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Special Issue "Auxin–New Insights into the modus operandi of a Well-Known Plant Hormone"

A special issue of International Journal of Molecular Sciences (ISSN 1422-0067). This special issue belongs to the section "Molecular Plant Sciences".

Deadline for manuscript submissions: closed (28 February 2022) | Viewed by 21086

Special Issue Editor

Dr. Stephan Pollmann
E-Mail Website1 Website2
Guest Editor
Centro de Biotecnología y Genómica de Plantas, Universidad Politécnica de Madrid (UPM)—Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA), 28223 Pozuelo de Alarcón, Madrid, Spain
Interests: auxin biosynthesis; plant hormonal networks; auxin-jasmonate crosstalk; metabolomics; mass spectrometry
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Decades of intensive scientific work have cemented our knowledge of the fundamental roles of plant hormones. Phytohormones represent signaling compounds that act at sub-micromolar concentrations controlling not only plant growth and development, but also their interactions with the environment. In contrast to animal hormones that mainly work in an isolated manner, plant hormones tend to act in intertwined networks in order to gradually steer adaptive processes.

Auxin is associated with virtually all plant growth processes, including cell elongation, division, and differentiation. It is also a driver of differential growth in response to gravity or light stimuli. Over recent years, our understanding of cellular auxin homeostasis and auxin signaling-related molecular mechanisms has massively improved, but we are still far from having a comprehensive picture of the complex molecular interactions of this pivotal plant hormone. Research published in the last few years has highlighted a plethora of processes that involve the crosstalk of auxin with other plant hormones to, for example, finetune responses to biotic and abiotic stresses. However, there is a great deal more to be learned about auxin crosstalk in plants.

This Special Issue calls for original research, reviews, and perspectives that address the progress and current knowledge in the research on auxin and its intertwined collaboration with other plant hormones.

Dr. Stephan Pollmann
Guest Editor

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Keywords

  • Auxin
  • Plant hormone crosstalk
  • Plant development
  • Plant growth
  • Stress responses
  • Regulatory mechanism

Published Papers (17 papers)

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Article
TaKLU Plays as a Time Regulator of Leaf Growth via Auxin Signaling
Int. J. Mol. Sci. 2022, 23(8), 4219; https://doi.org/10.3390/ijms23084219 - 11 Apr 2022
Viewed by 435
Abstract
The growth of leaves is subject to strict time regulation. Several genes influencing leaf growth have been identified, but little is known about how genes regulate the orderly initiation and growth of leaves. Here, we demonstrate that TaKLU/TaCYP78A5 contributes to a time regulation [...] Read more.
The growth of leaves is subject to strict time regulation. Several genes influencing leaf growth have been identified, but little is known about how genes regulate the orderly initiation and growth of leaves. Here, we demonstrate that TaKLU/TaCYP78A5 contributes to a time regulation mechanism in leaves from initiation to expansion. TaKLU encodes the cytochrome P450 CYP78A5, and its homolog AtKLU has been described whose deletion is detrimental to organ growth. Our results show that TaKLU overexpression increases leaf size and biomass by altering the time of leaf initiation and expansion. TaKLU-overexpressing plants have larger leaves with more cells. Further dynamic observations indicate that enlarged wheat leaves have experienced a longer expansion time. Different from AtKLU inactivation increases leaf number and initiation rates, TaKLU overexpression only smooths the fluctuations of leaf initiation rates by adjusting the initiation time of local leaves, without affecting the overall leaf number and initiation rates. In addition, complementary analyses suggest TaKLU is functionally conserved with AtKLU in controlling the leaf initiation and size and may involve auxin accumulation. Our results provide a new insight into the time regulation mechanisms of leaf growth in wheat. Full article
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Article
Mixed Transcriptome Analysis Revealed the Possible Interaction Mechanisms between Zizania latifolia and Ustilago esculenta Inducing Jiaobai Stem-Gall Formation
Int. J. Mol. Sci. 2021, 22(22), 12258; https://doi.org/10.3390/ijms222212258 - 12 Nov 2021
Viewed by 671
Abstract
The smut fungus Ustilago esculenta infects Zizania latifolia and induces stem expansion to form a unique vegetable named Jiaobai. Although previous studies have demonstrated that hormonal control is essential for triggering stem swelling, the role of hormones synthesized by Z. latifolia and U. [...] Read more.
