Salt cress (Thellungiella halophila) is a small winter annual crucifer with a short life cycle. It has a small genome (about 2 3 Arabidopsis) with high sequence identity (average 92%) with Arabidopsis, and can be genetically transformed by the simple floral dip procedure. It is capable of copious seed production. Salt cress is an extremophile native to harsh environments and can reproduce after exposure to extreme salinity (500 mM NaCl) or cold to 215°C. It is a typical halophyte that accumulates NaCl at controlled rates and also dramatic levels of Pro (.150 mM) during exposure to high salinity. Stomata of salt cress are distributed on the leaf surface at higher density, but are less open than the stomata of Arabidopsis and respond to salt stress by closing more tightly. Leaves of salt cress are more succulent-like, have a second layer of palisade mesophyll cells, and are frequently shed during extreme salt stress. Roots of salt cress develop both an extra endodermis and cortex cell layer compared to Arabidopsis. Salt cress, although salt and cold tolerant, is not exceptionally tolerant of soil desiccation. We have isolated several ethyl methanesulfonate mutants of salt cress that have reduced salinity tolerance, which provide evidence that salt tolerance in this halophyte can be significantly affected by individual genetic loci. Analysis of salt cress expressed sequence tags provides evidence for the presence of paralogs, missing in the Arabidopsis genome, and for genes with abiotic stressrelevant functions. Hybridizations of salt cress RNA targets to an Arabidopsis whole-genome oligonucleotide array indicate that commonly stress-associated transcripts are expressed at a noticeably higher level in unstressed salt cress plants and are induced rapidly under stress. Efficient transformation of salt cress allows for simple gene exchange between Arabidopsis and salt cress. In addition, the generation of T-DNA-tagged mutant collections of salt cress, already in progress, will open the door to a new era of forward and reverse genetic studies of extremophile plant biology.Salinity is a severe and increasing constraint on the productivity of agricultural crops. High concentrations of salts in the soil have a strong inhibitory effect on the growth and harvestable yield of all crop species. Secondary salinization significantly impairs crop production on at least 20% of irrigated land worldwide (Ghassemi et al., 1995), and irrigated agriculture contributes more than 30% of global agricultural production (Hillel, 2000). Salinization of arable land arising from poor water management has led to the decline of past civilizations, and it threatens the long-term sustainability of many current large-scale irrigation systems, especially those in Asia (Sharma and Goyal, 2003). Soil salinity almost always originates from previous exposure to seawater (Flowers et al., 1986). Although it is believed that, for most of the Earth's history, the salt level of the oceans was much lower than at present (Serrano et al., 1997), all plant spec...

We have identified a T-DNA insertion mutation of Arabidopsis (ecotype C24), named sto1 (salt tolerant), that results in enhanced germination on both ionic (NaCl) and nonionic (sorbitol) hyperosmotic media. sto1 plants were more tolerant in vitro than wild type to Na 1 and K 1 both for germination and subsequent growth but were hypersensitive to Li 1 . Postgermination growth of the sto1 plants on sorbitol was not improved. Analysis of the amino acid sequence revealed that STO1 encodes a 9-cis-epoxicarotenoid dioxygenase (similar to 9-cis-epoxicarotenoid dioxygenase GB:AAF26356 [Phaseolus vulgaris] and to NCED3 GB:AB020817 [Arabidopsis]), a key enzyme in the abscisic acid (ABA) biosynthetic pathway. STO1 transcript abundance was substantially reduced in mutant plants. Mutant sto1 plants were unable to accumulate ABA following a hyperosmotic stress, although their basal ABA level was only moderately altered. Either complementation of the sto1 with the native gene from the wild-type genome or supplementation of ABA to the growth medium restored the wild-type phenotype. Improved growth of sto1 mutant plants on NaCl, but not sorbitol, medium was associated with a reduction in both NaClinduced expression of the ICK1 gene and ethylene accumulation. Osmotic adjustment of sto1 plants was substantially reduced compared to wild-type plants under conditions where sto1 plants grew faster. The sto1 mutation has revealed that reduced ABA can lead to more rapid growth during hyperionic stress by a signal pathway that apparently is at least partially independent of signals that mediate nonionic osmotic responses.

