Tracks as frames of reference for academic self-concept.

This study compared the development of academic self-concepts between different educational programs. A longitudinal cohort study in Flanders (3205 students in 46 schools) was used to compare students' academic self-concepts during the first three years of secondary education. General academic self-concept, self-concept in mathematics and self-concept in Dutch were measured. The investigated educational programs, called tracks, differ in the extent they are academically or vocationally focused and differ in average student academic ability. To control for selection effects, students who are comparable across the four tracks were matched using propensity score matching, Mahalanobis distance matching and coarsened exact matching. By means of multiple indicator quadratic latent growth curves, pairs of tracks that are hierarchically consecutive were compared regarding the development in academic self-concepts. For the two highest tracks, it was beneficial to be allocated to the highest track, whereas the pairwise comparisons between the three lower tracks indicated a detrimental effect of being in a higher track. The findings from this study do not support the big-fish-little-pond hypothesis or the basking in reflected glory hypothesis. Differences between tracks for the development of self-concepts only became apparent after two years.

MGOUN1 and MGOUN2: two genes required for primordium initiation at the shoot apical and floral meristems in Arabidopsis thaliana.

We report two new recessive mutations in Arabidopsis, mgoun1 and mgoun2 which cause a reduction in the number of leaves and floral organs, larger meristems and fasciation of the inflorescence stem. Although meristem structure is affected in the mutants, we provide evidence that its overall organisation is normal, as shown by the expression patterns of two meristem markers. Microscopical analyses suggest that both mutations affect organ primordia production. mgo1 strongly inhibits leaf production in a weak allele of shoot meristemless, stm-2. In addition, mgo1 and 2 severely reduce the ability of the fasciata1 and 2 mutants to initiate organs, although meristem formation per se was not inhibited. The strong allele, stm-5, is epistatic to mgo1, showing that the presence of meristematic cells is essential for MGO1 function. These results suggest a role for the MGO genes in primordia initiation although a more general role in meristem function can not be excluded. We describe a form of fasciation which is radically different from that described for clavata, which is thought to have an increased size of the meristem centre. Instead of one enlarged central meristem mgo1 and 2 show a continuous fragmentation of the shoot apex into multiple meristems, which leads to the formation of many extra branches. The phenotype of mgo1 clv3 and mgo2 clv3 double mutants suggest that the MGO and CLV genes are involved in different events. In conclusion, our results reveal two new components of the regulatory network controlling meristem function and primordia formation. A model for MGO genes is discussed.

The homeobox gene ATK1 of Arabidopsis thaliana is expressed in the shoot apex of the seedling and in flowers and inflorescence stems of mature plants.

The homeodomain is a DNA-binding domain present in a large family of eukaryotic regulatory proteins. Homeodomain proteins have been shown to play key roles in controlling developmental programs in various organisms. Here we report the isolation and characterisation of a homeobox gene from Arabidopsis thaliana designated ATK1. The gene was isolated using as a probe the homeobox domain of the KN1 gene from maize. The homeodomain of ATK1 is highly homologous to the homeodomain of the KN1 gene of maize (81%) but shows only poor homology outside the homeodomain. Therefore ATK1 is probably not the Arabidopsis homologue of the KN1 gene from maize. It contains the four invariant amino acid residues present in the recognition helix 3 of all other homeodomain proteins. Outside the homeodomain a region rich in aspartate and glutamate residues is found suggesting that ATK1 is a transcriptional activator. The gene contains four introns which is similar in the KN1 gene of maize and the Osh1 gene of rice. Primer extension reveals the presence of two transcription initiation sites. The leader sequence of the genuine transcript is 342 nucleotides long and contains two upstream open reading frames. ATK1 is strongly expressed in the shoot apex of seedlings, while in mature plants the gene is primarily expressed in flowers and inflorescence stems. Such an expression pattern is reminiscent of that of the KN1 gene of maize and therefore ATK1 could similarly be involved in determining cell fate.

The homeobox gene ATH1 of Arabidopsis is derepressed in the photomorphogenic mutants cop1 and det1.

