Temperature-dependent internode elongation in vegetative plants of Arabidopsis thaliana lacking phytochrome B and cryptochrome 1

 Vegetative plants of Arabidopsis thaliana (L.) Heynh. form a compact rosette of leaves in which internode growth is virtually arrested. Rapid extension of the internodes occurs after flower buds are present in the reproductive apex. Under natural radiation, continuous light from fluorescent lamps, or short photoperiods of light from fluorescent lamps, plants of the phyB cry1 double mutant (lacking both phytochrome B and cryptochrome 1) did not form normal rosettes because all the internodes showed some degree of elongation. Internode elongation was weak in the phyB single mutant and absent in the cry1 mutant, indicating redundancy between phytochrome B and cryptochrome 1. The absence of phytochrome A caused no effects. The failure to form normal rosettes was conditional because internode elongation was arrested at low temperatures in all the mutant combinations. In contrast, the temperature dependence of phytochrome B and cryptochrome 1 effects on hypocotyl growth was weak. The elongation of the internodes in phyB cry1 was not accompanied by early flowering as showed by the lack of effects on the final number of leaves. Apex dissection indicated that in phyB cry1 double mutants internode elongation anticipated the transition from the vegetative to the reproductive stage. Thus, stem growth in Arabidopsis thaliana is not fully dependent on the program of reproductive development. Received: 2 June 1999 / Accepted: 13 August 1999     

Effects of Ethanol and Other Alkanols on Transport of Acetic Acid in Saccharomyces cerevisiae

In glucose-grown cells of Saccharomyces cerevisiae IGC 4072, acetic acid enters only by simple diffusion of the undissociated acid. In these cells, ethanol and other alkanols enhanced the passive influx of labelled acetic acid. The influx of the acid followed first-order kinetics with a rate constant that increased exponentially with the alcohol concentration, and an exponential enhancement constant for each alkanol was estimated. The intracellular concentration of labelled acetic acid was also enhanced by alkanols, and the effect increased exponentially with alcohol concentration. Acetic acid is transported across the plasma membrane of acetic acid-, lactic acid-, and ethanol-grown cells by acetate-proton symports. We found that in these cells ethanol and butanol inhibited the transport of labelled acetic acid in a noncompetitive way; the maximum transport velocity decreased with alcohol concentration, while the affinity of the system for acetate was not significantly affected by the alcohol. Semilog plots of V_max versus alcohol concentration yielded straight lines with negative slopes from which estimates of the inhibition constant for each alkanol could be obtained. The intracellular concentration of labelled acid was significantly reduced in the presence of ethanol or butanol, and the effect increased with the alcohol concentration. We postulate that the absence of an operational carrier for acetate in glucose-grown cells of S. cerevisiae, combined with the relatively high permeability of the plasma membrane for the undissociated acid and the inability of the organism to metabolize acetic acid, could be one of the reasons why this species exhibits low tolerance to acidic environments containing ethanol.     

Classification of rice germplasm: III. High-resolution fingerprinting of cytoplasmic genetic male-sterile (CMS) lines with AFLP

 The cytoplasmic genetic male-sterile (CMS) lines developed at the International Rice Research Institute are valuable in producing tropical rice hybrids. Efficient use of CMS lines in hybrid rice production will depend on their level of genetic diversity. Aside from morphological characterization, molecular analysis based on DNA markers can provide information on the genetic diversity of the germplasm. The Amplified Fragment Length Polymorphism (AFLP) technique was used to fingerprint 71 CMS lines and four rice cultivars, ‘IR64’, ‘Azucena’, ‘IR74’, and ‘FR13A’. Eleven primer pair combinations specific to the enzymes PstI and MseI were used to generate 530 AFLP markers, 176 of which were polymorphic. Each CMS line revealed a distinct fingerprint. The AFLP marker-based dendrogram depicted genetic variation among the CMS lines. The CMS lines developed in japonica background grouped with ‘Azucena’, a japonica cultivar. None of the CMS lines clustered with ‘FR13A’, a flood-tolerant traditional indica variety. ‘IR64’ was found to be distinct from the other indica CMS lines and clustered with lines developed in its background. The grouping of CMS lines into a few groups is useful for breeders in selecting genetically diverse CMS lines for hybrid rice production and in avoiding test crossing every CMS line empirically. This study demonstrated that AFLP is a powerful and reliable tool in determining the genetic relationships and in producing distinct fingerprints of rice cultivars. Received: 20 December 1996 / Accepted: 9 October 1997     

The genetic structure of fermentative vineyard-associated <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> populations revealed by microsatellite analysis