The smut fungus Ustilago esculenta infects Zizania latifolia and induces stem expansion to form a unique vegetable named Jiaobai. Although previous studies have demonstrated that hormonal control is essential for triggering stem swelling, the role of hormones synthesized by Z. latifolia and U. esculenta and the underlying molecular mechanism are not yet clear. To study the mechanism that triggers swollen stem formation, we analyzed the gene expression pattern of both interacting organisms during the initial trigger of culm gall formation, at which time the infective hyphae also propagated extensively and penetrated host stem cells. Transcriptional analysis indicated that abundant genes involving fungal pathogenicity and plant resistance were reprogrammed to maintain the subtle balance between the parasite and host. In addition, the expression of genes involved in auxin biosynthesis of U. esculenta obviously decreased during stem swelling, while a large number of genes related to the synthesis, metabolism and signal transduction of hormones of the host plant were stimulated and showed specific expression patterns, particularly, the expression of ZlYUCCA9 (a flavin monooxygenase, the key enzyme in indole-3-acetic acid (IAA) biosynthesis pathway) increased significantly. Simultaneously, the content of IAA increased significantly, while the contents of cytokinin and gibberellin showed the opposite trend. We speculated that auxin produced by the host plant, rather than the fungus, triggers stem swelling. Furthermore, from the differently expressed genes, two candidate Cys2-His2 (C2H2) zinc finger proteins, GME3058_g and GME5963_g, were identified from U. esculenta, which may conduct fungus growth and infection at the initial stage of stem-gall formation. Full article
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Article
Jasmonic Acid-Dependent MYC Transcription Factors Bind to a Tandem G-Box Motif in the YUCCA8 and YUCCA9 Promoters to Regulate Biotic Stress Responses
Int. J. Mol. Sci. 2021, 22(18), 9768; https://doi.org/10.3390/ijms22189768 - 09 Sep 2021
Cited by 3 | Viewed by 1267
Abstract
The indole-3-pyruvic acid pathway is the main route for auxin biosynthesis in higher plants. Tryptophan aminotransferases (TAA1/TAR) and members of the YUCCA family of flavin-containing monooxygenases catalyze the conversion of l-tryptophan via indole-3-pyruvic acid to indole-3-acetic acid (IAA). It has been described [...] Read more.
The indole-3-pyruvic acid pathway is the main route for auxin biosynthesis in higher plants. Tryptophan aminotransferases (TAA1/TAR) and members of the YUCCA family of flavin-containing monooxygenases catalyze the conversion of l-tryptophan via indole-3-pyruvic acid to indole-3-acetic acid (IAA). It has been described that jasmonic acid (JA) locally produced in response to mechanical wounding triggers the de novo formation of IAA through the induction of two YUCCA genes, YUC8 and YUC9. Here, we report the direct involvement of a small number of basic helix-loop-helix transcription factors of the MYC family in this process. We show that the JA-mediated regulation of the expression of the YUC8 and YUC9 genes depends on the abundance of MYC2, MYC3, and MYC4. In support of this observation, seedlings of myc knockout mutants displayed a strongly reduced response to JA-mediated IAA formation. Furthermore, transactivation assays provided experimental evidence for the binding of MYC transcription factors to a particular tandem G-box motif abundant in the promoter regions of YUC8 and YUC9, but not in the promoters of the other YUCCA isogenes. Moreover, we demonstrate that plants that constitutively overexpress YUC8 and YUC9 show less damage after spider mite infestation, thereby underlining the role of auxin in plant responses to biotic stress signals. Full article
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Article
Actin Isovariant ACT7 Modulates Root Thermomorphogenesis by Altering Intracellular Auxin Homeostasis
Int. J. Mol. Sci. 2021, 22(14), 7749; https://doi.org/10.3390/ijms22147749 - 20 Jul 2021
Cited by 1 | Viewed by 1294
Abstract
High temperature stress is one of the most threatening abiotic stresses for plants limiting the crop productivity world-wide. Altered developmental responses of plants to moderate-high temperature has been shown to be linked to the intracellular auxin homeostasis regulated by both auxin biosynthesis and [...] Read more.
