Beta-1, 3-glucanase and chitinase transgenes in hybrids show distinctive and independent patterns of posttranscriptional gene silencing.

Nicotiana sylvestris Speg. & Comes transformed with a tobacco class-I beta-1,3-glucanase (GLU I ) cDNA driven by CaMV 35S RNA expression signals exhibits posttranscriptional gene silencing (PTGS) which is triggered between the cotyledon and two-leaf stages of seedling development and is postmeiotically reset to the high-expressing state during seed development. The incidence of GLU I PTGS in sibling plants differed for the two different transformants tested and increased with the number of T-DNA loci. Comparison of host class-I and class-II beta-1,3-glucanase gene expression suggests that a similarity of 60-70% in the coding-region is required for PTGS of the homologous host genes. The GLU I transformants exhibited a spatial gradient in PTGS, in which expression of the silent phenotype gradually increased in successive leaves toward the bottom of the plant. In contrast, transformants carrying an unrelated tobacco class I chitinase (CHN I) cDNA in the same expression vector exhibited discontinuous patterns of PTGS with adjacent high-expressing and silent leaves. The GLU I- and CHN I-specific patterns were maintained in hybrids homozygous for both T-DNA's indicating that two different transgenes present in the same genome can exhibit independent and distinctive patterns of PTGS. This implies that the nature of the transgene rather than a general pre-pattern of competence for PTGS or propagation of the silent state are important for pattern determination.