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 exhi bits posttranscriptional gene silencing (PTGS) which is triggered between t he cotyledon and two-leaf stages of seedling development and is postmeiotic ally reset to the high-expressing state during seed development. The incide nce of GLU I PTGS in sibling plants differed for the two different transfor mants tested and increased with the number of T-DNA loci. Comparison of hos t 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 homol ogous host genes. The GLU I transformants exhibited a spatial gradient in P TGS, in which expression of the silent phenotype gradually increased in suc cessive leaves toward the bottom of the plant. In contrast, transformants c arrying an unrelated tobacco class I chitinase (CHN I) cDNA in the same exp ression vector exhibited discontinuous patterns of PTGS with adjacent high- expressing and silent leaves. The GLU I- and CHN I-specific patterns were m aintained in hybrids homozygous for both T-DNA's indicating that two differ ent transgenes present in the same genome can exhibit independent and disti nctive patterns of PTGS. This implies that the nature of the transgene rath er than a general pre-pattern of competence for PTGS or propagation of the silent state are important for pattern determination.