Elevations in cytosolic free calcium concentration ([Ca(2+)](cyt)) constitute a fundamental signal transduction mechanism in eukaryotic cells, but the molecular identity of Ca(2+) channels initiating this signal in plants is still under debate. Here, we show by pharmacology and loss-of-function mutants that in tobacco and Arabidopsis, glutamate receptor-like channels (GLRs) facilitate Ca(2+) influx across the plasma membrane, modulate apical [Ca(2+)](cyt) gradient, and consequently affect pollen tube growth and morphogenesis. Additionally, wild-type pollen tubes grown in pistils of knock-out mutants for serine-racemase (SR1) displayed growth defects consistent with a decrease in GLR activity. Our findings reveal a novel plant signaling mechanism between male gametophyte and pistil tissue similar to amino acid-mediated communication commonly observed in animal nervous systems.

RNA gel blot and reverse transcription-polymerase chain reaction experiments were used to identify a single K ؉ channel gene in Arabidopsis as expressed throughout the plant. Use of the ␤ -glucuronidase reporter gene revealed expression of this gene, AKT2/AKT3 , in both source and sink phloem tissues. The AKT2/AKT3 gene corresponds to two previously identified cDNAs, AKT2 (reconstructed at its 5 Ј end) and AKT3 , the open reading frame of the latter being shorter at its 5 Ј end than that of the former. Rapid amplification of cDNA ends with polymerase chain reaction and sitedirected mutagenesis was performed to identify the initiation codon for AKT2 translation. All of the data are consistent with the hypothesis that the encoded polypeptide corresponds to the longest open reading frame previously identified ( AKT2 ). Electrophysiological characterization (macroscopic and single-channel currents) of AKT2 in both Xenopus oocytes and COS cells revealed a unique gating mode and sensitivity to pH (weak inward rectification, inhibition, and increased rectification upon internal or external acidification), suggesting that AKT2 has enough functional plasticity to perform different functions in phloem tissue of source and sink organs. The plant stress hormone abscisic acid was shown to increase the amount of AKT2 transcript, suggesting a role for the AKT2 in the plant response to drought. INTRODUCTIONPlant growth and development are dependent on translocation of newly fixed photoassimilates from the sites of synthesis (sources) to the sites of consumption or storage (sinks) (Sonnewald and Willmitzer, 1992). Loading solutes into the sieve tubes at the source and unloading them at the sink are thought to result in a gradient of osmotic potential along the phloem tubes, from source to sink. The resulting gradient in turgor pressure gives rise to the movement of water and solutes (mass flow as great as 1 m hr Ϫ 1 ) toward the sink (Patrick, 1997;Sjölund, 1997;Oparka and Turgeon, 1999).Identifying membrane transport proteins that may be involved in the loading and unloading processes is a major focus of current research on phloem. Assimilate loading and unloading have only recently begun to be explored with molecular and cellular approaches (Rentsch and Frommer, 1996;Kühn et al., 1999). Until now, the focus has been on sucrose loading (Riesmeier et al., 1994; Truernit and Sauer, 1995;Kühn et al., 1997Kühn et al., , 1999) and, to a lesser extent, membrane energization by proton pumping (DeWitt et al., 1991; DeWitt and Sussman, 1995). In contrast, the molecular mechanisms responsible for K ϩ loading into and unloading from the phloem sap have been poorly investigated, although a large set of electrophysiological data supports the view that these transports play major roles, taking part in both the control of phloem sap flow rate and the integration of K ϩ fluxes in the whole plant. Briefly, K ϩ assays in phloem sap have revealed large differences in concentration between sources and sinks, the concentration near the unloading ...

The essential mineral nutrient potassium (K + ) is the most important inorganic cation for plants and is recognized as a limiting factor for crop yield and quality. Nonetheless, it is only partially understood how K + contributes to plant productivity. K + is used as a major active solute to maintain turgor and to drive irreversible and reversible changes in cell volume. K + also plays an important role in numerous metabolic processes, for example, by serving as an essential cofactor of enzymes. Here, we provide evidence for an additional, previously unrecognized role of K + in plant growth. By combining diverse experimental approaches with computational cell simulation, we show that K + circulating in the phloem serves as a decentralized energy storage that can be used to overcome local energy limitations. Posttranslational modification of the phloem-expressed Arabidopsis K + channel AKT2 taps this "potassium battery," which then efficiently assists the plasma membrane H + -ATPase in energizing the transmembrane phloem (re) loading processes.channel gating | energy limiting condition | phloem reloading | posttranslational regulation | potassium channel