SummaryArabidopsis TIP1;1 (cTIP) is a member of the tonoplast family of aquaporins (AQP). Using RNA interference (RNA i ) we reduced TIP1;1 to different extent in various lines. When most severely affected, miniature plants died, a phenotype partially complemented by the TIP1;1 homolog McMIP-F. Less severely affected lines produced small plants, early senescence, and showed lesion formation. The relative water content in TIP1;1 RNAi plants was not significantly affected. Global expression profiling suggested a disturbance in carbon metabolism in RNAi lines with upregulated transcripts for functions in carbon acquisition and respiration, vesicle transport, signaling and transcription, and radical oxygen stress. Metabolite profiles showed low glucose, fructose, inositol, and threonic, succinic, fumaric, and malic acids, but sucrose levels were similar to WT. Increased amounts were found for raffinose and several unknown compounds. TIP1;1 RNAi plants also contained high starch and apoplastic carbohydrate increased. A GFP-TIP1;1 fusion protein indicated tonoplast location in spongy mesophyll cells, and high signal intensity in palisade mesophyll associated with vesicles near plastids. Signals in vascular tissues were strongest not only in vesicle-like structures but also outlined large vacuoles. Compromised routing of carbohydrate and lack of sucrose provision for cell-autonomous functions seems to characterize this RNAi phenotype. We suggest a function for TIP1;1 in vesicle-based metabolite routing through or between pre-vacuolar compartments and the central vacuole. Phenotype and expression characteristics support a view of TIP1;1 functioning as a marker for vesicles that are targeted to the central vacuole.

A hyper-osmotically sensitive mutant of Arabidopsis thaliana, designated hos3-1 (high expression of osmotically responsive genes), was identified based on its hyper-luminescence of RD29A:LUC promoter fusion plants upon treatment with NaCl and ABA. These responses implicate the disrupted gene as a direct or indirect negative regulator of the RD29A stress-responsive pathway. By sequencing the flanking regions of the T-DNA borders, it was determined that the disrupted gene is at locus At4g36830, annotated as encoding a putative protein with high homology to CIG30 (ELO2/FEN1). CIG30 has been implicated in synthesis of very long chain fatty acids (VLCFA), which are essential precursors for sphingolipids and ceramides. Altered stress responses characteristic of ABA-hypersensitivity, including reduced root growth inhibition and reduced germination with ABA treatment and reduced water loss from leaves, were exhibited by allelic hos3-1 and hos3-2 mutants. The hos3-2 mutant is partially suppressed in its transcript abundance and is inherited as a recessive trait. Further, the HOS3 ORF under the control of the 35SCaMV promoter restored wild-type NaCl- and ABA-root growth sensitivity as well as RD29A:LUC luminescence in mutant plants. We also show here that the HOS3 wild-type gene functionally complements the sensitivity of elo2 and elo3 yeast mutants to monensin. Furthermore, both hos3-1 and hos3-2 alleles shared increased sensitivity to the herbicide Metolachlor, which inhibits acyl chain elongation in synthesis of VLCFA, and HOS3 functionally complemented both elo2 and elo3 and restored levels of VLCFA. Together, these data establish that HOS3 inhibits ABA-mediated stress responses and implicate the VLCFA pathway and products as control points for several aspects of abiotic stress signaling and responses. The results also provide support for a role of ceramide in the control of stomatal behavior.

Increasing demand for limited water supplies in populated arid regions over the next decade may require implementation of new water-use practices. Eliminating use of high-quality water for landscape irrigation by using low-quality water delivered through secondary systems is an ideal option for conserving potable water. However, irrigation of woody landscape plants using waters high in inorganic salts may adversely affect soil fertility, structure, plant growth and appearance. Twelve woody ornamentals commonly used in landscapes in Salt Lake County, Utah, were treated with three blends of Utah Lake and Provo River water to assess the quality of plants produced. Three irrigation treatments, designated high-, medium-, and low-quality water were blended to maintain sodium concentrations of 15, 80 and 120 mg/liter respectively. Soils irrigated with medium-and low-quality water developed significantly higher adjusted sodium absorption ratio (SAR) and salinity than soils irrigated with high quality water and the effect varied with time. Except for four species, medium-and low-quality water did not significantly lower scores for plant appearance. Results of this two-year study support development of secondary water systems and use of lower-quality water for landscape irrigation.