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PubMed2002

Complex regulation of ABA biosynthesis in plants.

Abscisic acid (ABA) is a plant hormone that plays important roles during many phases of the plant life cycle, including seed development and dormancy, and in plant responses to various environmental stresses. Because many of these physiological processes are correlated with endogenous ABA levels, the regulation of ABA biosynthesis is a key element facilitating the elucidation of these physiological characteristics. Recent studies on the identification of genes encoding enzymes involved in ABA biosynthesis have revealed details of the main ABA biosynthetic pathway. At the same time, the presence of gene families and their respective organ-specific expression are indicative of the complex mechanisms governing the regulation of ABA biosynthesis in response to plant organ and/or environmental conditions. There have been recent advances in the study of ABA biosynthesis and new insights into the regulation of ABA biosynthesis in relation to physiological phenomena.

Abscisic Acid↗

PubMed1993

Arabidopsis auxin-resistance gene AXR1 encodes a protein related to ubiquitin-activating enzyme E1.

The plant hormone auxin has a central role in many aspects of plant growth and development. By screening for mutants of Arabidopsis that are resistant to exogenous auxin, we have identified several genes that are required for normal auxin response. One of these genes, AXR1, is defined by recessive mutations that confer auxin resistance to the roots, rosettes and inflorescences of mutant plants. In addition, axr1 mutants display a variety of morphological defects that are consistent with a reduction in auxin sensitivity. Here we isolate the AXR1 gene using a map-based approach and report that AXR1 encodes a new protein with significant sequence similarity to the ubiquitin-activating enzyme E1. The AXR1 protein is highly diverged from previously characterized E1 enzymes, however, and lacks a key cysteine residue that is essential for E1 activity. AXR1 may therefore define a new class of enzymes in the ubiquitin pathway or it may have a novel function in cellular regulation which is unrelated to ubiquitin conjugation.

Amino Acid Sequence↗

PubMed2003

The case for morphogens in plants.

Plants and animals have evolved as multicellular organisms independently of one another. This raises the intriguing question of whether plants and animals have developed similar or distinct patterning strategies to establish their body plans. Animals use concentration gradients of signals termed morphogens for tissue patterning, but whether they are also used by plants is unclear. Here we compare and contrast the plant hormone auxin with animal morphogens, and speculate as to whether plants have independently evolved similar mechanisms to regulate pattern formation.

Animals↗

PubMed1992

Isolation of a wheat cDNA clone for an abscisic acid-inducible transcript with homology to protein kinases.

Increases in the plant hormone abscisic acid (ABA) initiate water-stress responses in plants. We present evidence that a transcript with homology to protein kinases is induced by ABA and dehydration in wheat. A 1.2-kilobase cDNA clone (PKABA1) was isolated from an ABA-treated wheat embryo cDNA library by screening the library with a probe developed by polymerase chain reaction amplification of serine/threonine protein kinase subdomains VIb to VIII. The deduced amino acid sequence of the PKABA1 clone contains the features of serine/threonine protein kinases, including homology with all 12 conserved regions of the catalytic domain. PKABA1 transcript levels are barely detectable in growing seedlings but are induced dramatically when plants are subjected to dehydration stress. The PKABA1 transcript can also be induced by supplying low concentrations of ABA, and coordinate increases in ABA levels and PKABA1 mRNA occur when seedlings are water-stressed. Identification of this ABA-inducible transcript with homology to protein kinases provides a basis for examining the role of protein phosphorylation in plant responses to dehydration.

Abscisic Acid↗

PubMed2002

Generation and possible roles of NO in plant roots and their apoplastic space.

In recent years, three different enzymatic pathways and a few non-enzymatic reactions have been proposed for the generation of NO in plant roots. Two of the enzymatic pathways are located in the cytosol of the plant cells, whereas the third is exclusively located in the root plasma membrane facing the apoplast from where it seems to interact with nitrate metabolism by producing signals. A response of the NO pathways to external nitrate concentrations by preventing excess nitrate nutrition, particularly in the apoplast, as well as a regulatory role in root morphogenesis of NO in interaction with plant hormones is suggested. Other functions of NO, those in stimulating plant defence reactions against pathogens and against abiotic stress are reported. In addition to enzymatic NO formation by the plant, sources of NO in the soil, and hence in the rhizosphere from bacterial nitrification and denitrification, are discussed in view of their possible interaction with the plant roots. A synoptical perspective is given on the assumed roles of apoplastic NO in plant roots, based upon known facts and with some assumptions about the gaps in current knowledge.

