A gene coding for the cell wall hydroxyproline-rich glycoprotein extensin was isolated by constructing a size fractionated genomic library of oilseed rape (Brassica napus) in the plasmid vector pSK⁺. The library was screened with the coding sequence of the extA extensin gene. The isolated gene (extC) contains a coding sequence of 780 nucleotides specifying a protein of 259 amino acid residues. The protein contains the Ser-Pro-Pro-Pro-Pro repeat motif, which is characteristic of extensins. Southern blots of Hind Ill-digested oilseed rape genomic DNA hybridised against the extC probe revealed six hybridising fragments of sizes 2.5, 3.1, 3.4, 3.8, 7.7 and 10.7 kb. This suggests that extC is a member of an extensin multigene family ~resent in the oilseed rape genome which is distinct from the extA gene family. Expression studies showed that the extC gene was not expressed in the leaves, petioles, stems and roots of healthy plants. Application of a wounding stimulus and treatment of leaves by incubation in abscisic acid (ABA), sodium salicylate and methyl jasmonate (MeJ) solutions also failed to induce expression of the gene. In contrast, the extA gene was induced by treatment
of leaves with ABA, sodium salicylate and MeJ. Significant levels of extA transcripts were detected 12 h after leaves were treated with ABA, and stayed at a constant level for 36 h before increasing at 48 h. Treatment of leaves with sodium salicylate induced the accumulation of extA transcripts after 12 h and continued to increase until 48 h. In MeJ treated leaves, significant levels of extA transcripts were detected at 12 h, increased and reached a maximumat 24 h before decreasing at 36 h. These results suggest that the extC gene is regulated differently from the extA gene. extC may be transcriptionally activated by non-wounding stresses or may be expressed at developmentalstages and organ and tissue types which have not been identified.