Cytokinin biosynthesis: A black box?

The biosynthetic origin of cytokinins (CKs) in plants has been debated ever since the recognition of CKs as plant hormones. Although several possible biosynthetic pathways have been suggested, none of the data published to date are conclusive. The enzymes involved in CK biosynthesis have not yet been purified and characterized in detail, nor have plant genes encoding CK biosynthetic enzymes been identified.     

Light-induced expression of ipt from Agrobacterium tumefaciens results in cytokinin accumulation and osmotic stress symptoms in transgenic tobacco

Cytokinins are plant growth regulators that induce shoot formation, inhibit senescence and root growth. Experiments with hydroponically grown tobacco plants, however, indicated that exogenously applied cytokinin led to the accumulation of proline and osmotin. These responses were also associated with environmental stress reactions, such as salt stress, in many plant species. To test whether increased endogenous cytokinin accumulation led to NaCl stress symptoms, the gene ipt from Agrobacterium tumefaciens, encoding isopentenyl transferase, was transformed into Nicotiana tabacum cv. SR-1 under the control of the light-inducible rbcS-3A promoter from pea. In high light (300 mol PPFD m^-2 s^-1), ipt mRNA was detected and zeatin/zeatin glucoside levels were 10-fold higher than in control plants or when transformants were grown in low light (30 mol PPFD m^-2 s^-1). High light treatment was accompanied by increased levels of proline and osmotin when compared to low light grown transformed and untransformed control plants. Elevated in planta cytokinin levels induced responses also stimulated by salt stress, suggesting either common or overlapping signaling pathways are initiated independently by cytokinin and NaCl, setting in motion gene expression normally elicited by developmental processes such as flowering or environmental stress.Abbreviations IPT isopentenyl, transferase - rbcS-3A gene encoding a small subunit protein (SSU) of Rubisco from Pisum sativum - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase     

Anther-specific expression of ipt gene in transgenic tobacco and its effect on plant development

Isopentenyl transferase (ipt) gene from Agrobacterium tumefaciens T-DNA was placed under the control of a TA29 promoter which expresses specifically in anther. The chimeric TA29-ipt gene was transferred to tobacco plants. During flowering, mRNA of the ipt gene in the anthers of the transgenic plants accumulated and the level of iPA + iPs increased 3–4-fold in the leaves, petals, pistils, and stamens compared with those in the wild type plants. This cytokinin increase affected various aspects in development indicating that the alterations of endogenous cytokinin level by using anther-specific expression of the TA29-ipt gene affected morphology, floral organ systems and reproductivity of the transgenic plants.     

Regulatable endogenous production of cytokinins up to ‘toxic’ levels in transgenic plants and plant tissues

The effects of expressing a chimeric gene consisting of a soybean heat shock gene promoter and a sequence that encodes an enzyme catalyzing the synthesis of a potent phytohormone, the cytokinin iPMP, have been analyzed in transgenic tobacco plants. The production of cytokinin endogenously produced several effects previously undocumented. The differentiation of shoots independent of exogenous cytokinin from heat-treated transgenic plant leaf explants demonstrates that long-term heat treatments do not interfere with complex developmental processes. This extends the potential usefulness of heat shock gene promoters to conditionally express genes during windows of development that span several weeks.     

激活标签法及其在植物基因工程上的应用

激活标签法是新近发展起来的一种用于基因分离和鉴定的方法。它通过诱变使特定内源基因发生过量表达,而产生显性功能获得型突变,从而对基因进行鉴定和分析。因为具有独特的性质已使其成为发现新基因和进行基因功能分析的有效工具。本文综述了激活标签法的原理、研究现状和在植物基因工程上的应用。Abstract:Activation tagging is a new method for isolation and functional identification.It can generate dominant gain-of-function mutants by overexpression of a particular endogenous gene.Due to this special characteristics of activation tagging,this method has been a powerful tool for new gene discovery and gene functional analysis.This paper reviewed the principle and study conditions of activation tagging,as well as its use in plant genetic engineering.     

Probing the distribution of plant hormones by immunocytochemistry

Taking advantage of the highly specific structuralrequirements of antibodies for binding, the detectionof plant hormones in tissue by immunolocalisationoffers a powerful tool to study the distribution ofthese signalling molecules. For instance, specificmonoclonal and polyclonal antibodies have been raisedfor abscisic acid, indole-3-acetic acid and a varietyof cytokinins. Immobilisation by chemical fixation orfreezing minimises diffusion of these low molecularweight compounds in plant tissues. Associated primaryantibodies in sections or permeabilised cells can bedetected by secondary antibodies linked to enzymes,fluorescent molecules or electron opaque markers,which allow detection by either light or electronmicroscopy. These techniques have already found theirapplication in various studies related to thephysiology of these plant hormones.     

