and Ecker, 1990). Some of these features have been utilized in the isolation of mutants with altered responses to ethylene . Ethylene-resistant mutants (ein or etr mutants) were identified by selecting seedlings that fail to express the triple response in the presente of ethylene. Other mutants were identified that pr0duc-d a triple-response phenotype constitutively, i.e. when no ethylene was applied. These mutants have been grouped into two classes, ethylene overproducers (eto) and constitutive triple-response mutants (ctr), distinguished on the basis of whether the phenotype is suppressed by inhibitors Of ethYlene biosynthesis Or action (Guzmán and Eckerr l990). Through epistasis analysis of the mutants, ethylene-response genes have been ordered on an unbranched pathway . Interestingly, ein or etr mutants appear to have very little effect on the normal growth and development of light-grown Arabidopsis seedlings in the absence of supplied ethylene.We have examined the relationship between cytokinin and ethylene responses in Arabidopsis seedlings and have found that ethylene largely mediates a number of responses to exogenous cytokinin, such as the inhibition of root and hypocotyl elongation. Cytokinin stimulates ethylene production, which, in turn, inhibits root and darkgrown hypocotyl elongation. Because of that, the effects of cytokinin on root and hypocotyl elongation can be blocked, in part, by the action of ethylene inhibitors or by ethylene mutants. These results constitute genetic proof of the idea put forth by Lieberman (1979) that cytokinin is coupled to ethylene action in seedlings.Cytokinins have profound effects on seedling development in Arabidopsis thaliana. Benzyladenine (BA) inhibits root elongation in light-or dark-grown seedlings, and in dark-grown seedlings BA inhibits hypocotyl elongation and exaggerates the curvature of apical hooks. The latter are characteristic ethylene responses and, therefore, the possible involvement of ethylene in BA responses was examined in seedlings. It was found that the inhibitory effects of BA on root and hypocotyl elongation were partially blocked by the action of ethylene inhibitors or ethylene-resistant mutations (ein7-7 and ein2-7). Ethylene production was stimulated by submicromolar concentrations of BA and could account, in part, for the inhibition of root and hypocotyl elongation. It was demonstrated further that BA did not affect the sensitivity of seedlings to ethylene. Thus, the effect of cytokinin on root and hypocotyl elongation in Arabidopsis appears to be mediated largely by the production of ethylene. The coupling between cytokinin and ethylene responses is further SUPported by the discovery that the cytokinin-resistant mutant ckrl is resistant to ethylene and is allelic to the ethylene-resistant mutant ein2.The responses of seedlings to developmental and environmental cues have been useful in studying signal transduction pathways in plants. Arabidopsis thaliana seedlings respond to exogenously added or endogenous cytokinins, and in light-grown se...
SummaryCritical developmental and gene expression profiles were charted during the formation of shoots from root explants in Arabidopsis tissue culture. Shoot organogenesis is a two-step process involving pre-incubation on an auxin-rich callus induction medium (CIM) during which time root explants acquire competence to form shoots during subsequent incubation on a cytokinin-rich shoot induction medium (SIM). At a histological level, the organization of shoot apical meristems (SAMs) appears to occur during incubation on SIM about the time of shoot commitment, i.e. the transition from hormone-dependent to hormone-independent shoot development. Genes involved in SAM formation, such as SHOOTMERISTEMLESS (STM ) and CLA-VATA1 (CLV1), were upregulated at about the time of shoot commitment, while WUSCHEL (WUS) was upregulated somewhat earlier. Genes required for STM expression, such as CUP-SHAPED COTYLEDON 1 and 2 (CUC1 and 2 ) were upregulated prior to shoot commitment. Gene expression patterns were determined for two GFP enhancer trap lines with tissue-specific expression in the SAM, including one line reporting on CUC1 expression. CUC1 was generally expressed in callus tissue during early incubation on SIM, but later CUC1 was expressed more locally in presumptive sites of shoot formation. In contrast, the expression pattern of the enhancer trap lines during zygotic embryogenesis was more localized to the presumptive SAM even in early stages of embryogenesis.
BackgroundMolecular feedback loops involving transcription and translation and several key genes are at the core of circadian regulatory cycles affecting cellular pathways and metabolism. These cycles are active in most adult animal cells but little is known about their expression or influence during development.Methodology/Principal FindingsTo determine if circadian cycles are active during mammalian development we measured the expression of key circadian genes during embryogenesis in mice using quantitative real-time RT-PCR. All of the genes examined were expressed in whole embryos beginning at the earliest age examined, embryonic day 10. In contrast to adult tissues, circadian variation was absent for all genes at all of the embryonic ages examined in either whole embryos or individual tissues. Using a bioluminescent fusion protein that tracks translation of the circadian gene, per2, we also analyzed protein levels. Similar to mRNA, a protein rhythm was observed in adult tissue but not in embryonic tissues collected in-vivo. In contrast, when tissues were placed in culture for the continuous assay of bioluminescence, rhythms were observed in embryonic (E18) tissues. We found that placing embryonic tissues in culture set the timing (phase) of these rhythms, suggesting the importance of a synchronizing signal for the expression of circadian cycles in developing tissues.Conclusions/SignificanceThese results show that embryonic tissues express key circadian genes and have the capacity to express active circadian regulatory cycles. In vivo, circadian cycles are not expressed in embryonic tissues as they are in adult tissues. Individual cells might express oscillations, but are not synchronized until later in development.
Shoots and roots can be regenerated through organogenesis in tissue culture by subjecting plant explants to the appropriate regime of hormone treatments. In an effort to understand the control of shoot organogenesis, we screened for mutants in Arabidopsis thaliana (L.) Heynh. Columbia ecotype for enhanced shoot development at sub-optimal concentrations of cytokinin. Mutants in four different complementation groups were identified, one of which represents a new locus named increased organ regeneration1 (ire1) and another that is allelic to the previously identified pom1/erh2 mutant. Although the mutants were selected for their response to cytokinin, they were neither hypersensitive to, nor were they over-producers of cytokinins. The mutations identified in this study not only promote more robust shoot production in tissue culture, but also enhance green-callus and root formation. We interpret this to mean that, in tissue culture, IRE genes act before organ specification during the time when root explants acquire the competency to respond to organ formation signals. In normal plant development, IRE genes may down-regulate the competency of vegetative tissue to respond to hormonal signals involved in shoot and root organogenesis.
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