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The cotton sesquiterpene cyclase,(+)-δ-cadinene synthase,is encoded by a gene family,which can be divided into two subfamilies:CAD1-A and CAD1-C.The gene CAD1-A was isolated from G.arboreum.In situ hybridization performed on seven-day-old cotton seedlings localized transcripts of both the CAD1-A and CAD1-C mainly in lateral root primordium and apical ground meristem,vascular tissues of emerging lateral roots,and also in procambium and some subepidermal cells of the hypocotyl.The CAD1-A promoter showed a similar tissue-specificity in transgenic tobacco plants.Histochemistry showed occurrence of sesquiterpene aldehydes in outer cells of the lateral root tips,as well as in pigment glands.The CAD1 gene expression in G.arboreum seedlings and the spatial pattern of sesquiterpene biosynthesis constitute a chemical defense machinery in cotton seedlings.