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Co Suppression of Genes -

In case of many endogenous plant genes, an overexpression of the sense RNA or mRNA surprisingly leads to a drastic reduction in the level of expression of the genes concerned; this is called co-suppression.

One way of achieving an overexpression of the mRNA is to introduce a homologous sense construct of the gene concerned so that it also produces sense RNA or mRNA (in addition to the endogenously present gene).

The efficiency of co-suppression seems to vary among plant genes. Co-suppression has never been observed for the petunia chalcone isomerase gene, while tobacco glutamine synthetase nuclear gene is always co-repressed; CHS gene (petunia) represents the intermediate situation.

The mechanism of co-suppression is not understood. According to a threshold model, when RNA transcripts of a get1e accumulate beyond a critical, threshold level, they are selectively degraded by RNases. An accumulation of high levels of RNA transcripts of a gene may lead to the production of aberrant sense RNA transcripts of the transgene.

An accumulation of aberrant RNA transcripts is proposed to activate RNA-dependent RNA polymerase of plant origin, which transcribes the RNA transcripts to produce antisense RNA. The antisense RNA transcripts would associate with the accumulated normal and aberrant RNA transcripts of the transgene as well as the endogenous gene.

This will produce RNA duplexes, which present targets for double-stranded RNA specific RNases like RNase H. Degradation of the RNA transcripts of a gene is postulated to somehow lead to a hypermethylation of the DNA sequences homologous to the degraded RNA sequences.
This often leads to a drastic reduction in the level of expression of the transgene in question and also of homologous endogenous gene(s), if any; this is called gene silencing.
Ethylene is an important phytohormone and is involved, among other things, in fruit ripening, e.g., in banana, tomato, avocado, etc., leaf abscission and flower senescence. It is produced from amino acid methionine, the terminal two reactions of ethylene biosynthesis being as follows.
A reduced ethylene production results in delayed petal senescence in carnation and slow ripening of tomato fruits. Drastically reduced ethylene production has been achieved in one of the following ways:

(i) expression of antisense constructs of ACC synthase or ACC oxidase,
(ii) co-suppression of either of these enzymes, and
(iii) expression of enzymes that metabolize S-adenosyl methionine (SAM), e.g., SAM hydrolase from bacteriophase T3 (in tomato), or ACC, e.g., ACC deaminase (over-expression in tomato). A carnation variety with longer vase life has its ACC synthase gene co-suppressed. A similar co-suppression approach has been used to block the onset of fruit ripening in tomatoes.

 

 
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