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...

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.

Modulation of the activity of the molecular chaperone HSP90 has been extensively discussed as a means to alter phenotype in many traits and organisms. Such changes can be due to the exposure of cryptic genetic variation, which in some instances may also be accomplished by mild environmental alteration. Should such polymorphisms be widespread, natural selection may be more effective at producing phenotypic change in suboptimal environments. However, the frequency and identity of buffered polymorphisms in natural populations are unknown. Here, we employ quantitative genetic dissection of an Arabidopsis thaliana developmental response, hypocotyl elongation in the dark, to detail the underpinnings of genetic variation responsive to HSP90 modulation. We demonstrate that HSP90-dependent alleles occur in continuously distributed, environmentally responsive traits and are amenable to quantitative genetic mapping techniques. Furthermore, such alleles are frequent in natural populations and can have significant effects on natural phenotypic variation. We also find that HSP90 modulation has both general and allele-specific effects on developmental stability; that is, developmental stability is a phenotypic trait that can be affected by natural variation. However, effects of revealed variation on trait means outweigh effects of decreased developmental stability, and the HSP90-dependent trait alterations could be acted on by natural selection. Thus, HSP90 may centrally influence canalization, assimilation, and the rapid evolutionary alteration of phenotype through the concealment and exposure of cryptic genetic variation.cryptic variation ͉ morphological evolution

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