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Structural Basis for DNA Binding Specificity by the Auxin-Dependent ARF Transcription Factors

Structural Basis for DNA Binding Specificity by the Auxin-Dependent ARF Transcription Factors | plant developments | Scoop.it
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Finally structural basis of ARFs for DNA binding is now revealed.

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WOX5–IAA17 Feedback Circuit-Mediated Cellular Auxin Response Is Crucial for the Patterning of Root Stem Cell Niches in Arabidopsis

WOX5–IAA17 Feedback Circuit-Mediated Cellular Auxin Response Is Crucial for the Patterning of Root Stem Cell Niches in Arabidopsis | plant developments | Scoop.it
PMG's insight:

feedback loop linking WOX5-mediated auxin production to IAA17-dependent repression of auxin responses. This WOX5–IAA17 feedback circuit further assures the maintenance of auxin response maximum in the root tip and thereby contributes to the maintenance of distal stem cell (DSC) populations. Our experimental studies and in silico computer simulations both demonstrate that the WOX5–IAA17 feedback circuit is essential for the maintenance of auxin gradient in the root tip and the auxin-mediated root DSC differentiation.

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Arabidopsis KANADI1 Acts as a Transcriptional Repressor by Interacting with a Specific cis-Element and Regulates Auxin Biosynthesis, Transport, and Signaling in Opposition to HD-ZIPIII Factors

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Myb-like domain in KAN1 binds the 6-bp motif GNATA(A/T) and that this motif alone is sufficient to squelch transcription of a linked reporter in vivo. In addition, we report that KAN1 acts as a transcriptional repressor. Among its targets are genes involved in auxin biosynthesis, auxin transport, and auxin response. Furthermore, we find that the adaxializing HD-ZIPIII transcription factor REVOLUTA has opposing effects on multiple components of the auxin pathway. We hypothesize that HD-ZIPIII and KANADI transcription factors pattern auxin accumulation and responsiveness in the embryo. Specifically, we propose the opposing actions of KANADI and HD-ZIPIII factors on cotyledon formation (KANADI represses and HD-ZIPIII promotes cotyledon formation) occur through their opposing actions on genes acting at multiple steps in the auxin pathway.

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A novel protective function for cytokinin in the light stress response is mediated by the AHK2 and AHK3 receptors

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The efficiency of photoprotective mechanisms, such as non-enzymatic and enzymatic scavenging systems, was decreased in plants with a reduced cytokinin status which could be a cause for the increased photodamage and subsequent D1 degradation. Mutant analysis revealed that the protective function of cytokinin during light stress depends on the AHK2 and AHK3 receptors and the type-B response regulators ARR1 and ARR12. We conclude that proper cytokinin signaling and regulation of specific target genes is necessary to protect plants efficiently from light stress.

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A Spatial Accommodation by Neighboring Cells Is Required for Organ Initiation in Arabidopsis

A Spatial Accommodation by Neighboring Cells Is Required for Organ Initiation in Arabidopsis | plant developments | Scoop.it

Lateral root formation in plants can be studied as the process of interaction between chemical signals and physical forces during development. Lateral root primordia grow through overlying cell layers that must accommodate this incursion. Here, we analyze responses of the endodermis, the immediate neighbor to an initiating lateral root. Endodermal cells overlying lateral root primordia lose volume, change shape, and relinquish their tight junction–like diffusion barrier to make way for the emerging lateral root primordium. Endodermal feedback is absolutely required for initiation and growth of lateral roots, and we provide evidence that this is mediated by controlled volume loss in the endodermis. We propose that turgidity and rigid cell walls, typical of plants, impose constraints that are specifically modified for a given developmental process.


