The diversity of cytokinin (CK) metabolites suggests their interconversions are the predominant regulatory mechanism of CK action. Nevertheless, little is known about their directionality and kinetics in planta. CK metabolite levels were measured in 2-wk-old Arabidopsis thaliana plants at several time points up to 100 min following exogenous application of selected CKs. The data were then evaluated qualitatively and by mathematical modeling. Apart from elevated levels of trans-zeatin (tZ) metabolites upon application of N6 -(Δ2 -isopentenyl)adenine (iP), we observed no conversions between the individual CK-types - iP, tZ, dihydrozeatin (DHZ) and cis-zeatin (cZ). In particular, there was no sign of isomerization between tZ and cZ families. Also, no increase of DHZ-type CKs was observed after application of tZ, suggesting low baseline activity of zeatin reductase. Among N-glucosides, those of iP were not converted back to iP while tZ N-glucosides were cleaved to tZ bases, thus affecting the whole metabolic spectrum. We present the first large-scale study of short-term CK metabolism kinetics and show that tZ N7- and N9-glucosides are metabolized in vivo. We thus refute the generally accepted hypothesis that N-glucosylation irreversibly inactivates CKs. The subsequently constructed mathematical model provides estimates of the metabolic conversion rates.

中文翻译：

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异戊烯腺嘌呤和反式玉米素的N-葡萄糖苷的独特代谢决定了拟南芥中细胞分裂素的代谢谱。

细胞分裂素（CK）代谢产物的多样性表明它们的相互转化是CK作用的主要调节机制。然而，关于它们在植物中的方向性和动力学知之甚少。在外源施用选定CKs后直至100分钟的几个时间点，在2周龄的拟南芥植物中测量CK代谢物水平。然后通过数学建模对数据进行定性评估。除了施用N6-（Δ2-异戊烯基）腺嘌呤（iP）后反式玉米素（tZ）代谢产物水平升高外，我们还没有观察到单个CK型-iP，tZ，二氢玉米素（DHZ）和顺式玉米素之间的转化（cZ）。特别地，在tZ和cZ家族之间没有异构化的迹象。另外，施用tZ后未观察到DHZ型CK的增加，提示玉米醇溶蛋白还原酶的基线活性较低。在N-葡糖苷中，iP的那些未转换回iP，而tZ N-葡糖苷被切割成tZ碱基，从而影响了整个代谢谱。我们提出了第一个短期CK代谢动力学的大规模研究，并表明tZ N7-和N9-葡萄糖苷在体内被代谢。因此，我们驳斥了公认的N-糖基化不可逆地使CKs失活的假设。随后构建的数学模型提供了代谢转化率的估计值。因此，我们驳斥了公认的N-糖基化不可逆地使CK失活的假设。随后构建的数学模型提供了代谢转化率的估计值。因此，我们驳斥了公认的N-糖基化不可逆地使CK失活的假设。随后构建的数学模型提供了代谢转化率的估计值。