Two pairs of sucrose transporters in Ipomoea batatas (L.) Lam are predominantly expressed in sink leaves and source leaves respectively

In sweet potato (Ipomoea batatas L. Lam), sucrose is the major component of photosynthate transported through phloem from source organs to sink ones. In this transport process, sucrose transporters (SUTs) play important roles. In the present study, two SUT genes, designated as IbSUT1 and IbSUT2, were cloned from sweet potato by degenerate PCR and rapid amplification of cDNA ends techniques. Further studies indicated that there are two highly homologous genes for both IbSUT1 and IbSUT2, i.e. IbSUT1x and IbSUT1y, IbSUT2x and IbSUT2y, respectively. When expressed in yeast (Saccharomyces cerevisiae), the two IbSUT2 proteins exhibited sucrose uptake activity, whereas the two IbSUT1 proteins did not. Semi-quantitative reverse transcription-PCR analysis revealed that IbSUT1 was expressed predominantly in sink leaves, but IbSUT2 exhibited the strongest expression in source leaves. Immunolocalization study of IbSUT2 showed that the protein was localized to companion cells of minor veins of source leaves, suggesting that IbSUT2 may be involved in phloem loading in source leaves of sweet potato.