Communication between the Plasma Membrane and Tonoplast Is an Emergent Property of Ion Transport

Abstract

Dear Editor, A very substantial body of data now exists for ion transport at both the plasma membrane and the tonoplast of several plant cell models, notably for guard cells, root hairs, and epidermal cells of several species to name a few (Grierson et al., 2014; Jezek and Blatt, 2017; Wang et al., 2019). This knowledge has its foundation in detailed electrophysiological and flux studies that have provided quantitative biophysical and kinetic information. Our understanding has expanded through molecular biology to include the genetic identities of many transporters that operate at both membranes. Among these studies, it is common to deconstruct the mechanics and genetics of transport and to characterize each transporter in isolation. In many cases, this work has included the cloning and heterologous expression of specific transport gene products and their analysis under voltage clamp and by radiotracer flux measurements. Where such studies often struggle is in phenotypic analysis in vivo to associate a genetic lesion with a function for the transport gene of interest. We suggest that, in focusing on single gene products, the deconstructionist approach can lose sight of the unique feature of transport, namely its physiological integration and apparent communication with other transport processes in situ, what is often referred to as emergent properties arising from transport interactions. This communication is particularly evident when the transporter in question moves charge across the membrane. Although less often appreciated, similar considerations apply to transport across serial membranes when these operate on common pools of solutes within an enclosed and finite compartment. Both situations are common to plants. Communication is especially important between serial membranes, for example between the

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