Participation of the plant vacuole in the early events of gravitropism has been suggested in Arabidopsis thaliana (Morita et al., 2002). Moreover, in the mushroom Flammulina velutipes, it has been observed that the fastest ultrastructural response to changes in the direction of the gravitational force is the accumulation of cytosolic vesicles contributing to the expansion of the central vacuole, which consequently causes the differential enlargement of cells (Kern & Hock, 1996). The product encoded by the upregulated clone U043 (Table

2) was highly homologous (E-value: 10−32) to the subtilisin-like serine protease (subtilase) SPM1 of the fungus Magnaporthe grisea; this protein was predicted to be translocated into the endoplasmic reticulum and to be localized in the vacuole (Fukiya et al., 2002). Fungal vacuole subtilases found in Saccharomyces see more cerevisiae MLN8237 and Aspergillus fumigatus are involved in spore morphogenesis (Moehle et al., 1987) and conidiogenesis (Reichard et al., 2000), respectively. These data could indicate that the product encoded by U043 might be localized in the vacuole or be involved in the morphogenesis of cells or spores

in P. ostreatus. It has been proposed that the ornithine cycle enzymes including arginase in the fruiting bodies of mushrooms are important for urea accumulation because members of the family Agaricaceae are known to accumulate substantial amounts of urea in their fruiting bodies (Hammond, 1979), which are required for the production of basidiospores (Donker Rutecarpine & van As, 1999). In the mushroom Agaricus bisporus, arginase expression was found to correlate with the urea contents in the tissues of fruiting bodies (Wagemaker

et al., 2005). The clone D039, which encodes a putative arginase, was slightly downregulated under simulated microgravity condition (Fig. 2), possibly implying that urea requirement might decrease under simulated microgravity. There seemed to be more downregulated than upregulated genes, despite the fact that we analyzed more than twice as many upregulated clones (108 samples) as downregulated clones (43 samples) (refer to the column ‘Number of genes cloned,’Tables 2 and 3). Based on the effects of the microgravity conditions, the view that the microgravity-induced decrease in gravitational stress might affect gene expressions is still under discussion. In an actual spaceflight, in low-Earth orbit, it was found that the effects of microgravity negatively impacted the immune system of mammalian cells (Lesnyak et al., 1996), and some metabolic activities in bacterial cells were found to be decreased (Nickerson et al., 2003).