Cell division and DNA topisomerase I activity in root meristems of pea seedlings during water stress

ABSTRACT

Low water potential, generated by PEG addition to the liquid medium of hydroponically grown pea seedlings, induces a fall in moisture content in the roots, followed by the arrest of elongation. This water stress reduces the mitotic index of root meristems during the treatment and induces the appearance of a peak of mitosis at 12 hours from the beginning of recovery. This peak suggests that during water stress the cell cycle is blocked in G2 or late S phase. In a first attempt to understand the biochemical events leading to cell cycle arrest, we tested the in vitro activity of DNA topoisomerase I extracted from stressed or control root meristems. The activity of this enzyme in extracts from stressed seedlings was lower than in controls, whereas it was higher in extracts from seedlings which had recovered from water stress for a few hours. The highest specific activity was observed with seedlings at 24 hours from the start of recovery. The fact that during stress treatments and recovery there was no variation in the synthesis of a 45 kDa protein, indicated as DNA topoisomerase I, suggested that the activity of this enzyme could be posttranslationally regulated. The hypothesis that variations in the concentration of unknown endogenous regulators of the activity of this enzyme may take place during water loss or uptake in the cytosol of meristematic cells is discussed.     

Structural and Cytochemical Characteristics of the Stigma and Style in Vitis vinifera L. var. Sangiovese (Vitaceae)

Studies were carried out on structural and cytochemical aspectsof the stigma and style ofVitis vinifera . The stigma is ofthe wet papillate type with a continuous cuticle and pellicle.During the development of the papillae, the cell walls increasein thickness and produce a secretion product constituted oflipids that pass through the wall forming the exudate. The styleis solid with a central core of transmitting tissue which hasconspicuous intercellular spaces that increase remarkably fromthe periphery to the centre where the cuticle is present. Theintercellular spaces, where the pollen tubes grow, contain amatrix that includes polysaccharides, pectic substances andscattered areas of lipidic nature. Cytochemistry; stigma; style; ultrastructure; Vitis vinifera     

牛脊髓神经丝的体外组装与结构

利用差速离心法从牛脊髓中分离神经丝 ,在电镜下观察了其形态 ;应用扫描隧道显微镜 (STM)研究了它的结构 ,发现神经丝具有长短 2种侧臂 ,二者相间排列 ,相邻长侧臂或相邻短侧臂的间距都是 2 0～ 2 2nm ;由此推测神经丝内部存在 3 /4分子交错 ;还研究了神经丝蛋白的体外组装 ,以胶体金标记的方法证明 ,中等分子量与高分子量的神经丝蛋白 ,都能同低分子量的神经丝蛋白共同装配成 10nm的纤维 ;同时发现 ,中等分子量与高分子量的神经丝蛋白能够组装成一种较细的纤维 ,不同于中间纤维 .     

Foliar modifications induced by inhibition of polar transport of auxin

The effects of auxin polar transport inhibitors,9-hydroxy-fluorene-9-carboxylic acid (HFCA);2,3,5-triiodobenzoic acid(TIBA) and trans-cinnamic acid (CA) on leaf pattern formation were investigated with shoots formed from cultured leaf explants of tobacco and cultured pedicel explants of Orychophragmus violaceus,and the seedlings of tobacco and Brassica chinensis,Although the effective concentration varies with the inhibitors used,all of the inhibitors induced the formation of trumpet-shaped and/or fused leaves.The frequency of trumpet-shaped leaf formation was related to the concentration of inhibitors in the medium.Histological observation of tobacco seedlings showed that there was only one main vascular bundle and several minor vascular bundles in normal leaves of the control,but there were several vascular bundles of more or less the same size in the trumpet-shaped leaves of treated ones.These results indicated that auxin polar transport played an important role on bilateral symmetry of leaf growth.     

A LONG-SNOUTED DYROSAURID (CROCODYLIFORMES, MESOEUCROCODYLIA) FROM THE PALEOCENE OF MOROCCO: PHYLOGENETIC AND PALAEOBIOGEOGRAPHIC IMPLICATIONS

Abstract:  New material of a long-snouted dyrosaurid has been discovered in the Paleocene of Morocco. It consists of a well-preserved skull with embedded mandible and four dorsal vertebrae. The particularly elongate snout, proportionally the longest of all known dyrosaurids, allows precise identification of this material as Atlantosuchus coupatezi Buffetaut, 1979 a , and presentation of an emended diagnosis for this species previously known only from a mandibular symphysis. A phylogenetic analysis of the dyrosaurids indicates a close relationship between A. coupatezi and Rhabdognathus . It also confirms a previous hypothesis that Congosaurus is distinct from Hyposaurus . It is more closely related to Atlantosuchus than Hyposaurus . The analysis also allows palaeobiogeographic interpretations to be made. Dyrosaurids ranged from North Africa to other areas. They were rare during the Maastrichtian and endemic to each continent at this time. Competition with large marine reptiles, such as mosasaurs, limited their dispersal during the Late Cretaceous. The disappearance of these rivals during the 'K-T crisis' enabled their diversification and widespread dispersal during the Paleocene, with the same genera present on several continents.