Salinity significantly increased trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS) uptake and decreased the K þ /Na þ ratio in salt-sensitive rice (Nipponbare) but did not markedly in salt-tolerant rice (Pokkali). Proline and glycinebetaine (betaine) suppressed the increase in PTS uptake and the decrease in the K þ /Na þ ratio in Nipponbare, but did not affect PTS uptake or the K þ /Na þ ratio in Pokkali.Key words: betaine; proline; rice; salt tolerance Salinity is one of the major abiotic stresses adversely affecting crop growth and productivity. 1) Although salt tolerance mechanisms are not fully understood, it is thought that the main reasons for losses in higher-plant growth are osmotic damage and ion cytotoxicity. 2)Rice is moderately sensitive to salinity.3) In rice plants, not only a symplastic flow but also an apoplastic flow are involved in Na þ uptake from the roots to the leaves. 4,5) Faiyue et al. recently found that the apoplastic pathway is a valid screening technique for the salinity resistance of rice. 6) We found that exogenous proline and betaine suppress apoplastic flow, resulting in reducing Na þ uptake in rice plants under salt stress. 7)There is increasing evidence that proline and betaine improve salt tolerance in plants and cultured cells via the maintenance of osmotic potentials, membrane integrity, and enzyme activities, and the activation of antioxidant systems. [8][9][10][11][12] In this study, we investigated the effects of exogenous proline and betaine on the growth, chlorophyll content, apoplastic flow, and K þ /Na þ ratio in rice plants under saline conditions, and assessed the effectiveness of proline and betaine on the salt tolerance of two rice cultivars.Seeds of rice (Oryza sativa L.) cultivars Nipponbare (salt-sensitive) and Pokkali (salt-tolerant) were hydroponically grown as described previously.7) Proline and betaine were added to the hydroponic solution. The chlorophyll content of the leaves was measured as previously reported.13) Apoplastic flow was evaluated using a membrane-impermeable fluorescent dye, trisodium salt of 8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), as previously described.7) The Na þ and K þ contents of the leaves were determined as described previously. 14) Significance of difference between data sets was assessed by Student's t-test. We regarded differences at a level of p < 0:05 as significant.In rice, exogenous application of proline and betaine mitigates growth inhibition due to NaCl. 15,16) The effects of exogenous proline and betaine on the growth of rice cultivars under saline conditions were investigated. NaCl at 100 mM decreased shoot fresh weight of Nipponbare by 39% and of Pokkali by 10% (Fig. 1A and B). Exogenous proline and betaine at 1 mM significantly improved the shoot growth of salt-stressed Nipponbare, whereas neither proline nor betaine affected the shoot growth of salt-stressed Pokkali (Fig. 1A and B).NaCl at 100 mM significantly reduced the chlorophyll contents of Nipponbare but not of Pokkali (Fig. 1C). Exogenous proline...

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We investigated the mechanism of selenium (Se) tolerance using an Arabidopsis thaliana knockout mutant of a sulfate transporter, sultr1;2. Se stress inhibited plant growth, decreased chlorophyll contents, and increased protein oxidation and lipid peroxidation in the wild type, whereas the sultr1;2 mutation mitigated damage of these forms, indicating that sultr1;2 is more tolerant of Se than the wild type is. The accumulation of symplastic Se was suppressed in sultr1;2 as compared to the wild type, and the chemical speciation of Se in the mutant was different from that in the wild type. Regardless of Se stress, the activities of ascorbate peroxidase, catalase, and peroxidase in the mutant were higher than in the wild type, while the activity of superoxide dismutase in the mutant was the same as in the wild type. These results suggest that the sultr1;2 mutation confers Se tolerance on Arabidopsis by decreasing symplastic Se and maintaining antioxidant enzyme activities.

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