High temperature stress is one of the most threatening abiotic stresses for plants limiting the crop productivity world-wide. Altered developmental responses of plants to moderate-high temperature has been shown to be linked to the intracellular auxin homeostasis regulated by both auxin biosynthesis and transport. Trafficking of the auxin carrier proteins plays a major role in maintaining the cellular auxin homeostasis. The intracellular trafficking largely relies on the cytoskeletal component, actin, which provides track for vesicle movement. Different classes of actin and the isovariants function in regulating various stages of plant development. Although high temperature alters the intracellular trafficking, the role of actin in this process remains obscure. Using isovariant specific vegetative class actin mutants, here we demonstrate that ACTIN 7 (ACT7) isovariant plays an important role in regulating the moderate-high temperature response in Arabidopsis root. Loss of ACT7, but not ACT8 resulted in increased inhibition of root elongation under prolonged moderate-high temperature. Consistently, kinematic analysis revealed a drastic reduction in cell production rate and cell elongation in act7-4 mutant under high temperature. Quantification of actin dynamicity reveals that prolonged moderate-high temperature modulates bundling along with orientation and parallelness of filamentous actin in act7-4 mutant. The hypersensitive response of act7-4 mutant was found to be linked to the altered intracellular auxin distribution, resulted from the reduced abundance of PIN-FORMED PIN1 and PIN2 efflux carriers. Collectively, these results suggest that vegetative class actin isovariant, ACT7 modulates the long-term moderate-high temperature response in Arabidopsis root. Full article
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Article
The AtCRK5 Protein Kinase Is Required to Maintain the ROS NO Balance Affecting the PIN2-Mediated Root Gravitropic Response in Arabidopsis
Int. J. Mol. Sci. 2021, 22(11), 5979; https://doi.org/10.3390/ijms22115979 - 01 Jun 2021
Cited by 6 | Viewed by 1508
Abstract
The Arabidopsis AtCRK5 protein kinase is involved in the establishment of the proper auxin gradient in many developmental processes. Among others, the Atcrk5-1 mutant was reported to exhibit a delayed gravitropic response via compromised PIN2-mediated auxin transport at the root tip. Here, [...] Read more.
The Arabidopsis AtCRK5 protein kinase is involved in the establishment of the proper auxin gradient in many developmental processes. Among others, the Atcrk5-1 mutant was reported to exhibit a delayed gravitropic response via compromised PIN2-mediated auxin transport at the root tip. Here, we report that this phenotype correlates with lower superoxide anion (O2•−) and hydrogen peroxide (H2O2) levels but a higher nitric oxide (NO) content in the mutant root tips in comparison to the wild type (AtCol-0). The oxidative stress inducer paraquat (PQ) triggering formation of O2•− (and consequently, H2O2) was able to rescue the gravitropic response of Atcrk5-1 roots. The direct application of H2O2 had the same effect. Under gravistimulation, correct auxin distribution was restored (at least partially) by PQ or H2O2 treatment in the mutant root tips. In agreement, the redistribution of the PIN2 auxin efflux carrier was similar in the gravistimulated PQ-treated mutant and untreated wild type roots. It was also found that PQ-treatment decreased the endogenous NO level at the root tip to normal levels. Furthermore, the mutant phenotype could be reverted by direct manipulation of the endogenous NO level using an NO scavenger (cPTIO). The potential involvement of AtCRK5 protein kinase in the control of auxin-ROS-NO-PIN2-auxin regulatory loop is discussed. Full article
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Article
Bootstrapping and Pinning down the Root Meristem; the Auxin–PLT–ARR Network Unites Robustness and Sensitivity in Meristem Growth Control
Int. J. Mol. Sci. 2021, 22(9), 4731; https://doi.org/10.3390/ijms22094731 - 29 Apr 2021
Viewed by 756
Abstract
After germination, the meristem of the embryonic plant root becomes activated, expands in size and subsequently stabilizes to support post-embryonic root growth. The plant hormones auxin and cytokinin, together with master transcription factors of the PLETHORA (PLT) family have been shown to form [...] Read more.