Nitric Oxide↗

PubMed2006

Global effect of indole-3-acetic acid biosynthesis on multiple virulence factors of Erwinia chrysanthemi 3937.

Production of the plant hormone indole-3-acetic acid (IAA) is widespread among plant-associated microorganisms. The non-gall-forming phytopathogen Erwinia chrysanthemi 3937 (strain Ech3937) possesses iaaM (ASAP16562) and iaaH (ASAP16563) gene homologues. In this work, the null knockout iaaM mutant strain Ech138 was constructed. The IAA production by Ech138 was reduced in M9 minimal medium supplemented with l-tryptophan. Compared with wild-type Ech3937, Ech138 exhibited reduced ability to produce local maceration, but its multiplication in Saintpaulia ionantha was unaffected. The pectate lyase production of Ech138 was diminished. Compared with wild-type Ech3937, the expression levels of an oligogalacturonate lyase gene, ogl, and three endopectate lyase genes, pelD, pelI, and pelL, were reduced in Ech138 as determined by a green fluorescent protein-based fluorescence-activated cell sorting promoter activity assay. In addition, the transcription of type III secretion system (T3SS) genes, dspE (a putative T3SS effector) and hrpN (T3SS harpin), was found to be diminished in the iaaM mutant Ech138. Compared with Ech3937, reduced expression of hrpL (a T3SS alternative sigma factor) and gacA but increased expression of rsmA in Ech138 was also observed, suggesting that the regulation of T3SS and pectate lyase genes by IAA biosynthesis might be partially due to the posttranscriptional regulation of the Gac-Rsm regulatory pathway.

Bacterial Proteins↗

PubMed1995

Synthesis of phytohormones by plant-associated bacteria.

The plant hormones, auxins and cytokinins, are involved in several stages of plant growth and development such as cell elongation, cell division, tissue differentiation, and apical dominance. The biosynthesis and the underlying mechanism of auxins and cytokinins action are subjects of intense investigation. Not only plants but also microorganisms can synthesize auxins and cytokinins. The role of phytohormone biosynthesis by microorganisms is not fully elucidated: in several cases of pathogenic fungi and bacteria these compounds are involved in pathogenesis on plants; auxin and cytokinin production may also be involved in root growth stimulation by beneficial bacteria and associative symbiosis. The genetic mechanism of auxin biosynthesis and regulation by Pseudomonas, Agrobacterium, Rhizobium, Bradyrhizobium, and Azospirillum, are well studied; in these bacteria several physiological effects have been correlated to the bacterial phytohormones biosynthesis. The pathogenic bacteria Pseudomonas and Agrobacterium produce indole-3-acetic acid via the indole-3-acetamide pathway, for which the genes are plasmid borne. However, they do possess also the indole-3-pyruvic acid pathway, which is chromosomally encoded. In addition, they have genes that can conjugate free auxins or hydrolyze conjugated forms of auxins and cytokinins. In Agrobacterium there are also several genes, located near the auxin and cytokinin biosynthetic genes, that are involved in the regulation of auxins and cytokinins sensibility of the transformed plant tissue. Symbiotic bacteria Rhizobium and Bradyrhizobium synthesize indole-3-acetic acid via indole-3-pyruvic acid; also the genetic determinants for the indole-3-acetamide pathway have been detected, but their activity has not been demonstrated. In the plant growth-promoting bacterium Azospirillum, as in Agrobacterium and Pseudomonas, both the indole-3-pyruvic acid and the indole-3-acetamide pathways are present, although in Azospirillum the indole-3-pyruvic acid pathway is of major significance. In addition, biochemical evidence for a tryptophan-independent indole-3-acetic acid pathway in Azospirillum has been presented.

Azospirillum↗

PubMed1983

Genotoxic activity of plant growth-regulating hormones in Aspergillus nidulans.

Three plant growth-regulating hormones, indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and kinetin (6-furfuryl-aminopurine), were tested for their genetic activity in Aspergillus nidulans in a plate test. The first two hormones were found to greatly increase somatic segregation in the fungus whereas kinetin was not effective. Several concentrations of the plant hormones were used and it was found that increasing concentrations of IAA and IBA increased mitotic segregation of the fungus with most of the segregants being produced by mitotic crossing-over, together with non-disjunctional segregants at a lower level. The metabolic activation technique was also used and it was shown that when S9 mixture was added to IAA and IBA a further 3- to 5-fold increase in the number of segregants was obtained. In the case of kinetin the S9 had no effect.

