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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Phloem loading in Verbascum phoeniceum L depends on the synthesis of raffinose- family oligosaccharides
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Phloem loading in Verbascum phoeniceum L depends on the synthesis of raffinose- family oligosaccharides

机译：凤尾草中韧皮部的负载量取决于棉子糖家族寡糖的合成

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摘要

Phloem loading is the initial step in photoassimilate export and the one that creates the driving force for mass flow. It has been proposed that loading occurs symplastically in species that translocate carbohydrate primarily as raffinose family oligosaccharides (RFOs). In these plants, dense fields of plasmodesmata connect bundle sheath cells to specialized companion cells (intermediary cells) in the minor veins. According to the polymer trap model, advanced as a mechanism of symplastic loading, sucrose from the mesophyll diffuses into intermediary cells and is converted there to RFOs. This process keeps the sucrose concentration low and, because of the larger size of the RFOs, prevents back diffusion. To test this model, the RFO pathway was down-regulated in Verbascum phoeniceum L. by suppressing the synthesis of galactinol synthase (GAS), which catalyzes the first committed step in RFO production. Two GAS genes (VpGAS1 and VpGAS2) were cloned and shown to be expressed in intermediary cells. Simultaneous RNAi suppression of both genes resulted in pronounced inhibition of RFO synthesis. Phloem transport was negatively affected, as evidenced by the accumulation of carbohydrate in the lamina and the reduced capacity of leaves to export sugars during a prolonged dark period. In plants with severe down-regulation, additional symptoms of reduced export were obvious, including impaired growth, leaf chlorosis, and necrosis and curling of leaf margins.
机译：韧皮部负载是光同化作用输出的第一步，也是产生质量流的驱动力。已经提出在主要以棉子糖家族寡糖（RFO）转运碳水化合物的物种中共生地发生负载。在这些植物中，线虫的密集区域将束鞘细胞连接到小静脉中的专门陪伴细胞（中间细胞）。根据聚合物陷阱模型，作为共生性加载的机制，蔗糖从叶肉中扩散到中间细胞，并在那里转化为RFO。该过程使蔗糖浓度保持较低，并且由于RFO的尺寸较大，因此可以防止向后扩散。为了测试该模型，通过抑制半乳糖醇合酶（GAS）的合成，在凤尾草中下调了RFO途径，这催化了RFO生产的第一步。克隆了两个GAS基因（VpGAS1和VpGAS2）并显示在中间细胞中表达。同时抑制两个基因的RNAi导致明显抑制RFO合成。韧皮部的运输受到不利影响，在较长的黑暗时期，叶片中糖分的积累和叶片出口糖分的能力降低证明了这一点。在严重下调的植物中，还明显出现了出口减少的其他症状，包括生长受损，叶绿化，叶片边缘坏死和卷曲。

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