Expressed sequence tag (EST) libraries for cultivated peanut (Arachis hypogaea L.) were developed from two cDNA libraries constructed by means of mRNA prepared from leaves of peanut line C34‐24 (resistant to leaf spots and Tomato spotted wilt virus) and immature pods of peanut line A13 (tolerant to drought stress and preharvest aflatoxin contamination). Randomly selected cDNA clones were partially sequenced to generate a total of 1825 ESTs, 769 from the C34‐24 cDNA library and 1056 from the A13 cDNA library, in which 536 and 769 unique ESTs were identified, respectively. Results of BLASTx search showed that 52.8% of the ESTs from leaf tissue and 78.6% of the ESTs from the pod tissue have homology to genes of known function. Approximately 27.3 and 22.1% of ESTs matching homologous sequences in dbEST of GenBank on the basis of BLASTn algorithm have unknown functions. The ESTs were queried against MIPS functional catalog criteria and sorted according to putative function into 15 categories. A total of 1345 ESTs have been released to GenBank1 Four hundred unigenes have been selected from these ESTs and arrayed on glass slides for gene expression analysis, and 44 EST‐derived simple sequence repeat (SSR) markers have been characterized for cultivated peanut, in which over 20% of the SSRs produced polymorphic markers among 24 cultivated peanut genotypes. This is the first report of ESTs in cultivated peanut, and further characterization of resistance and stress genes may explain mechanisms functioning in these two peanut lines.

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Late leaf spot disease caused by Cercosporidium personatum is one of the most destructive foliar diseases of peanut (Arachis hypogaea) worldwide. The objective of this research was to identify resistance genes in response to leaf spot disease using microarray and real-time polymerase chain reaction (PCR). To identify transcripts involved in disease resistance, we studied the gene expression profiles in two peanut genotypes, resistant or susceptible to leaf spot disease, using cDNA microarray containing 384 unigenes selected from two expressed sequenced tag (EST) cDNA libraries challenged by abiotic and biotic stresses. A total of 112 spots representing 56 genes in several functional categories were detected as up-regulated genes (log(2) ratio > 1). Seventeen of the top 20 genes, each matching gene with known function in GenBank, were selected for validation of their expression levels using real-time PCR. The two peanut genotypes were also used to study the functional analysis of these genes and the possible link of these genes to the disease resistance trait. Microarray technology and real-time PCR were used for comparison of gene expression. The selected genes identified by microarray analysis were validated by real-time PCR. These genes were more greatly expressed in the resistant genotype as a result of response to the challenge of C. personatum than in the susceptible genotype. Further investigations are needed to characterize each of these genes in disease resistance. Gene probes could then be developed for application in breeding programs for marker-assisted selection.

Plant caffeic acid 3-O-methyltransferase (COMT) has been implicated in the lignin biosynthetic pathway through catalyzing the multi-step methylation reactions of hydroxylated monomeric lignin precursors. However, genetic evidence for its function in plant disease resistance is poor. Sharp eyespot, caused primarily by the necrotrophic fungus Rhizoctonia cerealis, is a destructive disease in hexaploid wheat (Triticum aestivum L.). In this study, a wheat COMT gene TaCOMT-3D, is identified to be in response to R. cerealis infection through microarray-based comparative transcriptomics. The TaCOMT-3D gene is localized in the long arm of the chromosome 3D. The transcriptional level of TaCOMT-3D is higher in sharp eyespot-resistant wheat lines than in susceptible wheat lines, and is significantly elevated after R. cerealis inoculation. After R. cerealis inoculation and disease scoring, TaCOMT-3D-silenced wheat plants exhibit greater susceptibility to sharp eyespot compared to unsilenced wheat plants, whereas overexpression of TaCOMT-3D enhances resistance of the transgenic wheat lines to sharp eyespot. Moreover, overexpression of TaCOMT-3D enhances the stem mechanical strength, and lignin (particular syringyl monolignol) accumulation in the transgenic wheat lines. These results suggest that TaCOMT-3D positively contributes to both wheat resistance against sharp eyespot and stem mechanical strength possibly through promoting lignin (especially syringyl monolignol) accumulation.