The potassium battery: a mobile energy source for transport processes in plant vascular tissues

Contents   Summary 1049 I. Introduction 1049 II. The ‘normal’ phloem (re)loading mechanism 1050 III. A futile cycling of potassium? 1050 IV. Potassium channels of the AKT2-type are involved in sugar retrieval by the transport phloem 1050 V. The role of the K+ battery in phloem reloading 1051 VI. Remote control of phloem reloading 1051 VII. Regulation of AKT2 1052 VIII. Conclusions and future outlook 1052   Acknowledgements 1053   References 1053 Summary Plant roots absorb potassium ions from the soil and transport them in the xylem via the transpiration stream to the shoots. There, in source tissues where sufficient chemical energy (ATP) is available, K+ is loaded into the phloem and then transported with the phloem stream to other parts of the plant; in part, transport is also back to the roots. This, at first sight, futile cycling of K+ has been uncovered to be part of a sophisticated mechanism that (1) enables the shoot to communicate its nutrient demand to the root, (2) contributes to the K+ nutrition of transport phloem tissues and (3) transports energy stored in the K+ gradient between phloem cytosol and the apoplast. This potassium battery can be tapped by opening AKT2-like potassium channels and then enables the ATP-independent energization of other transport processes, such as the reloading of sucrose. Insights into these mechanisms have only been possible by combining wet-lab and dry-lab experiments by means of computational cell biology modeling and simulations.