Tungngoen Kessarin, Kongsawadworakul Panida, Viboonjun Unchera, Katsuhara Maki, Brunel Nicole, Sakr Soulaiman, Narangajavana Jarunya, Chrestin Hervé
Department of Biotechnology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand.
Plant Physiol. 2009 Oct;151(2):843-56. doi: 10.1104/pp.109.140228. Epub 2009 Aug 5.
Natural rubber is synthesized in specialized articulated cells (laticifers) located in the inner liber of Hevea brasiliensis. Upon bark tapping, the laticifer cytoplasm (latex) is expelled due to liber tissue turgor pressure. In mature virgin (untapped) trees, short-term kinetic studies confirmed that ethylene, the rubber yield stimulant used worldwide, increased latex yield, with a concomitant decrease in latex total solid content, probably through water influx in the laticifers. As the mature laticifers are devoid of plasmodesmata, the rapid water exchanges with surrounding liber cells probably occur via the aquaporin pathway. Two full-length aquaporin cDNAs (HbPIP2;1 and HbTIP1;1, for plasma membrane intrinsic protein and tonoplast intrinsic protein, respectively) were cloned and characterized. The higher efficiency of HbPIP2;1 than HbTIP1;1 in increasing plasmalemma water conductance was verified in Xenopus laevis oocytes. HbPIP2;1 was insensitive to HgCl(2). In situ hybridization demonstrated that HbPIP2;1 was expressed in all liber tissues in the young stem, including the laticifers. HbPIP2;1 was up-regulated in both liber tissues and laticifers, whereas HbTIP1;1 was down-regulated in liber tissues but up-regulated in laticifers in response to bark Ethrel treatment. Ethylene-induced HbPIP2;1 up-regulation was confirmed by western-blot analysis. The promoter sequences of both genes were cloned and found to harbor, among many others, ethylene-responsive and other chemical-responsive (auxin, copper, and sulfur) elements known to increase latex yield. Increase in latex yield in response to ethylene was emphasized to be linked with water circulation between the laticifers and their surrounding tissues as well as with the probable maintenance of liber tissue turgor, which together favor prolongation of latex flow.
天然橡胶在巴西橡胶树内皮层中特化的有节细胞（乳管）中合成。割胶时，由于皮层组织的膨压，乳管细胞质（乳胶）被挤出。在成熟的未开割树中，短期动力学研究证实，乙烯作为全球通用的橡胶产量刺激剂，可提高乳胶产量，同时乳胶总固体含量降低，这可能是由于水分流入乳管所致。由于成熟的乳管没有胞间连丝，与周围皮层细胞的快速水分交换可能通过水通道蛋白途径发生。克隆并鉴定了两个全长水通道蛋白cDNA（分别为HbPIP2;1和HbTIP1;1，分别代表质膜内在蛋白和液泡膜内在蛋白）。在非洲爪蟾卵母细胞中验证了HbPIP2;1比HbTIP1;1在增加质膜水导率方面效率更高。HbPIP2;1对HgCl₂不敏感。原位杂交表明HbPIP2;1在幼茎的所有皮层组织中表达，包括乳管。在皮层组织和乳管中，HbPIP2;1均上调，而HbTIP1;1在皮层组织中下调，但在响应树皮乙烯利处理时在乳管中上调。蛋白质免疫印迹分析证实了乙烯诱导的HbPIP2;1上调。克隆了这两个基因的启动子序列，发现其中含有许多已知可提高乳胶产量的乙烯响应元件和其他化学响应元件（生长素、铜和硫）。强调乙烯诱导的乳胶产量增加与乳管及其周围组织之间的水分循环以及皮层组织膨压的可能维持有关联，这共同有利于延长乳胶流动时间。
