REGULATED EXPRESSION OF A WHEAT GERMIN GENE IN TOBACCO - OXALATE OXIDASE ACTIVITY AND APOPLASTIC LOCALIZATION OF THE HETEROLOGOUS PROTEIN

Wheat (Triticum aestivum) germin is a homopentameric glycoprotein whos e synthesis is allied with seed germination. Germin pentamers show an unusual resistance to dissociation and possess an oxalate oxidase (OxO ) activity. In order to increase our knowledge of germin gene expressi on, the function(s) of germin during development and possible uses in plant genetic engineering, an in vivo expression system is required. T o this end, a gene for germin, named gf-2.8, was studied by expressing either promoter-GUS fusions or the intact gene in transgenic tobacco (Nicotiana tabacum) plants. Heterologous gene transcription was monito red in vitro and in vivo by GUS or OxO activity and was found to occur in developing seeds and in seedlings. This transcription was stimulat ed by auxins, as would be expected because of the presence of putative auxin-responsive elements in the promoter of the gf-2.8 Auxin stimula tion also extended to young leaves since OxO activity could be detecte d in treated but not in untreated leaves. The biochemical characterist ics of wheat germin were also conserved in a transgenic host: the OxO activity was present under the form of a doublet co-migrating with ger min G and G' isoforms. Also, germin distributed between a soluble and an apoplastic fractions despite the fact that wheat cell wall substant ially differs from tobacco cell wall. Therefore, tobacco constitutes a suitable host for in vivo studies of this monocotyledon gene.