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The role of plasma membrane-localized sucrose transporter (NtSUT1) was investigated using cultured tobacco cell (Nicotiana tabacum L.) line BY-2. The wild type (WT) cells were first transformed with the NtSUT1 gene or its fragments cloned from tobacco cell line SL to form the over-expression (OX) and suppression (RNAi) cell lines, respectively. Using OX and RNAi transgenics, the role of NtSUT1 in growth capacity of actively growing cells and in aluminum (Al)-treated cells was examined. During the logarithmic phase of growth in nutrient medium containing 2,4-dichlorophenoxyacetic acid (2,4-D), both the rate of sucrose uptake measured with radio-tracer and the content of soluble sugars were higher in OX and lower in RNAi cell lines compared to WT. Overall, the content of soluble sugars negatively correlated with the time necessary for doubling mass (fresh weight). When cells were treated without (control) or with Al in a simple medium containing calcium, sucrose and 2-(N-morpholino)ethanesulfonic acid (MES; pH 5.0) for up to 18 h, the expression of NtSUT1 under its native promoter, or under the control of strong constitutive cauliflower mosaic virus (CaMV) 35S promoter, was strongly dependent on the presence of 2,4-D. Thereafter, the cells were preferentially treated in the presence of 2,4-D. During 6 h after a start of the control treatment, sucrose uptake rates were, compared to WT, slightly higher and lower in OX and RNAi lines respectively. The addition of Al reduced the sucrose uptake rates of OX and WT to the level of RNAi line, indicating that Al inhibits sucrose uptake via NtSUT1. During the post-Al culture of control and Al-treated cells in a nutrient medium, sucrose uptake rates were much higher in OX compared to WT and RNAi lines, which closely and positively correlated with the growth capacity of the cells. Judging from the growth capacity of Al-treated cells relative to that of control cells, OX cells were more tolerant to Al than WT and RNAi. In summary, we conclude that over-expression of NtSUT1 confers higher growth capacity in actively growing cells as well as in Al-treated cells.
The role of plasma membrane-localized sucrose transporter (NtSUT1) was investigated using cultured tobacco cell (Nicotiana tabacum L.) line BY-2. The wild type (WT) cells were first transformed with the NtSUT1 gene or its fragments cloned from tobacco cell line SL to form the over-expression (OX) and suppression (RNAi) cell lines, respectively. Using OX and RNAi transgenics, the role of NtSUT1 in growth capacity of actively growing cells and in aluminum (Al)-treated cells was examined. During the logarithmic phase of growth in nutrient medium containing 2,4-dichlorophenoxyacetic acid (2,4-D), both the rate of sucrose uptake measured with radio-tracer and the content of soluble sugars were higher in OX and lower in RNAi cell lines compared to WT. Overall, the content of soluble sugars negatively correlated with the time necessary for doubling mass (fresh weight). When cells were treated without (control) or with Al in a simple medium containing calcium, sucrose and 2-(N-morpholino)ethanesulfonic acid (MES; pH 5.0) for up to 18 h, the expression of NtSUT1 under its native promoter, or under the control of strong constitutive cauliflower mosaic virus (CaMV) 35S promoter, was strongly dependent on the presence of 2,4-D. Thereafter, the cells were preferentially treated in the presence of 2,4-D. During 6 h after a start of the control treatment, sucrose uptake rates were, compared to WT, slightly higher and lower in OX and RNAi lines respectively. The addition of Al reduced the sucrose uptake rates of OX and WT to the level of RNAi line, indicating that Al inhibits sucrose uptake via NtSUT1. During the post-Al culture of control and Al-treated cells in a nutrient medium, sucrose uptake rates were much higher in OX compared to WT and RNAi lines, which closely and positively correlated with the growth capacity of the cells. Judging from the growth capacity of Al-treated cells relative to that of control cells, OX cells were more tolerant to Al than WT and RNAi. In summary, we conclude that over-expression of NtSUT1 confers higher growth capacity in actively growing cells as well as in Al-treated cells.
Carbohydrate partitioning is the process of carbon assimilation and distribution from source tissues, such as leaves, to sink tissues, such as stems, roots and seeds. Sucrose, the primary carbohydrate transported long distance in many plant species, is loaded into the phloem and unloaded into distal sink tissues. However, many factors, both genetic and environmental, influence sucrose metabolism and transport. Therefore, understanding the function and regulation of sugar transporters and sucrose metabolic enzymes is key to improving agriculture. In this review, we highlight recent findings that (i) address the path of phloem loading of sucrose in rice and maize leaves; (ii) discuss the phloem unloading pathways in stems and roots and the sugar transporters putatively involved; (iii) describe how heat and drought stress impact carbohydrate partitioning and phloem transport; (iv) shed light on how plant pathogens hijack sugar transporters to obtain carbohydrates for pathogen survival, and how the plant employs sugar transporters to defend against pathogens; and (v) discuss novel roles for sugar transporters in plant biology. These exciting discoveries and insights provide valuable knowledge that will ultimately help mitigate the impending societal challenges due to global climate change and a growing population by improving crop yield and enhancing renewable energy production.
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