Transgenic tobacco plants expressing the ascorbate oxidase (AAO) gene in sense and antisense orientations, and an Arabidopsis mutant in which the T-DNA was inserted into a putative AAO gene, were used to examine the potential roles of AAO for salt-stress tolerance in plants. AAO activities in the transgenic tobacco plants expressing the gene in sense and antisense orientations were, respectively, about 16-fold and 0.2-fold of those in the wild type. Under normal growth conditions, no significant differences in phenotypes were observed, except for a delay in flowering time in the antisense plants. However, at high salinity, the percentage germination, photosynthetic activity, and seed yields were higher in antisense plants, with progressively lower levels in the wild type and the sense plants. The redox state of apoplastic ascorbate in sense plants was very low even under normal growth conditions. Upon salt stress, the redox state of symplastic and apoplastic ascorbate decreased among the three types of plants, but was lowest in the sense plants. The hydrogen peroxide contents in the symplastic and apoplastic spaces were higher in sense plants, progressively lower in the wild type, followed by the antisense plants. The Arabidopsis T-DNA inserted mutant exhibited very low ascorbate oxidase activity, and its phenotype was similar to that of antisense tobacco plants. These results suggest that the suppressed expression of apoplastic AAO under salt-stress conditions leads to a relatively low level of hydrogen peroxide accumulation and a high redox state of symplastic and apoplastic ascorbate which, in turn, permits a higher seed yield.
Betaine is an important osmoprotectant, synthesized by many plants in response to abiotic stresses. Almost all known biosynthetic pathways of betaine are two-step oxidations of choline. Recently, a biosynthetic pathway of betaine from glycine, catalyzed by two N-methyltransferase enzymes, was found. Here, the potential role of N-methyltransferase genes for betaine synthesis was examined in a freshwater cyanobacterium, Synechococcus sp. PCC 7942, and in Arabidopsis plants. It was found that the coexpression of N-methyltransferase genes in Synechococcus caused accumulation of a significant amount of betaine and conferred salt tolerance to a freshwater cyanobacterium sufficient for it to become capable of growth in seawater. Arabidopsis plants expressing N-methyltransferase genes also accumulated betaine to a high level in roots, stems, leaves, and flowers and improved seed yield under stress conditions. Betaine levels were higher than those produced by choline-oxidizing enzymes. These results demonstrate the usefulness of glycine N-methyltransferase genes for the improvement of abiotic stress tolerance in crop plants.cyanobacteria ͉ methyltransferase ͉ osmoprotectant ͉ stress resistance T oday, Ϸ20% of the world's cultivated land and nearly half of all irrigated lands are affected by high salinity (1). High salinity causes ion imbalance and hyperosmotic stress in plants. Organisms that thrive in hypersaline environments possess specific mechanisms for the adjustment of their internal osmotic status. One such mechanism is the ability to accumulate low-molecular-weight organic-compatible solutes such as sugars, some amino acids, and quaternary ammonium compounds (2-4). Glycine betaine (N,N,Ntrimethylglycine, hereafter betaine) is a major osmolyte (2-4). Another mechanism for adaptation to high salinity is the exclusion of the Na ϩ ion from sodium-sensitive sites (5). Genetic engineering techniques have been applied to improve the salt tolerance of plants (6-13). Considerable success has been demonstrated by manipulating the Na ϩ ͞H ϩ antiporter genes (6-8). By contrast, the genetic engineering of betaine synthesis has been hampered by low accumulation levels of betaine (9-13). Most known biosynthetic pathways of betaine include a two-step oxidation of choline: choline 3 betaine aldehyde 3 betaine. The first step is catalyzed by choline monooxygenase (CMO) in plants (14), choline dehydrogenase (CDH) in animals and bacteria (15,16), and choline oxidase in some bacteria (11,17). The second step is catalyzed by NAD ϩ -dependent betaine aldehyde dehydrogenase in all organisms (15,18,19), although in some bacteria, CDH and choline oxidase also catalyze the second step (15-17). Hitherto, all attempts at betaine synthesis have been carried out by using choline-oxidizing enzymes (9-13). The supply and transport of betaine precursors such as choline, ethanolamine, and serine to plastids may be of importance, because these precursors have been suggested to be limiting (12,13).Recently, we showed that a halotolerant cyanobacter...
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