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PubMed1993

On plant growth regulators and their metabolites: a changing perspective.

From seed germination to vegetative growth and flowering virtually all aspects of plant growth and development are influenced by structurally relatively simple substances, termed phytohormones. It has been argued that the wide range of responses elicited by these substances requires a mode of action that is radically different from those of animal hormones. In contrast to animal hormones, it is often very difficult to distinguish between the site of synthesis and the site of action of phytohormones. Hence, plants may have developed their own mechanisms for synthesis, sequestration and release of active hormones. Current evidence indicates that enzymes that can synthesize and modify phytohormones and their antagonists or hydrolyze phytohormone conjugates to release active hormones which play a role in initiating important regulatory pathways. They are also likely to provide invaluable tools for studying the mechanisms underlying growth and development in plants.

Bacteria↗
PubMed2006

Alpha-tocopherol may influence cellular signaling by modulating jasmonic acid levels in plants.

Most studies on the function of tocopherols in plants have focused on their photo-protective and antioxidant properties, and it has been recently suggested, though not yet demonstrated, that they may also play a role in cellular signaling. By using vte1 mutants of Arabidopsis thaliana, with an insertion in the promoter region of the gene encoding tocopherol cyclase, we demonstrate here for the first time that tocopherol deficiency may alter endogenous phytohormone levels in plants, thereby reducing plant growth and triggering anthocyanin accumulation in leaves. In plants grown under a combination of high light and low temperature conditions to induce anthocyanin accumulation, we evaluated age-dependent changes in tocopherols, indicators of photo-oxidative stress, phytohormone levels, plant growth and anthocyanin levels in wild type and vte1 mutants. These mutants showed lower tocopherol levels, reduced growth and enhanced anthocyanin accumulation compared with the wild type, while both the maximum and relative efficiencies of PSII, chlorophylls, and carotenoids were not significantly altered. Analyses of phytohormone levels revealed that reduced growth and enhanced anthocyanin accumulation in tocopherol-deficient plants were preceded by increased jasmonic acid levels. This is the first study suggesting a direct effect of tocopherols on phytohormones levels in plants and will undoubtedly help us to better understand the multiple functions tocopherols play in plants, as well as the cellular signaling mechanisms responsible for the phenotypes thus far described in tocopherol-deficient plants.

Anthocyanins↗
PubMed1993

Genes involved in the control of growth and differentiation in plants.

The mechanisms underlying totipotency, the unique ability of isolated plant cells to regenerate into plants, offer developmental biology a unique challenge. While it has been recognised for some time that phytohormones, such as auxin and cytokinin, play a role in this process by inducing a variety of growth patterns in both isolated cells, unorganised callus and intact plants, the molecular basis of their action remains unknown. The molecular and biochemical analysis of the novel interaction between tumour-inducing soil bacteria and the wounded plant has provided a valuable insight into how plants respond to phytohormones. During tumour formation, the bacteria transfer to the genome of the host plant a variety of genes which either short circuit the normal pathways of accumulation of phytohormones or modify how the plant cell responds to them. In parallel to these studies, we have been investigating plant genes involved directly or indirectly in the mechanism of phytohormone action. Auxin-binding proteins (putative receptors) have been localised in various cellular locations and the genes encoding them are currently undergoing analysis. Recently, a novel form of T-DNA has been devised by which mutant plant cell lines can be generated which grow in culture in the absence of exogenously applied auxin. The tagged genes, which are in effect plant cellular proto-oncogenes, are likely to shed more light on how auxin serves to regulate growth and development.

Cell Differentiation↗
PubMed1991

Hormonal modulation of plant growth: the role of auxin perception.

