Functional characterization of the Arabidopsis AtSUC2 sucrose/H+ symporter by tissue-specific complementation reveals an essential role in phloem loading but not in long-distance transport
Functional characterization of the Arabidopsis AtSUC2 sucrose/H+ symporter by tissue-specific complementation reveals an essential role in phloem loading but not in long-distance transport
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DOI：
10.1104/pp.108.124776
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发表时间：
2008-09-01
期刊：
影响因子：
7.4
通讯作者：
Ayre, Brian G.
中科院分区：
文献类型：
--
作者：
Srivastava, Avinash C.;Ganesan, Savita;Ayre, Brian G.
AtSUC2 (At1g22710) 编码拟南芥 (Arabidopsis thaliana) 中韧皮部定位的蔗糖 (Suc)/H+ 同向转运蛋白，这是将 Suc 从源组织有效转运至库组织所必需的。 AtSUC2 在成熟叶片的收集韧皮部中高度表达，其在韧皮部装载中的功能已得到充分证实。然而，AtSUC2 在运输韧皮部中也强烈表达，其作用更加模糊，并且它涉及介导通过质外体往返于侧翼组织的流出和回收。为了表征 AtSUC2 在控制沿着韧皮部路径的碳分配中的作用，在 Atsuc2 突变体背景中从组织特异性启动子表达 AtSUC2 cDNA。该突变体中的 Suc 运输受到严重损害，如营养组织中生长迟缓和大量糖和淀粉的积累所表明的那样。 AtSUC2 cDNA 从 2-kb AtSUC2 启动子的表达足以将生长和碳分配恢复到接近野生型的水平。甜瓜的半乳糖醇合酶启动子 (CmGAS1p) 仅在成熟叶的小脉中表达，而不在较大叶脉和茎的运输韧皮部中表达。从 CmGAS1p 表达 AtSUC2 cDNA 的突变体植物具有中等生长并积累糖和淀粉，但除此之外它们具有正常形态。这些特征支持 AtSUC2 在回收中发挥作用，但不沿着运输韧皮部流出，并表明 AtSUC2 在光同化物分布中的唯一重要功能是韧皮部装载。此外，Atsuc2突变植物虽然衰弱，但确实生长，并且讨论了AtSUC2独立的韧皮部运输模式，包括从叶肉细胞到库组织的完全共生途径。
AtSUC2 (At1g22710) encodes a phloem-localized sucrose (Suc)/H+ symporter necessary for efficient Suc transport from source tissues to sink tissues in Arabidopsis (Arabidopsis thaliana). AtSUC2 is highly expressed in the collection phloem of mature leaves, and its function in phloem loading is well established. AtSUC2, however, is also expressed strongly in the transport phloem, where its role is more ambiguous, and it has been implicated in mediating both efflux and retrieval to and from flanking tissues via the apoplast. To characterize the role of AtSUC2 in controlling carbon partitioning along the phloem path, AtSUC2 cDNA was expressed from tissue-specific promoters in an Atsuc2 mutant background. Suc transport in this mutant is highly compromised, as indicated by stunted growth and the accumulation of large quantities of sugar and starch in vegetative tissues. Expression of AtSUC2 cDNA from the 2-kb AtSUC2 promoter was sufficient to restore growth and carbon partitioning to nearly wild-type levels. The GALACTINOL SYNTHASE promoter of Cucumis melo (CmGAS1p) confers expression only in the minor veins of mature leaves, not in the transport phloem of larger leaf veins and stems. Mutant plants expressing AtSUC2 cDNA from CmGAS1p had intermediate growth and accumulated sugar and starch, but otherwise they had normal morphology. These characteristics support a role for AtSUC2 in retrieval but not efflux along the transport phloem and show that the only vital function of AtSUC2 in photoassimilate distribution is phloem loading. In addition, Atsuc2 mutant plants, although debilitated, do grow, and AtSUC2-independent modes of phloem transport are discussed, including an entirely symplastic pathway from mesophyll cells to sink tissues.