Transport characteristics of ion channels as influenced by apoplastic properties K+ channels of the phloem

Abstract

Our studies demonstrate that members of the AKT2/3 subfamily exhibit unique features among the plant Shaker-like K+ channels. They show weak voltage dependence and rectification, are blocked by calcium or protons and all members investigated so far are phloem located. AKT2/3-type channels are involved in the control of sugar and potassium (K) translocation in the phloem from source to sink. The membrane potential (Em) is controlled by the channels via a voltage clamp near the equilibrium potential for K+ (EK), to maintain sugar/H+ co-transport. This function could be demonstrated by co-expression of AKT2/3-type channels with sucrose transporters in Xenopus oocytes and confirmed the findings in planta performed by the aphid stylectomy technique and analysis of an AKT2/3-loss-of-function mutant. Furthermore, channel regulation in vivo is accomplished by changes in the number and activity of K+ channels in response to changes in the sink/source relation, extarcellular sugar concentration and pH. Some members of this channel subfamily are highly expressed in the source phloem, while others are restricted to the sink, indicating specialised functions in different plants, e.g. rosette plants (Arabidopsis), leguminoses (Vicia faba) or perennial plants like poplar.