SummaryTo investigate the uptake and long-distance translocation of sulphate in plants, we have characterized three cell-type-speci®c sulphate transporters, Sultr1;1, Sultr2;1 and Sultr2;2 in Arabidopsis thaliana. Heterologous expression in the yeast sulphate transporter mutant indicated that Sultr1;1 encodes a high-af®nity sulphate transporter (K m for sulphate 3.6 K 0.6 mM), whereas Sultr2;1 and Sultr2;2 encode low-af®nity sulphate transporters (K m for sulphate 0.41 K 0.07 mM and > 1.2 mM, respectively). In Arabidopsis plants expressing the fusion gene construct of the Sultr1;1 promoter and green¯uorescent protein (GFP), GFP was localized in the lateral root cap, root hairs, epidermis and cortex of roots. bglucuronidase (GUS) expressed with the Sultr2;1 promoter was speci®cally accumulated in the xylem parenchyma cells of roots and leaves, and in the root pericycles and leaf phloem. Expression of the Sultr2;2 promoter±GFP fusion gene showed speci®c localization of GFP in the root phloem and leaf vascular bundle sheath cells. Plants continuously grown with low sulphate concentrations accumulated high levels of Sultr1;1 and Sultr2;1 mRNA in roots and Sultr2;2 mRNA in leaves. The abundance of Sultr1;1 and Sultr2;1 mRNA was increased remarkably in roots by short-term stress caused by withdrawal of sulphate. Addition of selenate in the sulphate-suf®cient medium increased the sulphate uptake capacity, tissue sulphate content and the abundance of Sultr1;1 and Sultr2;1 mRNA in roots. Concomitant decrease of the tissue thiol content after selenate treatment was consistent with the suggested role of glutathione (GSH) as a repressive effector for the expression of sulphate transporter genes.

SummaryThe completion of the Arabidopsis thaliana genome has revealed that there are nine members of the Pht1 family of phosphate transporters in this species. As a step towards identifying the role of this gene family in phosphorus nutrition, we have isolated the promoter regions from each of these genes, and fused them to the reporter genes b-glucuronidase and/or green¯uorescent protein. These chimeric genes have been introduced into A. thaliana, and reporter gene expression has been assayed in plants grown in soil containing high and low concentrations of inorganic phosphate (Pi). Four of these promoters were found to direct reporter gene expression in the root epidermis, and were induced under conditions of phosphate deprivation in a manner similar to previously characterised Pht1 genes. Other members of this family, however, showed expression in a range of shoot tissues and in pollen grains, which was con®rmed by RT-PCR. We also provide evidence that the root epidermally expressed genes are expressed most strongly in trichoblasts, the primary sites for uptake of Pi. These results suggest that this gene family plays a wider role in phosphate uptake and remobilisation throughout the plant than was previously believed.

Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time-resolved-reflectance techniques, a subpicosecond (~0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at-200°C and annealed at 600 "C. With the same material as a photoconductive switch we have measured electrical pulses with a full-width at half-maximum of 0.6 ps using the technique of electro-optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of-120-150 cm"/V s. GaAs grown by MBE at 200 "C! and annealed at 600 "C is also semi-insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of. subpicosecond photoconductive applications.

Putative phosphate transporters have been identified in a barley (Hordeum vulgare L.) genomic library by their homology to known phosphate transporters from dicot species. The genes designated HORvu;Pht1;1 and HORvu;Pht1;6 encode proteins of 521 and 535 amino acids respectively with 12 predicted membrane-spanning domains and other motifs common to the Phtl family of phosphate transporters. HORvu;Pht1;1 is expressed exclusively in roots and is strongly induced by phosphate deprivation. HORvu;Pht1;6 is expressed in the aerial parts of the plant with strongest expression in old leaves and flag leaves. In situ hybridization showed that HORvu;Pht1;6 is expressed in the phloem of vascular bundles in leaves and ears. In order to study the biochemical properties of HORvu;Pht1;1 and HORvu;Pht1;6, the genes were expressed in transgenic rice (Oryza sativa L.) plants under the control of the rice actin promoter and suspension cell cultures were generated. Cells derived from transgenic plants were able to take up phosphate at a much higher rate than control cells, demonstrating that both genes encode functional phosphate transporters. The estimated Km for phosphate for cells expressing HORvu;Pht1;1 was 9.06 +/- 0.82 microM, which is characteristic of a high-affinity transporter. The rate of phosphate uptake decreased with increasing pH, suggesting that HORvu;Pht1;1 operates as a H+/H2PO4(-) symporter. In contrast, the estimated Km for phosphate for cells expressing HORvu;Pht1;6 was 385 +/- 61 microM, which is characteristic of a low-affinity transporter. Taken together, the results suggest that HORvu;Pht1;1 functions in uptake of phosphate at the root surface, while HORvu;Pht1;6 probably functions in remobilization of stored phosphate from leaves.

