DEVELOPMENT, HORMONAL, AND PATHOGENESIS-RELATED REGULATION OF THE TOBACCO CLASS-I BETA-1,3-GLUCANASE-B PROMOTER

The class I beta-1,3-glucanases are antifungal vacuolar proteins impli cated in plant defense that show developmental, hormonal, and pathogen esis-related regulation. The tobacco enzymes are encoded by a small ge ne family with members derived from ancestors related to the present-d ay species Nicotiana sylvestris and N. tomentosiformis, We studied the expression in transgenic tobacco plants of a chimeric beta-glucuronid ase (GUS) reporter gene fused to 1.6 kb of upstream sequence of the to bacco class I beta-1,3-glucanase B (GLB) gene, which is of N. tamentos iformis origin. Expression of the GUS reporter gene and the accumulati on of class I beta-1,3-glucanase and its mRNA showed very similar patt erns of regulation. In young seedlings the reporter gene was expressed in the roots. In mature tobacco plants it was preferentially expresse d in lower leaves and roots and was induced in leaves by ethylene trea tment and by infection with tobacco mosaic virus (TMV). Furthermore, i t was down-regulated in cultured leaf discs by combinations of the hor mones auxin and cytokinin. Histological studies of GUS activity showed that the GLB promoter shows highly localized expression in roots of s eedlings. It is also expressed in a ring of cells around necrotic lesi ons induced by TMV infection, but not in cells immediately adjacent to the lesions or in the lesions themselves. The results of deletion ana lyses suggest that multiple positive and negative elements in the GLB promoter regulate its activity. The region from -1452 to -1193 contain ing two copies of the heptanucleotide AGCCGCC, which is highly conserv ed in plant-stress and defense-related genes, is necessary for high le vel expression in leaves. Additional regions important for organ-speci fic and regulated expression were: -568 to -402 for ethylene induction of leaves; -402 to -211 for expression in lower leaves and cultured l eaf discs and for TMV induction of leaves; and -211 to -60 for express ion in roots.