Nitrate reductase and nitrite reductase as targets to study gene silencing phenomena in transgenic plants

Nitrate assimilation is a fundamental function in any plant including those that can fix atmospheric nitrogen in symbiosis with soil bacteria. In recent years, attempts have been made to understand the biological significance of the complex regulation of this pathway using genetic engineering techniques. Transgenic plants that either over- or under-express genes of the nitrate assimilation pathway were created in order to determine whether such directed changes affect the regulation of the metabolism. Apart from interesting physiological results, unexpected gene silencing phenomena have been observed resulting from the introduction of five different transgenes derived from either the tobacco Nia or Nii genes encoding nitrate reductase and nitrite reductase, respectively. In this review, each of these five silencing phenomena is described, with the emphasis on the advantages provided by the use of both Nia and Nii genes to analyze the molecular and genetic basis of gene silencing in transgenic plants.