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Fig. 7 | BMC Plant Biology

Fig. 7

From: New insights into the evolution and functional divergence of the SWEET family in Saccharum based on comparative genomics

Fig. 7

Schematic models for roles of SWEET proteins in phloem loading and unloading based on gene expression profiles in sugarcane. Notes: In the leave tissues, SWEET13a/13b/13c were the dominant expressional genes in mature and maturing zones, SWEET1b was expressed predominantly in the mature zone, SWEET2b was expressed in the transitional zone and the maturing zone, SWEET1a/2a were expressed in the basal zone (sink tissues), transitional zone and maturing zone, and SWEET4b/11a were expressed in the basal zone (sink tissues). These observations suggested that SWEET1b/2b/13a/13b/13c are associated with sucrose efflux in the photosynthetic tissues, while, SWEET1a/4b/11a are associated with sucrose unloading from the leaf to the stem. Sucrose is synthesized by the Calvin cycle from carbon dioxide and water in mesophyll cells. SWEET13a/b/c was the most abundantly expressed gene in the maturing and mature zones of leaves, and was suggested to play a role in the efflux of photosynthesized sucrose to the leaf apoplast on the basis of the function of its homologs in Arabidopsis (AtSWEET11 and AtSWEET12) [95]. Moreover, SWEET1b/13a/13b presented a diurnal expression pattern, indicating that these three genes are regulated by sunlight. In the stem tissues, SWEET1a/4a/4b were constitutively expressed in the whole stalk, SWEET2a was mainly expressed in the immature stem, SWEET13c was expressed in the maturing and mature stem, suggesting that SWEET1a/4a/4b are involved in sucrose unloading and accumulation in sink tissues, whereas SWEET2a is associated with sucrose efflux from the source to the sink tissues, and SWEET13c is likely to be involved in sugar accumulation in the mature stem. Synthesized sucrose is transported from the leaf through sieve elements and accumulates in the parenchyma cells of sugarcane stalks. Therefore, SoSWEET2a may be involved in the accumulation of sugars in parenchyma cells and sugar transporting in sclerenchyma cells. SoSWEET4a, SoSWEET4b, SoSWEET13c may contribute to the sugar transport in sclerenchyma cells. SoSWEET16a had higher expression in parenchyma cells than in sclerenchyma cells, indicating that these genes may be involved in the accumulation of sugars in parenchyma. Reference: Chen LQ, Cheung LS, Feng L, Tanner W, Frommer WB. Transport of sugars. Annu Rev. Biochem. 2015; 84(84):865–94

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