Kinetics of determination in the differentiation of isolated mesophyll cells of Zinnia elegans to tracheary elements.
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Mechanically isolated mesophyll cells of Zinnia elegans L. cv Envy differentiate to tracheary elements when cultured in inductive medium containing 0.5 micromolar alpha-naphthaleneacetic acid and 0.5 micromolar benzyladenine. The cells do not differentiate when cultured in medium in which the concentration of auxin and/or cytokinin has been reduced to 0.005 micromolar. Cells require an initial 24-hour exposure to inductive cytokinin and 56-hour exposure to inductive auxin for differentiation at 72 hours of culture. Freshly isolated Zinnia cells can be maintained in medium having low concentrations of both auxin and cytokinin for only 1 day without significant loss of potential to differentiate upon transfer to inductive medium. Initial culture for up to 2 days in medium having high auxin and low cytokinin, or low auxin and high cytokinin, allows full differentiation on the third day after transfer to inductive medium and potentiates the early differentiation of some cells. (+info)
The senescence of oat leaf segments is promoted under simulated microgravity condition on a three-dimensional clinostat.
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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. (+info)
The role of amyloplasts during gravity perception in gynophores of the peanut plant (Arachis hypogaea).
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Gravitropic perception and response are essential for the completion of the reproductive life cycle of the peanut plant (Arachis hypogaea L.). The developing seeds are buried in the soil by a specialized organ, the gynophore, allowing the fruit to mature underground. Controversy exists about the site of graviperception in the gynophore: previous workers suggested that the intercalary meristem was the zone where gravity was perceived. Taking the starch statolith hypothesis for graviperception as a framework, we explored the possibility that the starch-grain filled plastids (amyloplasts) in the starch sheath of the gynophore may be acting as gravisensors. We show that these amyloplasts sediment readily with respect to the gravity vector within 30 min of reorientation, and before there is a measurable gravitropic response. Gynophore explants were incubated with gibberellic acid and kinetin, in darkness, to remove starch from the amyloplasts. Destarching the gynophores did not inhibit overall growth of the organ, but reduced the gravitropic response curvature by 82% compared to water-treated controls. In addition, gynophores placed on a rotating clinostat (without hormone treatment) also showed a reduced gravitropic response. In conclusion, the evidence presented in this work strongly suggests that the amyloplasts of the starch sheath are responsible for gravitropic perception in the peanut gynophore. A model for graviperception in the gynophore is presented. (+info)
Cell cycle-dependent control of polarised development by a cyclin-dependent kinase-like protein in the Fucus zygote.
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Although iterative development can be uncoupled from morphogenesis in plant organs, the relationship between the cell cycle and developmental events is not well established in embryos. Zygotes of fucoid algae, including Fucus and Pelvetia are particularly well suited for studying the interaction(s) between cell cycle progression and the early morphogenetic events, as the establishment of polarity and its morphogenetic expression, i.e. germination, and the first cell cycle are concomitant. We have previously demonstrated that, in Fucus zygotes, various aspects of cell cycle progression are tightly controlled by cyclin-dependent kinase (CDK)-like proteins, including two PSTAIRE CDK-like proteins, p34 and p32, which are synthesised after fertilisation. We show that specific inhibition of CDK-like proteins, either with purine derivatives such as olomoucine and amino-purvalanol or by microinjection of the CDK inhibitor p21(cip1), prevents germination and cell division. Whereas direct inhibition of DNA replication by aphidicolin did not affect polarised development, olomoucine, which has previously been shown to prevent entry in S phase, and other purine derivatives also inhibited photopolarisation. Early microinjection of a monoclonal anti-PSTAIRE antibody also prevented germination and cell division. Only p34 had affinity for amino-purvalanol, suggesting that among PSTAIRE CDKs, this protein is the main target of purine derivatives. Models to account for the simultaneous control of early cell cycle progression and polarisation are proposed. (+info)
ARR1, a transcription factor for genes immediately responsive to cytokinins.