Via Jean-Pierre Zryd
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PLOS ONE: Inducible Expression of Arabidopsis Response Regulator 22 (ARR22), a Type-C ARR, in Transgenic Arabidopsis Enhances Drought and Freezing Tolerance

PLOS ONE: Inducible Expression of Arabidopsis Response Regulator 22 (ARR22), a Type-C ARR, in Transgenic Arabidopsis Enhances Drought and Freezing Tolerance | plant developments | Scoop.it
PLOS ONE: an inclusive, peer-reviewed, open-access resource from the PUBLIC LIBRARY OF SCIENCE. Reports of well-performed scientific studies from all disciplines freely available to the whole world.
PMG's insight:

ARR22 is induced by cold and dehydration. Here, we show that inducible overexpression of ARR22 inArabidopsis enhanced dehydration, drought, and cold tolerance in a dexamethasone-dependent manner, whereas mutation of the putative phospho-accepting Asp to Asn in ARR22 (ARR22D74N) abolished these tolerance phenotypes. Overexpression of ARR22 decreased electrolyte leakage in dehydration-, drought-, or cold-stressed transgenic Arabidopsis plants compared with that of ARR22D74N or compared with wild-type plants. Transpiration rates and stomatal apertures were not affected by ARR22 overexpression. No significant difference in both dehydration and freezing tolerance was observed between wild-type and arr22 mutants with or without cytokinin preincubation, consistent with the lack of phenotypes of arr22 mutants in their vegetative development.

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RETINOBLASTOMA-RELATED Protein Stimulates Cell Differentiation in the Arabidopsis Root Meristem by Interacting with Cytokinin Signaling

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RETINOBLASTOMA-RELATED (RBR) protein regulates the differentiation of meristematic cells at the transition zone by allowing mRNA accumulation of AUXIN RESPONSE FACTOR19 (ARF19), a transcription factor involved in cell differentiation. We show that both RBR and the cytokinin-dependent transcription factor ARABIDOPSIS RESPONSE REGULATOR12 are required to activate the transcription of ARF19, which is involved in promoting cell differentiation and thus root growth.

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Lateral root formation and growth of Arabidopsis is redundantly regulated by cytokinin metabolism and signalling genes

PMG's insight:

cytokinin biosynthesis genes IPT3 and IPT5 and all three cytokinin receptor genes (AHK2, AHK3, and CRE1/AHK4) act redundantly during LR initiation. Mutation of AHK2 and AHK3 caused increased auxin sensitivity of LR formation, corroborating the functional relevance of auxin–cytokinin interaction during LR formation. In contrast, LR development of cytokinin receptor mutants in response to other hormones was mostly similar to that of the wild type, which is consistent with separate response pathways. A noticeable exception was an increased sensitivity of LR elongation to brassinolide in ahk2 ahk3 mutants indicating antagonistic action of cytokinin and brassinosteroid.

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The Arabidopsis Eukaryotic Translation Initiation Factor eIF5A-2 Regulates Root Protoxylem Development by Modulating Cytokinin Signaling

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FBR12 genetically interacts with the cytokinin receptor CYTOKININ RESPONSE1 (CRE1) and downstream AHP genes, as double mutants show enhanced phenotypes. FBR12 forms a protein complex with CRE1 and AHP1, and cytokinin regulates formation of this protein complex. Intriguingly, ahp6 partially suppresses the fbr12 mutant phenotype, and the fbr12 mutation causes increased expression of AHP6, indicating that FBR12 negatively regulates AHP6. Consistent with this, ectopic expression of FBR12 in the CRE1-expressing domain partially rescues defective protoxylem development in fbr12, and overexpression of AHP6 causes an fbr12-like phenotype.

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Current Biology - Cytokinin Induces Cell Division in the Quiescent Center of the Arabidopsis Root Apical Meristem

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Cytokinin downregulates the expression of several key regulatory genes in the root tip, including SCARECROW, WOX5, and the auxin influx carriers AUX1 and LAX2. The decrease in LAX2 expression in response to cytokinin requires ARR1 and ARR12, two type B ARRs that mediate the primary transcriptional response to cytokinin. ARR1 was found to bind directly to the LAX2 gene in vivo, which indicates that type B ARRs directly regulate genes that are repressed by cytokinin. Disruption of the LAX2 gene results in a phenotype similar to that observed in response to cytokinin, including increased division of the cells in the QC and decreased expression of WOX5 and the auxin response reporter DR5.