Aspergillus nidulans↗

PubMed2003

Simple hormones but complex signalling.

It has not been easy to make sense of the pleiotropic effects of plant hormones, especially of auxins; but now, it has become possible to study these effects within the framework of what we know about signal transduction in general. Changes in local auxin concentrations, perhaps even actively maintained auxin gradients, signal to networks of transcription factors, which in turn signal to downstream effectors. Transcription factors can also signal back to hormone biosynthetic pathways.

Biological Transport↗

PubMed2006

The presence of jasmonate-inducible lectin genes in some but not all Nicotiana species explains a marked intragenus difference in plant responses to hormone treatment.

Tobacco (Nicotiana tabacum L. cv Samsun NN) leaves accumulate a cytoplasmic/nuclear lectin, called Nictaba, in response to methyl jasmonate. To check whether, and if so to what extent, the specific induction of this lectin applies to related species, a collection of 19 Nicotiana species--covering 12 Nicotiana sections and eight Nicotiana tabacum cultivars--was screened for their capability to synthesize the jasmonate-inducible lectin. Protein analyses by agglutination assays and western blot confirmed that only nine out of the 19 species examined synthesize lectin after jasmonate treatment. Remarkably, all allotetraploid cultivars of the N. tabacum L. species tested express the lectin after jasmonate treatment. PCR analyses demonstrated that all responsive species possess one or more lectin genes, whereas no lectin gene(s) could be traced in the non-responding species. The number of introns present in the lectin genes varies between zero and two. Four tobacco species/cultivars contain both intronless Nictaba genes as well as lectin genes with introns. These findings provide the first firm evidence for a striking intragenus difference with respect to the activation of a well-defined jasmonate-inducible gene that can be correlated with the presence/absence of orthologous genes in the genomes of closely related species from a single plant genus. In addition, the differential response of closely related tobacco species illustrates that in the field of plant hormone research, care must be taken when extrapolating results obtained with a particular model system to other--even taxonomically closely related--species.

Acetates↗

PubMed2000

MAP kinases in plant signal transduction.

Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular targets in all eukaryotes. Distinct MAPK pathways are regulated by different extracellular stimuli and are implicated in a wide variety of biological processes. In plants there is evidence for MAPKs playing a role in the signaling of abiotic stresses, pathogens, plant hormones, and cell cycle cues. The large number and divergence of plant MAPKs indicates that this ancient mechanism of bioinformatics is extensively used in plants and their study promises to give molecular answers to old questions.

MAP Kinase Signaling System↗

PubMed1991

The discovery of novel auxin transport inhibitors by molecular modeling and three-dimensional pattern analysis.

Molecular modeling techniques and three-dimensional (3D) pattern analysis have been used to investigate the chemical and steric properties of compounds that inhibit transport of the plant hormone auxin. These compounds bind to a specific site on the plant plasma membrane characterized by its affinity for the herbicide N-1-naphthylphthalamic acid (NPA). A 3D model was derived from critical features of a set of ligands for the NPA receptor, a suggested binding conformation is proposed, and implications for the topographical features of the NPA receptor are discussed. This model, along with 3D structural analysis techniques, was then used to search the Abbott corporate database of chemical structures. Of the 467 compounds that satisfied the criteria of the model, 77 representative molecules were evaluated for their ability to compete for the binding of [3H]NPA to corn microsomal membranes. Nineteen showed activity that ranged from 16 to 85% of the maximum NPA binding. Four of the most active of these, representing chemical classes not included in the original compound set, were also found to inhibit polar auxin transport through corn coleoptile sections. Thus, this study demonstrates that 3D analysis techniques can identify active, novel ligands for biochemical target sites with concomitant physiological activity.

Binding Sites↗

PubMed1998

Unusual membrane-associated protein kinases in higher plants.

