Epigenetic Modifications and Gene Silencing in Plants

Abstract

During the development of plants, meristems and organ primordia become progressively committed to form specific and definite structures. This process, called determination, results from stable changes in phenotype that persist in the absence of the agent that originally induced the change. As a consequence, parts of an organism can “remember” their past, and this permits new formation of structures. Nevertheless, plants also show remarkable developmental plasticity. The shape and size of organs are strongly influenced by the environment, and plants have a pronounced capacity for regeneration. The developmental plasticity of determined structures illustrates the important point that stable changes are not necessarily irreversible. Determination is a relative process: Developmental states stable in one environment are not necessarily stable in some other environment (Meins and Wenzler 1986).

Little is known about the molecular mechanisms underlying stable reversible changes in plants. Recent studies show that multiple copies of transgenes introduced into plant cells can interact with one another or with homologous host genes in trans, resulting in the inactivation of expression of both genes. This phenomenon, called silencing (Jorgensen 1992) or cosuppression (Napoli et al. 1990), is of particular interest because it provides well-defined experimental systems for investigating the molecular basis for stable reversible changes. The aim of this chapter is to call attention to the similarities between stable changes in plant development and gene silencing and to propose general biochemical switch models that can account for many features of both forms of variation.

THE CONCEPT OF EPIGENETIC CHANGE
Stable Changes in...