Mg2+ Is a Missing Link in Plant Cell Ca2+ Signalling and Homeostasis-A Study on Vicia faba Guard Cells.

Fouad Lemtiri-Chlieh,Chris Gehring,Stefan T. Arold

doi:10.3390/ijms21113771

Abstract

Hyperpolarization-activated calcium channels (HACCs) are found in the plasma membrane and tonoplast of many plant cell types, where they have an important role in Ca2+-dependent signalling. The unusual gating properties of HACCs in plants, i.e., activation by membrane hyperpolarization rather than depolarization, dictates that HACCs are normally open in the physiological hyperpolarized resting membrane potential state (the so-called pump or P-state); thus, if not regulated, they would continuously leak Ca2+ into cells. HACCs are permeable to Ca2+, Ba2+, and Mg2+; activated by H2O2 and the plant hormone abscisic acid (ABA); and their activity in guard cells is greatly reduced by increasing amounts of free cytosolic Ca2+ ([Ca2+]Cyt), and hence closes during [Ca2+]Cyt surges. Here, we demonstrate that the presence of the commonly used Mg-ATP inside the guard cell greatly reduces HACC activity, especially at voltages ≤ −200 mV, and that Mg2+ causes this block. Therefore, we firstly conclude that physiological cytosolic Mg2+ levels affect HACC gating and that channel opening requires either high negative voltages (≥−200 mV) or displacement of Mg2+ away from the immediate vicinity of the channel. Secondly, based on structural comparisons with a Mg2+-sensitive animal inward-rectifying K+ channel, we propose that the likely candidate HACCs described here are cyclic nucleotide gated channels (CNGCs), many of which also contain a conserved diacidic Mg2+ binding motif within their pores. This conclusion is consistent with the electrophysiological data. Finally, we propose that Mg2+, much like in animal cells, is an important component in Ca2+ signalling and homeostasis in plants.

Highlights

Ca2+ has long been recognized as an essential component in many plant cellular processes
Ca2+ influx into plant cells is achieved by three main types of Ca2+ channels [2,3]: firstly, channels that show little or no voltage sensitivity, referred to as non-selective calcium channels (NSCCs), primarily for being active at all voltages and for being less selective for calcium over monovalent cations, such as K+ and Na+ [4]; secondly, channels that show high voltage-dependence, such as those activated by depolarization (DACCs) [5]; thirdly, others that are activated only by membrane hyperpolarization (HACCs) [6], which are the focus of this report
Using the patch clamp technique in both whole cell (WC) and excised configurations (EC), two types of Hyperpolarization-activated calcium channels (HACCs) were identified in the plasma membrane (PM)

Summary

Introduction

Ca2+ has long been recognized as an essential component in many plant cellular processes. The proteins responsible for this type of channel activity have not yet been identified This is not the case for the other type of HACCs, where a family of 20 genes is known and their translation products were originally described as being mostly gated open by cyclic nucleotides, such as 3 ,5 -cyclic adenosine monophosphate (cAMP) or 3 ,5 - cyclic guanosine monophate (cGMP) [12]; the reason for their name—cyclic nucleotide gated channels (CNGCs). These plant channels, to their animal functional homologs, poorly discriminate between divalent and monovalent cations. In plants, the rectification of IK,in was found not to be due to Mg2+-dependent blockage [23] but rather due to an intrinsic property of the channel protein itself [24]

Methods

Results

Discussion

Conclusion