The diverse physiological role of sugar transporters has been revealed in many plants. In rice, cloned cDNAs, OsSUT2 and OsMST5, correspond to di- and monosaccharide transporter, respectively, during flowering and grain development. Both are expressed during heading and play an important role at the early stage of seed development (Takeda et al. 2001).

Here, we further characterized OsSUT2 cDNA, whether it is a functional sucrose transporter. A heterologous expression of this clone in yeast mutant SUSY7/ura3 was established (Riesmeier et al. 1992). The full length of OsSUT2 was subcloned to pDR195 vector at XhoI site and transformed into that of yeast mutant. A media containing 1.7 g L^-1 yeast nitrogen base without amino acids, 2% sucrose, 5 g L^-1 ammonium sulfate, 20 mg L^-1 tryptophan and 2% agar, was used to assay yeast growth. SUSY7/ura3 transformed with OsSUT2 can grow in this media and show better growth than yeast transformed with the empty pDR195 vector (data not shown, the heterologous expression system was provided by Dr. Wolf Frommer at Universitaet Tuebingen). The growth of yeast with OsSUT2 on a media containg sucrose as the sole carbon, indicates that OsSUT2 encodes a functional sucrose transporter.

To investigate the expression of OsSUT2, samples of panicles were obtained during flowering as well as after flowering, 0, 1, 3 and 5 DAP (days after pollination). Total RNA was extracted and used to perform gel blot analysis. The transcription of OsSUT2 was detected in panicle tissues before flowering stage whereas no signal appeared in others after flowering (Fig. 1). Developing reproductive organs including pollen grains, require carbohydrates to support their maturation, germination and growth (Williams et al. 2000). Thus, OsSUT2 might be involved in this physiological process.

In situ hybridization was performed to localize OsSUT2 mRNA. OsSUT2 mRNA was found in developing pollen grains (Fig. 2) as well as detection of mRNA accumulation in panicle

tissues before flowering stage by Northern blot (Fig. 1). This finding suggests that sucrose is the major source of carbon during pollen development. Our results indicates that OsSUT2 is associated with the developmental processes of the spikelets and pollen grains. However, other sugar transporters might be involved in these processes. Further investigations are needed to clarify sugar transport and regulation during flowering and pollen development.

[Note] Sequence alignment of OsSUT2 is identical to that of OsSUT3 (accession no. AB071809 by Hirose et al).

References

Riesmeier, J. W., L. Willmitzer and W. B. Frommer, 1992. Isolation and characterization of a sucrose carrier cDNA from spinach by functional expression in yeast. EMBO J. 11: 4705-4713.

Takeda, T., K. Toyofuku, C. Matsukura and J. Yamaguchi, 2001. Sugar transporters involved in flowering and grain development of rice. J. Plant. Physiol. 158: 465-470.

Williams, L.E., R. Lemoine and N. Sauer, 2000. Sugar transporters in higher plants - a diversity of role and complex regulation. Trends Plant Sci. 5: 283-290.