Auxin-mediated Growth Research Articles

Genome-wide analysis reveals the TCP-miR159-miR319 module is crucial for rice (Oryza sativa L.) growth and response to drought and salinity.

The TCP are key plant-specific proteins responsible for regulating various developmental activities. Herein, we retrieved 21 OsTCP genes from the genome database of rice. The bioinformatics analysis, including conserved motif, domain, interactive proteins, gene ontology (GO) analysis, target micro-RNAs (miRNA) prediction, and the physiochemical properties, highlight the key aspects of OsTCP genes in Oryza sativa L. The phylogeny analysis disclosed that OsTCP was further clustered into five clades (Clade I - Clade V) based on the structural similarities and functional diversities. The GO analysis of OsTCP proteins showed they are primarily exercising the rice developmental processes, reproductive stages in particular. The microarray expression analysis showed transcription in twelve rice tissues, and the differential expression pattern of OsTCP genes revealed their possible involvement in auxin-mediated growth and response against salinity and drought. The predicted miRNA that includes miR159 and miR319 families further the engagement of OsTCP genes in the post-translational regulation of rice growth. Owing to the above-mentioned outcomes, we speculate that OsTCP genes in the O. sativa genome play an essential role in growth and development and fine-tuning the immune responses to various environmental stresses, thus providing major indications for further functional studies.

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

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

Arabidopsis NHX5 and NHX6 regulate PIN6-mediated auxin homeostasis and growth

NHX5 and NHX6, endosomal Na+,K+/H+ antiporters in Arabidopsis thaliana, play a vital role in growth and development. Our previous study has shown that NHX5 and NHX6 function as H+ leak to regulate auxin-mediated growth in Arabidopsis. In this report, we investigated the function of NHX5 and NHX6 in controlling PIN6-mediated auxin homeostasis and growth in Arabidopsis. Phenotypic analyses found that NHX5 and NHX6 were critical for the function of PIN6, an auxin transporter. We further showed that PIN6 depended on NHX5 and NHX6 in regulating auxin homeostasis. NHX5 and NHX6 were colocalized with PIN6, but they did not interact physically. The conserved acidic residues that are vital for the activity of NHX5 and NHX6 were critical for PIN6 function. Together, NHX5 and NHX6 may regulate PIN6 function by their transport activity.

MiRNA160 is associated with local defense and systemic acquired resistance against Phytophthora infestans infection in potato.

To combat pathogen infection, plants employ local defenses in infected sites and elicit systemic acquired resistance (SAR) in distant tissues. MicroRNAs have been shown to play a significant role in local defense, but their association with SAR is unknown. In addition, no such studies of the interaction between potato and Phytophthora infestans have been reported. We investigated the role of miR160 in local and SAR responses to P. infestans infection in potato. Expression analysis revealed induced levels of miR160 in both local and systemic leaves of infected wild-type plants. miR160 overexpression and knockdown plants exhibited increased susceptibility to infection, suggesting that miR160 levels equivalent to those of wild-type plants may be necessary for mounting local defense responses. Additionally, miR160 knockdown lines failed to elicit SAR, and grafting assays indicated that miR160 is required in both local and systemic leaves to trigger SAR. Consistently, SAR-associated signals and genes were dysregulated in miR160 knockdown lines. Furthermore, analysis of the expression of defense and auxin pathway genes and direct regulation of StGH3.6, a mediator of salicylic acid-auxin cross-talk, by the miR160 target StARF10 revealed the involvement of miR160 in antagonistic cross-talk between salicylic acid-mediated defense and auxin-mediated growth pathways. Overall, our study demonstrates that miR160 plays a crucial role in local defense and SAR responses during the interaction between potato and P. infestans.

The M3 Phosphorylation Site Is Required for Trafficking and Biological Roles of PIN-FORMED1, 2, and 7 in Arabidopsis.