From the analysis of six polymorphic microsatellite loci performed in 361 Saccharomyces cerevisiae isolates, 93 alleles were identified, 52 of them being described for the first time. All these isolates have a distinct mtDNA RFLP pattern. They are derived from a pool of 1620 isolates obtained from spontaneous fermentations of grapes collected in three vineyards of the Vinho Verde Region in Portugal, during the 2001–2003 harvest seasons. For all loci analyzed, observed heterozygosity was 3–4 times lower than the expected value supposing a Hardy–Weinberg equilibrium (random mating and no evolutionary mechanisms acting), indicating a clonal structure and strong populational substructuring. Genetic differences among S. cerevisiae populations were apparent mainly from gradations in allele frequencies rather than from distinctive “diagnostic” genotypes, and the accumulation of small allele-frequency differences across six loci allowed the identification of population structures. Genetic differentiation in the same vineyard in consecutive years was of the same order of magnitude as the differences verified among the different vineyards. Correlation of genetic differentiation with the distance between sampling points within a vineyard suggested a pattern of isolation-by-distance, where genetic divergence in a vineyard increased with size. The continuous use of commercial yeasts has a limited influence on the autochthonous fermentative yeast population collected from grapes and may just slightly change populational structures of strains isolated from sites very close to the winery where they have been used. The present work is the first large-scale approach using microsatellite typing allowing a very fine resolution of indigenous S. cerevisiae populations isolated from vineyards.     

Light and ultrastructural description of Meglitschia mylei n. sp. (myxozoa) from Myleus rubripinnis (Teleostei: Serrasalmidae) in the Amazon River system

Meglitschia mylei n. sp. found in the gall bladder of the teleostean fish Myleus rubripinnis (Serrasalmidae) from the middle Amazonian region of Brazil is described using light and transmission electron microscopy. The spores observed in the bile averaged 24.6±0.8 μm long, 8.7±0.4 μm wide and 5.1±0.3 μm thick and were strongly furcate and arcuate ∩-shaped composed of two symmetric equal-sized valves, up to ～70 nm thick. Each valve possessed one opposed tapering appendage, 20.1±0.7 μm long, oriented parallel towards the basal tip of the appendages and joined along a right suture line forming a thick strand. The strand goes around the central part of the spore, which in turn surrounds two equal and symmetric spherical polar capsules (PC), 2.1±0.3 μm in diameter, located at the same level. Each capsule contains a polar filament with five (rarely six) coils. The binucleate sporoplasm was irregular in shape, contained several sporoplasmosomes, ～175 nm in diameter and filled all the space of the two caudal appendages. Based on the arc shape of the spore with two tapering caudal appendages oriented to the basis of spores, on the number and position of the PC and of the polar filament coils and arrangements, and on the host specificity, we propose the name M. mylei n. sp. for this new myxozoan. Accordingly, this is the second described species of this genus.     

Agent-based modeling of the context dependency in T cell recognition

Antigen recognition by T cells is a key event in the adaptive immune response. T cells scan the surface of antigen-presenting cells (APCs) or target cells for specific peptides bound to MHC molecules. In the physiological setting, a typical APC presents tens of thousands of diverse endogenous self-derived peptides complexed to MHC (pMHC complexes). When 'foreign' peptides are presented, they constitute a small fraction of the total surface peptide repertoire. As T cells seem to be capable of discerning minute amounts of 'foreign' peptides among a complex background of self-peptides, endogenous peptides are generally assumed to play no role in recognition. However, recent results suggest that these background peptides may alter the sensitivity of T cells to foreign peptides. Current experimental limitations preclude analysis of peptide mixtures approaching physiological complexity, making it difficult to further address the role of complex background peptides. In this paper, we present a computational model to test how complex, varied peptide populations on an APC could potentially modulate a T cell's ability to detect the presence of small numbers of agonist peptides among a diverse population. We use the model to investigate the notion that under physiological conditions, T cell recognition of foreign peptides is context dependent, that is, T cells process signals gathered from all pMHC interactions, not just from a few agonist peptides while ignoring all others.     

Developmental compartments and planar polarity in Drosophila

BACKGROUND: Planar polarity refers to the asymmetry of a cell within the plane of the epithelium; for example, cells may form hairs that point in a posterior direction, or cilia may beat in one way. This property implies that cells have information about their orientation; we wish to understand the nature of this information. Relevant also is the body plan of insects, which, in the ectoderm and somatic mesoderm, consists of a chain of alternating anterior and posterior compartments - basic units of development with independent cell lineage and subject to independent genetic control.RESULTS: Using the abdomen of adult Drosophila, we have taken genes required for normal polarity and either removed the gene or constitutively expressed it in small clones of cells and observed the effects on polarity. Hitherto, all such studies of polarity genes have not found any difference of behavior between the different compartments. We report here that the three genes, four-jointed, dachsous, and fat, cause opposite effects in anterior and posterior compartments. For example, in anterior compartments, clones ectopically expressing four-jointed reverse the polarity of cells in front of the clone, while, in posterior compartments, they reverse behind the clone. These three genes have been reported by others to be functionally linked.CONCLUSIONS: This discovery impacts on models of how cells read polarity. At the heart of one class of models is the hypothesis that cell polarity is determined by the vector of a morphogen gradient. Here, we present evidence that cell polarity in the abdomen depends on at least two protein gradients (Fj and Ds), each of which is reflected at compartment borders. Consequently, these gradients have opposing slopes in the two compartments. Because all polarized structures made by abdominal cells point posteriorly, we surmise that cells in each compartment are programmed to interpret these protein gradients with opposite signs, pointing up the gradient in one compartment and down the gradient in the other.