Mechanism for formation of the secondary wall thickening in tracheary elements: Microtubules and microfibrils of tracheary elements of Pisum sativum L. and Commelina communis L. and the effects of amiprophosmethyl

Arrangements of microfibrils (MFs) and microtubules (MTs) were examined in tracheary elements (TEs) of Pisum sativum L. and Commelina communis L. by production of replicas of cryo-sections, and by immunofluorescence microscopy, respectively. The secondary wall thickenings of TEs of Pisum and Commelina roots have pitted and latticed patterns, respectively. Most MFs in the pitted thickening of Pisum TEs retain a parallel alignment as they pass around the periphery of pits. However, some groups of MFs grow into the pits but then terminate at the edge of the thickening, indicating that cellulose-synthase complexes are inactivated in the plasma membrane under the pit. Microtubules of TEs of both Pisum and Commelina are localized under the secondary thickening and few MTs are detected in the areas between wall thickenings. In the presence of the MT-disrupting agent, amiprophosmethyl, cellulose and hemicellulose, which is specific to secondary thickening, are deposited in deformed patterns in TEs of Pisum roots, Pisum epicotyls and Commelina roots. This indicates that the localized deposition of hemicellulose as well as cellulose involves MTs. The deformed, but heterogeneous pattern of secondary thickening is still visible, indicating that MTs are involved in determining and maintaining the regular patterns of the secondary thickening but not the spatial heterogeneous pattern of the wall deposition. A working hypothesis for the formation of the secondary thickening is proposed.Abbreviations APM amiprophosmethyl - DMSO dimethyl sulfoxide - F-WGA fluorescein-conjugated wheat-germ agglutinin - M F microfibril - MT microtubule - PEG polyethyleneglycol - TE tracheary element I thank Ms. Aiko Hirata (Institute of Applied Microbiology, University of Tokyo, Japan) for help in taking stereomicrographs. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science and Culture of Japan.     

Studies on graft unions

Summary De novo formation of cytoplasmic cell connections are studied at the graft interface of 5 day old in vitro heterografts ofVicia faba onHelianthus annuus. Continuous and half plasmodesmata, both branched and unbranched, are described at various stages of development in non-division walls between unlike and like dedifferentiated callus cells. In apical portions of protruding callus cells and in the contact zone between opposing cells extremely thin wall parts with a striking ER/plasmalemma contact are observed. During subsequent thickening of the modified wall parts cytoplasmic strands enclosing constricted ER cisternae are entrapped within the newly deposited wall material. These cytoplasmic strands represent half plasmodesmata which—in case of fusion with corresponding structures of adjoining cells across the loosened wall matrix — form continuous cell connections. Golgi vesicles secreting wall material are involved in the process of forming half and continuous plasmodesmata, thus following the same mechanism of plasmodesmata development as described for isolated protoplasts in cell cultures. The findings suggest the existence of a unifying mechanism of secondary formation of plasmodesmata showing far-reaching similarities with the establishment of primary cell connections.     

Nonlever action of the mandible

This study considers the current concept of the mandible as a lever of the third order. The concept requires a fulcrum, and this function has been ascribed to the condyle region, but it tends to be overlooked that the fulcrum of a third-order lever in this case would sometimes have to bear a considerable stress. Certain changes, attributed to stress, have been observed in anatomical components of the articulation, but they cannot be explained in terms of the lever concept. They are accounted for by the changing anatomical relations in the working and contralateral sides during mandibular function. They arise from minor stress, especially when dental conditions indicate a period of abnormal function.     

Phylogeny of the 5S ribosomal RNA fromSynechococcus lividus II: The cyanobacterial/chloroplast 5S RNAs form a common structural class

Summary The complete nucleotide sequence of the 5S ribosomal RNA from the cyanobacteriumSynechococcus lividus II has been determined. The sequence is 5-UGCCUAGUGUUUAUGGCGCG-GUGGAACCACGCUGAUCCAUCCCGAACUC-AGAGGUGAAACAUCGCAGCGGUGAAGAU-AGUUGGAGGGUAGCCUCCUGCAAAAAUA-GCUCAAUGCUAGGCA_OH-3. This 5S RNA has the cyanobacterial- and chloroplast-specific nucleotide insertion between positions 30 and 31 (using the numbering system of the generalized eubacterial 5S RNA) and the chloroplast-specific nucleotide-deletion signature between positions 34 and 39. The 5S RNA ofS. lividus II has 27 base differences compared with the 5S RNA of the related strainS. lividus III. This large difference may reflect an ancient divergence between these two organisms. The electrophoretic mobilities on nondenaturing polyacrylamide gels of renatured 5S RNAs fromS. lividus II,S. lividus III, and spinach chloroplasts are identical, but differ considerably from that ofEscherichia coli 5S RNA. This most likely reflects differences in higher-order structure between the 5S RNA ofE. coli and these cyanobacterial and chloroplast 5S RNAs.     

