Selection and characterization of a feedback-insensitive tissue culture of maize

Tissue culture selection techniques were used to isolate a maize (Zea mays L.) variant D33, in which the aspartate family pathway was less sensitive to feedback inhibition by lysine. D33 was recovered by successively subculturing cultures originally derived from immature embryos on MS medium containing growth-inhibitory levels of lysine+threonine. The ability of D33 to grow vigorously on lysine+ threonine medium was retained after growth for 12 months on nonselection medium. New cultures initiated from shoot tissues of plants regenerated from D33 also were resistant to lysine+threonine inhibition. The K_i of aspartokinase for its feedback inhibitor, lysine, was about 9-fold higher in D33 than for the enzyme from unselected cultures. The free pools of lysine, threonine, isoleucine and methionine were increased 2–9-fold in D33 cultures. This was consistent with the observed change in feedback regulation of aspartokinase, the first enzyme common to the biosynthesis of these amino acids in the aspartate pathway. The accumulated evidence including the stability of resistance in the cultures, the resistance of cultures initiated from regenerated plants, the altered feedback regulation, and the increased free amino acids, indicates a mutational origin for these traits in line D33.Abbreviation LT lysine+threonine in equimolar concentration Paper No. 10880, Scientific Journal Series, Minnesota Agricultural Expertment Station     

Ultrastructure and taxonomy of Paraphysomonas (Chrysophyceae) and related genera 3

New observations are presented on the internal ultrastructure of the scale–bearing chrysophycean genera Chromophysomonas, Chrysosphaerella , the new genus Polylepidomonas and 15 species of Paraphysomonas. These data show that the pigmented genera Chromophysomonas, Chrysosphaerella and Polylepidomonas have a generally similar internal structure and that their taxonomic separation is based only on differences in scale structure. The structure of Paraphysomonas resembles that of these genera but the cells always possess a leucoplast rather than a chloroplast. In cell structure, the pigmented genera resemble the naked genus Ochromonas while Paraphysomonas resembles Spumella , the colourless counterpart of Ochromonas. Evaluation of the differences between these genera and the scale–bearing genera Mallomonas and Synura has led to the conclusion that Chromophysomonas, Chrysosphaerella, Polylepidomonas and Paraphysomonas should no longer be classified within the family Mallomonadaceae. The new family Paraphysomonadaceae is established to include Chrysophyceae with an Ochromonas type of cell structure but which also produce silica scales.     

Inhibition of Megakaryocyte Development in the Bone Marrow Underlies Dengue Virus-Induced Thrombocytopenia in Humanized Mice

A characteristic clinical feature of dengue virus infection is thrombocytopenia, though its underlying mechanism is not definitively determined. By adoptive transfer of human CD34⁺ fetal liver cells into immunodeficient mice, we have constructed humanized mice with significant levels of human platelets, monocytes/macrophages, and hepatocytes. Infection of these mice with both lab-adapted and clinical strains of dengue virus induces characteristic human hematological changes, including transient leukopenia and thrombocytopenia. We show that the specific depletion of human platelets is not mediated by antibodies in the periphery or reduced production of human thrombopoietin in the liver but reduction of human megakaryocytes and megakaryocyte progenitors in the bone marrow of the infected mice. These findings identify inhibition of platelet production in the bone marrow as a key mechanism underlying dengue-induced thrombocytopenia and suggest the utility of the improved humanized mouse model in studying dengue virus infection and pathogenesis in a human cell context.     

Transgenic maize phosphoenolpyruvate carboxylase alters leaf–atmosphere CO2 and 13CO2 exchanges in Oryza sativa

Photosynthesis Research - The engineering process of C4 photosynthesis into C3 plants requires an increased activity of phosphoenolpyruvate carboxylase (PEPC) in the cytosol of leaf mesophyll...     

Solute flux into parasitic plants

Parasitic plants form intimate contacts with host tissue in order to gain access to host solutes. There are a variety of cell types within the host which parasitic plants could access to extract solutes. Depending on the degree to which the parasite has embraced the parasitic lifestyle, the extent of solute flux and the pathways used to transfer solutes from host to parasite will vary. To date, a variety of experimental approaches argue for diversity in the mechanisms and the routes by which parasites accumulate host solutes. Contact between host and parasite ranges from direct lumen-to-lumen links between host and parasite xylem and continuity between the sieve elements of host and parasite, to the involvement of transfer cells between host and parasite. Progress has been slow since Solms-Laubach distinguished types of parasitic plants that fed from host phloem or xylem in 1867, but advances in clearly delineating the pathways that link host and parasite should now be possible using fluorescent proteins expressed and restricted to particular cell types of the host. This will initially necessitate using Arabidopsis, but should allow the types of connection, i.e. symplasmic or apoplasmic, to be determined and then the identification of parasite transporters responsible for solute flux.     

The TIME FOR COFFEE gene maintains the amplitude and timing of Arabidopsis circadian clocks

Plants synchronize developmental and metabolic processes with the earth's 24-h rotation through the integration of circadian rhythms and responses to light. We characterize the time for coffee (tic) mutant that disrupts circadian gating, photoperiodism, and multiple circadian rhythms, with differential effects among rhythms. TIC is distinct in physiological functions and genetic map position from other rhythm mutants and their homologous loci. Detailed rhythm analysis shows that the chlorophyll a/b-binding protein gene expression rhythm requires TIC function in the mid to late subjective night, when human activity may require coffee, in contrast to the function of EARLY-FLOWERING3 (ELF3) in the late day to early night. tic mutants misexpress genes that are thought to be critical for circadian timing, consistent with our functional analysis. Thus, we identify TIC as a regulator of the clock gene circuit. In contrast to tic and elf3 single mutants, tic elf3 double mutants are completely arrhythmic. Even the robust circadian clock of plants cannot function with defects at two different phases.     

Localization of photosynthetic metabolism in the parasitic angiosperm Cuscuta reflexa

Cells capable of photosynthesis in the parasitic angiosperm Cuscuta reflexa Roxb. (dodder) are highly localized. Immunolocalization of ribulose-1,5 bisphosphate carboxylase-oxygenase (Rubisco) and autofluorescence of chlorophyll in transverse sections of stems showed that they were largely restricted to a band of cells adjacent to the vascular bundles, consequently, the concentrations of Rubisco and chlorophyll were low per unit area or fresh weight. When ¹⁴CO₂ was supplied to stem segments of C. reflexa it preferentially accumulated in these cells adjacent to the vasculature. Although the conductance for CO₂ movement to the cells containing chlorophyll and Rubisco was very low, both the light reactions and dark reactions of photosynthesis appeared to be functional. De-epoxidation of the xanthophyll-cycle pigments after exposure to high light, and the chlorophyll fluorescence parameters, photochemical quenching (qP), non-photochemical quenching (NPQ) and the quantum efficiency of photosystem II (φPSII) responded normally to changes in photon flux density, indicating functional light-driven electron transport. The response of CO₂ exchange to photon flux density followed a typical hyperbolic curve, and positive rates of CO₂ fixation occurred when external CO₂ was increased to 5%. We propose that CO₂ for carbon assimilation is derived from internally respired CO₂ and that this layer of photosynthetic cells makes a positive contribution to the carbon budget of C. reflexa. Received: 23 October 1997 / Accepted: 16 December 1997