In SATrends issues 62 (Jan 2006) and 75 (Feb 2007), we reported that when the DREB1A gene driven by the promoter of a stress-inducible gene, rd29A, was genetically introduced into groundnuts, it not only improved the transpiration efficiency, but also induced a positive root response under water stress conditions. Here we report that the expression of DREB1A transcription factor also manifests in a differential gene expression under drought stress, thereby, acting as a master switch to counter the effects of drought.
The total RNA from the leaves of five selected transgenic events expressing the DREB1A gene under stress were subjected to differential display of transcripts for comparing the expression profiles of transgenic and wild type groundnut plants under water stress. This enabled the detection of gene transcripts that were expressed in response to the stress. A total of 51 differentially expressed transcripts were identified, where 35 transcripts were newly expressed, 11 were up-regulated, and five were down-regulated. Analysis of the partial sequences of 40 cDNAs indicated that most were from abiotic stress responsive genes (SRGs) that could be classified broadly in four groups as follows:
- cDNAs showing similarity with drought responsive Arachis cDNAs.
- cDNAs showing similarity with expressed sequenced tags (ESTs) responding to biotic and abiotic stresses that may be from regulatory genes.
- cDNAs showing similarity with mitochondrial genes involved in alternative respiratory pathways that have been postulated to play a key role in plant stress responses.
- cDNAs showing low level of sequence similarity with stress responsive ESTs which might be considered novel stress inducible genes.
Nevertheless, only 17 cDNA clones showed a strong similarity with the published ESTs, thereby indicating that a majority of these may be novel. This would indicate that the expression of DREB1A in transgenic plants might influence the expression of these genes either directly or indirectly. Further analysis of the transcript levels may provide insights into the regulatory pathways of these genes.
Analysis of the newly expressed genes indicate that their products might function co-operatively to protect the cells from dehydration and may also play an important role in plant adaptive mechanisms under water stress conditions. Cloning and characterization of full-length cDNAs and promoter regions of the genomic sequences corresponding to the stress-regulated clones could provide further insights into the mechanism of expression of an individual gene, as well as its potential role in stress response. Future activities would involve the identification and functional analysis of these transcripts for use in both genetic engineering as well as marker-assisted selection for plant breeding.