After germination, the meristem of the embryonic plant root becomes activated, expands in size and subsequently stabilizes to support post-embryonic root growth. The plant hormones auxin and cytokinin, together with master transcription factors of the PLETHORA (PLT) family have been shown to form a regulatory network that governs the patterning of this root meristem. Still, which functional constraints contributed to shaping the dynamics and architecture of this network, has largely remained unanswered. Using a combination of modeling approaches we reveal how the interplay between auxin and PLTs enables meristem activation in response to above-threshold stimulation, while its embedding in a PIN-mediated auxin reflux loop ensures localized PLT transcription and thereby, a finite meristem size. We furthermore demonstrate how this constrained PLT transcriptional domain enables independent control of meristem size and division rates, further supporting a division of labor between auxin and PLT. We subsequently reveal how the weaker auxin antagonism of the earlier active Arabidopsis response regulator 12 (ARR12) may arise from the absence of a DELLA protein interaction domain. Our model indicates that this reduced strength is essential to prevent collapse in the early stages of meristem expansion while at later stages the enhanced strength of Arabidopsis response regulator 1 (ARR1) is required for sufficient meristem size control. Summarizing, our work indicates that functional constraints significantly contribute to shaping the auxin–cytokinin–PLT regulatory network. Full article
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Article
OsARF11 Promotes Growth, Meristem, Seed, and Vein Formation during Rice Plant Development
Int. J. Mol. Sci. 2021, 22(8), 4089; https://doi.org/10.3390/ijms22084089 - 15 Apr 2021
Cited by 5 | Viewed by 1118
Abstract
The plant hormone auxin acts as a mediator providing positional instructions in a range of developmental processes. Studies in Arabidopsis thaliana L. show that auxin acts in large part via activation of Auxin Response Factors (ARFs) that in turn regulate the expression of [...] Read more.
The plant hormone auxin acts as a mediator providing positional instructions in a range of developmental processes. Studies in Arabidopsis thaliana L. show that auxin acts in large part via activation of Auxin Response Factors (ARFs) that in turn regulate the expression of downstream genes. The rice (Oryza sativa L.) gene OsARF11 is of interest because of its expression in developing rice organs and its high sequence similarity with MONOPTEROS/ARF5, a gene with prominent roles in A. thaliana development. We have assessed the phenotype of homozygous insertion mutants in the OsARF11 gene and found that in relation to wildtype, osarf11 seedlings produced fewer and shorter roots as well as shorter and less wide leaves. Leaves developed fewer veins and larger areoles. Mature osarf11 plants had a reduced root system, fewer branches per panicle, fewer grains per panicle and fewer filled seeds. Mutants had a reduced sensitivity to auxin-mediated callus formation and inhibition of root elongation, and phenylboronic acid (PBA)-mediated inhibition of vein formation. Taken together, our results implicate OsARF11 in auxin-mediated growth of multiple organs and leaf veins. OsARF11 also appears to play a central role in the formation of lateral root, panicle branch, and grain meristems. Full article
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Article
Pickle Recruits Retinoblastoma Related 1 to Control Lateral Root Formation in Arabidopsis
Int. J. Mol. Sci. 2021, 22(8), 3862; https://doi.org/10.3390/ijms22083862 - 08 Apr 2021
Cited by 3 | Viewed by 1492
Abstract
Lateral root (LR) formation is an example of a plant post-embryonic organogenesis event. LRs are issued from non-dividing cells entering consecutive steps of formative divisions, proliferation and elongation. The chromatin remodeling protein PICKLE (PKL) negatively regulates auxin-mediated LR formation through a mechanism that [...] Read more.
