The effect of endogenous mRNA levels on co-suppression in tomato

Introduction of truncated polygalacturonase (PG) transgenes into tomato plants caused the production of small interfering RNAs (siRNAs) and co-suppression of both the endogenous and PG transgenes in ripening fruits by post-transcriptional gene silencing. In order to test the possible effect on co-suppression of the endogenous PG mRNA level, we transferred the PG transgenes from a PG-silenced line (wild type background) by crossing to two ripening regulatory mutants with reduced PG: Never-ripe (Nr, approximately 10% endogenous PG mRNA compared to wild type) and ripening-inhibitor (rin, approximately 1% endogenous PG mRNA) and to wild type (as a control). The PG transgenes caused strong co-suppression of the transgenes and the endogenous PG gene in cells with high PG mRNA background (wild type) and silencing appeared to be linked with higher transgene copy number and/or a particular transgene locus. In cells with low endogenous PG mRNA accumulation (Nr), the endogenous PG gene was preferentially suppressed compared to the transgenes, whose expression was not reduced significantly. The expression of the transgenes was also not reduced in the very low PG background (rin), in which endogenous PG was barely detectable. In all the analysed lines with all three PG background levels, siRNAs accumulated in leaves and green fruits, in which the endogenous PG gene is not transcribed. The relatively abundant production of siRNAs in most of the lines was linked with a particular transgene insert. These results suggest that a certain threshold level of endogenous PG mRNA is required for the co-suppression of the truncated PG transgenes and the endogenous PG gene or for extensive silencing of the transgenes.