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.

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SUMMARY -The early phases of basidiospore-derived infection of Me/ampsora pulcherrima (Bub.) Maire on the leaves of Mercurialis annua L. were studied by light microscopy, SEM and TEM. The fine morphology of the basidiospore germling penetration and intraepidermal infection structures is discussed in comparison with that of other rusts recently described. The direct penetration through the epidermal cell wall, characteristic of the rust basidiospore-derived germlings, is confirmed. The absence of an extrahyphal matrix around the intraepidermal vesicle and the presence of a collar around the vesicle neck are pointed out.

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A cyto-histological study was carried out on Pinus sylvestris seedlings artificially infected with basidiospores of Melampsora pinitorqua and Melampsora laricitremulae, in order to differentiate between the two rust fungi and delineate their taxonomical relationships. The general morphology of the early phases of the majority of infections of Melampsora larici-tremulae on P. sylvestris show similar characteristics to those of M. pinitorqua. Moreover, in M. larici-tremulae / P. sylvestris interactions, the following were the specific characteristics: abnormal germlings and secondary basidiospore production, «cell wall-like appositions» encasing the fungal structures in the host, necrosis of the infection structures, necrosis of the host tissue colonized by mycelium from epidermal to parenchymal cells. Such reaction types reveal the coexistence of pre-and posthaustorial defence mechanisms, typical, respectively, of a nonhost and host resistance. This coexistence which is useful in understanding the evolution of this host-rust interaction, was examined. The described defence mechanisms of P. sylvestris vs. M. larici-tremulae could indicate that the two rust fungi are still closely related entities, however diverging, which evolve from «formae speciales» to «species».

SUMMARY-It 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 T axus 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 new result of this study shows that frequent plasmodesmata, previously observed only in the walls of the other nucellus cells, are present in the megasporocyte wall, as well. The peculiar fine structure and the delayed occlusion of these cytoplasmic connections suggest that the additional multi-directional simplastic circulation of nutrients before and during meiosis takes place. This condition may allow for the maturation of some meiocytes and may even allow for at least the initial development of all the megaspores which arise from a single meiotic process. However, the megasporocyte chalazal cytoplasm is usually the most privileged in relation to the development of the female gametophyte because of its nearness to the conducting tissues of the ovules.

Vol. 63, no. 2: 197-210, 2010 The John Bingham Laboratory, NIAB. Huntingdon Road, Cambridge CB3 0LE, UK.Abstract -These observations are the fi rst 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 previously 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 accumulation stage in E. sativa cv. Nemat appears to occur early (Heart stage of embryo development, as Brassica napus). 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 chloroplasts 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 fl attened 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.