Szczerba Mark W, Britto Dev T, Kronzucker Herbert J
Department of Life Sciences, University of Toronto, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4.
J Exp Bot. 2006;57(12):3293-300. doi: 10.1093/jxb/erl088. Epub 2006 Aug 7.
The existence of distinct high- and low-affinity transport systems (HATS and LATS) is well established for major nutrient ions. However, influx mediated by these systems is usually estimated using uniformly simple tracer protocols. Two (42)K radiotracer methods to measure potassium influxes in the HATS and LATS ranges in intact barley (Hordeum vulgare L.) roots are compared here: a direct influx (DI) method, and an integrated flux analysis (IFA), which is designed to account for tracer efflux from labelled roots and differential tracer accumulation along the plant axis. Methods showed only minor discrepancies for influx values in the HATS range, but large discrepancies in the LATS range, revealing striking distinctions in the cellular exchange properties dominated by the operation of the two transport systems. It is shown that accepted DI protocols are associated with very large errors in the high-conductance LATS range, underestimating influx at least 6-fold due to four characteristics of this transport mode: (i) accelerated cellular (42)K exchange; (ii) a greatly increased ratio of efflux to influx; (iii) increased (42)K loss during the removal of water from roots in preweighing centrifugation or blotting protocols; and (iv) increased (42)K retention at the root-shoot interface, a region of the plant frequently disregarded in DI determinations. The findings warrant a re-evaluation of a large body of literature reporting influx in the LATS range, and are of fundamental importance to ion flux experimentation in plant physiology.
主要营养离子存在明显的高亲和性和低亲和性转运系统（HATS和LATS），这一点已得到充分证实。然而，这些系统介导的流入通常使用统一简单的示踪剂方案进行估算。本文比较了两种用于测量完整大麦（Hordeum vulgare L.）根中HATS和LATS范围内钾流入的（42）K放射性示踪剂方法：直接流入（DI）法和综合通量分析（IFA）法，后者旨在考虑示踪剂从标记根中的流出以及沿植物轴的示踪剂差异积累。两种方法在HATS范围内的流入值仅显示出微小差异，但在LATS范围内差异很大，这揭示了由两种转运系统的运作主导的细胞交换特性的显著区别。结果表明，公认的DI方案在高传导性LATS范围内存在非常大的误差，由于这种转运模式的四个特征，流入量至少被低估了6倍：（i）细胞（42）K交换加速；（ii）流出与流入的比例大幅增加；（iii）在预称重离心或印迹方案中从根中去除水分期间（42）K损失增加；（iv）在根 - 茎界面处（42）K保留增加，这是DI测定中经常被忽视的植物区域。这些发现值得对大量报道LATS范围内流入量的文献进行重新评估，并且对植物生理学中的离子通量实验具有至关重要的意义。
