Preparation of edible vaccines

Introduction of foreign DNA into plant's genome can either be done by
a. bombarding embryonic suspension cell cultures using gene-gun (biolistic method)- pellets of metal (usually tungsten) coated with the desirable DNA are fired at plant cells. Those cells that take up the DNA (again, this is confirmed with a marker gene) are then allowed to grow into new plants, and may also be cloned to produce more genetically identical crop.
b. more commonly through Agrobacterium tumefaciens, a naturally occurring soil bacterium, which has the ability to get into plants through some kind of wound (scratch, etc.).
Most bacteria contain plasmids- extra chromosomal, double stranded, circular, self-replicating DNA. Agrobacterium tumefaciens possesses a circular "Ti plasmid" (tumour inducing), which enables it to infect plant cells by integrating into their genome and producing a hollow tumour, where it can live. This ability is exploited to insert foreign DNA into plant genome. Usually prior to this, the plasmid is made harmless by deleting the genes for auxin and cytokinin synthesis, so that it does not produce any cancerous growth in the host plant. This is carried out using restriction endonucleases.
While Agrobacterium-mediated transformation still remains the method of choice for dicots, biolistic method is followed for monocots. Various strategies for expression of foreign genes in high amounts in plants include use of strong and organ-specific plant promoters, targeting of the protein into endoplasmic reticulum (ER) by incorporating ER-targeting and ER-retention signals, creation of optimized translation start site context as well as alteration of codons to suit the expression of prokaryotic genes in a plant. Targeting of the protein to appropriate cellular compartment may be helpful in stabilizing the protein. Retention of heat labile E. coli enterotoxin in ER of potato by using ER-retention signal has been reported to elevate the expression levels of the recombinant protein.
Genes for antibiotic resistance are used to select out the transformed cells and whole plants, which contain the foreign gene; and for expressing the desired product, which can then be regenerated from them. The DNA integrates randomly into plant genome, resulting in a different antigen expression level for each independent line, so that 50-100 plants are transformed together at a time, from which one can choose the plant expressing the highest levels of antigen and least number of adverse effects. Thus the process is also economical.
Production of transgenic plants depends on the plant species and takes 3-9 months. Reducing this time to 6-8 weeks is currently under investigation. Some antigens, like viral capsid proteins, have to self-assemble into VLPs (virus-like particles). VLPs mimic the virus without carrying DNA or RNA and therefore are not infectious.
Each single antigen expressed in plants must be tested for its proper assembly and can be verified by animal studies, Western blot; and quantified by enzyme-linked immunosorbent assay (ELISA) etc.

References:
a.Lal P, Ramachandran VG, Goyal R, Sharma R. Edible vaccines: Current status and future. Indian J Med Microbiol [serial online] 2007 [cited 2008 May 2];25:93-102.
Available from: http://www.ijmm.org/text.asp?2007/25/2/93/32713
b.http://www.ias.ac.in/currsci/aug25/articles20.htm
c.http://library.thinkquest.org/C004367/be9.shtml