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Frontiers | A robust family of Golden Gate Agrobacterium vectors for plant synthetic biology | Frontiers in Plant Systems Biology

Tools that allow for rapid, accurate and inexpensive assembly of multi-component combinatorial libraries of DNA for transformation into plants will accelerate the progress of synthetic biology research.
PMG's insight:

a new set of binary vectors for use in Agrobacterium-mediated plant transformation that utilizes the Golden-Gate Cloning approach. Our optimized protocol facilitates the rapid and inexpensive generation of multi-component transgenes for later introduction into plants

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Auxin and cytokinin control formation of the quiescent centre in the adventitious root apex of arabidopsis

Auxin and cytokinin control formation of the quiescent centre in the adventitious root apex of arabidopsis | plant developments | Scoop.it
PMG's insight:

The accumulation of auxin and the expression of the QC marker WOX5 characterized the early derivatives of the AR founder cells, in planta and in in vitro cultured TCLs. By determination of PIN1 auxin efflux carrier and LAX3 auxin influx carrier activities, an auxin maximum was determined to occur at the AR tip, to which WOX5 expression was restricted, establishing the positioning of the QC. Cytokinin caused a restriction of LAX3 and PIN1 expression domains, and concomitantly the auxin biosynthesis YUCCA6 gene was expressed in the apex.

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Current Biology - Cytokinins Control Endocycle Onset by Promoting the Expression of an APC/C Activator in Arabidopsis Roots

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cytokinin-activated ARR2 directly upregulates the expression of CCS52A1, which encodes an activator of an E3 ubiquitin ligase, anaphase-promoting complex/cyclosome (APC/C) [6], thereby promoting the onset of the endocycle and restricting meristem size. Our genetic data revealed that CCS52A1 function is independent of SHY2-mediated control of auxin signaling, indicating that downregulation of auxin signaling and APC/C-mediated degradation of cell-cycle regulators cooperatively promote endocycle onset, and thus fine tune root growth.

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Developmental Cell - Auxin-Callose-Mediated Plasmodesmal Gating Is Essential for Tropic Auxin Gradient Formation and Signaling

Developmental Cell - Auxin-Callose-Mediated Plasmodesmal Gating Is Essential for Tropic Auxin Gradient Formation and Signaling | plant developments | Scoop.it
PMG's insight:

 operation of an auxin-GSL8 feedback circuit that regulates the level of plasmodesmal-localized callose in order to locally downregulate symplasmic permeability during hypocotyl tropic response. This system likely involves a plasmodesmal switch that would prevent the dissipation of a forming gradient by auxin diffusion through the symplasm. This regulatory system may represent a mechanism by which auxin could also regulate symplasmic delivery of a wide range of signaling agents.

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Yucasin is a potent inhibitor of YUCCA, a key enzyme in auxin biosynthesis - Nishimura - 2014 - The Plant Journal - Wiley Online Library

Yucasin is a potent inhibitor of YUCCA, a key enzyme in auxin biosynthesis - Nishimura - 2014 - The Plant Journal - Wiley Online Library | plant developments | Scoop.it
PMG's insight:

5–(4–chlorophenyl)-4H-1,2,4–triazole-3–thiol (yucasin) as a potent inhibitor of IAA biosynthesis inYUC-expressing coleoptile tips. Enzymatic analysis of recombinant AtYUC1-His suggested that yucasin strongly inhibited YUC1-His activity against the substrate IPyA in a competitive manner. Phenotypic analysis of Arabidopsis YUC1 over-expression lines (35S::YUC1) demonstrated that yucasin acts in IAA biosynthesis catalyzed by YUC. In addition, 35S::YUC1 seedlings showed resistance to yucasin in terms of root growth. A loss-of-function mutant of TAA1, sav3–2, was hypersensitive to yucasin in terms of root growth and hypocotyl elongation of etiolated seedlings. Yucasin combined with the TAA1 inhibitor l–kynurenine acted additively in Arabidopsis seedlings, producing a phenotype similar to yucasin-treated sav3–2 seedlings, indicating the importance of IAA biosynthesis via the IPyA pathway in root growth and leaf vascular development. 

