Vacuoles release sucrose via tonoplast-localised SUC4-type transporters

Arabidopsis thaliana has seven genes for functionally active sucrose transporters. Together with sucrose transporters from other dicot and monocot plants, these proteins form four separate phylogenetic groups. Group-IV includes the Arabidopsis protein SUC4 (synonym SUT4) and related proteins from monocots and dicots. These Group-IV sucrose transporters were reported to be either tonoplast- or plasma membrane-localised, and in heterologous expression systems were shown to act as sucrose/H(+) symporters. Here, we present comparative analyses of the subcellular localisation of the Arabidopsis SUC4 protein and of several other Group-IV sucrose transporters, studies on tissue specificity of the Arabidopsis SUC4 promoter, phenotypic characterisations of Atsuc4.1 mutants and AtSUC4 overexpressing (AtSUC4-OX) plants, and functional comparisons of Atsuc4.1 and AtSUC4-OX vacuoles. Our data show that SUC4-type sucrose transporters from different plant families (Brassicaceae, Cucurbitaceae and Solanaceae) localise exclusively to the tonoplast, demonstrating that vacuolar sucrose transport is a common theme of all SUC4-type proteins. AtSUC4 expression is confined to the stele of Arabidopsis roots, developing anthers and meristematic tissues in all aerial parts. Analyses of the carbohydrate content of WT and mutant seedlings revealed reduced sucrose content in AtSUC4-OX seedlings. This is in line with patch-clamp analyses of AtSUC4-OX vacuoles that characterise AtSUC4 as a sucrose/H(+) symporter directly in the tonoplast membrane.     

The role of pre-symbiotic auxin signaling in ectendomycorrhiza formation between the desert truffle <Emphasis Type="Italic">Terfezia boudieri</Emphasis> and <Emphasis Type="Italic">Helianthemum sessiliflorum</Emphasis>

The ectendomycorrhizal fungus Terfezia boudieri is known to secrete auxin. While some of the effects of fungal auxin on the plant root system have been described, a comprehensive understanding is still lacking. A dual culture system to study pre mycorrhizal signal exchange revealed previously unrecognized root–fungus interaction mediated by the fungal auxin. The secreted fungal auxin induced negative taproot gravitropism, attenuated taproot growth rate, and inhibited initial host development. Auxin also induced expression of Arabidopsis carriers AUX1 and PIN1, both of which are involved in the gravitropic response. Exogenous application of auxin led to a root phenotype, which fully mimicked that induced by ectomycorrhizal fungi. Co-cultivation of Arabidopsis auxin receptor mutants tir1-1, tir1-1 afb2-3, tir1-1 afb1-3 afb2-3, and tir1-1 afb2-3 afb3-4 with Terfezia confirmed that auxin induces the observed root phenotype. The finding that auxin both induces taproot deviation from the gravity axis and coordinates growth rate is new. We propose a model in which the fungal auxin induces horizontal root development, as well as the coordination of growth rates between partners, along with the known auxin effect on lateral root induction that increases the availability of accessible sites for colonization at the soil plane of fungal spore abundance. Thus, the newly observed responses described here of the root to Terfezia contribute to a successful encounter between symbionts.     

Lysine oligopeptides. Preparation by ion-exchange chromatography

The preparation of L -lysine peptides (Lys_n, n = 2–14) from polyL -lysine is described. Fractionation by ion-exchange column chromatography of poly-L -lysine hydrolysates on a preparative scale resulted in 0.2–1.0 g quantities of individual members of the poly-L -lysine series. The peptides isolated proved to be analytically pure and the optical configuration was fully retained, as demonstrated by complete enzymic digestion. Peptides higher than n = 14 were also prepared. They consisted of oligolysine groups of narrow and accurately determined size distribution. Potentiometric titrations were used both to characterize the products and to demonstrate the characteristic dependence of the dissociation constants on size of the peptide.     

Purification of human tumor cell autocrine motility factor and molecular cloning of its receptor.

Tumor autocrine motility factor (AMF) has been detected in and purified from serum-free conditioned medium of human HT-1080 fibrosarcoma cells. Under nonreducing conditions, AMF migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a single band of 55 kDa but under reducing conditions as a band of 64 kDa. Two-dimensional polyacrylamide gel electrophoresis of the purified AMF resolved two groups of polypeptides with isoelectric points of 6.1 and 6.2 (majors), 6.35 and 6.4 (minors). Purified AMF stimulated HT-1080 cell migration in a dose-dependent fashion. The motility stimulation of the fibrosarcoma cells with AMF is associated with the phosphorylation of the AMF receptor, a 78-kDa cell surface glycoprotein (gp78), suggesting protein kinase participation in migratory signal transduction. The gene encoding gp78 was cloned from an HT-1080 fibrosarcoma complementary DNA library. The deduced sequence encodes a polypeptide of 323 amino acids. The nucleotide and predicted amino acid sequence of the gp78 reveals significant homology with the human suppressor/oncogene p53 protein.     

Plexin-A3 mediates semaphorin signaling and regulates the development of hippocampal axonal projections.

Plexins are receptors implicated in mediating signaling by semaphorins, a family of axonal chemorepellents. The role of specific plexins in mediating semaphorin function in vivo has not, however, yet been examined in vertebrates. Here, we show that plexin-A3 is the most ubiquitously expressed plexin family member within regions of the developing mammalian nervous system known to contain semaphorin-responsive neurons. Using a chimeric receptor construct, we provide evidence that plexin-A3 can transduce a repulsive signal in growth cones in vitro. Analysis of plexin-A3 knockout mice shows that plexin-A3 contributes to Sema3F and Sema3A signaling and that plexin-A3 regulates the development of hippocampal axonal projections in vivo.