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PubMed2005

The F-box protein TIR1 is an auxin receptor.

The plant hormone auxin regulates diverse aspects of plant growth and development. Recent studies indicate that auxin acts by promoting the degradation of the Aux/IAA transcriptional repressors through the action of the ubiquitin protein ligase SCF(TIR1). The nature of the signalling cascade that leads to this effect is not known. However, recent studies indicate that the auxin receptor and other signalling components involved in this response are soluble factors. Using an in vitro pull-down assay, we demonstrate that the interaction between transport inhibitor response 1 (TIR1) and Aux/IAA proteins does not require stable modification of either protein. Instead auxin promotes the Aux/IAA-SCF(TIR1) interaction by binding directly to SCF(TIR1). We further show that the loss of TIR1 and three related F-box proteins eliminates saturable auxin binding in plant extracts. Finally, TIR1 synthesized in insect cells binds Aux/IAA proteins in an auxin-dependent manner. Together, these results indicate that TIR1 is an auxin receptor that mediates Aux/IAA degradation and auxin-regulated transcription.

Amino Acid Sequence↗

PubMed1998

The arabidopsis thaliana AGRAVITROPIC 1 gene encodes a component of the polar-auxin-transport efflux carrier.

Auxins are plant hormones that mediate many aspects of plant growth and development. In higher plants, auxins are polarly transported from sites of synthesis in the shoot apex to their sites of action in the basal regions of shoots and in roots. Polar auxin transport is an important aspect of auxin functions and is mediated by cellular influx and efflux carriers. Little is known about the molecular identity of its regulatory component, the efflux carrier [Estelle, M. (1996) Current Biol. 6, 1589-1591]. Here we show that mutations in the Arabidopsis thaliana AGRAVITROPIC 1 (AGR1) gene involved in root gravitropism confer increased root-growth sensitivity to auxin and decreased sensitivity to ethylene and an auxin transport inhibitor, and cause retention of exogenously added auxin in root tip cells. We used positional cloning to show that AGR1 encodes a putative transmembrane protein whose amino acid sequence shares homologies with bacterial transporters. When expressed in Saccharomyces cerevisiae, AGR1 promotes an increased efflux of radiolabeled IAA from the cells and confers increased resistance to fluoro-IAA, a toxic IAA-derived compound. AGR1 transcripts were localized to the root distal elongation zone, a region undergoing a curvature response upon gravistimulation. We have identified several AGR1-related genes in Arabidopsis, suggesting a global role of this gene family in the control of auxin-regulated growth and developmental processes.

Amino Acid Sequence↗

PubMed1998

Abscisic acid-induced stomatal closure mediated by cyclic ADP-ribose.

Abscisic acid (ABA) is a plant hormone involved in the response of plants to reduced water availability. Reduction of guard cell turgor by ABA diminishes the aperture of the stomatal pore and thereby contributes to the ability of the plant to conserve water during periods of drought. Previous work has demonstrated that cytosolic Ca2+ is involved in the signal transduction pathway that mediates the reduction in guard cell turgor elicited by ABA. Here we report that ABA uses a Ca2+-mobilization pathway that involves cyclic adenosine 5'-diphosphoribose (cADPR). Microinjection of cADPR into guard cells caused reductions in turgor that were preceded by increases in the concentration of free Ca2+ in the cytosol. Patch clamp measurements of isolated guard cell vacuoles revealed the presence of a cADPR-elicited Ca2+-selective current that was inhibited at cytosolic Ca2+ >/= 600 nM. Furthermore, microinjection of the cADPR antagonist 8-NH2-cADPR caused a reduction in the rate of turgor loss in response to ABA in 54% of cells tested, and nicotinamide, an antagonist of cADPR production, elicited a dose-dependent block of ABA-induced stomatal closure. Our data provide definitive evidence for a physiological role for cADPR and illustrate one mechanism of stimulus-specific Ca2+ mobilization in higher plants. Taken together with other recent data [Wu, Y., Kuzma, J., Marechal, E., Graeff, R., Lee, H. C., Foster, R. & Chua, N.-H. (1997) Science 278, 2126-2130], these results establish cADPR as a key player in ABA signal transduction pathways in plants.

Journal Article↗

PubMed2006

Dynamic analysis of ABA accumulation in relation to the rate of ABA catabolism in maize tissues under water deficit.