A very large number of plant species are capable of forming symbiotic associations with arbuscular mycorrhizal (AM) fungi. The roots of these plants are potentially capable of absorbing P from the soil solution both directly through root epidermis and root hairs, and via the AM fungal pathway that delivers P to the root cortex. A large number of phosphate (P) transporters have been identified in plants; tissue expression patterns and kinetic information supports the roles of some of these in the direct root uptake pathways. Recent work has identified additional P transporters in several unrelated species that are strongly induced, sometimes specifically, in AM roots. The primary aim of the work described in this paper was to determine how mycorrhizal colonisation by different species of AM fungi influenced the expression of members of the Pht1 gene families in the cereals Hordeum vulgare (barley), Triticum aestivum (wheat) and Zea mays (maize). RT-PCR and in-situ hybridisation, showed that the transporters HORvu;Pht1;8 (AY187023), TRIae;Pht1;myc (AJ830009) and ZEAma;Pht1;6 (AJ830010), had increased expression in roots colonised by the AM fungi Glomus intraradices,Glomus sp. WFVAM23 and Scutellospora calospora. These findings add to the increasing body of evidence indicating that plants that form AM associations with members of the Glomeromycota have evolved phosphate transporters that are either specifically or preferentially involved in scavenging phosphate from the apoplast between intracellular AM structures and root cortical cells. Operation of mycorrhiza-inducible P transporters in the AM P uptake pathway appears, at least partially, to replace uptake via different P transporters located in root epidermis and root hairs.

Phosphate (P) is taken up by plants through high-affinity P transporter proteins embedded in the plasma membrane of certain cell types in plant roots. Expression of the genes that encode these transporters responds to the P status of the plants, and their transcription is normally tightly controlled. However, this tight control of P uptake is lost under Zn deficiency, leading to very high accumulation of P in plants. We examined the effect of plant Zn status on the expression of the genes encoding the HVPT1 and HVPT2 high-affinity P transporters in barley (Hordeum vulgare L. cv Weeah) roots. The results show that the expression of these genes is intimately linked to the Zn status of the plants. Zn deficiency induced the expression of genes encoding these P transporters in plants grown in either P-sufficient or -deficient conditions. Moreover, the role of Zn in the regulation of these genes is specific in that it cannot be replaced by manganese (a divalent cation similar to Zn). It appears that Zn plays a specific role in the signal transduction pathway responsible for the regulation of genes encoding high-affinity P transporters in plant roots. The significance of Zn involvement in the regulation of genes involved in P uptake is discussed.Although relatively large amounts of phosphate (P) are essential to plant growth, forms that can be taken up directly by plants are only found in low concentrations (0.01-3.0 m) in most soil solutions (Barber, 1995). Low availability of P in soils limits crop yields. Under normal growing conditions the uptake of P by plants is tightly controlled. Plants normally moderate their capacity to take up P to maintain the P concentration in their tissues within physiological limits (Mimura, 1999). They therefore restrict their capacity to take up P when grown under high-P conditions but enhance this capacity when grown under low-P conditions (Clarkson and Scattergood, 1982; Jungk et al., 1990). However, under Zn-deficient conditions, high levels of P accumulate in the tissues of both dicotyledon and monocotyledon plant species (Welch et al., 1982;Cakmak and Marschner, 1986;Webb and Loneragan, 1988;Welch and Norvell, 1993) and can reach levels that are toxic to the plants if high concentrations of P are supplied (Loneragan et al., 1982; Welch et al., 1982;Cakmak and Marschner, 1986;Norvell and Welch, 1993). This suggests that Zn-deficient plants somehow lose control over the P absorption mechanism (Safaya and Gupta, 1979;Marschner and Cakmak, 1986).The kinetic characterization of the P uptake system by whole plants indicates a high-affinity transporter activity operating at the micromolar range . The high-affinity transporters are a key component of P uptake by plants at the very low concentrations of P found in many soil solutions. When the P concentration is low in the root growth medium, expression of genes encoding high-affinity P transporters is up-regulated in plant roots (Muchhal et al., 1996;Smith et al., 1997Smith et al., , 1999 Leggewie et al., 1997; Liu et al., 1998;Smi...

GaAs layers grown by molecular beam epitaxy (MBE) at substrate temperatures between 200 and 300 °C were studied using transmission electron microscopy (TEM), x-ray diffraction, and electron paramagnetic resonance (EPR) techniques. High-resolution TEM cross-sectional images showed a high degree of crystalline perfection of these layers. For a layer grown at 200 °C and unannealed, x-ray diffraction revealed a 0.1% increase in the lattice parameter in comparison with bulk GaAs. For the same layer, EPR detected arsenic antisite defects with a concentration as high as 5×1018 cm−3. This is the first observation of antisite defects in MBE-grown GaAs. These results are related to off-stoichiometric, strongly As-rich growth, possible only at such low temperatures. These findings are of relevance to the specific electrical properties of low-temperature MBE-grown GaAs layers.

An analysis has been carried out of optical heterodyne conversion with an interdigitated-electrode photomixer made from low-temperature-grown (LTG) GaAs and pumped by two continuous-wave, frequency-offset pump lasers. The analytic prediction is in excellent agreement with the experimental results obtained recently on a photomixer having 1.0~pm-wide electrodes and gaps. The analysis predicts that a superior photomixer having 0.2~pm-wide electrodes and gaps would have a temperature-limited conversion efficiency of 2.0% at a low difference frequency, 1.6% at 94 GHz, and 0.5% at 300 GHz when connected to a broadband 100 fi load resistance and pumped at hv=2.0 eV by a total optical power of 50 mW. The predicted 3-dB bandwidth (193 GHz) of this photomixer is limited by both the electron-hole recombination time (0.6 ps) of the LTG-GaAs material and the RC time constant (0.5 ps) of the photomixer circuit.