Chestnut trees, once a keystone species in forests of the Eastern U . S . , were decimated by a fungus, C . parasitica, that was unintentionally introduced into the U . S . ~ 1 2 0 years ago, killing > 3 billion chestnut trees ( 9 0 % ) over the next 5 0 years. C . parasitica kills host plant cells via production of a toxin, oxalic acid. Some plants, such as wheat, have naturally occurring resistance to oxalic acid due to expression of a gene encoding oxalate oxidase ( OxO ) , an enzyme that inactivates oxalic acid. Scientists have genetically engineered C . parasitica - resistant chestnut trees ( line " Darling 5 8 " ) by introducing the wheat OxO gene into chestnuts via Agrobacterium - mediated transformation. If approved by U . S . regulatory agencies for release into the wild, Darling 5 8 may be able to restore the prominence of chestnut trees in Eastern U . S . forests. This is background information for this question and the next one.
Which of the following processes could be used to introduce purified recombinant Ti plasmid DNA directly ( not involving of use of another organism ) into Agrobacteria prior to infection of chestnut embryos or cells?