The Brassica napus extA extensin gene is expressed in regions of the plant subject to tensile stresses

The expression of extA, an extensin gene from Brassica napus L. (oilseed rape) was examined in transgenic Nicotiana tabacum L. (tobacco) and untransformed Brassica juncea L. and B. napus tissues. Northern analysis showed that this gene maintained its normal pattern of expression when transferred to tobacco. In transgenic tobacco plants containing an extA promoter/-glucuronidase coding sequence fusion, expression of extA was detected in the external and internal phloem of the main stem. High expression levels were seen in cortical parenchyma cells at the point where the axillary flowering branch joined the main stem. Expression was greatest in regions where the maximum tensile stress would seem to be exerted on the main stem by the weight of the axillary branch. It was confirmed that this expression pattern was due to tensile stress by using weights to induce expression of the fusion gene in axillary flowering stalks. In B. juncea pods, in-situ hybridisation studies showed that the extensin gene was strongly expressed in cells of the carpel walls within which considerable tensile stresses develop.Abbreviations GUS -glucuronidase - X-GLUC 5-bromo-4-chloro-3-indolyl glucuronide - SSC saline sodium citrate We thank the Biotechnology and Biological Sciences Research Council for funding this research, and Mrs. W. Grail for her expertise in histochemical technique. Ms. J. Spence is the recipient of a Biotechnology and Biological Sciences Research Council post-graduate studentship award.