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Cytokinin as a mediator for regulating root system architecture in response to environmental cues

Root system architecture (RSA) is developmentally controlled by genetic pathways and their interaction with various environmental cues, in particular soil conditions.
PMG's insight:

the transcript levels of cytokinin genes in the root responds to changes in nutrient availability in distinct ways. IPT3,IPT5, CYP735A2, LOG5 and CKX4 are particularly responsive cytokinin metabolism genes, genes encoding different type-A response regulators and the transcriptions factor genes ARR10 and CRF6 are among the most responsive signalling genes. This finding supports the hypothesis that environmental cues operate through fine-tuned transcriptional regulation of cytokinin genes to modulate root development.

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Cytokinin antagonizes abscisic acid-mediated inhibition of cotyledon greening by promoting the degradation of ABI5 protein in Arabidopsis

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cytokinin specifically antagonizes ABA-mediated inhibition on cotyledon greening with minimal effects on seed germination in Arabidopsis (Arabidopsis thaliana). We found that the cytokinin-antagonized ABA effect is dependent on a functional cytokinin signaling pathway, mainly involved in the cytokinin receptor gene CRE1/AHK4, downstream AHP2, 3, 5 genes, and a type-B response regulator gene ARR12, which genetically acts upstream of ABI5 to regulate cotyledon greening. Cytokinin has no apparent effect on the transcription of ABI5. However, cytokinin efficiently promotes the proteasomal degradation of ABI5 protein in a cytokinin signaling-dependent manner. These results define a genetic pathway, through which cytokinin specifically induces the degradation of ABI5 protein, thereby antagonizing ABA-mediated inhibition on postgerminative growth.

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BEX1/ARF1A1C is required for BFA-sensitive recycling of PIN auxin transporters and auxin-mediated development in Arabidopsis

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bfa-visualized exocytic trafficking defective1 (bex1), in which PM localization of PIN1-GFP as well as development is hypersensitive to BFA. We found that in bex1 a member ofARF1 gene family, ARF1A1C was mutated. ARF1A1C localizes to TGN/EE and Golgi apparatus, acts synergistically to BEN1/MIN7 ARF GEF and is important for PIN recycling to the PM. Consistent with the developmental importance of PIN proteins, functional interference with ARF1 resulted in impaired auxin response gradient and various developmental defects including embryonic patterning defects and growth arrest. Our results show that ARF1A1C is essential for recycling of PIN auxin transporters and for various auxin-dependent developmental processes.

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Biochemical characterization of the maize cytokinin dehydrogenase family and cytokinin profiling in developing maize plantlets in relation to the expression of cytokinin dehydrogenase genes

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ZmCKX6 and ZmCKX9 could only be expressed successfully after the removal of putative sequence-specific vacuolar sorting signals (LLPT and LPTS, respectively), suggesting that these proteins are localized to the vacuole. Substrate specificity analyses revealed that the CKX isoforms can be grouped into two subfamilies: members of the first strongly prefer cytokinin free bases while members of the second degrade a broad range of substrates. The most active isoform was found to be ZmCKX1. One of the studied isoforms, ZmCKX6, seemed to encode a nonfunctional enzyme due to a mutation in a conserved HFG protein domain at the C-terminus. Site-directed mutagenesis experiments revealed that this domain is essential for CKX activity. The roles of the maize CKX enzymes in the development of maize seedlings during the two weeks immediately after radicle emergence were also investigated. It appears that ZmCKX1 is a key regulator of active cytokinin levels in developing maize roots. However, the expression of individual CKX isoforms in the shoots varied and none of them seemed to have strong effects on the cytokinin pool.