The plant hormone abscisic acid (ABA) accumulates in plant tissues which experience water deficit (stress ABA). This study analysed its accumulation as a function of both synthesis and catabolism in maize tissues. By following the disappearance of the stress ABA when ABA synthesis was blocked by nordihydroguaiaretic acid (NDGA), the rate of the catabolism of stress ABA was determined. When compared with the catabolic rate of baseline (non-stress) ABA, stress ABA showed a catabolic rate >11 times higher. With such an elevated catabolic rate, it is proposed that the xanthophyll precursor pool may not be able to sustain the ABA accumulation, and such a proposition has been substantiated by further experiments where fluridone is used to limit the availability of upstream ABA precursors. When fluridone was used, stress ABA accumulation could only be sustained for a few hours, i.e. approximately 5 h for leaf and 1 h for root tissues. In detached roots, stress ABA accumulation could not be sustained even if fluridone was not used, suggesting that stress ABA accumulation in root systems requires the continuous import of ABA precursors from the shoots. Such an assumption was substantiated by the observation that defoliation or shading significantly reduced ABA accumulation in intact roots. The present study suggests that ABA catabolism is rapid enough to play an important role in the regulation of ABA accumulation.

Abscisic Acid↗

PubMed2002

Trp-dependent auxin biosynthesis in Arabidopsis: involvement of cytochrome P450s CYP79B2 and CYP79B3.

The plant hormone auxin regulates many aspects of plant growth and development. Although several auxin biosynthetic pathways have been proposed, none of these pathways has been precisely defined at the molecular level. Here we provide in planta evidence that the two Arabidopsis cytochrome P450s, CYP79B2 and CYP79B3, which convert tryptophan (Trp) to indole-3-acetaldoxime (IAOx) in vitro, are critical enzymes in auxin biosynthesis in vivo. IAOx is thus implicated as an important intermediate in auxin biosynthesis. Plants overexpressing CYP79B2 contain elevated levels of free auxin and display auxin overproduction phenotypes. Conversely, cyp79B2 cyp79B3 double mutants have reduced levels of IAA and show growth defects consistent with partial auxin deficiency. Together with previous work on YUCCA, a flavin monooxygenase also implicated in IAOx production, and nitrilases that convert indole-3-acetonitrile to auxin, this work provides a framework for further dissecting auxin biosynthetic pathways and their regulation.

Arabidopsis↗

PubMed1999

Identification of an SCF ubiquitin-ligase complex required for auxin response in Arabidopsis thaliana.

The plant hormone auxin regulates diverse aspects of plant growth and development. We report that in Arabidopsis, auxin response is dependent on a ubiquitin-ligase (E3) complex called SCFTIR1. The complex consists of proteins related to yeast Skp1p and Cdc53p called ASK and AtCUL1, respectively, as well as the F-box protein TIR1. Mutations in either ASK1 or TIR1 result in decreased auxin response. Further, overexpression of TIR1 promotes auxin response suggesting that SCFTIR1 is limiting for the response. These results provide new support for a model in which auxin action depends on the regulated proteolysis of repressor proteins.

Amino Acid Sequence↗

PubMed2002

Abscisic acid determines basal susceptibility of tomato to Botrytis cinerea and suppresses salicylic acid-dependent signaling mechanisms.

Abscisic acid (ABA) is one of the plant hormones involved in the interaction between plants and pathogens. In this work, we show that tomato (Lycopersicon esculentum Mill. cv Moneymaker) mutants with reduced ABA levels (sitiens plants) are much more resistant to the necrotrophic fungus Botrytis cinerea than wild-type (WT) plants. Exogenous application of ABA restored susceptibility to B. cinerea in sitiens plants and increased susceptibility in WT plants. These results indicate that ABA plays a major role in the susceptibility of tomato to B. cinerea. ABA appeared to interact with a functional plant defense response against B. cinerea. Experiments with transgenic NahG tomato plants and benzo(1,2,3)thiadiazole-7-carbothioic acid demonstrated the importance of salicylic acid in the tomato-B. cinerea interaction. In addition, upon infection with B. cinerea, sitiens plants showed a clear increase in phenylalanine ammonia lyase activity, which was not observed in infected WT plants, indicating that the ABA levels in healthy WT tomato plants partly repress phenylalanine ammonia lyase activity. In addition, sitiens plants became more sensitive to benzo(1,2,3)thiadiazole-7-carbothioic acid root treatment. The threshold values for PR1a gene expression declined with a factor 10 to 100 in sitiens compared with WT plants. Thus, ABA appears to negatively modulate the salicylic acid-dependent defense pathway in tomato, which may be one of the mechanisms by which ABA levels determine susceptibility to B. cinerea.