Lateral root (LR) formation is an example of a plant post-embryonic organogenesis event. LRs are issued from non-dividing cells entering consecutive steps of formative divisions, proliferation and elongation. The chromatin remodeling protein PICKLE (PKL) negatively regulates auxin-mediated LR formation through a mechanism that is not yet known. Here we show that PKL interacts with RETINOBLASTOMA-RELATED 1 (RBR1) to repress the LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16) promoter activity. Since LBD16 function is required for the formative division of LR founder cells, repression mediated by the PKL–RBR1 complex negatively regulates formative division and LR formation. Inhibition of LR formation by PKL–RBR1 is counteracted by auxin, indicating that, in addition to auxin-mediated transcriptional responses, the fine-tuned process of LR formation is also controlled at the chromatin level in an auxin-signaling dependent manner. Full article
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Article
Biochemical Characterization of Recombinant UDPG-Dependent IAA Glucosyltransferase from Maize (Zea mays)
Int. J. Mol. Sci. 2021, 22(7), 3355; https://doi.org/10.3390/ijms22073355 - 25 Mar 2021
Viewed by 866
Abstract
Here, we report a biochemical characterization of recombinant maize indole-3-acetyl-β-d-glucose (IAGlc) synthase which glucosylates indole-3-acetic acid (IAA) and thus abolishes its auxinic activity affecting plant hormonal homeostasis. Substrate specificity analysis revealed that IAA is a preferred substrate of IAGlc synthase; however, [...] Read more.
Here, we report a biochemical characterization of recombinant maize indole-3-acetyl-β-d-glucose (IAGlc) synthase which glucosylates indole-3-acetic acid (IAA) and thus abolishes its auxinic activity affecting plant hormonal homeostasis. Substrate specificity analysis revealed that IAA is a preferred substrate of IAGlc synthase; however, the enzyme can also glucosylate indole-3-butyric acid and indole-3-propionic acid with the relative activity of 66% and 49.7%, respectively. KM values determined for IAA and UDP glucose are 0.8 and 0.7 mM, respectively. 2,4-Dichlorophenoxyacetic acid is a competitive inhibitor of the synthase and causes a 1.5-fold decrease in the enzyme affinity towards IAA, with the Ki value determined as 117 μM, while IAA–Asp acts as an activator of the synthase. Two sugar-phosphate compounds, ATP and glucose-1-phosphate, have a unique effect on the enzyme by acting as activators at low concentrations and showing inhibitory effect at higher concentrations (above 0.6 and 4 mM for ATP and glucose-1-phosphate, respectively). Results of molecular docking revealed that both compounds can bind to the PSPG (plant secondary product glycosyltransferase) motif of IAGlc synthase; however, there are also different potential binding sites present in the enzyme. We postulate that IAGlc synthase may contain more than one binding site for ATP and glucose-1-phosphate as reflected in its activity modulation. Full article
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Article
Mode of Action of 1-Naphthylphthalamic Acid in Conspicuous Local Stem Swelling of Succulent Plant, Bryophyllum calycinum: Relevance to the Aspects of Its Histological Observation and Comprehensive Analyses of Plant Hormones
Int. J. Mol. Sci. 2021, 22(6), 3118; https://doi.org/10.3390/ijms22063118 - 18 Mar 2021
Cited by 1 | Viewed by 776
Abstract
The mode of action of 1-naphthylphthalamic acid (NPA) to induce conspicuous local stem swelling in the area of its application to the growing internode in intact Bryophyllum calycinum was studied based on the aspects of histological observation and comprehensive analyses of plant [...] Read more.
The mode of action of 1-naphthylphthalamic acid (NPA) to induce conspicuous local stem swelling in the area of its application to the growing internode in intact Bryophyllum calycinum was studied based on the aspects of histological observation and comprehensive analyses of plant hormones. Histological analyses revealed that NPA induced an increase in cell size and numerous cell divisions in the cortex and pith, respectively, compared to untreated stem. In the area of NPA application, vascular tissues had significantly wider cambial zones consisting of 5–6 cell layers, whereas phloem and xylem seemed not to be affected. This indicates that stem swelling in the area of NPA application is caused by stimulation of cell division and cell enlargement mainly in the cambial zone, cortex, and pith. Comprehensive analyses of plant hormones revealed that NPA substantially increased endogenous levels of indole-3-acetic acid (IAA) in the swelling area. NPA also increased endogenous levels of cytokinins, jasmonic acid, and its precursor, 12-oxo-phytodienoic acid, but did not increase abscisic acid and gibberellin levels. It was shown, using radiolabeled 14C-IAA, that NPA applied to the middle of internode segments had little effect on polar auxin transport, while 2,3,5-triiodobenzoic acid substantially inhibited it. These results strongly suggest that NPA induces changes in endogenous levels of plant hormones, such as IAA, cytokinins, and jasmonic acid, and their hormonal crosstalk results in a conspicuous local stem swelling. The possible different mode of action of NPA from other polar auxin transport inhibitors in succulent plants is extensively discussed. Full article
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Article
Bioprospecting Fluorescent Plant Growth Regulators from Arabidopsis to Vegetable Crops
Int. J. Mol. Sci. 2021, 22(6), 2797; https://doi.org/10.3390/ijms22062797 - 10 Mar 2021
Cited by 1 | Viewed by 1479
Abstract
The phytohormone auxin is involved in almost every process of a plant’s life, from germination to plant development. Nowadays, auxin research connects synthetic chemistry, plant biology and computational chemistry in order to develop innovative and safe compounds to be used in sustainable agricultural [...] Read more.