Plant genomes encode a variety of protein kinases, and while some are functional homologues of animal and fungal kinases, others have a novel structure. This review focuses on three groups of unusual membrane-associated plant protein kinases: receptor-like protein kinases (RLKs), calcium-dependent protein kinases (CDPKs), and histidine protein kinases. Animal RLKs have a putative extracellular domain, a single transmembrane domain, and a protein kinase domain. In plants, all of the RLKs identified thus far have serine/threonine signature sequences, rather than the tyrosine-specific signature sequences common to animals. Recent genetic experiments reveal that some of these plant kinases function in development and pathogen resistance. The CDPKs of plants and protozoans are composed of a single polypeptide with a protein kinase domain fused to a C-terminal calmodulin-like domain containing four calcium-binding EF hands. No functional plant homologues of protein kinase C or Ca2+/calmodulin-dependent protein kinase have been identified, and no animal or fungal CDPK homologues have been identified. Recently, histidine kinases have been shown to participate in signaling pathways in plants and fungi. ETR1, an Arabidopsis histidine kinase homologue with three transmembrane domains, functions as a receptor for the plant hormone ethylene. G-protein-coupled receptors, which often serve as hormone receptors in animal systems, have not yet been identified in plants.

Arabidopsis↗

PubMed1978

Cytotoxic effects of maleic hydrazide.

Since 1950, maleic hydrazide (MH) has been introduced into agriculture as a major commercial herbicide and a depressant of plant growth in numerous circumstances such as suppression of sprouting of vegetables and stored food crops, control of sucker growth on tobacco plants, ratardation of flowering and prolongation of dormancy period. Since 1951 MH has been known as an effective chromosome-breaking agent in higher plants, in sharp contrast with its low effect on the chromosomes and general health of tested mammals. The selectivity of action of MH in plants and animals was obviously the main reason of low interest devoted to the chemical by people working the field of environmental mutagenesis. In early works the inhibitory effects of MH on plant growth were mainly considered to result from the suppression of plant metabolism (inhibition of enzymic activity) and interference of the compound with plant hormones and growth regulators. More recently, numerous experiments performed with various plant species have shown that MH acts as an inhibitor of the synthesis of nucleic acids and proteins. Similar results have been obtained with animal tumour cells. The chromosome-breaking effect of MH on plant chromosomes resembles very closely the chromosome-breaking properties of alkylating agents and other mutagenic compounds such as mitomycin C. MH-induced chromosomal aberrations have also been recorded in grasshoppers, fish and mice, although tests with some mammalian cell lines gave negative results. Among higher plants, selective sensitivity to the toxic effects of MH is well proved. This phenomenon seems to be due to the differential ability of various plant species to detoxicate the chemical. Plants can break down MH into several products, one of which, hydrazine, is a well-known mutagen and carcinogen. MH does not seem to be toxic to bacteria and fungi. The compound is degraded by soil microflora and hence can be utilized as a source of nitrogen nutrition. MH proved to be of low toxicity to mammals, but in some instances it decreased the fertility of rats. The reported carcinogenic effects of MH in mice and rats raise the question of its risks to man.

Animals↗

PubMed2002

Expression of Oryza sativa MAP kinase gene is developmentally regulated and stress-responsive.

Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular targets in all eukaryotes. In plants, there is evidence for MAPKs playing a role in the signalling of abiotic stresses, pathogens, plant hormones, and cell cycle cues. The large number and divergence of plant MAPKs indicates that this ancient mechanism of signal transduction is extensively used in plants. However, there have been no reports of classical MAPK module in rice. In this report, we have isolated a MAPK from rice (Oryza sativa) termed OsMAPK2. The cloned cDNA is 1457 nucleotides long and the deduced amino acid sequence comprised 369 amino acid residues. Sequence analysis revealed that the predicted amino acid sequence is 72% identical to tobacco wound-induced protein kinase (WIPK). Southern analysis suggested a single OsMAPK2 gene in rice. Analysis at the mRNA level has shown that OsMAPK2 is expressed in all plant organs and high relative amounts of OsMAPK2 were detected in the mature panicles in comparison with in the immature panicles. In suspension-cultured cells, the OsMAPK2 mRNA transcript increased markedly upon temperature downshift from 26 degrees C to 4 degrees C and sucrose starvation. In contrast, the OsMAPK2 mRNA level rapidly declined in rice cell challenged by high temperature. A similarly rapid response of OsMAPK2 was observed in stress-treated seedlings, demonstrating that response of the MAPK pathway occurs also in intact plants. These results suggest that this OsMAPK2 may function in the stress-signalling pathway as well as panicle development in rice.

Journal Article↗

PubMed2002

Phosphate sensing in higher plants.