Abscisic Acid↗

PubMed2001

Flavonoids act as negative regulators of auxin transport in vivo in arabidopsis.

Polar transport of the plant hormone auxin controls many aspects of plant growth and development. A number of synthetic compounds have been shown to block the process of auxin transport by inhibition of the auxin efflux carrier complex. These synthetic auxin transport inhibitors may act by mimicking endogenous molecules. Flavonoids, a class of secondary plant metabolic compounds, have been suggested to be auxin transport inhibitors based on their in vitro activity. The hypothesis that flavonoids regulate auxin transport in vivo was tested in Arabidopsis by comparing wild-type (WT) and transparent testa (tt4) plants with a mutation in the gene encoding the first enzyme in flavonoid biosynthesis, chalcone synthase. In a comparison between tt4 and WT plants, phenotypic differences were observed, including three times as many secondary inflorescence stems, reduced plant height, decreased stem diameter, and increased secondary root development. Growth of WT Arabidopsis plants on naringenin, a biosynthetic precursor to those flavonoids with auxin transport inhibitor activity in vitro, leads to a reduction in root growth and gravitropism, similar to the effects of synthetic auxin transport inhibitors. Analyses of auxin transport in the inflorescence and hypocotyl of independent tt4 alleles indicate that auxin transport is elevated in plants with a tt4 mutation. In hypocotyls of tt4, this elevated transport is reversed when flavonoids are synthesized by growth of plants on the flavonoid precursor, naringenin. These results are consistent with a role for flavonoids as endogenous regulators of auxin transport.

Alleles↗

PubMed2003

IBR5, a dual-specificity phosphatase-like protein modulating auxin and abscisic acid responsiveness in Arabidopsis.

Auxin is an important plant hormone that plays significant roles in plant growth and development. Although numerous auxin-response mutants have been identified, auxin signal transduction pathways remain to be fully elucidated. We isolated ibr5 as an Arabidopsis indole-3-butyric acid-response mutant, but it also is less responsive to indole-3-acetic acid, synthetic auxins, auxin transport inhibitors, and the phytohormone abscisic acid. Like certain other auxin-response mutants, ibr5 has a long root and a short hypocotyl when grown in the light. In addition, ibr5 displays aberrant vascular patterning, increased leaf serration, and reduced accumulation of an auxin-inducible reporter. We used positional information to determine that the gene defective in ibr5 encodes an apparent dual-specificity phosphatase. Using immunoblot and promoter-reporter gene analyses, we found that IBR5 is expressed throughout the plant. The identification of IBR5 relatives in other flowering plants suggests that IBR5 function is conserved throughout angiosperms. Our results suggest that IBR5 is a phosphatase that modulates phytohormone signal transduction and support a link between auxin and abscisic acid signaling pathways.

Abscisic Acid↗

PubMed2003

SIR1, an upstream component in auxin signaling identified by chemical genetics.

Auxin is a plant hormone that regulates many aspects of plant growth and development. We used a chemical genetics approach to identify SIR1, a regulator of many auxin-inducible genes. The sir1 mutant was resistant to sirtinol, a small molecule that activates many auxin-inducible genes and promotes auxin-related developmental phenotypes. SIR1 is predicted to encode a protein composed of a ubiquitin-activating enzyme E1-like domain and a Rhodanese-like domain homologous to that of prolyl isomerase. We suggest a molecular context for how the auxin signal is propagated to exert its biological effects.

Adenosine Triphosphate↗

PubMed1997

A Legume Ethylene-Insensitive Mutant Hyperinfected by Its Rhizobial Symbiont

Development of the Rhizobium-legume symbiosis is controlled by the host plant, although the underlying mechanisms have remained obscure. A mutant in the annual legume Medicago truncatula exhibits an increase of more than an order of magnitude in the number of persistent rhizobial infections. Physiological and genetic analyses indicate that this same mutation confers insensitivity to the plant hormone ethylene for multiple aspects of plant development, including nodulation. These data support the hypothesis that ethylene is a component of the signaling pathway controlling rhizobial infection of legumes.

