PMC

Upon nutrient limitation, cells cease growth and initiate sporulation. Asymmetric cell division generates a smaller forespore compartment and a larger mother cell. Following completion of DNA segregation, the mother cell proceeds to engulf the forespore. During the initiation of spore metabolic dormancy and compaction of the spore DNA, the mother cell mediates the development of the forespore into the spore through the production of the cortex and the inner and outer coats. Next, the mother cell lyses and releases the mature spore. In the presence of the appropriate environmental stimulus, the dormant spore initiates the process of germination that leads to the re-initiation of vegetative growth. Abbreviations: MC, mother cell compartment; FS, forespore compartment; MS, mature spore.

The cellular organization of a sporulating Bacillus cell. The spore is shown at the left within its mother cell. The dehydrated cytoplasm of the spore is shaded in yellow. It is surrounded by the spore inner membrane, by complex peptidoglycan layers shown in orange, by the spore outer membrane, and by a thick proteinaceous spore coat shown in blue. The spore resides within the mother cell cytoplasm. The mother cell will subsequently rupture and release the spore which will remain dormant until conditions are right for it to germinate.

images of hornwort sporophytes. A to D, P. carolinianus. A, One millimeter of sporogenous tissue extracted from remaining sporophyte shows young tetrads (t) and pseudoelaters (pe) in mucilaginous mass around the columella (c). Stomata collapse at the base where mucilage surrounds tetrads, and through progressive drying of mucilage upwardly, spores and pseudoelaters separate. B, Sporogenous tissue where stomata collapse held together in mucilage (m) showing mature spores of tetrads embedded in the spore mother cell wall (sw) with an imprint of spore wall ornamentation and pseudoelaters. C, Tetrad with spore mother cell wall drying down on the papillate distal wall ornamentation. At this level, stomata are collapsed and internal air spaces are dry within the assimilate tissue. D, Two spores of separated tetrad with a veil of spore mother cell wall adhering to the spore wall. The arrow identifies a spore mother cell wall remnant from a lost spore. E, Large collapsed stoma (arrow) in dried epidermis of a dehiscing Anthoceros cristatus Stephani. sporophyte that contains pseudoelaters separating dried tetrads not surrounded by mucilage or spore mother cell wall. This figure appears courtesy of Silvia Pressel and Jeffrey Duckett. Bars = 100 μm (A), 20 μm (B–D), and = 50 μm (E).

Model of B. anthracis spore maturation. BclA and ExsK are deposited to the spore late in sporulation, with BclA deposited prior to ExsK. Both proteins assemble first to the mother-cell-proximal pole and then around the spore surface in an ExsFA/BxpB-dependent manner. Due to the partial occlusion of ExsK by BclA, it is likely that these two proteins are adjacent to each other on the spore surface. Once deposited to the spore surface, ExsK assembles into high-molecular-mass complexes in a BclA-dependent manner. Because ExsK assembles to the exosporium and underlying spore compartments after the formation of a complete basal layer, ExsK may be synthesized by ribosomes in the mother cell as well as by ribosomes trapped within the interspace. In mature spores, ExsK remains asymmetrically distributed and is more concentrated at the mother-cell-proximal pole (depicted as dark-red triangles at the pole and light-red triangles around the spore). Gray ovals, BclA; gray-striped arrows, assembly of BclA around the spore surface.