The organisation of growth and development in vascular plants appears to be highly adapted to meet the specific demands of a sessile, autotrophic habit. Many of the characteristic features of plant development are associated with the activities of five groups of phytohormones. Each of the phytohormones has the ability to influence fundamentally a remarkable variety of developmental and physiological processes. This ability has been widely documented but remains to be explained. Here we describe how recent breakthroughs in the analysis and understanding of eucaryotic signal transduction are being applied, in conjunction with technical advances in molecular genetics, to elucidate the molecular basis of the phytohormonal properties of auxin. Both auxin concentration, and the sensitivity of plant cells to this phytohormone have been implicated as important parameters in auxin action. We describe recent molecular biological approaches to assess the contribution made by each of these parameters. Emphasis is given to a description of recent genetic and biochemical progress towards identification of the molecular targets of the auxin signal and the molecular components involved in its subsequent transduction.

Endoplasmic Reticulum↗
PubMed2005

Regulatory and functional interactions of plant growth regulators and plant glutathione S-transferases (GSTs).

Plant glutathioneS-transferases (GSTs) are a heterogeneous superfamily of multifunctional proteins, grouped into six classes. The tau (GSTU) and phi (GSTF) class GSTs are the most represented ones and are plant-specific, whereas the smaller theta (GSTT) and zeta (GSTZ) classes are also found in animals. The lambda GSTs (GSTL) and the dehydroascorbate reductases (DHARs) are more distantly related. Plant GSTs perform a variety of pivotal catalytic and non-enzymatic functions in normal plant development and plant stress responses, roles that are only emerging. Catalytic functions include glutathione (GSH)-conjugation in the metabolic detoxification of herbicides and natural products. GSTs can also catalyze GSH-dependent peroxidase reactions that scavenge toxic organic hydroperoxides and protect from oxidative damage. GSTs can furthermore catalyze GSH-dependent isomerizations in endogenous metabolism, exhibit GSH-dependent thioltransferase safeguarding protein function from oxidative damage and DHAR activity functioning in redox homeostasis. Plant GSTs can also function as ligandins or binding proteins for phytohormones (i.e., auxins and cytokinins) or anthocyanins, thereby facilitating their distribution and transport. Finally, GSTs are also indirectly involved in the regulation of apoptosis and possibly also in stress signaling. Plant GST genes exhibit a diversity of expression patterns during biotic and abiotic stresses. Stress-induced plant growth regulators (i.e., jasmonic acid [JA], salicylic acid [SA], ethylene [ETH], and nitric oxide [NO] differentially activate GST gene expression. It is becoming increasingly evident that unique combinations of multiple, often interactive signaling pathways from various phytohormones and reactive oxygen species or antioxidants render the distinct transcriptional activation patterns of individual GSTs during stress. Underestimated post-transcriptional regulations of individual GSTs are becoming increasingly evident and roles for phytohormones (i.e., ABA and JA) in these processes are being anticipated as well. Finally, indications are emerging that NO may regulate the activity of specific plant GSTs. In this review, the current knowledge on the regulatory and functional interactions of phytohormones and plant GSTs are covered. We refer to a previous extensive review on plant GSTs (Marrs, 1996) for most earlier work. An introduction on the classification and roles of plant GSTs is included here, but these topics are more extensively discussed in other reviews (Dixon et al., 2002a; Edwards et al., 2000; Frova, 2003).

Gene Expression↗
PubMed1984

Changes in T-DNA methylation and expression are associated with phenotypic variation and plant regeneration in a crown gall tumor line.

Phenotypic variation of an octopine-type crown gall tumor line resulting from changes in the pattern of T-DNA methylation and expression is described. Variants that grow as unorganized callus always express T-DNA transcripts 1 and 2. In shoot-forming variants (teratomas) only T-DNA transcript 4 is expressed. This line also regenerates normal-appearing, rooted plants in which all T-DNA expression is suppressed. Tissues from these plants require phytohormones for growth in vitro. These plants are self-fertile and transmit T-DNA through meiosis, and T-DNA suppression is maintained in the next generation. After treatment of regenerated plant tissue with 5-azacytidine, an inhibitor of DNA methylation, T-DNA transcription and phytohormone-independent tumorous growth resume. The T-DNA of cell lines in which T-DNA is not expressed is highly methylated, whereas the level of T-DNA methylation is reduced in 5-azacytidine treated cells that resume T-DNA expression and phytohormone-independent growth. The correlation between the degree of T-DNA methylation and the level of T-DNA expression indicates that hypermethylation is responsible for the suppression of T-DNA transcription.

