Depuis le debut du siecle, les questions posees sur le developpement de l' embryon vegetal ont co-evolue avec les outils ou les techniques disponibles: du microscope et du devenir des cellules issues du zygote, au laser et a la communication intercellulaire. Jusqu' a present l' etude de l' embryogenese vegetale a fait un usage limite de toutes les techniques utilisees par ailleurs dans l' etude du developpement. La raison est a rechercher dans les limitations techniques propres aux vegetaux. Neanmoins, ces barrieres commencent a etre levees, ce qui devrait ineluctablement faire progresser notre comprehension des interaction impliquees dans le developpement de l' embryon. Ces nouveaux outils vont a leur tour faire evoluer la nature des questions posees.

Our understanding of plant meiosis is rapidly increasing thanks to the model Arabidopsis thaliana. Here we present the results of a screening for meiotic mutants carried out with a library containing 30 719 T-DNA insertion lines. An average of one mutant per 1000 lines was recovered. Several phenotypic classes could be distinguished and are presented. In parallel, 39 proteins known to be involved in meiosis in non-plant organisms were chosen and a search was performed for homologue sequences in the completed Arabidopsis thaliana genome. Approximately 30/ of the meiotic related sequences showed similarities with one or several Arabidopsis putative genes. The relevance of forward versus reverse genetics in order to characterize meiotic functions is discussed

Depuis le début du siècle, les questions posées sur le développement de l'embryon végétal ont co-évolué avec les outils ou les techniques disponibles: du microscope et du devenir des cellules issues du zygote, au laser et à la communication intercellulaire. Jusqu'à présent l'étude de l'embryogenèse végétale a fait un usage limité de toutes les techniques utilisées par ailleurs dans l'étude du développement. La raison est à rechercher dans les limitations techniques propres aux végétaux. Néanmoins, ces barrières commencent à être levées, ce qui devrait inéluctablement faire progresser notre compréhension des interaction impliquées dans le développement de l'embryon. Ces nouveaux outils vont à leur tour faire évoluer la nature des questions posées

Failure of one parent's chromosomes to organize nucleoli in an interspecific hybrid is an epigenetic phenomenon known as nucleolar dominance. Selective gene silencing on a scale of millions of bp is known to be involved, but the full extent to which nucleolus organizer region (NOR)-bearing chromosomes are inactivated beyond the NORs is unknown. Aided by genome sequence data for Arabidopsis thaliana, we have mapped the extent of nucleolar dominance-induced silencing in Arabidopsis suecica, the allotetraploid hybrid of A. thaliana and Arabidopsis arenosa. Using a sensitive reverse transcription PCR assay, we show that the four A. thaliana NORs, each approximately equal to 4 Mbp in size, are approximately equal to 99.5% silenced in A. suecica vegetative leaves, whereas the NORs inherited from A. arenosa remain fully active. The two A. thaliana NORs, NOR2 and NOR4, abut the telomeres on chromosomes 2 and 4, thus there are no genes distal to the NORs. The three protein-coding genes nearest NOR4 on its centromere-proximal side, the closest of which is only 3.1 kb from rRNA gene sequences, are shown to be transcribed in the hybrid despite the silencing of the adjacent approximately equal to 4-Mbp NOR. These data argue against hypotheses in which NOR inactivation is attributed to the spread of silencing from adjacent chromosomal regions, but favor models in which NORs or rRNA genes are the targets of regulation.

Arabidopsis thaliana has been widely used as a model system, in various aspects of biological studies, such as genomics, genetics, cellular, developmental and molecular biology. In order to reveal the molecular events and regulatory networks controlling Arabidopsis development and responses to genetic and environmental changes, we designed and used a high-density oligonucleotide probe array (GeneChip) to profile global gene expression patterns. The Arabidopsis oligonucleotide probe array consists of probes from 8 300 unique Arabidopsis genes, which covers approximately one-third of the genome. Global transcription profiles of A. thaliana in various developmental stages, and their responses to different environments were generated using this microarray, and archived. Here, we analyze data sets derived from nineteen independent experiments. Constitutively and differentially expressed genes in seedlings, roots, leaves, inflorescences, flowers and siliques at different developmental stages were identified. Functions of these genes based on homologs were determined and categorized. Our results provide insight into the coordinated transcriptional regulation of the genes during plant growth and development

