Plant K+ channels: structure, activity and function.

Introduction In plants as in animals, K+ channel activity is the major determinant of cell membrane conductance [1,2]. Kt channels are therefore involved in central physiological processes such as setting the resting cell membrane potential, transducing signals and shaping action-potential waveforms. In addition, in plant cells, K t channel activity is thought to be involved in long-term fluxes, such as Kt uptake sustained by hyperpolarizing H t extruding ATPases, and K+ efflux sustained by depolarizing anion efflux through voltage-gated anion channels [2-51. In relation to these roles, Kf channel activity controls the osmotic potential, cell turgor and related functions at the cell or whole plant level, e.g. stomata1 guard cell movements and control of transpirational water loss [3,4,6]. Our present knowledge of the role and regulation of K+ channel activity in plants mainly stems from electrophysiological studies of K+ fluxes in guard cells [3,4,6], and to a lesser extent Kf uptake by roots [S]. Although still rough and fragmentary, the results suggest that most of the control mechanisms described in animal cells have a counterpart in plant cells. For instance, K+ channel activity in guard cells is controlled by hormones probably via G-proteins, second messengers (inositol trisphosphate, Ca2+, H + ) and (de)phosphorylation events [3,4,7]. A phenomenon that evokes hormone desensitization has been reported [8]. For reviews of hormonal control of K+ channel activity in guard cells and the roles of these channels in signal transduction, see refs. [3,4,6]. In this paper, we shall focus on the molecular cloning, structure, functional properties, expression and role in plants of the K' channels identified so far. Within the large superfamily of ion channels, K+ channels constitute the most diverse group [1,9]. All the Kf channels cloned so far, from prokaryotes to eukaryotes, and fungi to plants and animals, exhibit a characteristic domain, named H5 or P (for Pore), that forms part of the aqueous pore of the channel and controls its ionic selectivity [9]. These channels may