Cellular traits for sodium tolerance in rice (Oryza sativa L.)

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The present review focuses on two important aspects of Na+ toxicity in rice (Oryza sativa L.), (i) that Na+ stress induces different changes in cytosolic Ca2+, [Ca2+]cyt, and pH, [pH]cyt, in tolerant and sensitive cultivars, and (ii) that cells from a tolerant cultivar can better maintain a low cytosolic Na+ and/or Na+/K+ ratio. Experiments with single rice protoplasts, fluorescence microscopy and specific ion-selective dyes suggest that Na+ must be sensed inside the cytosol, before any prolonged changes in [Ca2+]cyt and [pH]cyt occur. Inhibitor analyses show that Na+-induced increase in [pH]cyt in the tolerant cv. Pokkali, and a decrease in [pH]cyt in the sensitive cv. BRRI DHan29, likely are coupled to different H+-ATPases. Expression analysis of OsHKT2;1 (previous name OsHKT1), OsHKT2;2 (previous name OsHKT2) and OsVHA transcripts in rice using RT-PCR and fluorescence in situ-PCR, shows a variable and cell- specific induction in the two rice cultivars under salt stress condition. We conclude that the transient uptake of Na+, which occurs only in the tolerant cultivar, and the fast compartmentalization of Na+ into the vacuole, probably are the most important cellular traits for Na+-tolerance in rice. The low [Na+]cyt in cv. Pokkali might depend on the fast down-regulation of OsHKT2;1, causing less uptake of Na+, and fast up-regulation of the OsVHA transcript, and subsequent activation of the Na+/H+-anti-porter in the tonoplast. To decrease the cytosolic Na+/K+ ratio under Na+ toxicity, cv. Pokkali may also induce increased uptake of K+ through induction of OsHKT2;2, and other specific K+-transporter genes.

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