Isolation and analysis of candidate ascochyta blight defence genes in chickpea. I. Generation and analysis of an expressed sequence tag (EST) library

Chickpea (Cicer arietinum L.) is the world's third-most important pulse crop, but a major limiting factor to production is a severe and destructive fungal disease known as ascochyta blight (Ascochyta rabiei (Pass.) Labrousse). A chickpea accession (ICC3996) resistant to ascochyta blight, but uncultivated due to poor agronomic characters, was used to generate an enriched library of EST sequences. The library, consisting of 1021 ESTs, was characterised by homology searches in public databases, where 571 (56%) showed significant homology to existing database entries. The ESTs were clustered and assembled into 516 unigenes, of which 4% were defence-related, encoding lignin and phytoalexin biosynthesis enzymes, pathogenesis-related proteins, signalling proteins, and putative defensive proteins. These unigenes may be involved in chickpea defence against ascochyta blight, and are discussed in detail. The generation of an EST library represents the first step in a functional genomics approach aimed at elucidating the function of genes involved in ascochyta blight resistance and the pathway of their action. Applications of the EST library to microarray expression studies and molecular mapping are discussed, with emphasis on the development of agronomically viable chickpea cultivars with durable and broad-spectrum resistance to ascochyta blight.