The phytohormone auxin is involved in almost every process of a plant’s life, from germination to plant development. Nowadays, auxin research connects synthetic chemistry, plant biology and computational chemistry in order to develop innovative and safe compounds to be used in sustainable agricultural practice. In this framework, we developed new fluorescent compounds, ethanolammonium p-aminobenzoate (HEA-pABA) and p-nitrobenzoate (HEA-pNBA), and investigated their auxin-like behavior on two main commercial vegetables cultivated in Europe, cucumber (Cucumis sativus) and tomato (Solanumlycopersicum), in comparison to the model plant Arabidopsis (Arabidopsis thaliana). Moreover, the binding modes and affinities of two organic salts in relation to the natural auxin indole-3-acetic acid (IAA) into TIR1 auxin receptor were investigated by computational approaches (homology modeling and molecular docking). Both experimental and theoretical results highlight HEA-pABA as a fluorescent compound with auxin-like activity both in Arabidopsis and the commercial cucumber and tomato. Therefore, alkanolammonium benzoates have a great potential as promising sustainable plant growth stimulators to be efficiently used in vegetable crops. Full article
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Article
Indole-3-Acetic Acid Is Synthesized by the Endophyte Cyanodermella asteris via a Tryptophan-Dependent and -Independent Way and Mediates the Interaction with a Non-Host Plant
by , and
Int. J. Mol. Sci. 2021, 22(5), 2651; https://doi.org/10.3390/ijms22052651 - 06 Mar 2021
Cited by 5 | Viewed by 1218
Abstract
The plant hormone indole-3-acetic acid (IAA) is one of the main signals playing a role in the communication between host and endophytes. Endophytes can synthesize IAA de novo to influence the IAA homeostasis in plants. Although much is known about IAA biosynthesis in [...] Read more.
The plant hormone indole-3-acetic acid (IAA) is one of the main signals playing a role in the communication between host and endophytes. Endophytes can synthesize IAA de novo to influence the IAA homeostasis in plants. Although much is known about IAA biosynthesis in microorganisms, there is still less known about the pathway by which IAA is synthesized in fungal endophytes. The aim of this study is to examine a possible IAA biosynthesis pathway in Cyanodermella asteris. In vitro cultures of C. asteris were incubated with the IAA precursors tryptophan (Trp) and indole, as well as possible intermediates, and they were additionally treated with IAA biosynthesis inhibitors (2-mercaptobenzimidazole and yucasin DF) to elucidate possible IAA biosynthesis pathways. It was shown that (a) C. asteris synthesized IAA without adding precursors; (b) indole-3-acetonitrile (IAN), indole-3-acetamide (IAM), and indole-3-acetaldehyde (IAD) increased IAA biosynthesis; and (c) C. asteris synthesized IAA also by a Trp-independent pathway. Together with the genome information of C. asteris, the possible IAA biosynthesis pathways found can improve the understanding of IAA biosynthesis in fungal endophytes. The uptake of fungal IAA into Arabidopsis thaliana is necessary for the induction of lateral roots and other fungus-related growth phenotypes, since the application of the influx inhibitor 2-naphthoxyacetic acid (NOA) but not the efflux inhibitor N-1-naphtylphthalamic acid (NPA) were altering these parameters. In addition, the root phenotype of the mutation in an influx carrier, aux1, was partially rescued by C. asteris. Full article
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Article
Accumulation of the Auxin Precursor Indole-3-Acetamide Curtails Growth through the Repression of Ribosome-Biogenesis and Development-Related Transcriptional Networks
Int. J. Mol. Sci. 2021, 22(4), 2040; https://doi.org/10.3390/ijms22042040 - 18 Feb 2021
Cited by 4 | Viewed by 1817
Abstract
The major auxin, indole-3-acetic acid (IAA), is associated with a plethora of growth and developmental processes including embryo development, expansion growth, cambial activity, and the induction of lateral root growth. Accumulation of the auxin precursor indole-3-acetamide (IAM) induces stress related processes by stimulating [...] Read more.
