Amino acid modulation of Ca²⁺signaling guides growth of plant pollen tubes.

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.

The AKT2 K(+) channel is endowed with unique functional properties, being the only weak inward rectifier characterized to date in Arabidopsis. The gene is expressed widely, mainly in the phloem but also at lower levels in leaf epiderm, mesophyll, and guard cells. The AKT2 mRNA level is upregulated by abscisic acid. By screening a two-hybrid cDNA library, we isolated a protein phosphatase 2C (AtPP2CA) involved in abscisic acid signaling as a putative partner of AKT2. We further confirmed the interaction by in vitro binding studies. The expression of AtPP2CA (beta-glucuronidase reporter gene) displayed a pattern largely overlapping that of AKT2 and was upregulated by abscisic acid. Coexpression of AtPP2CA with AKT2 in COS cells and Xenopus laevis oocytes was found to induce both an inhibition of the AKT2 current and an increase of the channel inward rectification. Site-directed mutagenesis and pharmacological analysis revealed that this functional interaction involves AtPP2CA phosphatase activity. Regulation of AKT2 activity by AtPP2CA in planta could allow the control of K(+) transport and membrane polarization during stress situations.

Multiple, diverse, and complex: Calcium currents characterize the developing pollen tube in the small mustard plant Arabidopsis and correlate with growth at the tip of the pollen tube. This system constitutes a practical model for screening for Ca ²⁺ -signaling mechanisms in plants. Wudick et al. analyzed multiple variants of glutamate receptor–like (GLR) channels and discovered that some work alone and others work in pairs or trios. Subcellular localization of GLRs is a complex response to CORNICHON sorting proteins, which leave some GLRs at the plasma membrane and ferry others to internal calcium reservoirs. The calcium current at the tip of the growing pollen tube apparently integrates multiple intracellular currents. Science , this issue p. 533

Summary: Plant outward‐rectifying K⁺ channels mediate K⁺ efflux from guard cells during stomatal closure and from root cells into the xylem for root–shoot allocation of potassium (K). Intriguingly, the gating of these channels depends on the extracellular K⁺ concentration, although the ions carrying the current are derived from inside the cell. This K⁺ dependence confers a sensitivity to the extracellular K⁺ concentration ([K⁺]) that ensures that the channels mediate K⁺ efflux only, regardless of the [K⁺] prevailing outside. We investigated the mechanism of K⁺‐dependent gating of the K⁺ channel SKOR of Arabidopsis by site‐directed mutagenesis. Mutations affecting the intrinsic K⁺ dependence of gating were found to cluster in the pore and within the sixth transmembrane helix (S6), identifying an ‘S6 gating domain’ deep within the membrane. Mapping the SKOR sequence to the crystal structure of the voltage‐dependent K⁺ channel KvAP from Aeropyrum pernix suggested interaction between the S6 gating domain and the base of the pore helix, a prediction supported by mutations at this site. These results offer a unique insight into the molecular basis for a physiologically important K⁺‐sensory process in plants.

In order to cope with reproduction in a dry environment without any sort of motility, plants have developed a very specialized and unique sexual system. Of special notice, the two sperm cells that will perform the double fertilization typical of higher plants are carried by one of the fastest growing cells in nature, the pollen tube. This tube develops from the vegetative cell of the pollen grain upon germination on the female tissues. While it cannot be considered as a canonical excitable cell, pollen tubes depend for most of their fundamental functional features on a close regulation of ion dynamics, namely in terms of polarization of extracellular fluxes and formation of standing cytosolic free ion gradients, namely of calcium (Ca²⁺) and protons (H⁺). In turn, these imply that plasma membrane transporters are polarized, or polarly regulated, and that internal signaling cascades transduce this spatial information into the basic features of growth and morphogenesis needed for pollen tubes to target correctly the ovules and discharge the sperm cells. Because of the singularity of this organization, and the ease with which pollen tubes can be experimentally handled, recent years have witnessed an accumulation of data at many levels, from basic biophysical and cell biology characterization, to gene assignment and transcriptomic description of pollen development. In this review we aim to organize this information in terms of the basic biophysical features of membrane function and integrate it into conceptual testable hypotheses on how the dynamics of ion regulation may underlie fundamental properties of cell development.