Gabriele Tani

Megasporogenesis and programmed cell death in Tillandsia (Bromeliaceae)

The degeneration of three of four meiotic products is a very common process in the female gender of oogamous eukaryotes. In Tillandsia (and many other angiosperms), the surviving megaspore has a callose-free wall in chalazal position while the other three megaspores are completely embedded in callose. Therefore, nutrients and signals can reach more easily the functional megaspore from the nucellus through the chalazal pole with respect to the other megaspores. The abortion of three of four megaspores was already recognized as the result of a programmed cell death (PCD) process. We investigated the process to understand the modality of this specific type of PCD and its relationship to the asymmetric callose deposition around the tetrad. The decision on which of the four megaspores will be the supernumerary megaspores in angiosperms, and hence destined to undergo programmed cell death, appears to be linked to the callose layer deposition around the tetrad. During supernumerary megaspores degeneration, events leading to the deletion of the cells do not appear to belong to a single type of cell death. The first morphological signs are typical of autophagy, including the formation of autophagosomes. The TUNEL positivity and a change in morphology of mitochondria and chloroplasts indicate the passage to an apoptotic-like PCD phase, while the cellular remnants undergo a final process resembling at least partially (ER swelling) necrotic morphological syndromes, eventually leading to a mainly lipidic cell corpse still separated from the functional megaspore by a callose layer.

Structural and ultrastructural characterization of Selaginella lepidophylla, a desiccation-tolerant plant, during the rehydration process

Summary Selaginella lepidophylla is a pteridophyte resurrection plant from Central America deserts that has been studied hitherto only for microphyll ultrastructure and photosynthetic activity during rehydration. With the present work the anatomy and cell ultrastructures have been examined during progressive rehydration. Light microscope observations clearly showed the maintenance in the dry condition of chloroplasts although in a crushed form, of chlorophyll content, and the restoration of the normal anatomical structures in stem and microphylls after the complete rehydration process. Electron microscope analyses permitted us to observe a gradual reorganization in the different cells of stem and microphylls of all the cytoplasmic components including the membrane systems present either in the protoplasm or in the organelles. In particular, the chloroplasts appeared to be able to newly accumulate starch, and then to recover the normal photosynthetic capacity. Therefore, it is possible to say that S. lepidophylla retains in the dry condition the cell structures in a well conserved state or, at least, in a functional state by protective mechanisms that permit a total reorganization of the cells after rehydration. This statement is further supported by the absence of any structural disruption effects in the rehydrated cells of S. lepidophylla .

Ultrastructural Features of the Tillandsia Usneoides L. Absorbing Trichome During Conditions Moisture and Aridity

SUMMARYThe ultrastructure of the whole cellular system which forms the absorbing trichome of Tillandsia usneoides L. was studied under conditions of water stress and moisture. Cytological modification which are associated with so special a modality of functioning, were identified. The trichome is tipically divided into two parts: the «shield» whose cells form a central disk and a wing; the «stalk» whose structure includes a dome cell, two intermediate cells and two foot cells. All the walls of the absorbing system cells did not show any differences under the two physiological conditions assayed. No differences were observed also between the cytoplasm of the «shield» cells in the two experimental conditions. However, the cytoplasm of the central disk cells remained vital implying that they play an active role under both conditions, in this portion of the trichome, as regards the choise of nutrient solutions. The most important ultrastructural aspects were offered by the cytoplasm of «stalk» cells under bot...

Ultrastructural aspects of the embryo and different endosperm compartments, in Eruca sativa Hill cv. Nemat (Brassicaceae) during Heart and Torpedo stages

Abstract These observations are the first on the ultrastructure of the embryo and the endosperm of Eruca sativa Hill. We investigated the cv. Nemat, which is characterized by a particularly high amount of lipids and glucosinolates. Our observations suggested that the thick and abundant micropylar endosperm, completely surrounding the suspensor, may be the main active source of nutrients for the embryo. This endosperm, like the central chamber endosperm, is particularly rich in functional chloroplasts and cellularizes later with respect to the other previ- ously investigated Brassicaceae. The last (distal with respect to the embryo) suspensor cell exhibits important features related to the passage of nutrients, such as wall ingrowths. In fact these ingrowths appear as highly convoluted labyrinthine wall projections. Such ultrastructural features are typical of transfer cells. The accumu- lation stage in E. sativa cv. Nemat appears to occur early (Heart stage of embryo development, as Brassica na pus). The endosperm compartment called Chalazal Endosperm Cyst (CEC), contributes actively to the embryo trophism during the Heart and Torpedo stages. This function is evident because of the high number of chloro- plasts in the cyst and for the observed continuity between the CEC and the other endosperm compartments (CC endosperm and micropylar endosperm) in cv. Nemat. The morphology of the CEC appeared to be more similar to the pyriform shape sensu Brown et al., but with a more flattened base with respect to the proposed examples, and without labyrinthine wall. The Chalazal Chamber appeared to be more similar to the Brown’s type B in E. sativa. The presence of chloroplasts with a well developed thylakoid system indicates an active photosynthetic activity by the majority of the seed tissues. E. sativa leaves are normally harvested for food, while the seeds of cv. Nemat appear to be particularly rich in oil. The premature independence of seeds and fruits from the necessity of absorbing nutrients from the rest of the plant, could indicate the possibility of harvesting both leaves (earlier) and seeds (later) in this plant without compromising a full seed maturation.

Soft Dehydration in the Embedding Method for Transmission Electron Microscopy

SUMMARYA technique for the preparation of biological material for transmission electron microscopy has been successfully tested. After a partial dehydration with ethanol, GMA is used to complete dehydration and to favor the infiltration of the Spurrs resin used for embedding. This procedure enhances sectioning quality of problem materials and, at the same time, minimizes the loss of cell constituents.

