POSTTRANSCRIPTIONAL GENE SILENCING OF ACC SYNTHASE IN TOMATO RESULTS FROM CYTOPLASMIC RNA DEGRADATION

Transgenic tomato plants with the coding region of a predominantly fru it specific ACC synthase gene (LE-ACS2) driven by the CAMV35S promoter , fall into two phenotypic classes: (i) epinastic plants and (ii) plan ts which appear vegetatively normal with ripening impaired fruit. In l eaf tissue from epinastic plants the transgene RNA pool is high, and i n the vegetatively normal plants the transgene transcript level is ver y low. While the epinastic phenotype results from the expected high le vel expression of the ACC synthase transgene throughout the plant, the vegetatively normal, ripening impaired phenotype is a consequence of the silencing of both the transgene and the corresponding endogenous g ene. As in several other cases of gene silencing, the mechanism is pos t-transcriptional. Transgene RNA pools are similar in nuclei of epinas tic and vegetatively normal tissue. However, the transgene mRNA pool i s dramatically reduced in the cytoplasm of vegetatively normal plants, and furthermore, this reduction results from degradation from the 3' end of the transcript. This degradation probably occurs on the polysom es where the transgene transcripts are localized.