Babb V M, Haigler C H
Department of Biological Sciences, Texas Tech University, Lubbock, TX 79409, USA.
Plant Physiol. 2001 Nov;127(3):1234-42.
Based on work with cotton fibers, a particulate form of sucrose (Suc) synthase was proposed to support secondary wall cellulose synthesis by degrading Suc to fructose and UDP-glucose. The model proposed that UDP-glucose was then channeled to cellulose synthase in the plasma membrane, and it implies that Suc availability in cellulose sink cells would affect the rate of cellulose synthesis. Therefore, if cellulose sink cells could synthesize Suc and/or had the capacity to recycle the fructose released by Suc synthase back to Suc, cellulose synthesis might be supported. The capacity of cellulose sink cells to synthesize Suc was tested by analyzing the Suc phosphate synthase (SPS) activity of three heterotrophic systems with cellulose-rich secondary walls. SPS is a primary regulator of the Suc synthesis rate in leaves and some Suc-storing, heterotrophic organs, but its activity has not been previously correlated with cellulose synthesis. Two systems analyzed, cultured mesophyll cells of Zinnia elegans L. var. Envy and etiolated hypocotyls of kidney beans (Phaseolus vulgaris), contained differentiating tracheary elements. Cotton (Gossypium hirsutum L. cv Acala SJ-1) fibers were also analyzed during primary and secondary wall synthesis. SPS activity rose in all three systems during periods of maximum cellulose deposition within secondary walls. The Z. elegans culture system was manipulated to establish a tight linkage between the timing of tracheary element differentiation and rising SPS activity and to show that SPS activity did not depend on the availability of starch for degradation. The significance of these findings in regard to directing metabolic flux toward cellulose will be discussed.
基于对棉纤维的研究，有人提出蔗糖（Suc）合酶的一种颗粒形式通过将蔗糖降解为果糖和UDP-葡萄糖来支持次生壁纤维素的合成。该模型提出，UDP-葡萄糖随后被输送到质膜中的纤维素合酶，这意味着纤维素库细胞中蔗糖的可用性会影响纤维素合成的速率。因此，如果纤维素库细胞能够合成蔗糖和/或有能力将蔗糖合酶释放的果糖再循环回蔗糖，纤维素合成可能会得到支持。通过分析三个具有富含纤维素次生壁的异养系统的蔗糖磷酸合酶（SPS）活性，测试了纤维素库细胞合成蔗糖的能力。SPS是叶片和一些储存蔗糖的异养器官中蔗糖合成速率的主要调节因子，但其活性此前尚未与纤维素合成相关联。所分析的两个系统，即百日草（Zinnia elegans L. var. Envy）的培养叶肉细胞和菜豆（Phaseolus vulgaris）的黄化下胚轴，都含有正在分化的管状分子。在初生壁和次生壁合成过程中也对棉花（Gossypium hirsutum L. cv Acala SJ-1）纤维进行了分析。在次生壁内纤维素沉积量最大的时期，所有三个系统中的SPS活性都有所上升。对百日草培养系统进行了调控，以在管状分子分化时间和SPS活性上升之间建立紧密联系，并表明SPS活性不依赖于淀粉降解的可用性。将讨论这些发现对于引导代谢通量向纤维素方向发展的意义。
