Morpho-histodifferentiation of Billbergia Thunb. (Bromeliaceae) nodular cultures

Nodule cultures are formed through an intermediate morphogenetic route that lies between organogenesis and somatic embryogenesis. Although well described in many species, different aspects of the morphological and histological development of nodules remain to be clarified. Based on their threatened status and high ornamental value, Billbergia alfonsi-joannis and Billbergia zebrina, two epiphytic bromeliad species endemic to the South American Atlantic Forest, were studied. Nodular cultures were induced to grow from nodal segments taken from etiolated seedlings grown in vitro for 12 weeks in the dark on MS medium supplemented with 1 μM TDZ. Samples were taken for analysis weekly over 8 weeks of growth and analyzed under light, transmission electron, and scanning electron microscopes. Morphological and histological analysis showed that nodular clusters originated from stem pericycles and consisted of a polycenter, cambial tissue, cortical parenchyma, and a covering tissue. The polycenter consisted of an organizational center dispersed in parenchymal tissue. Each organizational center was formed by a vascular system surrounded by a bundle sheath. A cambial tissue surrounded these polycenters, promoting the regeneration of new nodules and leading to the formation of buds and roots. Primary nodules could generate secondary nodules in a repetitive process. Thus, histological analysis revealed the origin and formation of nodular cultures. These new data will support the establishment of micropropagation protocols and regeneration on a large scale for these species.

Classical macroautophagy in Lobivia rauschii (Cactaceae) and possible plastidial autophagy in Tillandsia albida (Bromeliaceae) tapetum cells

The tapetum in anthers is a tissue that undergoes programmed cell death (PCD) during the production of pollen. We observed two types of autophagy prior to cell death. In Lobivia rauschii (Cactaceae), tapetum cells showed plant-type autophagosomes-autolysosomes, which have been found previously exclusively in root meristem cells. The autophagic structures were formed by a network of tubules which apparently merged laterally, thereby sequestering a portion of the cytoplasm. The organelles observed in the sequestered material included multilamellar bodies, which have not been reported earlier in these organelles. By contrast, Tillandsia albida (Bromeliaceae) tapetum cells contained no such organelles but showed plastids that might possibly carry out autophagy, as they contained portions of the cytoplasm similar to the phenomenon reported earlier in Phaseolus and Dendrobium. However, the ultrastructure of the T. albida plastids was different from that in the previous reports. It is concluded that in L. rauschii classical plant macroautophagy was involved in degradation of the cytoplasm, while in T. albida such classical macroautophagy was not observed. Instead, the data in T. albida suggested the hypothesis that plastids are able to carry out degradation of the cytoplasm.

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.