Abscisic acid-dependent and -independent regulation of gene expression by progressive drought in Arabidopsis thaliana

Abstract

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 Arabidopsis ABSCISIC ACID-INSENSITIVE2 (ABI2) and ABI1 genes encode homologous protein phosphatases 2C involved in abscisic acid signal transduction.

“…Although a number of phenotypes between the abi7-7 and abi2-7 mutants are highly similar, the roles of their respective proteins in ABA signaling are unlikely to be completely redundant (Figure 7). lnduction of the RAB78 transcript by exogenous ABA in plantlets is similarly inhibited in theabil-7 and abi2-7 mutants ( Figure 5D; Gosti et al, 1995). Nevertheless, the ABA-dependent accumulation of this same transcript triggered by progressive drought in the roots of adult plants is severely inhibited in abil-7 but not in abi2-7 (Gosti et al, 1995).…”

“…They affect both seeds and vegetative tissues, suggesting that their gene products act in ABA signal transduction before major branch points that control tissue-specific cascades. Differential effects imparted by the abi7-7 and abi2-7 mutations are most evident in adaptive responses to drought by modifications of the root, a process known as drought rhizogenesis (Vartanian et al, 1994), and induction of certain genes (Gilmour and Thomashow, 1991;Gosti et al, 1995;de Bruxelles et al, 1996;Soderman et al, 1996). Nevertheless, these mutants also have several phenotypes in common.…”

“…In seed, both mutations reduce dormancy and the sensitivity of germination to the inhibitory effects of ABA (Koornneef et al, 1984;Finkelstein and Somerville, 1990). In vegetative tissues, both mutants display ABA-resistant seedling growth, abnormal stomatal regulation, and defects in various ABA-induced morphological and molecular responses (Koornneef et al, 1984;Finkelstein and Somerville, 1990;Schnall and Quatrano, 1992;Finkelstein, 1994b;Gosti et al, The cloning and characterization of the ABl7 gene (Leung et al, 1994;Meyer et al, 1994) revealed that it encodes a protein composed of a nove1 N-terminal segment and a C-terminal domain that is an active protein serine/threonine phosphatase 2C (PP2C) (Bertauche et al, 1996). In guard cells, ABI1 appears to relay, together with counteracting protein kinases, ABA signals to stomatal regulation in that defects in stomatal closure caused by the abi7-7 mutant gene can be partially suppressed by kinase antagonists (Armstrong et al, 1995).…”

Isolation, characterization, and chromosomal location of a gene encoding the Δ1‐pyrroline‐5‐carboxylate synthetase in Arabidopsis thaliana

“…thaliana ecotype Columbia. A 2-kb PCR fragment showing sequence similarity to the V aeonitifolia P5CS cDNA was cloned into pBluescript (Stratagene) and used as a probe to screen a )tZAP-cDNA library from drought-stressed roots [15]. Of 150,000 colonies screened, 10 positive clones were isolated and the clones containing the longest insert of 2.6 kb was converted to pBluescript SK(-) phagemid clone by in vivo excision according to the Statagene protocol.…”

Sugar/osmoticum levels modulate differential abscisic acid-independent expression of two stress-responsive sucrose synthase genes in Arabidopsis

“…It is not yet fully understood how plants can sense osmotic stress, but it is clear that abscisic acid (ABA) is involved in the expression of a number of stress-responsive genes [4]. However, studies conducted on ABA-deficient or ABA-insensitive mutants have indicated that some of these genes are expressed independently of ABA [5,6]. With regard to anoxia, it is known to quickly inhibit protein synthesis, except for a selected set of proteins called anaerobic proteins (' ANPs ') ; most of them are enzymes of glycolysis and fermentation [2].…”