Spatial And Temporal Expression Difference Of Sucrose Symporter Genes Of Hybrid Rice

Sucrose is the main transport form of organic matters. The secondary active transport of sucrose mediated by sucrose transporters (SUTs) or sucrose proton symporter, is a proton facilitated transport, which transport sucrose across the plasma membrane barrier into the cytosol. Since sucrose is the main carbohydrate transported through the phloem of most plants, it is believed that SUTs play key roles in carbon partitioning of plants. Intercellular sucrose transport is the pivotal step in controlling the partitioning of assimilates and thus crop yield. Since Riesmeier et al (1992), for the first time, identified a cDNA clone as sucrose carrier, named as SoSUT1, considerable progress has been made in the study of sucrose transporters of plants. Genes encoding SUT proteins have been identified in many higher plants. SUTs have twelve trans-membraneα-helices and a highly conserved domain. Transporting through all SUTs is dependent on the trans-membrane proton motive force created by H~+-ATPase on the plasma membrane.The first identified sucrose transporter gene from monocots is OsSUT1 from rice. Up to date there have been five SUT gene family members identified in rice, and their coded protein's properties have been deduced. Aoki et al (2003) studied the expression profiles of all the five members in japonica rice by RT-PCR, Northern and heterogous expression system in yeast, but the actual physiological roles of each member are still not clear.In this investigation, we studied, by semi-quantitative RT-PCR, the spatial and temporal expression profiles of hybrid rice "Fuyou No. 1 "in its whole life cycle, its young seedlings of infertility line II 32-A, maintainer line II 32-B and restorer line No.21 and 10 other hybrid rice F1 (GuohaoGuoxiang 12, Gangyou 364, KYoul30, II You 602, Yuyou No.6, Yixiang 725, Weitian No.9, DYou 13, DYou 11, BYou840 ) , in a hope to get insights into the spatial and temporal expression profiles of all the five OsSUTs in "Fuyou No. 1" and profiles in seedlings of its three lines and different hybrid rice F1, paving the way for further study of the molecular bases of the assimilates partitioning and high yield of hybrid rice. Results are as follows.1. OsSUT2 was strongly and roughly equally expressed in young leaves, young roots, source and sink leaves, source and sink leaf sheath, spikelets, germinating seeds, and caryopsis day after flowering (DAF1, DAF2, DAF3, DAF5, DAF7, DAF10, DAF15 and DAF20) of Fuyou No.1, suggesting that OsSUT2 is the main and universal sucrose transporter in all plant parts of hybrid rice Fuyou No.12. The expression profiles of the other members of the SUT gene family are quite complicated. OsSUT1, OsSUT3～5 are expressed in all plant parts at various levels. OsSUT1 is mainly expressed in young leaves, source and sink leaf sheath and spikelets, while weakly expressed in source leaves and no expression was detected in all caryopsis of different maturity, suggesting that OsSUT1 probably not participate in the assimilate transport in caryopsis but have roles in source and sink leaves and spikelets; OsSUT3 is expressed at roughly the same level in young leaves and roots, spikelets, and caryopsis of DAF1, DAF2, DAF3, DAF5, DAF10, DAF15 and DAF20, while in source and sink leaves, source and sink leaf sheath, germinating seeds, no expression was detected, indicating that OsSUT3 acts mainly in young leaves and roots, and reproductive organs (spikelets and caryopsis of different maturity); OsSUT4 is expressed at roughly the same level in young leaves and roots, sink leaves, source and sink leaf sheath, spikelets, caryopsis of DAF3 and DAF7, while weakly expressed in source leaf, germinating seeds, DAF1 and DAF5, having the trend of decreasing and finally disappearing with the maturing of caryopsis. These results suggest that OsSUT4 is expressed in vegetative organs and early stages of reproductive organs; OsSUT5 is mainly expressed in young leaves, spikelets and caryopsis, while extremely weak or even no expression was detected in source and sink leaves, source and sink leaf sheath. These differences in expression profiles of OsSUTl, OsSUT3～5 among different plant parts, may have something to do with the fluctuation of environmental factors in pot (field) cultivation.The expression profiles of Fuyou No.1 show basically the same results as those of Aoki et al (2003) in japonica rice. However, there are noticeable differences in the expression of OsSUT1 in germinating seeds and in caryopses of various days after flowering between their and our results: in Fuyou No.1 mainly OsSUT2 was expressed and no OsSUT1 expression was detected, while Aoki et al showed that both OsSUT1 and OsSUT2 were expressed in germinating seeds; in Fuyou No.1 OsSUT1 was abundantly expressed in spikelets three days after flowering but not expressed in caryopses after flowering, whereas Aoki et al (2003) showed that OsSUT1 was expressed 3—15 days after flowering. In addition, the expression patterns of OsSUT 3～5 in our investigation and in theirs are not quite the same. Whether this difference is the distinguish feature between hybrid and japonica rice, further studies are needed.3. Only OsSUT2 is relatively strongly expressed in the young leaves and roots of the three lines of Fuyou No.1. The other members of the OsSUT gene family are either weakly expressed or not detectable in expression.OsSUT1 in the young roots of the maintainer line, OsSUTS in the young leaves of both the infertility line and the restorer line, and OsSUT4 in the young leaves of restorer line are extremely weakly expressed. OsSUT1, OsSUT3 and OsSUTA have no expression in other plant parts; OsSUT5 are not noticeably expressed in the young leaves and roots of all the three lines of Fuyou No.1.4. The expression pattern of OsSUTs in the young leaves and roots of different hybrid rice F1 show remarkable similarity that OsSUT2 have relatively high level of expression in both young leaves and roots of all hybrid rice F1 and the other members of the OsSUTs family have extremely low level of expression, no clear pattern observed. This might be the difference among different hybrid rice F1.5. The semi-quantitative RT-PCR expression profiles of OsSUT1～5 in the young leaves and roots of hybrid rice Fuyou No.1 and 10 other hybrid rice combinations (F1) and the spatial and temporal expression profiles of Fuyou No.1 pot cultivated, all show remarkable resemblance in that OsSUT2 is uniformly expressed.It is concluded that OsSUT2 is the main and universal sucrose transporter in all plant parts of hybrid rice. The expression profiles of the other OsSUTs are somewhat unstable and have some degree of fluctuation, suggesting that the expressions of these gene members probably are affected by changes in environmental factors. OsSUT2 is potentially an ideal target for the artificial control of assimilate partition in hybrid rice.