Methods of gene transfer

In biolistics (the gene-gun method), cells are bombarded with high-velocity micro-particles of gold or tungsten coated with DNA.
The particles penetrate the thick cell wall and deliver the DNA into the cell, so the method is suitable for plant cells.
It is a direct, vector-less method of introducing alien DNA into the host, i.e., of transformation.
DNA fingerprinting uses probes and electrophoresis, disarming of pathogen vectors is done by removing their disease-causing genes, and recombinant DNA is constructed using restriction enzymes and DNA ligase.
Hence biolistics is suitable for the transformation of plant cells.

Polyethylene glycol (PEG) alters the cell membrane; it makes plant protoplasts take up DNA directly from the surrounding medium, and it also induces the fusion of protoplasts.
So PEG-mediated uptake is a chemical method of direct, vector-less gene transfer.
Other vector-less methods are micro-injection, electroporation and biolistics (gene gun).
It has no role in energy production from sewage, biodiesel production or seedless fruit production.
Hence the polyethylene glycol method is used for gene transfer without a vector.