Abscisic acid (ABA) participates in the control of diverse physiological processes. The characterization of deficient mutants has clarified the ABA biosynthetic pathway in higher plants. Deficient mutants also lead to a revaluation of the extent of ABA action during seed development and in the response of vegetative tissues to environmental stress. Although ABA receptor(s) have not yet been identified, considerable progress has been recently made in the characterization of more downstream elements of the ABA regulatory network. ABA controls stomatal aperture by rapidly regulating identified ion transporters in guard cells, and the details of the underlying signalling pathways start to emerge. ABA actions in other cell types involve modifications of gene expression. The promoter analysis of ABA-responsive genes has revealed a diversity of cis-acting elements and a few associated trans-acting factors have been isolated. Finally, characterization of mutants defective in ABA responsiveness, and molecular cloning of the corresponding loci, has proven to be a powerful approach to dissect the molecular nature of ABA signalling cascades.

Four clones corresponding to Arabidopsis thaliana transcripts regulated by progressive drought stress were isolated. Abundance of the AtDi8, AtDi19 and AtDi21 mRNAs increased in both roots and leaves during progressive drought. The AtDr4 mRNA was expressed in a root-specific manner in regularly watered plants, and became undetectable under drought conditions. In all cases, the drought-induced modifications of mRNA abundance could be reversed by subsequent rehydration. The predicted AtDr4 protein displays extensive similarity to various members of the Künitz protein family, suggesting that AtDr4 might be a root-specific protease inhibitor. Of these four genes, only AtDi8 and AtDi21 responded to an exogenous supply of abscisic acid (ABA). Analysis of the ABA-deficient aba mutant demonstrated that endogenous ABA indeed participates in the drought regulation of these two transcripts. This ABA-dependent response was differentially affected in the various classes of ABA-insensitive Arabidopsis mutants. The AtDi19 and AtDr4 mRNAs both responded to drought in an ABA-independent manner, but at distinct thresholds of the progressive drought stress. Regulation of these four target genes by progressive drought stress thus appears to be mediated by at least three distinct signals, only one of which is ABA.

The role of the phytohormone abscisic acid (ABA) in the regulation of proline synthesis was investigated by following the expression of the At-P5S and At-P5R proline biosynthesis genes in Arabidopsis thaliana wild type, in an ABA-deficient aba1-1 mutant as well as in ABA-insensitive abi1-1 and abi2-1 mutants after ABA, cold and osmotic stress treatments. In wild-type and in ABA mutant seedlings, 50 microM ABA or osmotic stress treatment triggered expression of At-P5S, whereas At-P5R accumulation was scarcely detectable. Expression of either gene was mediated by endogenous ABA since transcript levels were similar in wild-type and in ABA-deficient mutant plants. Proline accumulated to a greater extent after osmotic stress than upon ABA or cold treatment. Thus, ABA-treated abi1-1 mutant plants accumulated less proline than the ABA-treated wild type. Upon salt stress, proline accumulated to a lesser extent in aba1-1 and abi1-1 mutant plants, suggesting an indirect role of ABA on proline accumulation during salt adaptation of the plant. These results indicate that the expression of the genes of the proline biosynthetic pathway is ABA independent upon cold and osmotic treatments, although their expression can be triggered by exogenously applied ABA. However, the endogenous ABA content may affect proline accumulation upon salt stress, suggesting post-transcriptional control of proline biosynthesis in response to NaCl.

Mutations at the ABIl (abscisic acid insensitive 1) locus of the plant Arabidopsis thaliana cause a reduction in sensitivity to the plant hormone abscisic acid. The sequence of ABZl predicts a protein composed of an N-terminal domain that contains motifs for an EF-hand Caz+-binding site, and a Cterminal domain with similarities to protein serinekhreonine phosphatases 2C. We report here two sets of experimental evidence that indicate that ABIl has typical protein phosphatase 2C activity. First, expression of the ABIl C-terminal domain partially complemented the temperature-sensitive growth defect of a Saccharomyces cerevisiae protein phosphatase 2C mutant. Second, recombinant proteins that contained the ABIl C-terminal domain displayed in vitro phosphatase activity towards 'ZP-labelled casein, and this activity displayed Mg2+ or Mn'+ dependence and okadaic acid insensitivity typical of protein phosphatases 2C. Characterisation of recombinant proteins that contained various portions of ABIl indicated that the putative EF-hand motif is unlikely to mediate Caz+ regulation of the ABIl phosphatase activity at physiological CaZ+ concentrations, and may represent an EF-hand analogue rather than an EF-hand homologue. The abil-l mutation appeared to cause significant reduction in the phosphatase activity of ABII. These results are discussed in relation to the dominant phenotype of abil-1 over the wild-type allele in plants, and to the possible role of ABIl in abscisic acid signalling.