Cytological aspects of the hypocotyl correlated to the behavior of the embryo radicle of Tillandsia atmospheric species

SUMMARYGenerally in Bromeliaceae, as in all monocotyledons, upon germination of the seed, the embryo radicle grows very little. In the atmospheric species of Tillandsia, not even this minimal growth occurs. In the adult stage, these species use special hairs as their sole absorbing organ. The present work analyses the morphological and cytochemical events that are responsible for the lack of growth in the embryo radicle. Ultrastructural analysis and specific stainings for light microscopy highlighted a special area (that we have called the «constriction zone») in the median region of the hypocotyl-root axis of embryos of T. flabellata Back. and T. schiedeana Steud., both atmospheric species. During the final phase of seed maturation, the cells of the aforesaid zone thickened their walls by means of materials that, at least in part, consisted of acid polysaccharydes and neutral lipids. These materials were derived from the fused contents of characteristic cytoplasmic vesicles. Due to the presence of hydrop...

Ultrastructural observations on the megasporocyte of Taxus baccata L. (Taxaceae) in relation to megaspore behaviour

SUMMARYIt is known that in Taxus (Taxaceae) megasporogenesis is carried out, for the most part, in a single megasporocyte. Upon maturity, this cell presents a grouping of the greater part of both the mitochondria and the amylipherous reserves in its chalazal cytoplasm. The chalazal megaspore, which inherits a larger supply of the said organelles, gives rise to the female gametophyte. These aspects essentially agree with what has been generally observed in gymnosperms. Nevertheless, research carried out under the LM shows that in Taxus there is—even if sporadically—differentiation of several megasporocytes. Moreover, the behaviour of the kinoplasmic mass is variable and any of the megaspores arising from the same meiotic act can be the functional one. Finally, more than one megaspore, even up to four, can begin to develop. The aim of this investigation was to identify in the megasporocyte of Taxus baccata L. the possible ultrastructural bases for the afore-mentioned uncommon aspects. The most significant n...

Host surface tissues and basidiospore-derived infection strategies of some rust fungi

Abstract Some Authors belief that the basidiospore germ tubes of rust fungi penetrate angiosperms directly and gymnosperms indirectly, hypothesizing that the different strategies of basidiospore-derived penetrations could be ascribed to the heavily cutinized and thick walled epidermal cells of the leaves of gymnosperms. In this paper the surface tissues of the organs of some angiosperms and gymnosperm—which are directly and/or indirectly penetrated by basidiospore germlings of Melampsora spp., Cronartium flaccidum and Crysomyxa abietis—were examined, in order to: (i) evaluate the role of the cuticular and epidermal cell-wall thickness, still considered a barrier to a direct penetration, and (ii) search for other possible cyto-histological aspects of surface tissues which could explain the different penetration strategies of rust basidiospore germlings. The results show that the thickness of cuticle and epidermal cell walls of the organs onto which the monokaryotic phase of each rust species has fitted itself during evolution to penetrate is not important in determining the penetration strategy, whereas it seems that it is the presence of lignin in the surface tissues of these organs which probably limits the direct penetration of rust basidiospore germlings.

Vesiculation of thylakoids in Myxobaktron salinum Walsby strain PCC 8305

Myxobaktron salinum WALSBY strain PCC 8305 (also known as Dactylococcopsis salina) is a unicellular cyanobacterium isolated from hypersaline waters of Solar Lake (Sinai). This cyanobacterium is characterised by fusiform cells showing a netlike cytoplasm, i.e. keritomy. Transmission electron microscopy studies showed that the keritomy is due to the enlargement of thylakoid lumen in electron-transparent areas, i.e. intrathylakoidal vesicles. In young cells grown photo-autotrophically under standard conditions the content of intrathylakoidal vesicles appears mostly electron-transparent. With the culture ageing the content becomes more electron-dense, the intrathylakoidal vesicles are sometimes filled with small osmiophilic granules, and the vesiculation of thylakoids increases; under dark condition there is a progressive decrease of the electron-dense content up to complete disappearance of the osmiophilic granules. There is also a reduction of the dimension of intrathylakoidal vesicles. During growth under nitrogen depleted conditions there is a marked accumulation of low electron-dense granules inside the intrathylakoidal vesicles. These granules tend to disappear when the culture is transferred to dark conditions, and the intrathylakoidal vesicle content becomes completely electron-transparent. This behaviour is typical of carbohydrates, which are usually consumed under dark conditions. Using a specific staining (THIERY 1967) we confirmed the polysaccharidic nature of the content of the intrathylakoidal vesicles. The dynamic of the formation and role of these vesicles are discussed together with their occurrence among other unicellular cyanobacteria.

Ultrastructural features of megasporogenesis inTorreya nucifera (Taxaceae)

In megasporogenesis ofTorreya nucifera (Taxaceae) more than one product of meiosis can start to germinate, a process previously observed inTaxus. Ultrastructural analysis ofT. nucifera revealed that this behaviour is cytologically determined by the presence of a transfer cell type wall-membrane apparatus and by the presence of uncommonly complex plasmalemmasomes in the megasporocyte. These features may determine a uniform supply of nutrients to the entire megasporocyte cytoplasm, so that after meiosis the germination potential is not restricted to the chalazal megaspore. Even if more than one product of meiosis can originate the female gametophyte in bothTaxus andTorreya, this process involves different ultrastructural aspects in the two species. In conclusion, the extension of the germination potential is a common characteristic to both species but it is most probably the result of some evolutionary convergence.