Transformation Vectors and Expression of Foreign Genes in Higher Plants

Most common methods of the gene transfer in higher plants are Agrobacterium-mediated transformation and microparticle bombardment. The former can efficiently integrate lower numbers of transgene copies into plant chromosomes without much rearrangement while the latter can transform some plant species that are recalcitrant to the former method. The standard Escherichia coli cloning vectors with passenger genes used for microparticle bombardment are much simpler than binary vectors for Agrobacterium, which carry additional T-DNA borders and plasmid maintenance functions besides the common components for the both transformation methods. The common components include marker genes and some accessory elements such as Gateway® recombination sites to facilitate gene cloning. Many binary vectors have been constructed and used for plant transformation to date. Recent binary vectors have smaller size and provide a number of user-friendly features like convenient restriction sites, but classic vectors are still quite popular. Chimeric genes of interest are created by joining promoters, enhancers, introns, terminators and 5' and 3' untranslated regions. Each component significantly affects gene expression. A number of expression vectors with the common components are available and very useful. More elaborate constructions of gene cassettes will be useful for a desirable gene expression. A good option for a desirable gene expression is to mimic a gene known to be expressed in the plant species in the pattern of interest such as tapetal specific expression. When two gene cassettes are placed in a vector, the head-tohead arrangement is recommended in order to avoid transcriptional interference.