We identified three genes homologous to water channels in the plasma membrane type subfamily from roots of barley seedlings. These genes were designated HvPIP2;1, HvPIP1;3, and HvPIP1;5 after comparison to Arabidopsis aquaporins. Competitive reverse transcription (RT)-PCR was applied in order to distinguish and to quantify their transcripts. The HvPIP2;1 transcript was the most abundant among the three in roots. Salt stress (200 mM NaCl) down-regulated HvPIP2;1 (transcript and protein), but had almost no effect on the expressions of HvPIP1;3, or HvPIP1;5. Approximately equal amounts of the transcripts of the three were detected in shoots, and salt stress enhanced the expression of HvPIP2;1 but not of HvPIP1;3, or HvPIP1;5. HvPIP2;1 protein was confirmed to be localized in the plasma membrane. Functional expression of HvPIP2;1 in Xenopus oocytes confirmed that HvPIP2;1 encoded an aquaporin that transports water. This water permeability was reduced by HgCl(2), which is a typical water channel inhibitor. This activity was not modified by some inhibitors against protein kinase and protein phosphatase.

Barley HvPIP2;1 is a plasma membrane aquaporin and its expression was down-regulated after salt stress in barley [Katsuhara et al. (2002) Plant Cell Physiol. 43: 885]. We produced and analyzed transgenic rice plants over-expressing barley HvPIP2;1 in the present study. Over-expression of HvPIP2;1 increased (1) radial hydraulic conductivity of roots (Lp(r)) to 140%, and (2) the mass ratio of shoot to root up to 150%. In these transgenic rice plants under salt stress of 100 mM NaCl, growth reduction was greater than in non-transgenic plants. A decrease in shoot water content (from 79% to 61%) and reduction of root mass or shoot mass (both less than 40% of non-stressed plants) were observed in transgenic plants under salt stress for 2 weeks. These results indicated that over-expression of HvPIP2;1 makes rice plants sensitive to 100 mM NaCl. The possible involvement of aquaporins in salt tolerance is discussed.

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The present investigation was undertaken to study the effect of storage temperature in controlling ethylene production through the activity and gene expres-sion of ethylene biosynthetic enzymes of ' Irwin' mango during fruit storage. Ethylene evolution and activities of ethylene biosynthetic enzymes, 1-aminocyclo-propane-1-carboxylic acid (ACC) synthase and ACC oxidase were investigated using fruit harvested at commercial maturity and stored at 20 and 13°C, together with gene expression of these enzymes. Just after harvest respiration was high although it soon decreased. A climacteric peak was observed after 6 days at 20°C or 8 days at 13°C. Peak ethylene evolution occurred after 2 days at 20°C or 12 days at 13°C although ethylene evolution steadily increased after 4 days at 13°C. Highest activities of ACC synthase and ACC oxidase were recorded after 2 at 20°C and 8 days at 13°C, coinciding with ethylene climacteric peaks. Expression of both ACC synthase and ACC oxidase genes was higher for fruit stored at 13°C than at 20°C. The dynamism of ethylene biosynthetic enzyme activity and gene expression of those enzymes indicated enhanced effectiveness at lower storage temperatures.