Asymmetrically localized PIN-FORMED (PIN) auxin efflux carriers play key roles in regulating directional intercellular auxin movement, generating local auxin gradients, and diverse auxin-mediated growth and development. The polar localization of PINs is controlled by phosphorylation in the central hydrophilic loop (HL) of PINs. Although the M3 phosphorylation site, including phosphorylatable 5 Ser/Thr residues, is conserved among long HL-PINs, its native role has only been characterized in PIN3. In this study, we examined the role of M3 phosphorylation site of PIN1, PIN2, and PIN7 in intracellular trafficking, phosphorylation, and biological functions of those PINs in their native expressing tissues. Phosphorylation-defective mutations of the phosphorylatable residues in the M3 site of PIN1-HL led to alteration in subcellular polarity of PIN1 and caused defects in PIN1-mediated biological functions such as cotyledon development, phyllotaxy of vegetative leaves, and development of reproductive organs. The M3 mutations of PIN7 interfered with its polar recycling in the root columella cell in response to gravity stimulus and partially disrupted root gravitropism. On the other hand, the M3 site of PIN2 was shown to be necessary for its targeting to the plasma membrane. In vitro phosphorylation assay showed that the M3 phosphorylation residues of PIN1 are the partial targets by PINOID kinase. Our data suggest that the M3 phosphorylation site is functionally conserved among long HL-PINs by playing roles for their subcellular trafficking and auxin-mediated developmental processes.

Screen Identifying Arabidopsis Transcription Factors Involved in the Response to 9-Lipoxygenase-Derived Oxylipins.

13-Lipoxygenase-derived oxylipins, such as jasmonates act as potent signaling molecules in plants. Although experimental evidence supports the impact of oxylipins generated by the 9-Lipoxygenase (9-LOX) pathway in root development and pathogen defense, their signaling function in plants remains largely elusive. Based on the root growth inhibiting properties of the 9-LOX-oxylipin 9-HOT (9-hydroxy-10,12,15-octadecatrienoic acid), we established a screening approach aiming at identifying transcription factors (TFs) involved in signaling and/or metabolism of this oxylipin. Making use of the AtTORF-Ex (Arabidopsis thaliana Transcription Factor Open Reading Frame Expression) collection of plant lines overexpressing TF genes, we screened for those TFs which restore root growth on 9-HOT. Out of 6,000 lines, eight TFs were recovered at least three times and were therefore selected for detailed analysis. Overexpression of the basic leucine Zipper (bZIP) TF TGA5 and its target, the monoxygenase CYP81D11 reduced the effect of added 9-HOT, presumably due to activation of a detoxification pathway. The highly related ETHYLENE RESPONSE FACTORs ERF106 and ERF107 induce a broad detoxification response towards 9-LOX-oxylipins and xenobiotic compounds. From a set of 18 related group S-bZIP factors isolated in the screen, bZIP11 is known to participate in auxin-mediated root growth and may connect oxylipins to root meristem function. The TF candidates isolated in this screen provide starting points for further attempts to dissect putative signaling pathways involving 9-LOX-derived oxylipins.

Auxin response under osmotic stress.

The phytohormone auxin (indole-3-acetic acid, IAA) is a small organic molecule that coordinates many of the key processes in plant development and adaptive growth. Plants regulate the auxin response pathways at multiple levels including biosynthesis, metabolism, transport and perception. One of the most striking aspects of plant plasticity is the modulation of development in response to changing growth environments. In this review, we explore recent findings correlating auxin response-dependent growth and development with osmotic stresses. Studies of water deficit, dehydration, salt, and other osmotic stresses point towards direct and indirect molecular perturbations in the auxin pathway. Osmotic stress stimuli modulate auxin responses by affecting auxin biosynthesis (YUC, TAA1), transport (PIN), perception (TIR/AFB, Aux/IAA), and inactivation/conjugation (GH3, miR167, IAR3) to coordinate growth and patterning. In turn, stress-modulated auxin gradients drive physiological and developmental mechanisms such as stomata aperture, aquaporin and lateral root positioning. We conclude by arguing that auxin-mediated growth inhibition under abiotic stress conditions is one of the developmental and physiological strategies to acclimate to the changing environment.

