Williamson J D, Stoop J M, Massel M O, Conkling M A, Pharr D M
Department of Horticultural Science, North Carolina State University, Raleigh 27695-7609, USA.
Proc Natl Acad Sci U S A. 1995 Aug 1;92(16):7148-52. doi: 10.1073/pnas.92.16.7148.
Mannitol is the most abundant sugar alcohol in nature, occurring in bacteria, fungi, lichens, and many species of vascular plants. Celery (Apium graveolens L.), a plant that forms mannitol photosynthetically, has high photosynthetic rates thought to results from intrinsic differences in the biosynthesis of hexitols vs. sugars. Celery also exhibits high salt tolerance due to the function of mannitol as an osmoprotectant. A mannitol catabolic enzyme that oxidizes mannitol to mannose (mannitol dehydrogenase, MTD) has been identified. In celery plants, MTD activity and tissue mannitol concentration are inversely related. MTD provides the initial step by which translocated mannitol is committed to central metabolism and, by regulating mannitol pool size, is important in regulating salt tolerance at the cellular level. We have now isolated, sequenced, and characterized a Mtd cDNA from celery. Analyses showed that Mtd RNA was more abundant in cells grown on mannitol and less abundant in salt-stressed cells. A protein database search revealed that the previously described ELI3 pathogenesis-related proteins from parsley and Arabidopsis are MTDs. Treatment of celery cells with salicylic acid resulted in increased MTD activity and RNA. Increased MTD activity results in an increased ability to utilize mannitol. Among other effects, this may provide an additional source of carbon and energy for response to pathogen attack. These responses of the primary enzyme controlling mannitol pool size reflect the importance of mannitol metabolism in plant responses to divergent types of environmental stress.
甘露醇是自然界中最丰富的糖醇，存在于细菌、真菌、地衣和许多维管植物物种中。芹菜（Apium graveolens L.）是一种通过光合作用形成甘露醇的植物，其光合速率很高，被认为是由于己糖醇与糖类生物合成的内在差异所致。由于甘露醇作为渗透保护剂的功能，芹菜还表现出高耐盐性。已鉴定出一种将甘露醇氧化为甘露糖的甘露醇分解代谢酶（甘露醇脱氢酶，MTD）。在芹菜植株中，MTD活性与组织中甘露醇浓度呈负相关。MTD提供了转运的甘露醇进入中心代谢的第一步，并且通过调节甘露醇库的大小，在细胞水平上调节耐盐性方面很重要。我们现在已经从芹菜中分离、测序并鉴定了一个Mtd cDNA。分析表明，Mtd RNA在以甘露醇为生长培养基的细胞中含量更高，而在盐胁迫细胞中含量较低。蛋白质数据库搜索显示，之前描述的来自欧芹和拟南芥的ELI3病程相关蛋白是MTD。用水杨酸处理芹菜细胞会导致MTD活性和RNA增加。MTD活性增加导致利用甘露醇的能力增强。除其他作用外，这可能为应对病原体攻击提供额外的碳源和能源。控制甘露醇库大小的主要酶的这些反应反映了甘露醇代谢在植物对不同类型环境胁迫反应中的重要性。
