The EIR1 gene of Arabidopsis is a member of a family of plant genes with similarities to bacterial membrane transporters. This gene is expressed only in the root, which is consistent with the phenotypes of the eir1 mutants-the roots are agravitropic and have a reduced sensitivity to ethylene. The roots of eir1 mutants are also insensitive to the excess auxin produced by alf1-1 and fail to induce an auxin-inducible gene in the expansion zone. Although they fail to respond to internally generated auxin, they respond normally to externally applied auxin. Expression of the EIR1 gene in Saccharomyces cerevisiae confers resistance to fluorinated indolic compounds. Taken together, these data suggest that the EIR1 protein has a root-specific role in the transport of auxin.

Transgenic plants overexpressing the vacuolar H ؉ -pyrophosphatase are much more resistant to high concentrations of NaCl and to water deprivation than the isogenic wild-type strains. These transgenic plants accumulate more Na ؉ and K ؉ in their leaf tissue than the wild type. Moreover, direct measurements on isolated vacuolar membrane vesicles derived from the AVP1 transgenic plants and from wild type demonstrate that the vesicles from the transgenic plants have enhanced cation uptake. The phenotypes of the AVP1 transgenic plants suggest that increasing the vacuolar proton gradient results in increased solute accumulation and water retention. Presumably, sequestration of cations in the vacuole reduces their toxic effects. Genetically engineered drought-and salt-tolerant plants could provide an avenue to the reclamation of farmlands lost to agriculture because of salinity and a lack of rainfall. S alt-and drought-tolerant plants maintain their turgor at low water potentials by increasing the number of solute molecules in the cell (1-3). The active transport of solutes depends on the proton gradients established by proton pumps. In plants, three distinct proton pumps generate proton electrochemical gradients across cell membranes. The P-type ATPase pumps cytoplasmic H Plant vacuoles constitute 40-90% of the total intracellular volume of a mature plant cell (5) and, in concert with the cytosol, generate the cell turgor responsible for growth and plant rigidity. Cell turgor depends on the activity of vacuolar H ϩ pumps that maintain the H ϩ electrochemical gradient across the vacuolar membrane, which permits the secondary active transport of inorganic ions, organic acids, sugars, and other compounds. The accumulation of these solutes is required to maintain the internal water balance (6).In principle, increased vacuolar solute accumulation could confer salt and drought tolerance. The sequestration of ions such as sodium could increase the osmotic pressure of the plant and at the same time reduce the toxic effects of this cation. Exposure to NaCl has been shown to induce the H ϩ transport activity of vacuolar pumps in both salt-tolerant (7, 8) and salt-sensitive plants (9). In principle, enhanced expression of either of the vacuolar proton pumps should increase the sequestration of ions in the vacuole by increasing the availability of protons. However, overexpression of the plant vacuolar H ϩ -ATPase would be difficult because it consists of many subunits, each of which would have to be overexpressed at the correct level in a single transgenic plant to achieve higher activity of the multisubunit complex (10). By contrast, the vacuolar H ϩ -pyrophosphatase of Arabidopsis is encoded by a single gene, AVP1 (11). AVP1 can generate a H Here we show that transgenic plants expressing higher levels of the vacuolar proton-pumping pyrophosphatase, AVP1, are more resistant to salt and drought than are wild-type plants. These resistance phenotypes are associated with increased internal stores of solutes. Materials and MethodsGe...

Overexpression of the Arabidopsis thaliana vacuolar H ؉ -pyrophosphatase (AVP1) confers salt tolerance to the salt-sensitive ena1 mutant of Saccharomyces cerevisiae. Suppression of salt sensitivity requires two ion transporters, the Gef1 Cl ؊ channel and the Nhx1 Na ؉ ͞H ؉ exchanger. These two proteins colocalize to the prevacuolar compartment of yeast and are thought to be required for optimal acidification of this compartment. Overexpression of AtNHX1, the plant homologue of the yeast Na ؉ ͞H ؉ exchanger, suppresses some of the mutant phenotypes of the yeast nhx1 mutant. Moreover, the level of AtNHX1 mRNA in Arabidopsis is increased in the presence of NaCl. The regulation of AtNHX1 by NaCl and the ability of the plant gene to suppress the yeast nhx1 mutant suggest that the mechanism by which cations are detoxified in yeast and plants may be similar.