HSP90 regulates temperature-dependent seedling growth in Arabidopsis by stabilizing the auxin co-receptor F-box protein TIR1.

Recent studies have revealed that a mild increase in environmental temperature stimulates the growth of Arabidopsis seedlings by promoting biosynthesis of the plant hormone auxin. However, little is known about the role of other factors in this process. In this report, we show that increased temperature promotes rapid accumulation of the TIR1 auxin co-receptor, an effect that is dependent on the molecular chaperone HSP90. In addition, we show that HSP90 and the co-chaperone SGT1 each interact with TIR1, confirming that TIR1 is an HSP90 client. Inhibition of HSP90 activity results in degradation of TIR1 and interestingly, defects in a range of auxin-mediated growth processes at lower as well as higher temperatures. Our results indicate that HSP90 and SGT1 integrate temperature and auxin signalling in order to regulate plant growth in a changing environment.

Qa-SNARE Protein SYP22 Negatively Regulates Brassinosteroid Signaling in the Dark

Abstract Soluble N-ethyl-maleimide sensitive factor attachment adaptor protein receptor (SNARE) domain-containing proteins were mainly involved in vesicle-associated membrane fusion. Genetic screening has revealed the function of SNARE in different aspects of plant biology. Among them, Synthaxin-22 (SYP22) a Qa-SNARE has been reported to have a pleiotropic function in plant development including regulation of leaf waving, shoot gravitropism and flowering time. In this study, we identified a new role of SYP22 in regulation of brassinosteroid (BR) signaling, especially in the dark. SYP22 interacts with BR receptor, brassinosteroid insensitive 1 (BRI1), and overexpression of SYP22 enhanced a weak BRI1 mutant bri1-5 phenotype. syp22 mutant exhibits short hypocotyl and it is sensitive to exogenously treated BR while slightly insensitive to BR-biosynthesis inhibitor propiconazole (PCZ) in the dark. Expression levels of BR signaling maker genes ACS5, SAUR15 and IAA19 were slightly higher, while BR6OX2, a BR biosynthesis marker gene, was lower in syp22 compared to the wild-type. In addition, syp22 was sensitive to 2,4-D, a synthetic auxin, in the dark. In conclusion, SYP22 is involved in BR- and auxin-mediated hypocotyl growth inhibition in the dark, which might be via interaction with BR and auxin key regulators to alter their internalization in Arabidopsis.

Auxin-mediated growth of rice in cadmium-contaminated soil

Pollution from heavy metals is a global problem that is very dangerous to the environment. Among the heavy metals, cadmium is receiving more and more attention because it is one of the most ecotoxic, making it very harmful for biological activity in soil, biodiversity, plant metabolism, and human and animal health. Phytohormones, produced by plants, control or regulate germination, growth, metabolism, or other physiological activities, and they can alleviate the toxic effects of heavy metals on plant growth. Auxin is the most abundant natural growth hormone, controlling apical dominance, tropism, elongation of shoots, and initiation of roots in plants. However, cadmium stress causes the degradation of indigenous auxin. To cope with this, auxin may be applied exogenously. To evaluate the role of auxin in cadmium -contaminated soil, different levels of the auxin precursor L-TRP were applied to plants growing in contaminated soil. Rice seedlings were dipped in a L-TRP solution for 2 h and transplanted into potted soil contaminated with 30 mg kg1 cadmium. The results showed that the exogenous application of L-TRP improved the growth and yield of treated plants both in normal and contaminated soil. The data of different parameters showed that L-TRP enhanced plant height (12.54%), the number of tillers (25.53%), the number of panicles (19.04%), the 1000 grain weight (19.28%), and the paddy yield (11.78%) in cadmium-contaminated soil, as compared to plants grown in the absence of L-TRP with the same level of contamination. L-TRP increased the uptake of cadmium in rice straw while decreasing the translocation of cadmium towards grains, as compared to plants grown in contaminated soil without L-TRP. This experiment revealed the possibility of using L-TRP to improve plant growth and soil health in contaminated soil.

