Auxin-induced growth of coleoptiles depends on the presence of potassium and is suppressed by K ؉ channel blockers. To evaluate the role of K ؉ channels in auxin-mediated growth, we isolated and functionally expressed ZMK1 and ZMK2 (Zea mays K ؉ channel 1 and 2), two potassium channels from maize coleoptiles. In growth experiments, the time course of auxin-induced expression of ZMK1 coincided with the kinetics of coleoptile elongation. Upon gravistimulation of maize seedlings, ZMK1 expression followed the gravitropic-induced auxin redistribution. K ؉ channel expression increased even before a bending of the coleoptile was observed. The transcript level of ZMK2, expressed in vascular tissue, was not affected by auxin. In patch-clamp studies on coleoptile protoplasts, auxin increased K ؉ channel density while leaving channel properties unaffected. Thus, we conclude that coleoptile growth depends on the transcriptional up-regulation of ZMK1, an inwardly rectifying K ؉ channel expressed in the nonvascular tissue of this organ.

The phytohormone abscisic acid (ABA) plays important regulatory roles in many plant developmental processes including seed dormancy, germination,growth, and stomatal movements. These physiological responses to ABA are in large part brought about by changes in gene expression. To study genome-wide ABA-responsive gene expression we applied massively parallel signature sequencing (MPSS) to samples from Arabidopsis thaliana wildtype (WT)and abi1-1 mutant seedlings. We identified 1354 genes that are either up- or downregulated following ABA treatment of WT seedlings. Among these ABA-responsive genes, many encode signal transduction components. In addition,we identified novel ABA-responsive gene families including those encoding ribosomal proteins and proteins involved in regulated proteolysis. In the ABA-insensitive mutant abi1-1, ABA regulation of about 84.5% and 6.9%of the identified genes was impaired or strongly diminished, respectively;however, 8.6% of the genes remained appropriately regulated. Compared to other methods of gene expression analysis, the high sensitivity and specificity of MPSS allowed us to identify a large number of ABA-responsive genes in WT Arabidopsis thaliana. The database given in our supplementary materialprovides researchers with the opportunity to rapidly assess whether genes of interest may be regulated by ABA. Regulation of the majority of the genes by ABA was impaired in the ABA-insensitive mutant abi1-1. However, a subset of genes continued to be appropriately regulated by ABA, which suggests the presence of at least two ABA signaling pathways, only one of which is blocked in abi1-1.

The potassium-channel gene, AKT3, has recently been isolated from an Arabidopsis thaliana cDNA library. By using the whole-mount and in situ hybridization techniques, we found AKT3 predominantly expressed in the phloem. To study the physiological role of this channel type, AKT3 was heterologously expressed in Xenopus oocytes, and the electrical properties were examined with voltage-clamp techniques. Unlike the plant inward-rectifying guard cell K ؉ channels KAT1 and KST1, the AKT3 channels were only weakly regulated by the membrane potential. Furthermore, AKT3 was blocked by physiological concentrations of external Ca 2؉ and showed an inverted pH regulation. Extracellular acidification decreased the macroscopic AKT3 currents by reducing the single-channel conductance. Because assimilate transport in the vascular tissue coincides with both H ؉ and K ؉ f luxes, AKT3 K ؉ channels may be involved in K ؉ transport accompanying phloem loading and unloading processes.The plant vascular system, which consists of xylem and phloem, is specialized for long-distance solute and water transport. In both tissues, potassium represents one of the major mineral nutrients and is likely to assist in osmotic homeostasis. After uptake from the soil, K ϩ ions circulate between roots and shoots through the xylem and phloem to adopt the specific demands for this cation in the various tissues (1, 2). Recently, in vivo and in vitro analysis demonstrated the presence of K ϩ uptake and release channels in xylem parenchyma cells (3-6). In comparison, information about K ϩ transport across the plasma membrane of phloem cells, the underlying mechanisms, and the physiological role in long-distance solute transport is still limited. Phloem loading with assimilates is accompanied by ionic movements (7). Protons are pumped into the apoplast by a H ϩ -ATPase, generating transmembrane gradients in electropotential and pH that in turn enable the uphill transport of assimilates into the phloem through assimilate/ H ϩ -cotransporters (8-10). The phloem loading coincides with an increase in the symplastic K ϩ concentration likely to maintain electrical neutrality that is required for creating the pH gradient (2, 7). In addition, the K ϩ concentration in the sieve tube may affect the volume flow rate in the phloem (11). Furthermore, the membrane potential that transiently changes during phloem-propagating action potentials is possibly reestablished on K ϩ release from the sieve tube (12). Evidence for the expression of K ϩ channels in the phloem was recently provided (P.A., K. Philippar, and R.H., unpublished data) to support the idea that K ϩ channels may also mediate the transmembrane K ϩ fluxes in the phloem.In the present paper, we localized the cloned Arabidopsis K ϩ channel AKT3 to the phloem and revealed its unique dependence on voltage, Ca 2ϩ , and pH properties, which are well suited for meeting its supposed role in processes associated with the phloem. MATERIALS AND METHODSRNA Extraction and Northern Blot Analysis. Total RNA was isolate...

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Summary A new aquaporin from Nicotiana tabacum (cv. Samsun) was characterized. It shares sequence homology to the Arabidopsis thaliana PIP1 protein family. By two‐phase partitioning and immunoblot analysis, plasma membrane localization could be demonstrated. The corresponding mRNA is highly abundant in roots and flowers, while it is rarely expressed in leaves and stems. Functional expression in Xenopus oocytes revealed that NtAQP1 can mediate glycerol transport in addition to water flow. However, NtAQP1 is impermeable for Na + , K + and Cl – ions. The water permeability and selectivity could not be modulated by addition of mercurials or the activity of cAMP‐dependent protein kinases.