The major auxin, indole-3-acetic acid (IAA), is associated with a plethora of growth and developmental processes including embryo development, expansion growth, cambial activity, and the induction of lateral root growth. Accumulation of the auxin precursor indole-3-acetamide (IAM) induces stress related processes by stimulating abscisic acid (ABA) biosynthesis. How IAM signaling is controlled is, at present, unclear. Here, we characterize the ami1rooty double mutant, that we initially generated to study the metabolic and phenotypic consequences of a simultaneous genetic blockade of the indole glucosinolate and IAM pathways in Arabidopsisthaliana. Our mass spectrometric analyses of the mutant revealed that the combination of the two mutations is not sufficient to fully prevent the conversion of IAM to IAA. The detected strong accumulation of IAM was, however, recognized to substantially impair seed development. We further show by genome-wide expression studies that the double mutant is broadly affected in its translational capacity, and that a small number of plant growth regulating transcriptional circuits are repressed by the high IAM content in the seed. In accordance with the previously described growth reduction in response to elevated IAM levels, our data support the hypothesis that IAM is a growth repressing counterpart to IAA. Full article
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Article
Overexpression of the Auxin Receptor AFB3 in Arabidopsis Results in Salt Stress Resistance and the Modulation of NAC4 and SZF1
Int. J. Mol. Sci. 2020, 21(24), 9528; https://doi.org/10.3390/ijms21249528 - 15 Dec 2020
Cited by 7 | Viewed by 1024
Abstract
Soil salinity is a key problem for crop production worldwide. High salt concentration in soil negatively modulates plant growth and development. In roots, salinity affects the growth and development of both primary and lateral roots. The phytohormone auxin regulates various developmental processes during [...] Read more.
Soil salinity is a key problem for crop production worldwide. High salt concentration in soil negatively modulates plant growth and development. In roots, salinity affects the growth and development of both primary and lateral roots. The phytohormone auxin regulates various developmental processes during the plant’s life cycle, including several aspects of root architecture. Auxin signaling involves the perception by specialized receptors which module several regulatory pathways. Despite their redundancy, previous studies have shown that their functions can also be context-specific depending on tissue, developmental or environmental cues. Here we show that the over-expression of Auxin Signaling F-Box 3 receptor results in an increased resistance to salinity in terms of root architecture and germination. We also studied possible downstream signaling components to further characterize the role of auxin in response to salt stress. We identify the transcription factor SZF1 as a key component in auxin-dependent salt stress response through the regulation of NAC4. These results give lights of an auxin-dependent mechanism that leads to the modulation of root system architecture in response to salt identifying a hormonal cascade important for stress response. Full article
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Article
MtPIN1 and MtPIN3 Play Dual Roles in Regulation of Shade Avoidance Response under Different Environments in Medicago truncatula
Int. J. Mol. Sci. 2020, 21(22), 8742; https://doi.org/10.3390/ijms21228742 - 19 Nov 2020
Cited by 1 | Viewed by 830
Abstract
Polar auxin transport mediated by PIN-FORMED (PIN) proteins is critical for plant growth and development. As an environmental cue, shade stimulates hypocotyls, petiole, and stem elongation by inducing auxin synthesis and asymmetric distributions, which is modulated by PIN3,4,7 in Arabidopsis. Here, we [...] Read more.
