Important Roles of the DRE Binding Proteins During Drought and Cold Stresses

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Two cDNA clones that encode DRE binding proteins, DREB1A and DREB2A, were isolated by using the yeast one hybrid screening technique.14 The deduced amino acid sequences

Yeast One Hybrid

Fig. 20.2. Signal transduction pathways between initial dehydration stress signal and gene expression. There are at least four signal transduction pathways: two are ABA dependent (I and II) and two are ABA independent (III and IV). Protein synthesis is necessary for one of the ABA-dependent signal pathways (I). ABRE is involved in one of the ABA-dependent pathways (II). In one of the ABA-independent pathways, DRE is involved in the regulation of genes not only by drought and salt but also by cold stress (IV). Another ABA-independent pathway is controlled by drought and salt, but not by cold (III).

Fig. 20.2. Signal transduction pathways between initial dehydration stress signal and gene expression. There are at least four signal transduction pathways: two are ABA dependent (I and II) and two are ABA independent (III and IV). Protein synthesis is necessary for one of the ABA-dependent signal pathways (I). ABRE is involved in one of the ABA-dependent pathways (II). In one of the ABA-independent pathways, DRE is involved in the regulation of genes not only by drought and salt but also by cold stress (IV). Another ABA-independent pathway is controlled by drought and salt, but not by cold (III).

of DREB1A and DREB2A showed significant sequence similarity with the conserved DNA binding domains found in the EREBP and APETALA2 proteins that function in ethylene-responsive expression and floral morphogenesis, respectively.15,16 Each DREB protein contained a basic region in its N-terminal region that might function as a nuclear localization signal and an acidic C-terminal region that might act as an activation domain for transcription. These data suggest that each DREB cDNA encodes a DNA binding protein that might function as a transcriptional activator in plants.

The ability of the DREB1A and DREB2A proteins expressed in Escherichia coli to bind the wild type or mutated DRE sequences was examined using the gel retardation method.14 The results indicate that the binding of these two proteins to the DRE sequence is highly specific. To determine whether the DREB1A and DREB2A proteins are capable of trans-activating DRE-dependent transcription in plant cells, we performed transactivation experiments using protoplasts prepared from Arabidopsis leaves. Coexpression of the DREB1A or DREB2A proteins in protoplasts transactivated the expression of the GUS reporter gene. These results suggest that DREB1A and DREB2A proteins function as transcription activators involved in the cold-and dehydration-responsive expression, respectively, of the rd29A gene (Fig. 20.3).14

We isolated cDNA clones encoding two DREB1A homologs (named DREB1B and DREB1C). The DREB1B clone was identical to CBF1.17 We also isolated cDNA clones encoding a DREB2A homolog and named it

DREB2B. Expression of the DREB1A gene and its two homologs was induced by low temperature stress, whereas expression of the DREB2A gene and its single homolog was induced by dehydration.14,18 These results indicate that two independent families of DREB proteins, DREB1 and DREB2, function as trans-acting factors in two separate signal transduction pathways under low temperature and dehydration conditions, respectively (Fig. 20.3).14

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