A light-regulated Arabidopsis thaliana homeobox 1 gene (ATH1) was identified in a transcription factor gene collection. Primer extension analysis of this gene showed the presence of two major transcripts with unusually long 5' untranslated leader sequences. The leader sequence of the isolated cDNA clone contains several small open reading frames upstream of the initiation codon of the largest open reading frame coding for the homeodomain protein. This leader sequence was found to affect the translation efficiency negatively in an in vitro translation system. The expression of the ATH1 gene is dependent on the presence of light. After illuminating etiolated or dark-adapted Arabidopsis seedlings, the ATH1 mRNA level increased rapidly. Expression of ATH1 does not require the presence of active chloroplasts because photooxidative destruction of the chloroplast by norflurazon treatment did not influence the ATH1 mRNA level. In dark-grown seedlings of the photomorphogenic constitutive photomorphogenic 1 (cop1) and deetiolated 1 (det1) mutants, the ATH1 mRNA level was elevated. This shows that the gene products of these loci directly or indirectly repress ATH1 expression in etiolated wild-type seedlings. A correlation between the strength of the cop1 allele and the ATH1 mRNA level was found. This relationship suggests a role for the ATH1 protein in the signal transduction pathway downstream of COP1.

KNAT2: evidence for a link between knotted-like genes and carpel development.

The KNAT2 (for KNOTTED-like from Arabidopsis thaliana 2) homeobox gene is expressed in the vegetative apical meristem. It is also active during flower development, suggesting a function in the structuring of flowers. To investigate its role, we used a DEXAMETHASONE (DEX)-inducible system to generate transgenic plants that overexpressed a fusion of KNAT2 with the hormone binding domain of the glucocorticoid receptor. DEX-induced plants were similar to plants overexpressing the closely related KNAT1 gene, indicating overlapping functions, although we observed differences as well. In particular, KNAT2-GR activation induced ectopic carpel features. First, KNAT2 induced the homeotic conversion of nucellus into carpel-like structures. Second, KNAT2 induced stigmatic papillae on rosette leaves in the ap2-5 background. Third, ectopic expression of the carpel identity gene AGAMOUS (AG) was observed in carpels and ovules. Interestingly, the homeotic conversion was not dependent on AG activity, because it was maintained in the ag-1 ap2-5 double mutant. Therefore, our data indicate that KNAT2 also must activate other carpel regulators. Together, these results suggest that KNAT2 plays a role in carpel development.

Identification of a light-regulated MYB gene from an Arabidopsis transcription factor gene collection.

Seven different MYB-related genes have been isolated from a genomic Arabidopsis library with probes based on MYB DNA-binding motifs. The predicted amino acid sequence of these genes showed high similarity in the MYB domain but outside this region virtually no similarities were found. The set of MYB-related genes was used to identify differentially expressed genes following the transfer of etiolated seedlings to light. This differential screen resulted in the selection of the ATM4 gene which is induced by light within one hour of exposure of etiolated or dark-adapted seedlings.

cry IA(b) transcript formation in tobacco is inefficient.

Chimaeric PCaMV35Scry genes direct in tobacco mesophyll protoplasts mRNA levels of less than one transcript per cell. We provide evidence that this low cytoplasmic cry IA(b) mRNA level is not due to a rapid turnover but rather results from a marginal import flow of cry messenger into the cytoplasm. Run-on assays indicate that the frequency of transcription initiation is not limiting. However, the cry precursor mRNA carries at least three regions that are recognized as introns. The absence of high cytoplasmic levels of spliced cry mRNAs suggests that these mRNAs are unstable and/or not efficiently made. Point mutations in the 5' splice site of the most distal intron allows high accumulation levels of the full-length mRNA. This implies that the inefficient formation of full-size mRNA is a major cause of the low expression level of chimaeric cry IA(b) genes in tobacco.

Engineering herbicide resistance in plants by expression of a detoxifying enzyme.

Phosphinothricin (PPT) is a potent inhibitor of glutamine synthetase in plants and is used as a non-selective herbicide. The bar gene which confers resistance in Streptomyces hygroscopicus to bialaphos, a tripeptide containing PPT, encodes a phosphinothricin acetyltransferase (PAT) (see accompanying paper). The bar gene was placed under control of the 35S promoter of the cauliflower mosaic virus and transferred to plant cells using Agrobacterium-mediated transformation. PAT was used as a selectable marker in protoplast co-cultivation. The chimeric bar gene was expressed in tobacco, potato and tomato plants. Transgenic plants showed complete resistance towards high doses of the commercial formulations of phosphinothricin and bialaphos. These data present a successful approach to obtain herbicide-resistant plants by detoxification of the herbicide.