Polar auxin transport mediated by PIN-FORMED (PIN) proteins is critical for plant growth and development. As an environmental cue, shade stimulates hypocotyls, petiole, and stem elongation by inducing auxin synthesis and asymmetric distributions, which is modulated by PIN3,4,7 in Arabidopsis. Here, we characterize the MtPIN1 and MtPIN3, which are the orthologs of PIN3,4,7, in model legume species Medicago truncatula. Under the low Red:Far-Red (R:FR) ratio light, the expression of MtPIN1 and MtPIN3 is induced, and shadeavoidance response is disrupted in mtpin1 mtpin3 double mutant, indicating that MtPIN1 and MtPIN3 have a conserved function in shade response. Surprisingly, under the normal growth condition, mtpin1 mtpin3 displayed the constitutive shade avoidance responses, such as the elongated petiole, smaller leaf, and increased auxin and chlorophyll content. Therefore, MtPIN1 and MtPIN3 play dual roles in regulation of shadeavoidance response under different environments. Furthermore, these data suggest that PIN3,4,7 and its orthologs have evolved conserved and specific functions among species. Full article
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Article
Aux/IAA14 Regulates microRNA-Mediated Cold Stress Response in Arabidopsis Roots
Int. J. Mol. Sci. 2020, 21(22), 8441; https://doi.org/10.3390/ijms21228441 - 10 Nov 2020
Cited by 22 | Viewed by 1655
Abstract
The phytohormone auxin and microRNA-mediated regulation of gene expressions are key regulators of plant growth and development at both optimal and under low-temperature stress conditions. However, the mechanistic link between microRNA and auxin in regulating plant cold stress response remains elusive. To better [...] Read more.
The phytohormone auxin and microRNA-mediated regulation of gene expressions are key regulators of plant growth and development at both optimal and under low-temperature stress conditions. However, the mechanistic link between microRNA and auxin in regulating plant cold stress response remains elusive. To better understand the role of microRNA (miR) in the crosstalk between auxin and cold stress responses, we took advantage of the mutants of Arabidopsis thaliana with altered response to auxin transport and signal. Screening of the mutants for root growth recovery after cold stress at 4 °C revealed that the auxin signaling mutant, solitary root 1 (slr1; mutation in Aux/IAA14), shows a hypersensitive response to cold stress. Genome-wide expression analysis of miRs in the wild-type and slr1 mutant roots using next-generation sequencing revealed 180 known and 71 novel cold-responsive microRNAs. Cold stress also increased the abundance of 26–31 nt small RNA population in slr1 compared with wild type. Comparative analysis of microRNA expression shows significant differential expression of 13 known and 7 novel miRs in slr1 at 4 °C compared with wild type. Target gene expression analysis of the members from one potential candidate miR, miR169, revealed the possible involvement of miR169/NF-YA module in the Aux/IAA14-mediated cold stress response. Taken together, these results indicate that SLR/IAA14, a transcriptional repressor of auxin signaling, plays a crucial role in integrating miRs in auxin and cold responses. Full article
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Review

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Review
Auxin and Root Gravitropism: Addressing Basic Cellular Processes by Exploiting a Defined Growth Response
Int. J. Mol. Sci. 2021, 22(5), 2749; https://doi.org/10.3390/ijms22052749 - 09 Mar 2021
Cited by 12 | Viewed by 1675
Abstract
Root architecture and growth are decisive for crop performance and yield, and thus a highly topical research field in plant sciences. The root system of the model plant Arabidopsis thaliana is the ideal system to obtain insights into fundamental key parameters and molecular [...] Read more.
Root architecture and growth are decisive for crop performance and yield, and thus a highly topical research field in plant sciences. The root system of the model plant Arabidopsis thaliana is the ideal system to obtain insights into fundamental key parameters and molecular players involved in underlying regulatory circuits of root growth, particularly in responses to environmental stimuli. Root gravitropism, directional growth along the gravity, in particular represents a highly sensitive readout, suitable to study adjustments in polar auxin transport and to identify molecular determinants involved. This review strives to summarize and give an overview into the function of PIN-FORMED auxin transport proteins, emphasizing on their sorting and polarity control. As there already is an abundance of information, the focus lies in integrating this wealth of information on mechanisms and pathways. This overview of a highly dynamic and complex field highlights recent developments in understanding the role of auxin in higher plants. Specifically, it exemplifies, how analysis of a single, defined growth response contributes to our understanding of basic cellular processes in general. Full article
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