SWEET as Sugar: New Sucrose Effluxers in Plants
SWEET as Sugar: New Sucrose Effluxers in Plants
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DOI：
10.1093/mp/sss054
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发表时间：
2012-07-01
期刊：
影响因子：
27.5
通讯作者：
Braun, David M.
中科院分区：
文献类型：
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作者：
Baker, R. Frank;Leach, Kristen A.;Braun, David M.
光合作用导致叶子中碳的固定和碳水化合物（例如蔗糖、淀粉）的积累。这种同化的碳必须从叶子（源组织）运输到非光合器官（库组织），例如根和种子。尽管这个过程在植物中很重要，但对叶子中蔗糖输出的参与者缺乏完整的了解。 SWEET 蛋白是一类新发现的糖转运蛋白，可促进糖沿着浓度梯度跨细胞膜扩散融合。从这个角度来看，我们重点介绍 Chen 等人 (2012) 最近发表的论文，该论文报道 SWEET 蛋白跨细胞膜转运蔗糖，在韧皮部薄壁细胞的子集中表达，在蔗糖从叶子中输出的过程中发挥作用，并且可以被微生物病原体劫持以获得糖来促进其生长。这种长期寻找的蔗糖流出剂的鉴定是一项重大发现，增加了我们对植物内糖通量动态的了解（Braun，2012）。蔗糖在光合细胞中合成，并通过胞间连丝在细胞质中从一个细胞移动到另一个细胞，直到到达韧皮部薄壁细胞（图1）（Slewinski 和 Braun，2010）。假设这些血管细胞和/或束鞘细胞将蔗糖穿过质膜流出到韧皮部质外体中（Evert 等，1978；Geiger 等，1974），其中蔗糖随后通过蔗糖转运蛋白（SUT）加载到韧皮部筛元件伴随细胞复合体中（Ayre，2011；Braun 和斯莱文斯基, 2009）。由此产生的蔗糖在筛子元件中的积累产生静水压力梯度，导致蔗糖大量流过相邻筛子元件的导管，导致其到达并卸载到库组织中（Lalonde等，2003）。
Photosynthesis leads to the fixation of carbon in leaves and to the accumulation of carbohydrates (eg, sucrose, starch). This assimilated carbon must be transported from the leaves (source tissues) to non photosynthetic organs (sink tissues), such as roots and seeds. Despite the essentiality of this process in plants, a complete understanding of the actors involved in sucrose export from leaves is lacking. SWEET proteins are a newly identified class of sugar transporters that facilitate dif fusion of sugars across cell membranes down a concentration gradient. In this perspective, we highlight the recent publica tion of Chen et al.(2012), which reports that SWEET proteins transport sucrose across cell membranes, are expressed in a subset of phloem parenchyma cells, function in sucrose export out of leaves, and can be hijacked by microbial pathogens to obtain sugars for fueling their growth. Identification of this long sought sucrose effluxer is a significant discovery adding to our knowledge on the dynamics of sugar flux within plants (Braun, 2012).Sucrose is synthesized in photosynthetic cells and moves cytoplasmically from cell to cell via plasmodesmata until it reaches the phloem parenchyma cells (Figure 1)(Slewinski and Braun, 2010). These vascular cells and/or the bundle sheath cells have been hypothesized to efflux sucrose across the plasma membrane into the phloem apoplast (Evert et al., 1978; Geiger et al., 1974), where sucrose is subsequently loaded into the phloem sieve element companion cell com plexes by sucrose transporters (SUTs)(Ayre, 2011; Braun and Slewinski, 2009). The resultant accumulation of sucrose in the sieve elements produces a hydrostatic pressure gradient that results in the bulk flow of sucrose through a conduit of con tiguous sieve elements, leading to its arrival and unloading in sink tissues (Lalonde et al., 2003).