Mutants able to germinate and perform early growth in medium containing a high NaCl concentration were identified during the course of two independent screenings and named salt resistant (sre) and salobreño (sañ). The sre and sañ mutants also were able to germinate in high-osmoticum medium, indicating that they are osmotolerant in a germination assay. Complementation analyses revealed that sre1-1, sre1-2, sañ3-1, and sañ3-2 were alleles of the abscisic acid (ABA) biosynthesis ABA2 gene. A map-based cloning strategy allowed the identification of the ABA2 gene and molecular characterization of four new aba2 alleles. The ABA2 gene product belongs to the family of short-chain dehydrogenases/reductases, which are known to be NAD- or NADP-dependent oxidoreductases. Recombinant ABA2 protein produced in Escherichia coli exhibits a K_m value for xanthoxin of 19 μM and catalyzes in a NAD-dependent manner the conversion of xanthoxin to abscisic aldehyde, as determined by HPLC–mass spectrometry. The ABA2 mRNA is expressed constitutively in all plant organs examined and is not upregulated in response to osmotic stress. The results of this work are discussed in the context of previous genetic and biochemical evidence regarding ABA biosynthesis, confirming the xanthoxin→abscisic aldehyde→ABA transition as the last steps of the major ABA biosynthetic pathway.

Overexpression of a yeast gene, HAL2, allows the cells to tolerate higher than normal extracellular salt concentrations. HAL2 encodes a 3′(2′)5′-bisphosphate nucleotidase that serves to remove the end products of sulfate transfer during cellular metabolism. The enzyme is inhibited by lithium and sodium and is activated by potassium. Metabolic systems that are sensitive to salt, as well as those governing osmolyte synthesis and ion transport, offer routes by which genetic engineering can be used to improve the tolerance of various organisms to salt.

Summary: We have isolated two Arabidopsis thaliana genes, AtHAL3a and AtHAL3b, showing homology with HAL3, a yeast protein which regulates the cell cycle and tolerance to salt stress through inhibition of the PPZ1 type‐1 protein phosphatase. Expression of AtHAL3a in yeast hal3 mutants partially complements their LiCl sensitivity, suggesting possible conserved functions between both proteins. AtHAL3a and AtHAL3b are induced by salt stress and AtHAL3a is the most expressed in non‐stressed plants, particularly in seeds. In situ hybridization demonstrates enrichment of AtHAL3a mRNA in seed embryos and in the vascular phloem of different plant tissues. AtHAL3 proteins show striking homology with a group of proteins found in fungi, plants and animals and some homology with a large family of prokaryotic flavoproteins. Recombinant AtHAL3a protein purified from Escherichia coli was yellow because it contained a non‐covalently bound chromophore revealed as flavin mononucleotide. Trans‐ genic Arabidopsis plants, with gain of AtHAL3a function, show altered growth rates and improved tolerance to salt and osmotic stress.