Observations on Certain Plastids of the Ovule of Ginkyo Biloba L.

SUMMARYAs reported in the literature for several groups of algae and for certain cells of higher plants, among which the endosperm (CAMEFORT and SCHAEFFER 1965) and the oosphere of Ginkyo biloba L. (CAMEFORT 1965), the authors observe that there is also often a contiguity between endoplasmic reticulum and plastids in the tapetum and in the central cell of the archegonium of this plant. The functional significance of this relationships is hypothesized. Moreover, the authors note that in the central cell and in the coenocytic pro-embryo the outer membrane of the plastidial wall bears some evaginations that extend into the cytoplasm. These evaginations seem, in certain cases, to connect two plastids in the central cell, for which it is thought that their formation is due to a particular scission process of the plastids themselves

Passaggio di Corpi Figurati fra Cellule del Tappeto e Cellula Centrale Nell'Archegonio dei Pini

RIASSUNTONel ciclo dello sviluppo dell'archegonio di Pinus pinea si possono schematicamente distinguere tre periodi per quanto riguarda lo stato della membrana che circonda la cellula centrale.Nel primo periodo la cellula centrale ha una parete uniformemente sottile. Durante questo periodo diviene evidente il tappeto dell'archegonio, strato limite ben distinto per le sue caratteristiche dal rimanente dell'endosperma, in cui l'archegonio e immerso. L'apporto di sostanze dal tappeto alla cellula centrale in rapidissimo accrescimento dovrebbe avvenire senza particolari ostacoli attraverso a questa parete sottile, purche in forma fluida.Nel secondo periodo questa parete subisce un processo di ispessimento con apposizione di nuovi strati prospicienti verso la cellula centrale. Questo ispessimento non e uniforme, ma si riscontrano piccole aree, piu o meno frequenti, in cui la parete rimane sottile. E presumibile che causa dell'ispessimento sia la necessita di dare una impalcatura solida ad una cellula che ha ra..

On the Megasporogenesis in Chamaecyparis Lawsoniana Parl.

SUMMARYIn Chamaecyparis lawsoniana Parl, the megaspore mother cell (MMC) differentiates in the middle and innermost region of the nucellus as is generally the case in Gymnosperms. During its development the MMC attains a size much larger than that of the surrounding somatic cells and accumulates a considerable amount of food-storage substances. These consist of starch and droplets which could be, at least partly, of lipidic nature. During prophase I the MMC becomes a fully polarized cell since all or almost all the mitochondria are restricted to the chalazal cytoplasm. In the authors' opinion this is determined by the trophically privileged position of the chalazal cytoplasm. The polarization of the chondriome determines the course of the subsequent events in such a way that the meiotic divisions give rise to a linear triad, in which the two micropylar megaspores soon degenerate while the chalazal one inherits all of the mithochondria from the megasporocyte and remains functional. On the other hand the fo..

Structural adaptation of the subunit interface of oligomeric thermophilic and hyperthermophilic enzymes

Enzymes from thermophilic and, particularly, from hyperthermophilic organisms are surprisingly stable. Understanding of the molecular origin of protein thermostability and thermoactivity attracted the interest of many scientist both for the perspective comprehension of the principles of protein structure and for the possible biotechnological applications through application of protein engineering. Comparative studies at sequence and structure levels were aimed at detecting significant differences of structural parameters related to protein stability between thermophilic and hyperhermophilic structures and their mesophilic homologs. Comparative studies were useful in the identification of a few recurrent themes which the evolution utilized in different combinations in different protein families. These studies were mostly carried out at the monomer level. However, maintenance of a proper quaternary structure is an essential prerequisite for a functional macromolecule. At the environmental temperatures experienced typically by hyper- and thermophiles, the subunit interactions mediated by the interface must be sufficiently stable. Our analysis was therefore aimed at the identification of the molecular strategies adopted by evolution to enhance interface thermostability of oligomeric enzymes. The variation of several structural properties related to protein stability were tested at the subunit interfaces of thermophilic and hyperthermophilic oligomers. The differences of the interface structural features observed between the hyperthermophilic and thermophilic enzymes were compared with the differences of the same properties calculated from pairwise comparisons of oligomeric mesophilic proteins contained in a reference dataset. The significance of the observed differences of structural properties was measured by a t-test. Ion pairs and hydrogen bonds do not vary significantly while hydrophobic contact area increases specially in hyperthermophilic interfaces. Interface compactness also appears to increase in the hyperthermophilic proteins. Variations of amino acid composition at the interfaces reflects the variation of the interface properties. © 2008 Elsevier Ltd. All rights reserved