Prediction and comparison of the secondary structure of legume lectins

Secondary structure prediction for the 4 legume lectins: Concanavalin A, soybean agglutinin, favabean lectin and lentil lectin, was done by the method of Chou and Fasman. This prediction shows that these four lectins fall into a structurally distinct class of proteins, containing high amounts of β-sheet and β-turns. There is a notable similarity in the gross structure of these proteins; all four of them contain about 40–50% of β-sheet, 35–45 % β-turn and 0–10% of α-helix. When the secondary structure of corresponding residues in each pair of these lectins was compared, there was a striking similarity in the Concanavalin A-soybean agglutinin and favabean lectin-lentil lectin pairs, and considerably less similarity in the other pairs, suggesting that these legume lectins have probably evolved in a divergent manner from a common ancestor. A comparison of the predicted potential β-turn sites also supports the hypothesis of divergent evolution in this class of lectins.     

Structural implications of primary sequences from a family of Balbiani ring-encoded proteins inChironomus

Summary DNA sequencing has revealed an internal, tandemly repetitive structure in the family of giant polypeptides encoded by three types of Balbiani ring (BR) genes, in three different species ofChironomus. Each major BR repeat can be subdivided into two halves: a region consisting of short subrepeats and a more constant region that lacks obvious subrepeats. Comparative predictions of secondary structure indicate that an -helical segment is consistently present in the amino-terminal half of the constant region in all known BR proteins. Comparative predictions, coupled with consideration of the known phosphorylation of serine and threonine residues in BR proteins, suggest that the -helical structure may also extend into the carboxy-terminal half of the constant region, possibly interrupted by -turn(s). However, it is also possible that the structure is variable, and that a -strand is present in that half in some cases. All of the constant regions conserve one methionine and one phenylalanine residue, as well as all four cysteines; these residues presumably play roles in the packing or cross-linking of aligned constant regions. The structure of the subrepeat region is not clear, but the prevalence of a tripeptide pattern (basic-proline-acidic) suggests some type of structural regularity, possibly an extended helix. The possible significance of these conserved molecular features is discussed in the context of how they may serve the elasticity, insolubility, and hydrophilicity of the fibrils and threads formed by the BR polypeptides.     

油松茎次生结构中树脂道的结构分布和发育的研究

油松茎的次生结构中树脂道存在于次生维管组织中。其中,次生木质部内具有水平的和垂直的两类树脂道,而次生韧皮部内则仅有水平的树脂道。两类树脂道都由上皮细胞和鞘细胞包围着胞间道构成,其中木质部内的树脂道具有死鞘细胞,而韧皮部中的则都系生活细胞。在心材中,垂直树脂道形成拟侵填体。在次生木质部内,垂直树脂道常分布于早材的外部区域和最初形成的晚材中,它们与水平树脂道连接,腔道贯通,从而形成二维网状结构。垂直树脂道来源于纺锤状原始细胞的衍生细胞,而水平树脂道来源于射线原始细胞,两者都以裂生方式发生。     

The nature of the stable noncovalent dimers of band 3 protein from erythrocyte membranes in solutions of Triton X-100

Stable noncovalent dimers of band 3 protein from human erythrocyte membranes, in which state the protein is thought to exist after solubilization by the nonionic detergent Triton X-100, do not occur when purified batches of the detergent are used. Instead, the protein is in a monomer/dimer/tetramer association equilibrium. The stable dimers do appear, however, when the detergent has been 'aged'. They thus seem to be artifacts.     

Vegetation dynamics in the fens of Chautagne (Savoie,France) after the cessation of mowing

The mechanisms controlling secondary succession on eutrophic peat are analyzed on an area of 500 ha marsh near the Lac du Bourget in the Rhône floodplain between Geneva and Lyon. After describing the succession, the authors compare the succession with that described for fens in England.During succession, three categories of phenomena occur:-Predictable phenomena based on the occurrence of well-established relationships between species with a high cover density. When mowing ceases, populations with increasingly higher biomasses reach dominance one by one.-Phenomena which were to be expected although the manner in which they occurred were more difficult to predict, such as the conditions in which woody plants take over.-Phenomena hidden in the framework of a synchronic analysis. The lowering of the water table, for example, can modify the entire community of species occurring in the sequence as well as the competition between species. The pecularities of each type of ecological situation made it difficult to develop general models.