Plant hormones are transported across cell membranes during various physiological events. Recent identification of abscisic acid and strigolactone transporters suggests that transport of various plant hormones across membranes does not occur by simple diffusion but requires transporter proteins that are strictly regulated during development. Here, we report that a major glucosinolate transporter, GTR1/NPF2.10, is multifunctional and may be involved in hormone transport in Arabidopsis thaliana . When heterologously expressed in oocytes, GTR1 transports jasmonoyl-isoleucine and gibberellin in addition to glucosinolates. gtr1 mutants are severely impaired in filament elongation and anther dehiscence resulting in reduced fertility, but these phenotypes can be rescued by gibberellin treatment. These results suggest that GTR1 may be a multifunctional transporter for the structurally distinct compounds glucosinolates, jasmonoyl-isoleucine and gibberellin, and may positively regulate stamen development by mediating gibberellin supply.

Plant cells are sensitive to salinity stress and do not require sodium as an essential element for their growth and development. Saline soils reduce crop yields and limit available land. Research shows that HKT transporters provide a potent mechanism for mediating salt tolerance in plants. Knowledge of the molecular ion transport and regulation mechanisms and the control of HKT gene expression are crucial for understanding the mechanisms by which HKT transporters enhance crop performance under salinity stress. This review focuses on HKT transporters in monocot plants and in Arabidopsis as a dicot plant, as a guide to efforts toward improving salt tolerance of plants for increasing the production of crops and bioenergy feedstocks.

Since sodium, Na, is a non-essential element for the plant growth, the molecular mechanism of Na+ transport system in plants has remained elusive for the last two decades. The accumulation of Na+ in soil through irrigation for sustainable agricultural crop production, particularly in arid land, and by changes in environmental and climate conditions leads to the buildup of toxic level of salts in the soil. Since the latter half of the twentieth century, extensive molecular research has identified several classes of Na+ transporters that play major roles in the alleviation of ionic stress by excluding toxic Na+ from the cytosol or preventing Na+ transport to the photosynthetic organs, and also in osmotic stress by modulating intra/extracellular osmotic balance. In this review, we summarize the current knowledge of three major Na+ transporters, namely NHX, SOS1, and HKT transporters, including recently revealed characteristics of these transporters.

The tonoplast K ؉ membrane transport system plays a crucial role in maintaining K ؉ homeostasis in plant cells. Here, we isolated cDNAs encoding a two-pore K ؉ channel (NtTPK1) from Nicotiana tabacum cv. SR1 and cultured BY-2 tobacco cells. Two of the four variants of NtTPK1 contained VHG and GHG instead of the GYG signature sequence in the second pore region. All four products were functional when expressed in the Escherichia coli cell membrane, and NtTPK1 was targeted to the tonoplast in tobacco cells. Two of the three promoter sequences isolated from N. tabacum cv. SR1 were active, and expression from these was increased ϳ2-fold by salt stress or high osmotic shock. To determine the properties of NtTPK1, we enlarged mutant yeast cells with inactivated endogenous tonoplast channels and prepared tonoplasts suitable for patch clamp recording allowing the NtTPK1-related channel conductance to be distinguished from the small endogenous currents. NtTPK1 exhibited strong selectivity for K ؉ over Na ؉ . NtTPK1 activity was sensitive to spermidine and spermine, which were shown to be present in tobacco cells. NtTPK1 was active in the absence of Ca 2؉ , but a cytosolic concentration of 45 M Ca 2؉ resulted in a 2-fold increase in the amplitude of the K ؉ current. Acidification of the cytosol to pH 5.5 also markedly increased NtTPK1-mediated K ؉ currents. These results show that NtTPK1 is a novel tonoplast K ؉ channel belonging to a different group from the previously characterized vacuolar channels SV, FV, and VK.Plants take up potassium (K ϩ ) from the soil and plant cells accumulate K ϩ to regulate the membrane potential and turgor pressure. The cytoplasmic K ϩ concentration is tightly controlled at ϳ100 mM (1). Vacuoles are major subcellular reservoirs for controlling K ϩ homeostasis in plant cells (1). During cell expansion, for instance during stomata opening or cell growth, tonoplast transport system moves K ϩ into the vacuole, whereas, under conditions of salinity stress, K ϩ is replaced by Na ϩ (2-5).Several kinds of genes encoding K ϩ channels and K ϩ transporters have been identified in the Arabidopsis thaliana genome, and their function and tissue and cellular distribution have been extensively studied. They consist of two families, the Shaker-type channels, with six hydrophobic transmembrane domains and a single pore domain, and the two-pore K ϩ channel (TPK) 2 family, with four transmembrane and two pore domains. Six different genes encoding TPK-type channels are present in A. thaliana. AtTPK4 is targeted to the plasma membrane (6), while the other five, AtTPK1, AtTPK2, AtTPK3, AtTPK5, and AtKCO3, are localized in the vacuolar membrane (7). AtTPK1 and AtTPK4 have been functionally characterized. AtTPK4 shows a voltage-independent K ϩ profile in Xenopus laevis ooctyes and in yeast, and the K ϩ current is inhibited by extracellular Ca 2ϩ and reduced by shifting the cytosolic pH from 7.5 to 6.3, but is not affected by the external pH (6). AtTPK1 has different properties to AtTPK4 (7,8). In the yeast and plant ...

The group of voltage-independent K(+) channels in Arabidopsis thaliana consists of six members, five tandem-pore channels (TPK1-TPK5) and a single K(ir)-like channel (KCO3). All TPK/KCO channels are located at the vacuolar membrane except for TPK4, which was shown to be a plasma membrane channel in pollen. The vacuolar channels interact with 14-3-3 proteins (also called General Regulating Factors, GRFs), indicating regulation at the level of protein-protein interactions. Here we review current knowledge about these ion channels and their genes, and highlight open questions that need to be urgently addressed in future studies to fully appreciate the physiological functions of these ion channels.