Meristematic cell proliferation and ribosome biogenesis are decoupled in diamagnetically levitated Arabidopsis seedlings.

BackgroundCell growth and cell proliferation are intimately linked in the presence of Earth’s gravity, but are decoupled under the microgravity conditions present in orbiting spacecraft. New technologies to simulate microgravity conditions for long-duration experiments, with stable environmental conditions, in Earth-based laboratories are required to further our understanding of the effect of extraterrestrial conditions on the growth, development and health of living matter.ResultsWe studied the response of transgenic seedlings of Arabidopsis thaliana, containing either the CycB1-GUS proliferation marker or the DR5-GUS auxin-mediated growth marker, to diamagnetic levitation in the bore of a superconducting solenoid magnet. As a control, a second set of seedlings were exposed to a strong magnetic field, but not to levitation forces. A third set was exposed to a strong field and simulated hypergravity (2 g). Cell proliferation and cell growth cytological parameters were measured for each set of seedlings. Nucleolin immunodetection was used as a marker of cell growth. Collectively, the data indicate that these two fundamental cellular processes are decoupled in root meristems, as in microgravity: cell proliferation was enhanced whereas cell growth markers were depleted. These results also demonstrated delocalisation of auxin signalling in the root tip despite the fact that levitation of the seedling as a whole does not prevent the sedimentation of statoliths in the root cells.ConclusionsIn our model system, we found that diamagnetic levitation led to changes that are very similar to those caused by real- [e.g. on board the International Space Station (ISS)] or mechanically-simulated microgravity [e.g. using a Random Positioning Machine (RPM)]. These changes decoupled meristematic cell proliferation from ribosome biogenesis, and altered auxin polar transport.

SlmiR393 controls the auxin receptor homologous genes expression, and regulates sensitivity to auxin in tomato root growth

MicroRNAs (miRNAs), key posttranscriptional regulators of eukaryotic gene expression, regulate a wide variety of developmental and physiological processes in plant. Our understanding of miR393 regulatory function in tomato, however, is limited. In this study, we identified SlmiR393 from tomato (Solanum lycopersicum), and systematically investigated its regulatory function in auxin-mediated growth and development. Our results indicated that SlmiR393 is predominantly expressed in the stem and flower, and predicted to target three auxin receptor homologous genes mRNA, including SlTIR1a, SlTIR1b and SlAFB6. Accordingly, 5′RLM-RACE cleavage assay demonstrated SlmiR393-mediated cleavage of SlTIR1a, SlTIR1b and SlAFB6 transcripts in vivo. Moreover, transgenic tomato plants that constitutively expressed the SlmiR393 gene repressed expression of three auxin receptor homologue, and caused inhibition of primary root growth and hyposensitivity to auxin in root growth. Further analyses revealed that SlmiR393 over-expression alters the expression genes involved in auxin signal transduction. Take together, these data suggest that SlmiR393 functions in negative modulation of auxin response for plant growth and development of tomato.

Systems Analysis of Shoot Apical Meristem Growth and Development: Integrating Hormonal and Mechanical Signaling

The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. This review covers our current understanding of organ initiation by the SAM in Arabidopsis thaliana. Meristem function and maintenance involves two major hormones, cytokinins and auxins. Cytokinins appear to play a major role in meristem maintenance and in controlling meristematic properties, such as cell proliferation. Self-organizing transport processes, which are still only partially understood, lead to the patterned accumulation of auxin at particular positions, where organs will grow out. A major downstream target of auxin-mediated growth regulation is the cell wall, which is a determinant for both growth rates and growth distribution, but feedbacks with metabolism and the synthetic capacity of the cytoplasm are crucial as well. Recent work has also pointed at a potential role of mechanical signals in growth coordination, but the precise mechanisms at work remain to be elucidated.