Azacitidine↗
PubMed1998

Cytokinin stimulates and abscisic acid inhibits greening of etiolated Lupinus luteus cotyledons by affecting the expression of the light-sensitive protochlorophyllide oxidoreductase.

Plastid biogenesis in etiolated lupine (Lupinus luteus L.) cotyledons is highly sensitive to cytokinins and abscisic acid. In the presence of the synthetic cytokinin N6-benzylaminopurine, greening and plastid biogenesis is substantially promoted as compared to untreated controls, whereas abscisic acid has an inhibitory effect. Faster greening in cytokinin-treated cotyledons is accompanied by a higher level and slower degradation of the light-sensitive protochlorophyllide-oxidoreductase (POR); while ABA has the opposite effect. The phytohormones appear to modulate POR gene expression, since the steady-state levels of POR mRNA, as well as transcripts of other nuclear genes for plastid proteins, are strongly increased by cytokinin and reduced by abscisic acid treatment. When etiolated lupine cotyledons were illuminated with far-red light prior to phytohormone application, the POR level substantially decreased; this was accompanied by the loss of the phytohormone's effect on greening. Based on these findings it is concluded that the level of POR and the integrity of the prolamellar body is crucial for cytokinin- and abscisic acid-controlled greening following transfer of etiolated lupine cotyledons into the light.

Abscisic Acid↗
PubMed2000

The Roles of Auxins and Cytokinins in Mycorrhizal Symbioses.

Most land plant species that have been examined exist naturally with a higher fungus living in and around their roots in a symbiotic partnership called a mycorrhiza. Several types of mycorrhizal symbiosis exist, defined by the host/partner combination and the morphology of the symbiotic structures. The arbuscular mycorrhiza (AM) is ancient and may have co-evolved with land plants. Emerging results from gene expression studies have suggested that subsets of AM genes were co-opted during the evolution of other biotrophic symbioses. Here we compare the roles of phytohormones in AM symbiosis and ectomycorrhizas (EC), a more recent symbiosis. To date, there is little evidence of physiologic overlap between the two symbioses with respect to phytohormone involvement. Research on AM has shown that cytokinin (CK) accumulation is specifically enhanced by symbiosis throughout the plant. We propose a pathway of events linking enhanced CK to development of the AM. Additional and proposed involvement of other phytohormones are also described. The role of auxin in EC symbiosis and recent research advances on the topic are reviewed. We have reflected the literature bias in reporting individual growth regulator effects. However, we consider that gradients and ratios of these molecules are more likely to be the causal agents of morphologic changes resulting from fungal associations. We expect that once the individual roles of these compounds are explained, the subtleties of their function will be more clearly addressed.

Journal Article↗
PubMed2000

Plant Parasitic Nematodes: Habitats, Hormones, and Horizontally-Acquired Genes.

Plant parasitic nematodes are ubiquitous and cosmopolitan pathogens of vascular plants and exploit all parts of the roots and shoots, causing substantial crop damage. Nematodes deploy a broad spectrum of feeding strategies, ranging from simple grazing to the establishment of complex cellular structures (including galls) in host tissues. Various models of feeding site formation have been proposed, and a role for phytohormones has long been speculated, although whether they perform a primary or secondary function is unclear. On the basis of recent molecular evidence, we present several scenarios involving phytohormones in the induction of giant cells by root-knot nematode. The origin of parasitism by nematodes, including the acquisition of genes to synthesize or modulate phytohormones also is discussed, and models for horizontal gene transfer are presented.

Journal Article↗