Plant gene silencing regularized

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Abstract
Two surprising and potentially useful phenomena of transgenic plants originated from two distinct lines of research but may converge in underlying mechanism or mechanisms (1–3). Plant transformation technology, the staple of the crop biotechnology industry, overcomes the species barrier and allows introduction into plants of genes from other plant species, genes from species of other kingdoms, and even sequences derived from the DNA synthesizer. Subsequently, the introduced sequences often, but not always, are inherited as simple Mendelian characters and exhibit an expected phenotype. It is one group of annoying exceptions to expected phenotype and inheritance that constitutes the first surprising phenomenon. Investigators, seeking to increase the expression, and thereby enhance the phenotype, of an endogenous gene, introduced additional copies of the endogenous gene as a transgene. Usually the constructions use promoters more active than the promoter of the endogenous gene. In some instances, the result was not the overexpression of the gene, as expected, but a drastically reduced or undetected expression of both the endogenous and the introduced sequences (4, 5). Much of the early research used a chalcone synthase gene, the silencing of which sometimes resulted in flowers with spectacular patterns of nonpigmented and pigmented areas. The reduced expression phenomenon is termed gene silencing. Gene silencing results in no expression or very low expression of a gene or RNA sequence that formerly was expressed, or likely would be expressed, absent the gene-silencing phenomenon. Paradoxically, plants with multiple copies of the transgene and/or high levels of transgene transcription are more likely than plants with a single copy and low-level transcription to exhibit gene silencing (6). However, the correlation has not been consistent for some systems (7, 8). Gene silencing can be induced not only by expression of a transgene but also by transient expression from DNA …