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Cytokinins are a class of phytohormones involved in various physiological events of plants. The Arabidopsis sensor histidine kinase CRE1 was recently reported to be a cytokinin receptor. We used a steroid-inducible system to show that the transcription factor-type response regulator ARR1 directs transcriptional activation of the ARR6 gene, which responds to cytokinins without de novo protein synthesis. This fact, together with characteristics of ARR1-overexpressing plants and arr1 mutant plants, indicates that the phosphorelay to ARR1, probably from CRE1, constitutes an intracellular signal transduction occurring immediately after cytokinin perception. (+info)
Rapid increase of NO release in plant cell cultures induced by cytokinin.
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4,5-Diaminofluorescein, a fluorescence indicator for NO, was applied to detect the release of NO from plant cells. NO production was increased within 3 min when plant cell cultures (Arabidopsis, parsley, and tobacco) were treated by cytokinin and was dose-dependent and signal-specific in that other plant hormones and inactive cytokinin analog were not effective in stimulating of NO release. The response was quenched by addition of 2-(aminoethyl)-2-thiopseudourea, an inhibitor of the animal NO synthase, and by addition of an NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-1-oxy-3-oxide. These results imply that NO may act in cytokinin signal transduction. (+info)
The late-domain-containing protein p6 is the predominant phosphoprotein of human immunodeficiency virus type 1 particles.
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The Gag-derived protein p6 of human immunodeficiency virus type 1 (HIV-1) plays a crucial role in the release of virions from the membranes of infected cells. It is presumed that p6 and functionally related proteins from other viruses act as adapters, recruiting cellular factors to the budding site. This interaction is mediated by so-called late domains within the viral proteins. Previous studies had suggested that virus release from the plasma membrane shares elements with the cellular endocytosis machinery. Since protein phosphorylation is known to be a regulatory mechanism in these processes, we have investigated the phosphorylation of HIV-1 structural proteins. Here we show that p6 is the major phosphoprotein of HIV-1 particles. After metabolic labeling of infected cells with [ortho-32P]phosphate, we found that phosphorylated p6 from infected cells and from virus particles consisted of several forms, suggesting differential phosphorylation at multiple sites. Apparently, phosphorylation occurred shortly before or after the release of p6 from Gag and involved only a minor fraction of the total virion-associated p6 molecules. Phosphoamino acid analysis indicated phosphorylation at Ser and Thr, as well as a trace of Tyr phosphorylation, supporting the conclusion that multiple phosphorylation events do occur. In vitro experiments using purified virus revealed that endogenous or exogenously added p6 was efficiently phosphorylated by virion-associated cellular kinase(s). Inhibition experiments suggested that a cyclin-dependent kinase or a related kinase, most likely ERK2, was involved in p6 phosphorylation by virion-associated enzymes. (+info)
Cytokinin growth responses in Arabidopsis involve the 26S proteasome subunit RPN12.
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The 26S proteasome is an ATP-dependent eukaryotic protease responsible for degrading many important cell regulators, especially those conjugated with multiple ubiquitins. Bound on both ends of the 20S core protease is a multisubunit regulatory particle that plays a crucial role in substrate selection by an as yet unknown mechanism(s). Here, we show that the RPN12 subunit of the Arabidopsis regulatory particle is involved in cytokinin responses. A T-DNA insertion mutant that affects RPN12a has a decreased rate of leaf formation, reduced root elongation, delayed skotomorphogenesis, and altered growth responses to exogenous cytokinins, suggesting that the mutant has decreased sensitivity to the hormone. The cytokinin-inducible genes CYCD3 and NIA1 are upregulated constitutively in rpn12a-1, indicating that feedback-inhibitory mechanisms also may be altered. rpn12a-1 seedlings also showed changes in auxin-induced growth responses, further illustrating the close interaction between auxin and cytokinin regulation. In yeast, RPN12 is necessary for the G1/S and G2/M transitions of the cell cycle, phases that have been shown to be under cytokinin control in plants. We propose that RPN12a is part of the Arabidopsis 26S proteasome that controls the stability of one or more of the factors involved in cytokinin regulation. (+info)