Question

8) Why wouldn't the introduction of viral DNA via a plasmid to transform a plant also induce a viral infection?

Answer 1

In eucaryotic cells the cytoplasm—that part of the cell surrounding the nucleus—contains organelles that have their own separate DNA. In plants, protoplast fusion is used to transfer genes from both the nucleus and the cytoplasm. Fusion combines the genomes of two parents, as in traditional breeding, but results can sometimes be obtained faster, even though the fusion product must be backcrossed to the recipient line for several generations to create a new, stable line possessing the one trait desired from the donor. Protoplast fusion can be used for transferring genes that are hard to identify, isolate, and clone or for polygenic traits. Furthermore, protoplast fusion can be used for plants that cannot be crossed sexually (although plants regenerated from such fused hybrids may sometimes be sterile).

Most commonly, cells from closely related plants are fused in order to transfer one particular trait from the donor plant into the recipient. For example, a single dominant nuclear gene for resistance to tobacco mosaic virus and a polygenic trait for hornworm resistance were transferred into tobacco lines by this method. Traits from a wild species can be introduced into a related cultivated species. Cells of wild and cultivated potato plants were fused to transfer the wild species' resistance to potato leaf roll virus. The hybrids were fertile, bore tubers like those of the cultivated species, and were resistant to the virus.

Cytoplasmic (mitochondrial and chloroplast) traits can be transferred by fusing a donor cell whose nucleus has been inactivated, usually by irradiation, with an intact recipient cell to form a ''cybrid." Initially, the cybrid contains the active nucleus of the recipient cell along with mitochondria and chloroplasts from both the donor and recipient cells. However, progeny cells that contain mitochondrial or chloroplast genotypes from one parent only quickly segregate. Plants are then regenerated from cells that harbor the desired donor cytoplasmic genotypes. Both cytoplasmic male sterility (mitochondria) and resistance to the triazine class of herbicides (chloroplast) have been transferred into a single Brassica line via cybrid formation