Molecular biology of K+ transport across the plant cell membrane: What do we learn from comparison between plant species?

Cloning and characterizations of plant K+ transport systems aside from Arabidopsis have been increasing over the past decade, favored by the availability of more and more plant genome sequences. Information now available enables the comparison of some of these systems between species. In this review, we focus on three families of plant K+ transport systems that are active at the plasma membrane: the Shaker K+ channel family, comprised of voltage-gated channels that dominate the plasma membrane conductance to K+ in most environmental conditions, and two families of transporters, the HAK/KUP/KT K+ transporter family, which includes some high-affinity transporters, and the HKT K+ and/or Na+ transporter family, in which K+-permeable members seem to be present in monocots only. The three families are briefly described, giving insights into the structure of their members and on functional properties and their roles in Arabidopsis or rice. The structure of the three families is then compared between plant species through phylogenic analyses. Within clusters of ortologues/paralogues, similarities and differences in terms of expression pattern, functional properties and, when known, regulatory interacting partners, are highlighted. The question of the physiological significance of highlighted differences is also addressed.