many protein clients involved in the biosynthesis or signaling pathways of the main plant hormones, while increasing functional evidence indicates that 14-3-3-target interactions play pivotal regulatory roles. These advances provide a framework of our understanding of plant hormone action, suggesting that 14-3-3 proteins act as hubs of a cellular web encompassing different signaling pathways, transducing and integrating diverse hormone signals in the regulation of physiological processes.
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Mechanistic basis for the activation of plant membrane receptor kinases by SERK-family coreceptors

Mechanistic basis for the activation of plant membrane receptor kinases by SERK-family coreceptors | Plant hormones and  signaling peptides | Scoop.it
Plant-unique membrane receptor kinases with leucine-rich repeat ectodomains (LRR-RKs) can sense small molecule, peptide, and protein ligands. Many LRR-RKs require SERK-family coreceptor kinases for high-affinity ligand binding and receptor activation. How one coreceptor can contribute to the specific binding of distinct ligands and activation of different LRR-RKs is poorly understood. Here we quantitatively analyze the contribution of SERK3 to ligand binding and activation of the brassinosteroid receptor BRI1 and the peptide hormone receptor HAESA. We show that while the isolated receptors sense their respective ligands with drastically different binding affinities, the SERK3 ectodomain binds the ligand-associated receptors with very similar binding kinetics. We identify residues in the SERK3 N-terminal capping domain, which allow for selective steroid and peptide hormone recognition. In contrast, residues in the SERK3 LRR core form a second, constitutive receptor–coreceptor interface. Genetic analyses of protein chimera between BRI1 and SERK3 define that signaling-competent complexes are formed by receptor–coreceptor heteromerization in planta. A functional BRI1–HAESA chimera suggests that the receptor activation mechanism is conserved among different LRR-RKs, and that their signaling specificity is encoded in the kinase domain of the receptor. Our work pinpoints the relative contributions of receptor, ligand, and coreceptor to the formation and activation of SERK-dependent LRR-RK signaling complexes regulating plant growth and development.
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Gibberellin antagonizes jasmonate‐induced defense against Meloidogyne graminicola in rice - Yimer - 2018 - New Phytologist - Wiley Online Library

Gibberellin antagonizes jasmonate‐induced defense against Meloidogyne graminicola in rice - Yimer - 2018 - New Phytologist - Wiley Online Library | Plant hormones and  signaling peptides | Scoop.it
Gibberellin (GA) regulates various plant growth and developmental processes, but its role in pathogen attack, and especially nematode–plant interactions, still remains to be elucidated.
An in‐depth characterization of the role of GA in nematode infection was conducted using mutant lines of rice, chemical inhibitors, and phytohormone measurements.
Our results showed that GA influences rice–Meloidogyne graminicola interactions in a concentration‐dependent manner. Foliar spray of plants with a low concentration of gibberellic acid enhanced nematode infection. Biosynthetic and signaling mutants confirmed the importance of gibberellin for rice susceptibility to M. graminicola infection. Our study also demonstrates that GA signaling suppresses jasmonate (JA)‐mediated defense against M. graminicola, and likewise the JA‐induced defense against M. graminicola requires SLENDER RICE1 (SLR1)‐mediated repression of the GA pathway. In contrast to observations from other plant–pathogen interactions, GA plays a dominant role over JA in determining susceptibility to M. graminicola in rice. This GA‐induced nematode susceptibility was largely independent of auxin biosynthesis, but relied on auxin transport.
In conclusion, we showed that GA–JA antagonistic crosstalk is at the forefront of the interaction between rice and M. graminicola, and SLR1 plays a central role in the JA‐mediated defense response in rice against this nematode.
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Same tune, different song—cytokinins as virulence factors in plant–pathogen interactions?

Same tune, different song—cytokinins as virulence factors in plant–pathogen interactions? | Plant hormones and  signaling peptides | Scoop.it
Virulence factors are molecules that enable plant pathogens to infect and colonize host tissues successfully. These molecules co-evolve with host genes to ensure functionality and to avoid recognition by the host immune system. Some pathogens also produce the plant growth hormone cytokinin (CK) and other plant hormones that contribute to virulence without being subjected to the molecular arms race. Here, we summarize recent findings regarding the role of CKs during infection and the establishment of plant diseases. We discuss commonalities and differences in CK biosynthesis, perception, and activity in infections by different phytopathogenic bacteria, fungi, nematodes and parasitic plants. Finally, we attempt to answer the question if CKs can be classified as bona fide virulence factors.
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Frontiers | Antagonistic Regulation of ABA and GA in Metabolism and Signaling Pathways | Plant Science

Frontiers | Antagonistic Regulation of ABA and GA in Metabolism and Signaling Pathways | Plant Science | Plant hormones and  signaling peptides | Scoop.it
The phytohormones gibberellic acid (GA) and abscisic acid (ABA) are widely recognized as essential endogenous regulators that mostly play antagonistic roles in plant developmental processes and environmental responses. A variety of both internal and external cues oppositely regulate GA and ABA biosynthesis and catabolism, which directly and indirectly affect their signaling pathways and subsequent responses. Recent discoveries have revealed direct molecular links between GA- and ABA-signaling components, which provide novel insights into their antagonistic regulation. In this review, we mainly focus on these recent reports and the growing understanding of GA and ABA antagonism in metabolic regulation and signaling interactions, and attempt to clarify the problems and challenges involved in exploring the complicated regulatory events associated with these two phytohormones.
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