The tobacco extensin gene Ext 1.4 is expressed in cells submitted to mechanical constraints and in cells proliferating under hormone control

The promoter region of a Nicotiana tabacum extensin gene (Ext 1.4) was stud ied in tobacco transgenic plants carrying Ext 1.4/GUS (beta-glucuronidase) chimeric genes. The pattern of expression could be defined and cis-regulato ry elements were localized in small regions of the promoter. In healthy pla nts, expression was essentially found in cells under mechanical stress, tha t is at the emergence of lateral roots, at the junction between stem and pe tiole and at the fusion of carpels. In roots of germinating plantlets, expr ession was found in the piliferous zone. In flowers, expression was found o n the one hand in the placenta, in the locular tissue of ovaries and in the zone of carpel fusion, and on the other hand in the connective tissue of a nthers, in mature and in germinating pollen. A developmental regulation dur ing seed germination, where the gene fusion is transiently expressed in the endosperm and in the root tip before its expression becomes similar to tha t found in mature plants has also been shown. The expression of the Ext 1.4 /GUS chimeric gene was also induced during cell proliferation under hormone control, for example, in response to Agrobacterium tumefaciens infection a nd in calli. However, when organogenesis occurred under hormone control, ex pression was never found in root or shoot primordia. Cis-regulatory element s important for expression of the Ext 1.4/GUS gene fusion in germinating se eds, in mature plants or in proliferating cells have been localized in the proximal promoter region whereas enhancer elements have been located furthe r upstream.