We generated transgenic plants in which DREB1A or DREB2A cDNAs were introduced to overproduce DREB proteins to analyze the effects of overproduction of DREB1A and DREB2A proteins on the expression of the target rd29A gene. Arabidopsis plants were transformed with vectors carrying fusions of the enhanced CaMV 35S promoter and the DREB1A (35S:DREB1A) or DREB2A (35S: DREB2A) cDNAs in the sense orientation.14,19 All of the transgenic plants carrying the 35S:DREB1A transgene (the 35S:DREB1A plants) showed growth retardation phenotypes under normal growth conditions. The 35S: DREB1A plants showed variations in pheno-typic changes in growth retardation that may have been due to the different levels of expression of the DREB1A transgenes for the position effect.14
To analyze whether overproduction of the DREB1A protein caused the expression of the target gene in unstressed plants, we compared the expression of the rd29A gene in control plants carrying the pBI121 vector. Transcription of the rd29A gene was low in the unstressed wild type plants, but high in the unstressed 35S:DREB1A plants.14 The level of the rd29A transcripts under the unstressed control condition was found to depend on the level of the DREB1A transcripts.14 To analyze whether overproduction of the DREB1A protein caused the expression of other target genes, we evaluated the expression of its target stress induc-ible genes. In the 35S:DREB1A plants the kinl, cor6.6/kin2, cor15a, cor47/rd17and erdlOgenes were expressed strongly under unstressed control conditions, as was the rd29A gene.3,20
In contrast, the transgenic plants carrying the 35S:DREB2A transgene (the 35S:DREB2A
plants) showed little phenotypic change. In 35S:DREB2A transgenic plants, the rd29A mRNA did not accumulate significantly, although the DREB2A mRNA accumulated even under unstressed conditions.14 Expression of the DREB2A protein is not sufficient for the induction of the target stress inducible gene. Modification, such as phosphorylation of the DREB2A protein, seems to be necessary for its function in response to dehydration (Fig. 20.3). However, DREB1 proteins can function without modification.
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