Activation tagging: a means of isolating genes implicated as playing a role in plant growth and development

Activation T-DNA tagging has been used to generate a variety of tobacco cell lines selected by their ability to grow either in the absence of auxin or cytokinin in the culture media, or under selective levels of an inhibitor of polyamine biosynthesis. The majority of the cell lines studied in detail contain single T-DNA inserts genetically co-segregating with the selected phenotype. While most of the plants regenerated from the mutant cell lines appear phenotypically normal, several display phenotypes which could be inferred to result from disturbances in the content, or the metabolism, of auxins and cytokinins, or polyamines. The tagging vector is designed to allow the isolation of tagged plant genes by plasmid rescue. Confirmation that the genomic sequence responsible for the selected phenotype has indeed isolated is provided by PEG-mediated protoplast DNA uptake of rescued plasmids followed by selection for protoplast growth under the original selective conditions. Several plasmids have been rescued from the mutant lines which confer on transfected protoplasts the ability to grow either in the absence of auxin or cytokinin in the culture media, or under selective levels of an inhibitor of polyamine biosynthesis. This review describes the background to activation tagging and our progress in characterizing the genes that have been tagged in the mutant lines we have generated.     

Crosstalk between auxin, cytokinins, and sugars in the plant cell cycle

Plant meristems are utilization sinks, in which cell division activity governs sink strength. However, the molecular mechanisms by which cell division activity and sink strength are adjusted to a plant's developmental program in its environmental setting are not well understood. Mitogenic hormonal as well as metabolic signals drive and modulate the cell cycle, but a coherent idea of how this is accomplished, is still missing. Auxin and cytokinins are known as endogenous mitogens whose concentrations and timing, however, can be externally affected. Although the sites and mechanisms of signal interaction in cell cycle control have not yet been unravelled, crosstalk of sugar and phytohormone signals could be localized to several biochemical levels. At the expression level of cell cycle control genes, like cyclins, Cdks, and others, synergistic but also antagonistic interactions could be demonstrated. Another level of crosstalk is that of signal generation or modulation. Cytokinins affect the activity of extracellular invertases and hexose-uptake carriers and thus impinge on an intracellular sugar signal. With tobacco BY-2 cells, a coordinated control of cell cycle activity at both regulatory levels could be shown. Comparison of the results obtained with the root cell-representing BY-2 cells with literature data from shoot tissues or green cell cultures of Arabidopsis and Chenopodium suggests opposed and tissue-specific regulatory patterns of mitogenic signals and signal crosstalk in root and shoot meristems.     

Synthesis and function of isopentenyl adenosine derivatives in tRNA

Isopentenyl adenosine derivatives can be found next to the anticodon (position 37) in tRNA from both the Bacteria and Eucarya domains. These modified nucleosides improve the efficiency of tRNA in translation, can increase and decrease translational fidelity, and make the tRNA less codon context sensitive. In bacteria the synthesis of isopentenyl adenosine derivatives seems to be linked to iron metabolism and central metabolic pathways.     

Role of calcium in plant hormone action

Abstract

The role of calcium as a second messenger in plant cells has been recognized in a number of physiological processes. As described for animal systems, plant cells contain all the elements necessary for coupling the external signals to a specific response by regulation of calcium levels. However, the evidence that Ca₂₊ can be considered a second messenger for hormone response in plants is still circumstantial, besides several reports on the subject have been produced. All the hormone effects may in some tissues be regulated by calcium metabolism, but only for few of them a precise role of this cation has been established. The studies on the different hormones will be reviewed and discussed.     

转基因抗早衰棉的获得

利用花粉管通道技术将含有抑制衰老嵌合基因PCSAG12-ipt的pBG121质粒转入早衰型陆地棉品种中棉所10号中,通过对T1代植株进行卡那霉素田间筛选、PCR检测及GUS组织化学染色,获得了12株转基因棉花。在棉花发育进入衰老时期,对转基因植株进行叶绿素和细胞分裂素含量的测定及形态观察,结果表明转基因植株的衰老得到延迟。