Cedzich Anna, Stransky Harald, Schulz Burkhard, Frommer Wolf B
Center for Plant Molecular Biology, University of Tuebingen, D-72071 Tuebingen, Germany.
Plant Physiol. 2008 Dec;148(4):1857-67. doi: 10.1104/pp.108.128454. Epub 2008 Oct 3.
Cytokinins are distributed through the vascular system and trigger responses of target cells via receptor-mediated signal transduction. Perception and transduction of the signal can occur at the plasma membrane or in the cytosol. The signal is terminated by the action of extra- or intracellular cytokinin oxidases. While radiotracer studies have been used to study transport and metabolism of cytokinins in plants, little is known about the kinetic properties of cytokinin transport. To provide a reference dataset, radiolabeled trans-zeatin (tZ) was used for uptake studies in Arabidopsis (Arabidopsis thaliana) cell culture. Uptake kinetics of tZ are multiphasic, indicating the presence of both low- and high-affinity transport systems. The protonophore carbonyl cyanide m-chlorophenylhydrazone is an effective inhibitor of cytokinin uptake, consistent with H(+)-mediated uptake. Other physiological cytokinins, such as isopentenyl adenine and benzylaminopurine, are effective competitors of tZ uptake, whereas allantoin has no inhibitory effect. Adenine competes for zeatin uptake, indicating that the degradation product of cytokinin oxidases is transported by the same systems. Comparison of adenine and tZ uptake in Arabidopsis seedlings reveals similar uptake kinetics. Kinetic properties, as well as substrate specificity determined in cell cultures, are compatible with the hypothesis that members of the plant-specific purine permease family play a role in adenine transport for scavenging extracellular adenine and may, in addition, be involved in low-affinity cytokinin uptake.
细胞分裂素通过维管系统分布，并通过受体介导的信号转导触发靶细胞的反应。信号的感知和转导可发生在质膜或胞质溶胶中。信号通过细胞外或细胞内细胞分裂素氧化酶的作用而终止。虽然放射性示踪研究已被用于研究植物中细胞分裂素的运输和代谢，但对细胞分裂素运输的动力学特性了解甚少。为了提供一个参考数据集，放射性标记的反式玉米素（tZ）被用于拟南芥（Arabidopsis thaliana）细胞培养中的吸收研究。tZ的吸收动力学是多相的，表明存在低亲和力和高亲和力运输系统。质子载体羰基氰化物间氯苯腙是细胞分裂素吸收的有效抑制剂，这与H(+)介导的吸收一致。其他生理细胞分裂素，如异戊烯基腺嘌呤和苄基腺嘌呤，是tZ吸收的有效竞争者，而尿囊素没有抑制作用。腺嘌呤竞争玉米素的吸收，表明细胞分裂素氧化酶的降解产物通过相同的系统运输。拟南芥幼苗中腺嘌呤和tZ吸收的比较揭示了相似的吸收动力学。在细胞培养中确定的动力学特性以及底物特异性与以下假设一致：植物特异性嘌呤通透酶家族的成员在清除细胞外腺嘌呤的腺嘌呤运输中起作用，此外，可能还参与低亲和力细胞分裂素的吸收。