The fate of redundant genes resulting from genome duplication is poorly understood. Previous studies indicated that ribosomal RNA genes from one parental origin are epigenetically silenced during interspecific hybridization or polyploidization. Regulatory mechanisms for protein-coding genes in polyploid genomes are unknown, partly because of difficulty in studying expression patterns of homologous genes. Here we apply amplified fragment length polymorphism (AFLP)-cDNA display to perform a genome-wide screen for orthologous genes silenced in Arabidopsis suecica, an allotetraploid derived from Arabidopsis thaliana and Cardaminopsis arenosa. We identified ten genes that are silenced from either A. thaliana or C. arenosa origin in A. suecica and located in four of the five A. thaliana chromosomes. These genes represent a variety of RNA and predicted proteins including four transcription factors such as TCP3. The silenced genes in the vicinity of TCP3 are hypermethylated and reactivated by blocking DNA methylation, suggesting epigenetic regulation is involved in the expression of orthologous genes in polyploid genomes. Compared with classic genetic mutations, epigenetic regulation may be advantageous for selection and adaptation of polyploid species during evolution and development.

Seedlings of Arabidopsis thaliana were treated with Cd, and transcript populations that changed their levels were screened by a fluorescent differential display method. Among the 170 cDNAs initially identified, 31 were further characterized for their identity, expression profile and response to other stresses. Sequencing revealed 10, 12 and 15 to be related to signal transduction, protein denaturing stress and responses to active oxygen species, respectively. Many of these genes responded not only to Cd, but also to oxidative stress, Cu ions and a protein denaturation agent. Upon introduction into yeast cells, two genes, encoding ATMEKK1 and a putative farnesylated protein that has two metal binding motifs, endowed marked toleration of Cd toxicity. These results suggest that oxidative stress and protein denaturation are important components of Cd toxicity, and that to cope with such stresses, plants activate a set of genes involved in metal detoxification, protein refolding and wound healing. The results also suggested temporarily and spatially well-regulated protein phosphorylation and activation of transcription factors, accompanied by their transcription.

Plant transformation is now a routine practice in many laboratories around the world. However it has been observed that sometimes the introduced gene becomes inactive. This inactivation could be reversible or irreversible. In this study the genetic behavior of an inactive transgene was observed. For this purpose a wild type chlorophyll gene CH-42 was introduced into a pale (ch-42) mutant Arabidopsis by Agrobacterium mediated transformation. After transformation most of the plants showed green wild type phenotype- However, some transgenic lines did not show the phenotype of the CH-42 transgene and appeared pale in color. Four transgenic lines showing inactive CH-42 transgene were selected to study the segregation of the inactive transgene in the next generations. These transgenic lines with inactive insert were crossed to the wildtype and the CH-42 mutant plants. The F1, plants produced from these crosses were allowed to self fertilize and some of the seeds of these F1 were germinated for analysis of the F2 generation. The results of the reciprocal crosses of all four inactive lines were similar. The inactivation of transgene CH-42 was irreversible and the cellular gene at the CH-42 locus did not have any effect oil the expression of the transgene.

It is usually assumed that aquaporins present in the cellular membranes could be an important route in the control of water flux in plants, but evidence for this hypothesis is scarce. In this paper, we report measurements of the osmotic permeability (P(os)) of protoplasts isolated from hypocotyls of wild-type and mutant Arabidopsis thaliana (L.) Heynh. Mutants were affected in their growth and exhibited different sensitivities to the phytohormone, brassinolide. For the two mutants studied (cpd: constitutive photomorphogenesis and dwarfism; bril: brassinosteroid insensitive), hypocotyl length was correlated to P(os) for the protoplasts. Under experimental conditions where hypocotyl growth had ceased, restoration of root, hypocotyl and petiole growth by brassinolide was correlated with an increase in P(os) of the hypocotyl protoplasts. We consider that the increase in P(os) of the hypocotyl cells was needed because these cells were part of the transcellular water pathway of the plant. This is the first time, to our knowledge, that brassinolide has been shown to be involved in the modification of the water-transport properties of cell membranes. Our results also emphasize the importance of aquaporins and the transcellular pathway in water transport under normal growth conditions.