The tolerance to freezing and dehydration of the transgenic plant was analyzed using the 35S:DREB1A plants grown in pots at 22°C for 3 weeks. When plants were exposed to a temperature of -6°C for 2 days, returned to 22°C and grown for 5 days, all of the wild type plants died, whereas the 35S:DREB1A plants survived at high frequency.14,21 Freezing tolerance was correlated with the level of expression of the stress inducible genes under unstressed control conditions (Fig. 20.4; between 80 and 30% survival). 14, 21
To test whether the introduction of the DREB1A gene enhances tolerance to dehydration stress, we did not water the plants for 2 weeks. Although all of the wild type plants died within 2 weeks, between 40 and 20% of the 35S:DREB1A plants survived and continued to grow after rewatering. Drought tolerance was also dependent on the level of expression of the target genes in the 35S:DREB1A plants under unstressed conditions (Fig. 20.4).14,21
Overexpression of the DREB1A cDNA, driven by the constitutive 35S CaMV promoter in transgenic plants, activated strong expression of the target stress inducible genes under unstressed conditions, which, in turn, increased tolerance of freezing, salt and drought stresses.14,21 Jaglo-Ottosen et al reported that CBF1 overexpression also enhances freezing tolerance.22 However, the overexpression of stress inducible genes controlled by the DREB1A protein caused severe growth retardation under normal growth con-ditions.14,21
Fig. 20.3. A model of the induction of the rd29A gene and cis- and trans-acting elements involved in stress-responsive gene expression. Two cis-acting elements, DRE and ABRE, are involved in the ABA-independent and ABA-responsive induction of rd29A, respectively. Two types of different DRE binding proteins, DREB1 and DREB2, separate two different signal transduction pathways in response to cold and drought stresses, respectively. DREB1s/CBF1 are transcriptionally regulated whereas DREB2s are controlled posttranslationally as well as transcriptionally. ABRE binding proteins encode bZIP transcription factors.
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