Biology Department, University of York, YO1 5DD, Heslington, York, UK.
Planta. 1989 May;178(2):215-22. doi: 10.1007/BF00393197.
Primary modes of action of cytokinins have been thought to involve stimulation of the electrogenic H(+) pump and-or opening of plasmamembrane Ca(2+) channels. In order to test these hypotheses, rapid changes in membrane transport in response to cytokinin application were studied in heterotrophic suspension-cultured callus of soybean (Glycine max (L.) Merr.) using electrophysiological techniques. Kinetin (N(6)-furfurylaminopurine; 2 μM) elicited membrane hyperpolarization of 13±1 mV. This effect occurred even at membrane poteintials more negative than the most negative ionic equilibrium potential, and therefore might have resulted either from stimulation of the electrogenic pump, or from closure of ionic channels. The former mechanism of action appears most likely because (i) kinetin-induced membrane hyperpolarization is not accompanied by a significant change in plasma-membrane resistivity and (ii) hyperpolarization is abolished by cyanide, which inhibits electrogenic pump activity by depletion of cellular ATP.Electrogenic pumping is also activated by two other cytokinins: N(6)-(benzyl)adenine and trans-zeatin. However, it is unlikely that the hormonal effect on electrogenesis is directly related to transduction of the cytokinin signal, for the following reasons: (i) hormonally inactive, but chemically related compounds (cis-zeatin, adenine) also elicited membrane hyperpolarization; (ii) hormonally active, N(9)-substituted cytokinins failed to stimulate electrogenesis; (iii) the chemically unrelated cytokinin N,N'-diphenylurea also failed to stimulate electrogenesis.The results imply that the kinetin effect on electrogenic pumping is related to adenine, or its metabolism, and not hormonal action. Adenine was absorbed by soybean cells, but not in sufficient quantities to have a significant effect on adeninenucleotide pools. It appears likely that the control of electrogenesis requires either the presence of a purine free base (i.e. no substituents at the N(9) position) or phosphoribosylation of the free base. No evidence was found for cytokinin-induced Ca(2+)-channel opening, though it is argued that such an event might be physiologically relevant, yet undetectable with the methods employed. It is essential that future studies on cytokinin signal transduction - especially as they relate to membrane transport - take into account the possibility that metabolic effects unrelated to hormone action are dominant.
细胞分裂素的主要作用模式被认为涉及刺激电致氢离子泵和/或打开质膜 Ca2+通道。为了检验这些假说，我们使用电生理学技术研究了异养悬浮培养的大豆（Glycine max (L.) Merr.）愈伤组织中细胞分裂素应用引起的膜转运的快速变化。2 μM 的激动素（N(6)-呋喃甲氨基嘌呤）引起 13±1 mV 的膜超极化。这种效应甚至在比最负的离子平衡电位更负的膜电位下发生，因此可能是由于电致氢离子泵的刺激，或者是由于离子通道的打开。前者的作用机制似乎更有可能，因为（i）激动素诱导的膜超极化不伴有质膜电阻的显著变化，并且（ii）氰化物抑制电致氢离子泵的活性，通过耗尽细胞内的 ATP 来抑制电致氢离子泵的活性。电致氢离子泵也被另外两种细胞分裂素：N(6)-(苄基)腺嘌呤和反式玉米素激活。然而，由于以下原因，激素对电生成的影响不太可能与细胞分裂素信号的转导直接相关：（i）激素不活跃但化学相关的化合物（顺式玉米素、腺嘌呤）也引起膜超极化；（ii）激素活性的、N(9)-取代的细胞分裂素不能刺激电生成；（iii）化学上不相关的细胞分裂素 N,N'-二苯基脲也不能刺激电生成。结果表明，激动素对电致氢离子泵的作用与腺嘌呤或其代谢有关，而与激素作用无关。腺嘌呤被大豆细胞吸收，但数量不足以对腺嘌呤核苷酸池产生显著影响。似乎控制电生成需要嘌呤的游离碱基的存在（即 N(9)位置没有取代基）或游离碱基的磷酸核糖基化。虽然有人认为这样的事件可能在生理上相关，但用所采用的方法却无法检测到，但没有发现细胞分裂素诱导的 Ca2+通道打开的证据。在未来关于细胞分裂素信号转导的研究中——特别是当它们与膜转运有关时——必须考虑到与激素作用无关的代谢作用占主导地位的可能性。
