The control of epidermal cell fate is a complex molecular process and requires the regulatory activity of different transcription factors. Here, we describe the isolation of a member of the Arabidopsis MYB transcription factor family, AtMYB23, that is involved in trichome development. Expression of the AtMYB23 gene under the control of the viral CaMV 35S promoter causes the development of ectopic trichomes. The formation of ectopic trichomes depends on TRANSPARENT TESTA GLABRA1 but not on GLABRA1. The absence of the negative regulator TRIPTYCHON leads to branching of the ectopic trichomes on cotyledons and the formation of ectopic trichomes in the leaf subepidermal cell layer. The CaMV 35S promoter-controlled expression of AtMYB23 can partially rescue the glabra1 mutant phenotype. Together, the presented data indicate that the AtMYB23 gene has partially overlapping functions with GLABRA1 in controlling the initiation of trichome development.

No abstract

The tissue-specific expression pattern and the intracellular distribution of the Ca(2+)-binding protein calreticulin at the mRNA and protein levels have been studied during somatic and zygotic embryogenesis of Nicotiana plumbaginifolia Viv. A full-length cDNA sequence encoding calreticulin was isolated from a lembda Zap cDNA library from early developmental stages of somatic embryogenesis. The deduced amino acid sequence of the calreticulin from N. plumbaginifolia shows high homology to the corresponding proteins of tobacco (98.2% identity), maize (80%) and barley (76.5%), and more than 55% homology to animal calreticulins, and the sequence motifs with established functions found in calreticulins of other species were quite conserved. Northern experiments revealed a developmental regulation of the calreticulin transcript with a maximum during the early stages of somatic embryogenesis and an auxin dependence during in-vitro cell culture. alpha-Naphthaleneacetic acid stimulated calreticulin expression whereas 2,4-dichlorophenoxyacetic acid reduced it. Immunohistological analysis of calreticulin distribution in the ovaries during zygotic embryogenesis showed that calreticulin biosynthesis started tissue specifically, with a high abundance in the endothelium of the integument in the ovules, followed by calreticulin accumulation in the embryo proper and in the associated endosperm at the late globular stage of embryogenesis. Using immunogold labeling, calreticulin was intracellularly localized with a high abundance to the Golgi compartment and to patches on the surface of dividing protoplasts. Smaller amounts were found in the endoplasmic reticulum and plasma membranes. The functional role of calreticulin in posttranslational processing and translocation processes, apart from its postulated function in cellular Ca2+ homeostasis, is discussed.

The amino-acid sequence of lac repressor from Escherichia coli has been determined. The sequence contains 347 residues in the subunit single peptide chain. It shows no similarities with the sequences of histones or the known part of f3-galactosidase.

Further evidence is provided that the Calvin cycle enzymes ribose-5-phosphate isomerase (EC 5.3