The transport of auxin controls developmental events in plants. Here, we report that in addition to maintaining vacuolar pH, the H+-pyrophosphatase, AVP1, controls auxin transport and consequently auxin-dependent development. AVP1 overexpression results in increased cell division at the onset of organ formation, hyperplasia, and increased auxin transport. In contrast, avp1-1 null mutants have severely disrupted root and shoot development and reduced auxin transport. Changes in the expression of AVP1 affect the distribution and abundance of the P-adenosine triphosphatase and Pinformed 1 auxin efflux facilitator, two proteins implicated in auxin distribution. Thus, AVP1 facilitates the auxin fluxes that regulate organogenesis.

Chemiosmotic circuits of plant cells are driven by proton (H + ) gradients that mediate secondary active transport of compounds across plasma and endosomal membranes. Furthermore, regulation of endosomal acidification is critical for endocytic and secretory pathways. For plants to react to their constantly changing environments and at the same time maintain optimal metabolic conditions, the expression, activity and interplay of the pumps generating these H + gradients have to be tightly regulated. In this review, we will highlight results on the regulation, localization and physiological roles of these H + -pumps, namely the plasma membrane H + -ATPase, the vacuolar H + -ATPase and the vacuolar H + -PPase.

SUMMARYPhosphorus (P) is one of the essential nutrients for plants, and is indispensable for plant growth and development. P deficiency severely limits crop yield, and regular fertilizer applications are required to obtain high yields and to prevent soil degradation. To access P from the soil, plants have evolved high-and low-affinity Pi transporters and the ability to induce root architectural changes to forage P. Also, adjustments of numerous cellular processes are triggered by the P starvation response, a tightly regulated process in plants. With the increasing demand for food as a result of a growing population, the demand for P fertilizer is steadily increasing. Given the high costs of fertilizers and in light of the fact that phosphate rock, the source of P fertilizer, is a finite natural resource, there is a need to enhance P fertilizer use efficiency in agricultural systems and to develop plants with enhanced Pi uptake and internal P-use efficiency (PUE). In this review we will provide an overview of continuing relevant research and highlight different approaches towards developing crops with enhanced PUE. In this context, we will summarize our current understanding of root responses to low phosphorus conditions and will emphasize the importance of combining PUE with tolerance of other stresses, such as aluminum toxicity. Of the many genes associated with Pi deficiency, this review will focus on those that hold promise or are already at an advanced stage of testing (OsPSTOL1, AVP1, PHO1 and OsPHT1;6). Finally, an update is provided on the progress made exploring alternative technologies, such as phosphite fertilizer.

A defect in the yeast GEF1 gene, a CLC chloride channel homolog leads to an iron requirement and cation sensitivity. The iron requirement is due to a failure to load Cu 2؉ onto a component of the iron uptake system, Fet3. This process, which requires both Gef1 and the Menkes disease Cu 2؉

Engineering drought -resistant crop plants is a critically important objective. Overexpression of the vacuolar H ؉ -pyrophosphatase (H ؉ -PPase) AVP1 in the model plant Arabidopsis thaliana results in enhanced performance under soil water deficits. Recent work demonstrates that AVP1 plays an important role in root development through the facilitation of auxin fluxes. With the objective of improving crop performance, we expressed AVP1 in a commercial cultivar of tomato. This approach resulted in (i) greater pyrophosphate-driven cation transport into root vacuolar fractions, (ii) increased root biomass, and (iii) enhanced recovery of plants from an episode of soil water deficit stress. More robust root systems allowed transgenic tomato plants to take up greater amounts of water during the imposed water deficit stress, resulting in a more favorable plant water status and less injury. This study documents a general strategy for improving drought resistance of crops.root development ͉ biotechnology ͉ water deficit stress ͉ tomato