Molecular cloning and expression analysis of a gene encoding KUP/HAK/KT-type potassium uptake transporter from Cryptomeria japonica

The molecular mechanism of potassium ion transport across membranes in conifers is poorly known. We isolated and analyzed a gene encoding a potassium transporter from the conifer Cryptomeria japonica. Potassium ion (K+) is an essential and the most abundant intracellular cation in plants. The roles of K+ in various aspects of plant life are closely linked to its transport across biological membranes such as the plasma membrane and the tonoplast, which is mediated by membrane-bound transport proteins known as transporters and channels. Information on the molecular basis of K+ membrane transport in trees, especially in conifers, is currently limited. In this study, we isolated one complementary DNA, CjKUP1, which is homologous to known plant K+ transporters, from Cryptomeria japonica. Complementation tests using an Escherichia coli mutant, which is deficient in K+ uptake activity, was conducted to examine the K+ uptake function of the protein encoded by CjKUP1. Transformation of the K+-uptake-deficient mutant with CjKUP1 complemented the deficiency of this mutant. This result indicates that CjKUP1 has a function of K+ uptake. The expression levels of CjKUP1 in male strobili were markedly higher from late September to early October than in other periods. The expression levels in male and female strobili were higher than those in other organs such as needles, inner bark, differentiating xylem, and roots. These results indicate that CjKUP1 is mainly involved in K+ membrane transport in the cells of reproductive organs of C. japonica trees, especially in male strobili during pollen differentiation.