13 Plant Cells and Viruses

Abstract

Replication of plant genomes, whether they belong to viruses or to cells, has generally similar requirements to replication of animal genomes. However, some aspects are plant-specific, and consequently, it is important to learn about the mechanisms operating during plant virus and nuclear DNA replication in order to establish differences and similarities. In the case of plant DNA viruses, constraints imposed by the presence of a rigid plant cell wall contribute to the absence of cell lysis to free viruses into the extracellular milieu. Instead, plant virus genomes are actively exported to the neighbor cells in the form of nucleoprotein complexes. Then, they are transported systemically through the vascular system. Cellular DNA replication in plant cells also conforms to the rule of duplicating the genome once, and only once, per cell cycle. However, repeated rounds of full genome replication in the absence of an intervening mitosis, a process known as endoreplication, also occurs in plant cells. Occurrence of endocycles is far more frequent in plants than in animals, and in many cases it is required for, or associated with, specific morphogenetic and developmental patterns.

In the paragraphs below, I review recent information and concepts on three main topics: first, the mechanism of DNA virus replication as it is being learned from model viruses (geminivirus and nanovirus) (pararetroviruses, which develop an RNA step during replication and can integrate into the genome, have been discussed elsewhere [Kobayashi et al. 2002]); second, the virus/host cell interactions needed to complete a productive replication cycle, largely...