Pommerrenig Benjamin, Papini-Terzi Flavia Stal, Sauer Norbert
Molekulare Pflanzenphysiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, D-91058 Erlangen, Germany.
Plant Physiol. 2007 Jun;144(2):1029-38. doi: 10.1104/pp.106.089151. Epub 2007 Apr 13.
Several plant families generate polyols, the reduced form of monosaccharides, as one of their primary photosynthetic products. Together with sucrose (Suc) or raffinose, these polyols are used for long-distance allocation of photosynthetically fixed carbon in the phloem. Many species from these families accumulate these polyols under salt or drought stress, and the underlying regulation of polyol biosynthetic or oxidizing enzymes has been studied in detail. Here, we present results on the differential regulation of genes that encode transport proteins involved in phloem loading with sorbitol and Suc under salt stress. In the Suc- and sorbitol-translocating species Plantago major, the mRNA levels of the vascular sorbitol transporters PmPLT1 and PmPLT2 are rapidly up-regulated in response to salt treatment. In contrast, mRNA levels for the phloem Suc transporter PmSUC2 stay constant during the initial phase of salt treatment and are down-regulated after 24 h of salt stress. This adaptation in phloem loading is paralleled by a down-regulation of mRNA levels for a predicted sorbitol dehydrogenase (PmSDH1) in the entire leaf and of mRNA levels for a predicted Suc phosphate synthase (PmSPS1) in the vasculature. Analyses of Suc and sorbitol concentrations in leaves, in enriched vascular tissue, and in phloem exudates of detached leaves revealed an accumulation of sorbitol and, to a lesser extent, of Suc within the leaves of salt-stressed plants, a reduced rate of phloem sap exudation after NaCl treatment, and an increased sorbitol-to-Suc ratio within the phloem sap. Thus, the up-regulation of PmPLT1 and PmPLT2 expression upon salt stress results in a preferred loading of sorbitol into the phloem of P. major.
几个植物科会产生多元醇（单糖的还原形式），作为其主要光合产物之一。这些多元醇与蔗糖（Suc）或棉子糖一起，用于韧皮部中光合固定碳的长距离分配。这些科的许多物种在盐胁迫或干旱胁迫下会积累这些多元醇，并且已经对多元醇生物合成或氧化酶的潜在调控进行了详细研究。在此，我们展示了在盐胁迫下，编码参与山梨醇和蔗糖韧皮部装载的转运蛋白的基因的差异调控结果。在蔗糖和山梨醇转运物种大车前草中，维管束山梨醇转运蛋白PmPLT1和PmPLT2的mRNA水平会因盐处理而迅速上调。相反，韧皮部蔗糖转运蛋白PmSUC2的mRNA水平在盐处理初期保持恒定，并在盐胁迫24小时后下调。韧皮部装载的这种适应性变化与整个叶片中预测的山梨醇脱氢酶（PmSDH1）的mRNA水平下调以及维管束中预测的蔗糖磷酸合酶（PmSPS1）的mRNA水平下调同时发生。对叶片、富集的维管组织以及离体叶片韧皮部分泌物中的蔗糖和山梨醇浓度进行分析发现，盐胁迫植物的叶片中山梨醇积累，蔗糖也有一定程度的积累，NaCl处理后韧皮部汁液渗出率降低，韧皮部汁液中山梨醇与蔗糖的比例增加。因此，盐胁迫下PmPLT1和PmPLT2表达的上调导致山梨醇优先装载到大车前草的韧皮部中。