Journal Article↗

PubMed2005

Elimination of marker genes from transgenic plants using MAT vector systems.

We have developed an efficient system (Multi-Auto-Transformation [MAT] vectors) for the removal of marker genes and to increase the regeneration frequency of transgenic crops without using antibiotic selection, reducing their possible environmental impact. The MAT vector system is designed to use the oncogenes (ipt, iaaM/H, rol) of Agrobacterium, which control the endogenous levels of plant hormones and the cell responses to plant growth regulators, to differentiate transgenic cells, and to select marker-free transgenic plants. The oncogenes are combined with the site-specific recombination system (R/RS). At transformation, the oncogenes regenerate transgenic plants and then are removed by the R/RS system to generate marker-free transgenic plants. The choice of a promoter for the oncogenes and the recombinase (R) gene, the state of plant materials and the tissue culture conditions greatly affect efficiency of both the regeneration of transgenic plants and the generation of marker-free plants. We have evaluated these conditions in several plant species to increase their generation efficiency. This chapter describes our transformation protocols using MAT vectors.

Cloning, Molecular↗

PubMed1995

The senescence of oat leaf segments is promoted under simulated microgravity condition on a three-dimensional clinostat.

Plants have evolved on the earth, indicating the morphology, growth and development, and life cycle of plants are highly influenced by gravity as well as other environmental stimuli. Indeed, simulated microgravity on a clinostat or hypergravity on a centrifuge has recently been reported to change the growth and development of plants (Hoson et al. 1992, 1993, 1995, Rasmussen et al. 1994, Kasahara et al. 1995). Senescence is a final drastic phenomenon in life cycle of plants, which is characterized by the loss of total chlorophyll and protein, and/or the formation of the abscission (Osborne 1973, Thimann 1977, Addicott 1982). Many environmental stimuli as well as the qualitative and quantitative changes of plant hormones have been reported to affect plant senescence. Among those stimuli, light is the most important factor to regulate plant senescence (Leopold 1964). Dark condition promotes leaf senescence due to the decrease in endogenous level of cytokinin and/or the increase in that of abscisic acid or ethylene (Tetley and Thimann 1974, Gepstein and Thimann 1980). However, there are few reports concerning the effect of gravity on leaf senescence. Strenuous effort to learn leaf senescence under microgravity condition has been done using a three-dimensional (3-D) clinostat. In this paper, we report that simulated microgravity condition on a 3-D clinostat promoted the senescence of oat leaf segments in the dark. A possible mechanism of microgravity condition on promoting the senescence is also discussed.

Abscisic Acid↗

PubMed1976

Considerations of geotropism in plants.

Four periods of research in the reaction of plants to gravity are distinguished: (1) An early period of purely mechanistic interpretation of the phenomena observed. This period, starting in 1700, is characterized by studies of French and British plant physiologists. They demonstrated that gravity influenced the growth direction of plant parts. (2) In 1868 Frank introduced the term geotropism and postulated an active, peculiar force within the plant which causes a physiological reaction of the plant organ. This is the beginning of the second period, directed by consideration of the physiology of irritability. The elucidation of the stimulus-reaction chain, a quantitative description of the phenomena observed, including terminological questions, and the search for geosensors (statolith starch) dominate this "German" period. (3) With the detection of plant hormones to regulate cell elongation in plants at the beginning of the 20th century, the third period begins with the lateral-auxin-distribution theory of Cholodny and Went (1926). (4) After a "silent" period of 25 years research in geotropism of plants was revived with modern, critical studies using refined methods. It was found that there might be different principles of geo-reactions in unicellular geotropic plant organs, in roots, and shoots, respectively. In shoots the asymmetric distribution of auxin might be caused by differences in the longitudinal auxin transport, possibly as a result of an asymmetry of active Golgi vesicles. This hypothesis is discussed in connection with some recent papers on this subject.

Biological Transport↗

PubMed2002

Transcriptional profiling reveals novel interactions between wounding, pathogen, abiotic stress, and hormonal responses in Arabidopsis.

