PNAS：植物糖运输理论终获验证
该研究成果对减轻全球变暖问题或有重要意义
众所周知，植物会通过光合作用在叶片中产生糖类，但这些糖类是如何运输到机体其它部位（如花、根、果实等）的却一直没有得到实验证实。美国科学家的一项最新研究，终于验证了关于植物糖运输长期以来的理论猜测。这一成果不但有助于加深人们对植物基本生理过程的理解，还有望让科学家在将来通过基因工程方法增加植物光合作用率，增加二氧化碳吸收。相关论文发表在12月4日的美国《国家科学院院刊》（PNAS）上。
1991年，美国康奈尔大学的植物生物学教授Robert Turgeon提出了植物糖运输的“聚合物阱模型”（polymer trap model）。该理论认为，植物光合作用产生的蔗糖会逐渐扩散到植物的管状传输组织——韧皮部，并和其他营养物质一道，输送到机体的各个部分。而在韧皮部时，这些较小的糖分子会聚合形成更大、更复杂的糖结构，由于尺寸的原因，它们便再无法流回到叶子中去。
为了检验上述理论，在最新的研究中，Turgeon及其实验室的Ashlee McCaskill利用基因工程手段，改造了一种名为紫色毛蕊（Verbascum phoeneceum L.，与元参科家族十分接近）的植物，从而使与蔗糖聚合成较大分子相关的两个基因沉默。结果发现，紫色毛蕊中的糖类会重新回到叶子中来。
在正常植物中，一旦糖类在叶中累积，光合作用的速度就会变慢，植物也就不再“卖力”地从空气中吸收二氧化碳。反之，如果能够将糖类有效地从植物的叶中转移出去，比如提高植物韧皮部的装载率，那么光合作用的速率和二氧化碳吸收都会增加。
不过，McCaskill表示，“这一想法目前这还处于理论阶段。”2006年《科学》杂志上的一篇论文表明，当大气中二氧化碳浓度增加时，由于一系列反馈回路的限制和约束，植物并不会摄入过量的二氧化碳，而“韧皮部装载就是制约植物吸收二氧化碳峰值能力的反馈之一”。（科学网 任霄鹏/编译）
（《国家科学院院刊》（PNAS），10.1073/pnas.0707368104，Ashlee McCaskill and Robert Turgeon）
原始出处:
Published online before print November 28, 2007, 10.1073/pnas.0707368104
PNAS | December 4, 2007 | vol. 104 | no. 49 | 19619-19624
Phloem loading in Verbascum phoeniceum L. depends on the synthesis of raffinose-family oligosaccharides
Ashlee McCaskill and Robert Turgeon*
Department of Plant Biology, Cornell University, Ithaca, NY 14853
Edited by Maarten J. Chrispeels, University of California at San Diego, La Jolla, CA, and approved September 17, 2007 (received for review August 5, 2007)
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.
plasmodesmata | polymer trap | stachyose | galactinol | RNAi
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