A cDNA clone corresponding to the gene (ZHA1) for a putative plasma membrane H+-ATPase of a seagrass (Zostera marina L.) was isolated and sequenced. Comparison of the amino acid predicted sequence from the nucleotide sequence of ZHAl with those encoded by known genes for plasma membrane H+-ATPases from other plants indicated that ZHAl is most similar to the gene (PMA4) for a plasma membrane H+-ATPase in a tobacco (84.4%). Northern hybridization indicated that ZHAl was strongly expressed in mature leaves, which are exposed to seawater and have the ability of tolerate salinity; ZHAl was weakly expressed in immature leaves, which are protected from seawater by tightly enveloping sheaths and are sensitive to salinity. In mature leaves, in situ hybridization revealed that ZHAl was expressed specifically in epidermal cells, the plasma membranes of which were highly invaginated and morphologically similar to those of typical transfer cells. Therefore, the differentiation of the transfer cell-like structures, accompanied by the high-leve1 expression of ZHA1, in the epidermal cells of mature leaves in particular may be important for the excretion of salt by these cells.The majority of higher plants are sensitive to a high-salt environment. In particular, almost a11 crops are unable to tolerate saline conditions. Nevertheless, some unusual angiosperms, such as halophytes and seagrasses, are able to thrive in saline environments. These two classes of plants are morphologically quite different, since halophytes and seagrasses are able to thrive on saline soil and in seawater, respectively. They seem to adapt to salinity by different, independent mechanisms. There are many reports about mechanisms of salt tolerance in halophytes (e.g. Atriplex nummularia L. [Braun et al., 1986; Niu et al., 19931 and Mesembryanthemum crystallinum L. [Bohnert et al., 1988, 19941). Most dicotyledonous halophytes adapt to salinity by accumulating inorganic ions in their vacuoles, and the osmotic potential of the cytoplasm is balanced by the synthesis and accumulation of biologically compatible solutes, such as Pro, betaine, sugars, or sugar alcohols (Hanson and Hitz, 1982;Jefferies and Rudmik, 1984;Rhodes and Hanson, 1993). By contrast, mechanisms of salinity tolerance in seagrasses have scarcely been studied to date. Seagrasses, which consist of fewer than 100 species, are monocotyledonous angiosperms (Phillips and Menez, 1988). It was reported previously that nonspherical protoplasts obtained from mature leaves of a seagrass (Zostera marina L.) are highly resistant to a wide range of osmotic potentials and salinities. By contrast, spherical protoplasts isolated from meristematic and immature leaf tissues, which are protected from seawater by tightly enveloping sheaths, as well as protoplasts isolated from terrestrial plants, are more sensitive to salinity (Arai et al., 1991). These results suggest that the structure of plasma membrane of cells in the seawater-resistant mature leaves of the seagrass (Z. marina) might be morphologically and ph...

We have found a linear, 16 kb, double-stranded RNA (dsRNA) in symptomless Japonica rice (Oryza sativa L.) that is not found in Indica rice (Oryza sativa L.). The dsRNA was detected in every tissue and at every developmental stage, and its copy number was approximately constant (about 20 copies/cell). Double-stranded RNA was also detected in two strains of Oryza rufipogon (an ancestor of O. sativa). Hybridization experiments indicated that the dsRNA of O. rufipogon was homologous but not identical to that of O. sativa. The sequence of about 13.2 kb of the dsRNA was determined and two open reading frames (ORFs) were found. The larger ORF (ORF B) was more than 12,351 nucleotides long and encoded more than 4,117 amino acid residues.

Linear dsRNAs (double-stranded RNAs) belonging to several distinct size classes were found to be localized in chloroplasts and mitochondria of Bryopsis spp., raising the possibility that these dsRNAs are prokaryotic in nature. The algal cytosol and nuclei did not contain dsRNAs. The amount of the dsRNAs in the organelles appeared constant, and there were about 500 copies per chloroplast. The four major dsRNAs from Bryopsis chloroplasts were about 2 kbp (kilobase pairs) in length and originated from discrete isometric particles of about 25 nm diameter. These virus-like particles were purified by CsCl density gradient centrifugation after extraction from isolated chloroplasts with chloroformbutanol and subsequent precipitation with polyethylene glycol. They had a buoyant density of about 1.40 g · cm(-3) and contained four major and three minor proteins. Mitochondrial dsRNAs were about 4.5 kbp in length and formed less-stable particles of about 40 nm in diameter with a buoyant density of 1.47 g · cm(-3). Some observations support the hypothesis that vertical transmission of the protein-coated, non-infectious dsRNAs occurs within cell organelles. Double-stranded RNAs of various sizes were found in most green, red, and brown algae. The characteristics of the algal dsRNAs are compared with those of dsRNAs from higher plants and the biological significance of the dsRNAs in cell organelles is discussed.

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