An early experiment showed that a plasmid from a bacterium that normally causes tumors in plants could be modified by replacing the tumor gene with desirable genes.
Such modified bacteria have been injected into plant cells where they expressed their "foreign" genes.
Genetically modified crop plants could increase food production or grow with greater resistance to pest attack. Genetically engineered plants may also produce human hemoglobin, melanin, even plastics. There are cotton plants that display resistance to herbicide, aspen plants that produce less lignin and more cellulose, tobacco plants that produce human proteins, and mustard plant cells that produce biodegradable plastic.
This animation (Audio - Important) describes engineering plants.