Biochemical and Molecular Responses of Peanut to Drought Stress and Their Role in Aflatoxin Contamination
Drought stress severely affects yield and predisposes peanut to preharvest aflatoxin contamination. Due to the carcinogenic nature of aflatoxins, elimination of aflatoxin production is the area of focus of many peanut research programs around the world. However, no known peanut genotype appears to be resistant to drought-induced pre-harvest aflatoxin contamination. Hence, developing a drought-tolerant cultivar appears to be the choice for reducing aflatoxin contamination of peanut. However, success in this effort has been slow due to poor understanding of the mechanism of drought/aflatoxin tolerance and lack of specific markers to assist in the selection of drought-tolerant peanut germplasm. Breeding for drought tolerant lines is one of the strategies in developing aflatoxin-tolerant peanut lines which is in progress in some of the peanut breeding programs. The prevailing method of screening peanut genotypes under field conditions is unreliable, expensive and time consuming. Hence, peanut breeders have expressed a desire at the national meetings for the identification of biochemical/molecular markers linked to drought/aflatoxin tolerance for expediting the development of a drought/aflatoxin-tolerant peanut cultivar. These markers will be helpful in developing an easy and reliable technique for screening peanut germplasm, and identify drought-tolerant lines exhibiting resistance to aflatoxin production. DNA markers have been used extensively in many crop plants in recent years to detect polymorphism among individuals, to construct genetic maps and to tag genes. However, the probability of identifying DNA marker/s linked to drought tolerance in peanut is extremely rare, as the most commonly used methods to detect the DNA polymorphism, showed very little or no polymorphism in peanut, especially among cultivated lines of Arachis hypogaea (Halward et al. 1991, 1992). Hence, the alternative is to monitor changes in gene expression following water stress to identify differentially expressed transcripts and gene products, and correlate the levels of expression with the degree of drought tolerance. Hypothesis: Drought stress is reported to alter gene expression. Some of the stress induced transcripts may be specific for either drought tolerance or susceptibility. We propose to test this hypothesis by detecting changes in mRNA transcripts by RNA fingerprinting using Differential Display Reverse Transcribed Polymerase Chain Reaction (DDRT-PCR),and monitoring qualitative and quantitative changes in selected plant metabolite levels. Detection of differentially expressed transcripts and gene products (metabolites) between drought-susceptible and drought-tolerant genotypes can be a useful tool in identifying the genotypes tolerant to drought and aflatoxin production.
The primary objective of the proposed research is to determine differences in the biochemical and genetic responses of drought-tolerant and drought-susceptible peanut genotypes to drought stress. Using the technique of Differential Display Reverse Transcribed-PCR, we would identify mRNA transcripts that are up- or down-regulated due to drought stress, followed by further characterization of these transcripts to identify gene/s related to drought tolerance. In addition, differences in the composition of selected metabolite levels between the drought-tolerant and -susceptible genotypes following drought stress will be determined. Studies will also include monitoring differential plant response to Aspergillus infestation by drought-tolerant and -susceptible peanut genotypes. The gene/s thus identified can be incorporated into commercial cultivars to determine the impact of genes/product/s on drought tolerance characteristics of the transgenic line. Such a cultivar will enhance farmer’s profitability, protect consumer health and export value of peanut crop.