AKT1, a putative inwardly directed K+ channel of Arabidopsis, restores long-term potassium uptake in a yeast mutant defective in K+ absorption. In this paper, the expression pattern of the gene encoding AKT1 is described. Northern blots indicate that AKT1 transcripts are preferentially accumulated in Arabidopsis roots. Owing to the difficulties in producing large quantities of Arabidopsis roots under hydroponic conditions, experiments were undertaken with Brassica napus, a related species. Potassium starvation experiments on B. napus plants show that changes in the K+ status of the organs do not modify AKT1 mRNA accumulation. Western blot analysis of B. napus proteins confirms the presence of AKT1 at the root plasma membrane. Tissue-specific expression directed by the Arabidopsis AKT1 gene promoter was investigated by analysis of beta-glucuronidase (GUS) activity in transgenic Arabidopsis containing an AKT1-GUS gene fusion. As determined by fluorimetric and histochemical tests, the AKT1 promoter directs preferential expression in the peripheral cell layers of root mature regions. The discrete activity found in leaves relates to leaf primordia and to small groups of cells, hydathodes, found on toothed margins of the Arabidopsis leaf lamina. These data are discussed with regard to a possible role of AKT1 in K+ nutrition of plants.

We have isolated and sequenced the genomic clone coding for the potassium transport system AKT1 of Arabidopsis thaliana. Southern blot analysis indicated that the gene is present in one copy in the Arabidopsis genome. The coding sequence is interrupted by ten introns. Sequence comparisons of AKT1 polypeptide with the voltage-gated inward rectifying Arabidopsis K+ channel KAT1, and with voltage- or cyclic nucleotide-gated channels from insects and mammals, revealed a highly conserved domain found specifically in both plant polypeptides, and corresponding to about the last 50 amino acids of their C-terminal region. Northern blot analysis of AKT1 expression in Arabidopsis seedlings indicated that AKT1 is preferentially expressed in roots. No transcript was detected in extracts from heterotrophic suspension culture cells. Depleting K+ in the Arabidopsis seedling culture medium for 4 days led to a strong decrease in K+ tissue content (ca. 50%), but did not affect AKT1 transcript level.

1997. Plasma membrane transport systems in higher plants: From black boxes to molecular physiology. -Physiol. Plant. 100: 1-15.Considerable progress in identifying transport systems of the plant plasma membrane has been made recently. The putative systems cloned to date comprise H*-ATPases, potassium, chloride and water channels, and carriers involved in the transport of glucose, sucrose, amino acids, peptides, potassium, nitrate, ammonium, phosphate, sulfate, iron and copper. Most of these systems were identified first in Arabidopsis thaliarm. Successful cloning strategies have involved the following variety of techniques: complementation of yeast mutants, screening of Arabidopsis mutants, immunoscreening of a cDNA expression library expressed in mammalian cells, screening of genomic and cDNA libraries with probes (or degenerate oligonucleotides) derived from yeast and/or animal genes, or database screening for sequence similarity to eukaryotic counterparts. Many related transport systems have subsequently been identified either by screening libraries directly, or by systematic cDNA sequencing programs. Surprisingly large gene families have been revealed. Heterologous expression systems, such as yeast, Xenopus oocytes or insect cells, provide tools for studying the transport activities, biochemical properties and structure-function relationships of these systems. Their expression and functions in pianta are investigated using northern blot analysis, in situ hybridization, and transgenic approaches. Individual systems encoded by the same gene family can differ in their transport properties and have distinct tissue expression patterns. Such diversity might be central to the integration of solute transport at the whole plant level, allowing the differential expression of sets of transport systems specifically tailored to the requirements of each tissue.