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

Abstract

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.