Mechanical Regulation of Auxin-Mediated Growth

The phytohormone auxin is a primary regulator of growth and developmental pattern formation in plants. Auxin accumulates at specific sites (e.g., organ primordia) and induces localized growth within a tissue. Auxin also mediates developmental responses to intrinsic and external physical stimuli; however, exactly how mechanics influences auxin distribution is unknown. Here we show that mechanical strain can regulate auxin transport and accumulation in the tomato shoot apex, where new leaves emerge and rapidly grow. Modification of turgor pressure, application of external force, and artificial growth induction collectively show that the amount and intracellular localization of the auxin efflux carrier PIN1 are sensitive to mechanical alterations. In general, the more strained the tissue was, the more PIN1 was present per cell and the higher the proportion localized to the plasma membrane. Modulation of the membrane properties alone was sufficient to explain most of the mechanical effects. Our experiments support the hypothesis that the plasma membrane acts as a sensor of tissue mechanics that translates the cell wall strain into cellular responses, such as the intracellular localization of membrane-embedded proteins. One implication of this fundamental mechanism is the mechanical enhancement of auxin-mediated growth in young organ primordia. We propose that growth-induced mechanical strain upregulates PIN1 function and auxin accumulation, thereby promoting further growth, in a robust positive feedback loop.

Reduced expression of a vesicle trafficking-related ATPase SKD1 decreases salt tolerance in Arabidopsis

In this study we present the functional characterisation of SKD1 (suppressor of K+ transport growth defect) in salt tolerance of higher plants. SKD1 participates in endosome-mediated protein sorting and expression of SKD1 is salt-induced in Na+ storage cells of halophyte ice plant. Transgenic Arabidopsis with reduced SKD1 expression were generated by expressing AtSKD1 in antisense orientation. Relative root growth rate of antisense seedlings was slower than that of wild-type seedlings under salt treatment. The Na+/K+ ratio doubled in the antisense seedlings compared with the wild-type seedlings indicating a loss in Na+/K+ homeostasis. The PSII activity dropped following one week of salt-stress in antisense plants whereas wild-type plants maintained normal activity. Upon germination, transgenic seedlings developed multiple roots where each root had lower density of lateral roots. Application of 1-naphthaleneacetic acid restored the ability of transgenic seedlings to form lateral roots. Expression profiling analyses revealed that expressions of one stress-related kinase, several salt-induced transcription factors and one auxin efflux transporter were altered in antisense seedlings. With decreased expression of SKD1, plants experience a reduced salinity response and altered root development indicating the importance of intracellular vesicular trafficking in both auxin-mediated plant growth and in maintaining ion homeostasis under salt stress.

How Arabidopsis Roots Differentiate Hydrotropism from Gravitropism?

Root hydrotropism is a response to moisture gradients considered to be important for drought avoidance. Recent re-evaluation of root hydrotropism has emphasised the dominating effect of root gravitropism on it. It has been suggested that amyloplast dynamics inside columella cells and auxin regulation play roles in this interacting mechanism, even though the existence of distinct pathways of two tropisms derived from different stimuli remained unclear. We have recently found two factors that separate the mechanism of hydrotropism from that of gravitropism in Arabidopsis seedling roots. One is the difference in the mode of auxin-mediated growth regulation between two tropisms, and the other is the identification of gene indispensable only for root hydrotropism. Here we summarize the recent progress on root hydrotropism research and discuss the remaining and emerging issues.

Glycine Betaine Biosynthesis Is Induced by Salt Stress but Repressed by Auxinic Herbicides in Kochia scoparia.

Kochia scoparia biotypes that are susceptible or resistant to the auxinic herbicide dicamba were used to characterize expression levels of choline monooxygenase (CMO) and glycine betaine accumulation in response to salt stress and herbicide treatment. A 1180-bp cDNA was isolated using differential display and 3′ RACE with a deduced amino acid sequence that was more than 90% similar to the carboxy terminal 290 residues of CMOs from four related plant species. Salt stress led to a substantial increase in CMO mRNA and enzyme levels in K. scoparia biotypes, and the accumulation of up to 80 μmol g−1 fresh weight glycine betaine. In contrast, dicamba treatment was followed by the rapid attenuation of CMO message and protein levels, with a recovery of expression in the resistant but not the susceptible biotype. CMO mRNA and enzyme levels similarly declined, and recovered in the resistant biotype, after dicamba treatment of plants that were previously salt stressed for 4 days. The opposing effects of these two stresses may represent a regulatory scheme in which competition for the substrate choline leads to a repression of glycine betaine biosynthesis to make sufficient choline available for auxin-mediated growth processes.

