A Novel AtKEA Gene Family, Homolog of Bacterial K+/H+ Antiporters, Plays Potential Roles in K+ Homeostasis and Osmotic Adjustment in Arabidopsis

AtKEAs, homologs of bacterial KefB/KefC, are predicted to encode K+/H+ antiporters in Arabidopsis. The AtKEA family contains six genes forming two subgroups in the cladogram: AtKEA1-3 and AtKEA4-6. AtKEA1 and AtKEA2 have a long N-terminal domain; the full-length AtKEA1 was inactive in yeast. The transport activity was analyzed by expressing the AtKEA genes in yeast mutants lacking multiple ion carriers. AtKEAs conferred resistance to high K+ and hygromycin B but not to salt and Li+ stress. AtKEAs expressed in both the shoot and root of Arabidopsis. The expression of AtKEA1, -3 and -4 was enhanced under low K+ stress, whereas AtKEA2 and AtKEA5 were induced by sorbitol and ABA treatments. However, osmotic induction of AtKEA2 and AtKEA5 was not observed in aba2-3 mutants, suggesting an ABA regulated mechanism for their osmotic response. AtKEAs' expression may not be regulated by the SOS pathway since their expression was not affected in sos mutants. The GFP tagging analysis showed that AtKEAs distributed diversely in yeast. The Golgi localization of AtKEA3 was demonstrated by both the stably transformed seedlings and the transient expression in protoplasts. Overall, AtKEAs expressed and localized diversely, and may play roles in K+ homeostasis and osmotic adjustment in Arabidopsis.