Phosphate (Pi) plays a central role as reactant and effector molecule in plant cell metabolism. However, Pi is the least accessible macronutrient in many ecosystems and its low availability often limits plant growth. Plants have evolved an array of molecular and morphological adaptations to cope with Pi limitation, which include dramatic changes in gene expression and root development to facilitate Pi acquisition and recycling. Although physiological responses to Pi starvation have been increasingly studied and understood, the initial molecular events that monitor and transmit information on external and internal Pi status remain to be elucidated in plants. This review summarizes molecular and developmental Pi starvation responses of higher plants and the evidence for coordinated regulation of gene expression, followed by a discussion of the potential involvement of plant hormones in Pi sensing and of molecular genetic approaches to elucidate plant signalling of low Pi availability. Complementary genetic strategies in Arabidopsis thaliana have been developed that are expected to identify components of plant signal transduction pathways involved in Pi sensing. Innovative screening methods utilize reporter gene constructs, conditional growth on organophosphates and the inhibitory properties of the Pi analogue phosphite, which hold the promise for significant advances in our understanding of the complex mechanisms by which plants regulate Pi-starvation responses.

Journal Article↗

PubMed2001

Ectopic expression of ABI3 gene enhances freezing tolerance in response to abscisic acid and low temperature in Arabidopsis thaliana.

The plant hormone abscisic acid (ABA) regulates several physiological and developmental processes in plants, including stress adaptation and seed maturation. ABA-mediated processes appear to be central in plant cold acclimation and expression of cold acclimation-related genes. Ectopic expression of ABI3 encoding a seed-specific transcriptional activator confers on Arabidopsis vegetative tissues the ability to accumulate seed-specific transcripts in response to ABA, and also influences some ABA-mediated vegetative responses. In the present study we characterized the effect of ectopic expression of ABI3 on cold acclimation and development of freezing tolerance in Arabidopsis. We first determined the effect of ABI3 on ABA-induced expression of cold acclimation-related genes. Expression of ABI3 increased the ABA-induced accumulation of transcripts for several ABA/cold/drought-responsive genes such as RAB18 and LTI78. Enhanced expression of these genes was evident even after transient application of ABA, and the enhanced expression was correlated with increased freezing tolerance in ABI3 transgenic plants. Ectopic expression of ABI3 also appeared to modulate low temperature-induced freezing tolerance. The ABI3 transgenic plants acclimated faster than the wild-type plants, and the maximum tolerance obtained was significantly higher. These data showed that lower levels of ABA were needed to trigger the expression of the genes and to maintain the freezing-tolerant state in the ABI3 transgenic plants, and indicate that ectopic expression of ABI3 leads to enhanced responsiveness to ABA. The ectopic expression of ABI3 could provide a new strategy for engineering plant stress tolerance.

Abscisic Acid↗

PubMed2005

Gibberellin regulates mitochondrial pyruvate dehydrogenase activity in rice.

Pyruvate dehydrogenase kinase (PDK) is a negative regulator of the mitochondrial pyruvate dehydrogenase complex (mtPDC) that plays a key role in intermediary metabolism. OsPDK1 was identified as a gibberellin-up-regulated gene using a cDNA microarray. The full-length cDNA for OsPDK1 was 1498 bp and encoded a predicted polypeptide of 363 amino acids. Genomic DNA analysis showed the presence of another isoform of PDK, OsPDK2, in rice. Reverse transcriptase-PCR analysis revealed differential expression of the two isoforms. OsPDK1 was expressed in leaf blade and leaf sheath but not in callus and root, while OsPDK2 was expressed constitutively in all tissues examined. Maximum expression of OsPDK1 in leaf sheath was detected by Northern blot analysis when seedlings were treated with 5 microM GA3 for 24 h. OsPDK1 expression was up-regulated by GA3, and there was little effect of other plant hormones. Mitochondrial pyruvate dehydrogenase (PDH) activity was reduced compared with control plants in 2-week-old seedlings treated with GA3. The beta-glucuronidase (GUS) reporter gene, driven by a 2,067 bp OsPDK1 promoter region fragment, was mainly expressed in the aleurone layer of germinating seed and leaf sheath. Transgenic rice expressing PDK1 RNAi had altered vegetative growth with reduced accumulation of vegetative tissues. These results suggest that gibberellin modulates the activity of mtPDC by regulating OsPDK1 expression and subsequently controlling plant growth and development.

Blotting, Northern↗