High concentrations of Na+ in saline soils inhibit plant growth and reduce agricultural productivity. We report here that CaMV 35S promoter driven overexpression of the Arabidopsis thaliana SOS1 gene, which encodes a plasma membrane Na+/H+ antiporter, improves plant salt tolerance in A. thaliana. Transgenic plants showed substantial upregulation of SOS1 transcript levels upon NaCl treatment, suggesting post-transcriptional control of SOS1 transcript accumulation. In response to NaCl treatment, transgenic plants overexpressing SOS1 accumulated less Na+ in the xylem transpirational stream and in the shoot. Undifferentiated callus cultures regenerated from the transgenic plants were also more tolerant of salt stress, which was correlated with reduced Na+ content in the transgenic cells. These results show that improved salt tolerance could be achieved by limiting Na+ accumulation in plant cells.

To begin to determine which genes are essential for salt tolerance in higher plants, we identified four salt-hypersensitive mutants of Arabidopsis by using a root-bending assay on NaCI-containing agar plates. These mutants (sosl-7, sosl-2, sosl-3, and sosl-4) are allelic to each other and were caused by single recessive nuclear mutations. The SOSl gene was mapped to chromosome 2 at 29.5 f 6.1 centimorgans. The mutants showed no phenotypic changes except that their growth was >20 times more sensitive to inhibition by NaCI. Salt hypersensitivity is a basic cellular trait exhibited by the mutants at all developmental stages. The sosl mutants are specifically hypersensitive to Na+ and Li+. The mutants were unable to grow on media containing low levels (below 4 mM) of potassium. Uptake experiments using B6Rb showed that sosl mutants are defective in high-affinity potassium uptake. sosl plants became deficient in potassium when treated with NaCI. The results demonstrate that potassium acquisition is a critical process for salt tolerance in glycophytic plants. INTRODUCTIONPlants experience constant fluctuations in the availability and quality of soil water. The quality of soil water is influenced mostly by the concentrations of essential plant mineral nutrients as well as nonessential ions. Nonessential ions, such as Na+, when present in high concentrations in the soil, adversely affect plant growth (Greenway and Munns, 1980). Excessive Na+ in the soil, often referred to as salinity, is a major problem for a substantial portion of agricultura1 land in the world (Epstein et al., 1980). Recently, much effort has been directed toward understanding the molecular and cellular mechanisms by which plants tolerate salinity stress, with the eventual goal of improving salt tolerance of crop plants (Binzel and Reuveni, 1994). One important objective is to determine which genes are important for plant salt tolerance. A widely used approach has been to identify genes whose expression is regulated by salt stress. Many salt-regulated genes have been identified in the last decade. ldentification of these genes has permitted a better understanding of the complexity of salt tolerance in higher plants (Cushman et al., 1990;Bray, 1993;Serrano and Gaxiola, 1994). However, the function of most of the gene products in salt tolerance has been difficult to establish (Bray, 1993;Serrano and Gaxiola, 1994). This approach is also unsuitable for identifying low-abundance mRNAs and proteins that might have important regulatory roles in salt responses. Clearly, a genetic approach is needed to determine which genes are necessary for salt tolerance.Current address: Department of Plant Sciences, University of AriTo whom correspondence should be addressed. zona, Tucson, A 2 85721.Potassium is a major monovalent cationic essential nutrient. Potassium uptake plays a vital role in plant growth, development, stomatal movements, enzyme activation, and osmoregulation (Epstein, 1972;Kochian and Lucas, 1988). It is generally accepted that potassium uptake into ...

DEAD-box RNA helicases belong to an RNA helicase family that plays specific roles in various RNA metabolism processes, including ribosome biogenesis, mRNA splicing, RNA export, mRNA translation and RNA decay. This study investigated a DEAD-box RNA helicase, AtRH7/PRH75, in Arabidopsis. Expression of AtRH7/PRH75 was ubiquitous; however, the levels of mRNA accumulation were increased in cell division regions and were induced by cold stress. The phenotypes of two allelic AtRH7/PRH75-knockout mutants, atrh7-2 and atrh7-3, resembled auxin-related developmental defects that were exhibited in several ribosomal protein mutants, and were more severe under cold stress. Northern blot and circular reverse transcription-PCR (RT-PCR) analyses indicated that unprocessed 18S pre-rRNAs accumulated in the atrh7 mutants. The atrh7 mutants were hyposensitive to the antibiotic streptomycin, which targets ribosomal small subunits, suggesting that AtRH7 was also involved in ribosome assembly. In addition, the atrh7-2 and atrh7-3 mutants displayed cold hypersensitivity and decreased expression of CBF1, CBF2 and CBF3, which might be responsible for the cold intolerance. The present study indicated that AtRH7 participates in rRNA biogenesis and is also involved in plant development and cold tolerance in Arabidopsis.

DEAD-box RNA helicases are involved in RNA metabolism, including pre-mRNA splicing, ribosome biogenesis, RNA decay and gene expression. In this study, we identified a homolog of the RH36 gene, AtRH36, which encodes a DEAD-box protein in Arabidopsis thaliana. The gene was expressed ubiquitously throughout the plant. The AtRH36 fused to green fluorescent protein was localized in the nucleus. Homozygosity for the Arabidopsis atrh36 mutants, atrh36-1 and atrh36-2, could not be obtained. Progeny of selfed Arabidopsis atrh36 heterozygote plants were obtained at a heterozygote to wild-type ratio of 1 : 1, which suggested that the AtRH36 gene was involved in gametogenesis. Therefore, we performed a reciprocal cross to determine whether AtRH36 was involved in female gametophyte development. Female gametogenesis was delayed in atrh36-1, and asynchronous development of the female gametophytes was found within a single pistil. Knock-down of AtRH36 gave a pleiotropic phenotype and led to the accumulation of unprocessed 18S pre-rRNA. These results suggest that AtRH36 is essential for mitotic division during female gametogenesis and plays an important role in rRNA biogenesis in Arabidopsis.

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