Physiological functions of sucrose (Suc) transporters (SUTs) localized to the tonoplast in higher plants are poorly understood. We here report the isolation and characterization of a mutation in the rice (Oryza sativa) OsSUT2 gene. Expression of OsSUT2-green fluorescent protein in rice revealed that OsSUT2 localizes to the tonoplast. Analysis of the OsSUT2 promoter::bglucuronidase transgenic rice indicated that this gene is highly expressed in leaf mesophyll cells, emerging lateral roots, pedicels of fertilized spikelets, and cross cell layers of seed coats. Results of Suc transport assays in yeast were consistent with a H + -Suc symport mechanism, suggesting that OsSUT2 functions in Suc uptake from the vacuole. The ossut2 mutant exhibited a growth retardation phenotype with a significant reduction in tiller number, plant height, 1,000-grain weight, and root dry weight compared with the controls, the wild type, and complemented transgenic lines. Analysis of primary carbon metabolites revealed that ossut2 accumulated more Suc, glucose, and fructose in the leaves than the controls. Further sugar export analysis of detached leaves indicated that ossut2 had a significantly decreased sugar export ability compared with the controls. These results suggest that OsSUT2 is involved in Suc transport across the tonoplast from the vacuole lumen to the cytosol in rice, playing an essential role in sugar export from the source leaves to sink organs.

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Two drought-tolerant transgenic rice lines, L-8 (single copy) and L-23 (two copy), expressing the Capsicum annuum methionine sulfoxide reductase B2 (CaMsrB2) gene were selected for stress tolerance phenotyping under drought stress conditions. The two transgenic lines were selected on the basis of laboratory experiments and for performing well against drought. Field assessment of CaMsrB2 transgenic lines L-23 and L-8 in a drought stress environment was conducted. For the assessments, small plots were prepared at the Department of Botany of the University of Karachi to test the physiological response of transgenic lines. Relative water content, quantum yield (Fv/Fm ratio), photochemical quenching (qP) photosynthetic pigments, and performance index were high in transgenic lines compared to the wild type (WT). Antioxidant enzymes as represented by catalase and superoxide dismutase activities were increased while H 2 O 2 production was decreased in transgenic lines compared to the WT. The results are discussed with special reference to physiological response of the transgenic lines against drought in field conditions.

The yeast two-hybrid system was used to investigate dimerization between proteins of Phz2 and Phz4 clones of the homeodomaln-leucine zipper family which were obtained by screening a ~ brach~arpa shoot-tip cDNA library. Assays showed that Phz4 formed a homo rather than a heterodimer with i~.. In addition, we isolated cDNA clones, PIrj,,bl, Ph,/b2, and Phyb3, that encode proteins interacting with Phz4. Although Pbybl is not a HD-Zip protein, the activity of interaction between Pbybl and Phz4 was, surprisingly, stronger than that of the homodimerization of Phz4