The α-Helical D1 Domain of the Tobacco bZIP Transcription Factor BZI-1 Interacts with the Ankyrin-repeat Protein ANK1 and Is Important for BZI-1 Function, Both in Auxin Signaling and Pathogen Response

The tobacco (Nicotiana tabacum) bZIP transcription factor BZI-1 is involved in auxin-mediated growth responses and in establishing pathogen defenses. Transgenic plants expressing a dominant-negative BZI-1-DeltaN derivative, which lacks the N-terminal activation domain, showed altered vegetative growth. In particular auxin-induced rooting and formation of tobacco mosaic virus-induced hypersensitive response lesions are affected. BZI-1-related proteins described in various plant species share the conserved domains D1, D2, BD, and D4. To define those BZI-1 domains involved in transcription factor function, BZI-1 deletion derivatives were expressed in transgenic plants. The domains D1 or BD are crucial for BZI-1-DeltaN function in planta. The basic BD domain is mediating DNA binding of BZI-1. Yeast two-hybrid and in vitro binding studies reveal the ankyrin-repeat protein ANK1, which specifically interacts with a part of the BZI-1 protein (amino acids 73-222) encoding the D1 domain. ANK1 does not bind DNA or act as a co-activator of BZI-1-mediated transcription. Moreover, green fluorescence protein localization studies propose that ANK1 is acting mainly inside the cytosol. Transcription analysis reveals that ANK1 is ubiquitously expressed, but after pathogen attack transcription is transiently down-regulated. Along these lines, ANK1 homologous proteins in Arabidopsis thaliana have been reported to function in pathogen defense. We therefore propose that the D1 domain serves as an interaction surface for ANK1, which appears to regulate BZI-1 function in auxin signaling as well as pathogen response.

Cell wall autolytic activities and distribution of cell wall glucanases in Zea mays L. seedlings

Exo- and endo-glucanases mediate specific degradation of cell wall (1,3)(1,4)-β- d-glucans and these enzymes have been related to auxin-mediated growth and development of cereal coleoptiles. However, their distribution and functions have not been well established in other tissues. In this study the glucanase activities and cell wall autolytic activities of different maize organs were determined. Autolysis assays serve to evaluate the hydrolysis of cell wall polymers in situ by measuring the sugars released from the insoluble cell wall matrix resulting from the action of bound enzymes. Autolytic activities were observed in the cell walls of elongating young leaves, mesocotyl and roots of maize. Wall proteins extracted from all of these structures are enriched in several types of glucanases and other wall polysaccharide hydrolases. These enzymes therefore appear to have a widespread and fundamental role in wall metabolism in growing tissues

Auxin-induced K+ channel expression represents an essential step in coleoptile growth and gravitropism.

Auxin-induced growth of coleoptiles depends on the presence of potassium and is suppressed by K+ channel blockers. To evaluate the role of K+ channels in auxin-mediated growth, we isolated and functionally expressed ZMK1 and ZMK2 (Zea mays K+ channel 1 and 2), two potassium channels from maize coleoptiles. In growth experiments, the time course of auxin-induced expression of ZMK1 coincided with the kinetics of coleoptile elongation. Upon gravistimulation of maize seedlings, ZMK1 expression followed the gravitropic-induced auxin redistribution. K+ channel expression increased even before a bending of the coleoptile was observed. The transcript level of ZMK2, expressed in vascular tissue, was not affected by auxin. In patch-clamp studies on coleoptile protoplasts, auxin increased K+ channel density while leaving channel properties unaffected. Thus, we conclude that coleoptile growth depends on the transcriptional up-regulation of ZMK1, an inwardly rectifying K+ channel expressed in the nonvascular tissue of this organ.