The tobacco Ext 1.4 extensin gene family is not regulated by auxins in cells proliferating under hormone control

It was previously shown that the tobacco Ext 1.4 extensin gene is expressed in cells proliferating under hormone control and during lateral root initiation. Now, the regulation of expression of the Ext 1.4 gene family is analyzed in response to the auxins 2,4-dichlorophenoxyacetic acid (2,4-D) and 1-naphthaleneacetic acid (NAA). On the one hand, it is shown that there is no accumulation of Ext 1.4 mRNAs after short periods of incubation of roots or leaf strips in the presence of either 2,4-D or NAA. On the other, germination of transgenic seeds carrying the (–1331/+46)Ext 1.4/β-glucuronidase construct on a 2,4-D-containing medium does not lead to ectopic expression of the chimeric gene. Altogether, these results demonstrate that the genes in Ext 1.4 family are not early auxin-responsive genes. They may be late auxin-responsive genes or their induction in proliferating cells may be the consequence of a higher requirement of structural proteins for the formation of their particular cell walls.