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Developmental Cell - Side-Chain Modification of Cytokinins Controls Shoot Growth in Arabidopsis

PMG's insight:

 The trans-hydroxylation that forms trans-zeatin (tZ)-type CK from N6-(Δ2-isopentenyl)adenine (iP)-type CK is catalyzed by the cytochrome P450 enzymes CYP735A1 and CYP735A2 in Arabidopsis. Deficiency in trans-hydroxylation activity results in dramatic retardation of shoot growth without affecting total CK quantity, while augmentation of the activity enhances shoot growth. Application of exogenous tZ but not iP recovers the wild-type phenotype in the mutants, indicating that trans-hydroxylation modifies the physiological function of CK. We propose that the control of cytokinin function by side-chain modification is crucial for shoot growth regulation in plants.

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Characterization of a Purine Permease Family Gene OsPUP7 Involved in Growth and Development Control in Rice - Qi - 2013 - Journal of Integrative Plant Biology - Wiley Online Library

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The mutant showed multiple phenotypic changes including increased plant height, big seeds, and delayed flowering. The mutant also showed increased sensitivity to drought and salt stresses and treatments with kinetin and abscisic acid. OsPUP7 is expressed mainly in the vascular bundle, pistil, and stamens. The measurement of cytokinins (CKs) showed that CK content in the mutant spikelets accumulated higher than that in the wild type. Moreover, uptake experiment in the yeast fcy2 mutant suggested that OsPUP7 has the ability to transport caffeine, a CK derivative. Our results indicate that the PUP transport system also exists in rice, and OsPUP7 has an important role in the transport of CK, thus affecting developmental process and stress responses.

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Auxin and cytokinin control formation of the quiescent centre in the adventitious root apex of arabidopsis

Auxin and cytokinin control formation of the quiescent centre in the adventitious root apex of arabidopsis | plant developments | Scoop.it
PMG's insight:

The accumulation of auxin and the expression of the QC marker WOX5 characterized the early derivatives of the AR founder cells, in planta and in in vitro cultured TCLs. By determination of PIN1 auxin efflux carrier and LAX3 auxin influx carrier activities, an auxin maximum was determined to occur at the AR tip, to which WOX5 expression was restricted, establishing the positioning of the QC. Cytokinin caused a restriction of LAX3 and PIN1 expression domains, and concomitantly the auxin biosynthesis YUCCA6 gene was expressed in the apex.

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Frontiers | Regulation of Arabidopsis root development by small signaling peptides | Frontiers in Plant Systems Biology

Plant root systems arise de novo from a single embryonic root. Complex and highly coordinated developmental networks are required to ensure the formation of lateral organs maximises plant fitness.
PMG's insight:

Small signaling peptides are a growing class of regulatory molecules involved in many aspects of root development including meristem maintenance, the gravitropic response, lateral root development, and vascular formation.

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Updates on the model and the evolution of cytokinin signaling

Updates on the model and the evolution of cytokinin signaling | plant developments | Scoop.it
PMG's insight:

In recent years several changes and extensions to the model were necessary to accommodate new findings concerning its components, such as subcellular localization, selective protein degradation and new modes of cross talk. In addition phylogenetic analyses of components of the cytokinin circuitry started to reveal the origin and evolution of the cytokinin regulatory system.

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ABCG9, ABCG11 and ABCG14 ABC transporters are required for vascular development in Arabidopsis - Hir - The Plant Journal - Wiley Online Library

ABCG9, ABCG11 and ABCG14 ABC transporters are required for vascular development in Arabidopsis - Hir - The Plant Journal - Wiley Online Library | plant developments | Scoop.it
PMG's insight:

Mutations in the genes encoding these dimerizing “half transporters” are semidominant and result in vascular patterning defects in cotyledons and floral stem. Coimmunoprecipitation and bimolecular fluorescence complementation experiments demonstrated that these proteins dimerize, either by flexible pairing (ABCG11 and ABCG9) or by forming strict heterodimers (ABCG14). In addition, metabolome analyses and measurement of sterol ester contents in the mutants suggested that ABCG9, ABCG11 and ABCG14 are involved in lipid/sterol homeostasis regulation.

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