Mechanical wounding not only damages plant tissues, but also provides pathways for pathogen invasion. To understand plant responses to wounding at a genomic level, we have surveyed the transcriptional response of 8,200 genes in Arabidopsis plants. Approximately 8% of these genes were altered by wounding at steady-state mRNA levels. Studies of expression patterns of these genes provide new information on the interactions between wounding and other signals, including pathogen attack, abiotic stress factors, and plant hormones. For example, a number of wound-responsive genes encode proteins involved in pathogen response. These include signaling molecules for the pathogen resistance pathway and enzymes required for cell wall modification and secondary metabolism. Many osmotic stress- and heat shock-regulated genes were highly responsive to wounding. Although a number of genes involved in ethylene, jasmonic acid, and abscisic acid pathways were activated, many in auxin responses were suppressed by wounding. These results further dissected the nature of mechanical wounding as a stress signal and identified new genes that may play a role in wounding and other signal transduction pathways.

Abscisic Acid↗

PubMed2004

Chemical nursing: phytosulfokine improves genetic transformation efficiency by promoting the proliferation of surviving cells on selective media.

The relative growth rate of plant cells in vitro is considerably affected by initial cell density. This troublesome effect has interfered with the establishment of efficient plant cell culture systems, especially when only a small number of cells are expected to survive, such as in the genetic transformation of cells under antibiotic selection. To improve the recovery of antibiotic-resistant cells, we examined the use of the peptide plant hormone phytosulfokine (PSK), which has been shown to promote cellular growth and development in vitro. The addition of PSK to selective media increased the recovery of transformed callus from Agrobacterium-infected carrot hypocotyl explants from 7% to 39%, which is more than a fivefold improvement over the control. Most calluses developed into normal plantlets with cotyledons and primary roots and, eventually, formed foliage leaves. Thus, chemical nursing using PSK shows promise as a tool for basic research in plant biology and biotechnological applications.

Anti-Bacterial Agents↗

PubMed1997

The ethylene signal transduction pathway in Arabidopsis.

The gaseous hormone ethylene is an important regulator of plant growth and development. Using a simple response of etiolated seedlings to ethylene as a genetic screen, genes involved in ethylene signal transduction have been identified in Arabidopsis. Analysis of two of these genes that have been cloned reveals that ethylene signalling involves a combination of a protein (ETR1) with similarity to bacterial histidine kinases and a protein (CTR1) with similarity to Raf-1, a protein kinase involved in multiple signalling cascades in eukaryotic cells. Several lines of investigation provide compelling evidence that ETR1 encodes an ethylene receptor. For the first time there is a glimpse of the molecular circuitry underlying the signal transduction pathway for a plant hormone.

Arabidopsis↗

PubMed2026

Genome-Wide Analysis of the PYL Gene Family and Its Expression Dynamics in Response to Abscisic Acid in Tomato.

The plant hormone abscisic acid (ABA) plays a crucial role throughout the plant life cycle and in adaptive responses to environmental stresses. The pyrabactin resistance 1-like (PYR/PYL/RCAR) proteins act as key regulators in the ABA signal transduction pathway by functioning as direct receptors for ABA. Although PYL genes have been identified in a variety of plant species, their evolutionary and structural characteristics in tomatoes (Solanum lycopersicum) remain elusive. To address this gap, we identified nine SlPYL genes, which were classified into three subfamilies: I (two genes), II (three genes), and III (four genes), and their encoded proteins were predicted to be primarily localized in the cytosol and chloroplast. Structural analysis revealed diverse exon-intron organizations along with five conserved motifs. All identified SlPYLs contained the START domain (PF10604), validating their identity as actual PYL proteins. Prediction of cis-acting regulatory elements in SlPYL's promoter regions was found to be associated with light responsiveness, hormone signaling, stress responses, and plant growth and development. Prediction of post-translational modification sites indicated that SlPYLs are predominantly phosphorylated and acetylated at serine and lysine residues, respectively. Tertiary structure modeling demonstrated conserved three-dimensional architectures among SlPYL proteins, supporting their functional conservation. Expression profiling revealed that specific SlPYL genes exhibit distinct expression patterns across different tissues (root, leaf, and bud) following ABA treatment, indicating functional diversification. Considering the well-established negative correlation between ABA accumulation and bud outgrowth, the ABA-induced differential expression (3~5-fold) of some SlPYL genes (SlPYL3, SlPYL4, SlPYL7, and SlPYL8), particularly in bud tissues after 24 hpt, suggests a potential role in ABA-mediated suppression of bud outgrowth. However, these functional inferences are primarily based on genome-wide computational analyses and expression profiling and therefore require further experimental validation.

Solanum lycopersicum↗