Engineered plant minichromosomes: a resurrection of B chromosomes?

A number of crop species of commercial interest have been transformed using either Agrobacterium-mediated, biolistic, or other systems. However, these methods have several limitations. For example, they allow insertion of single or a few genes at random genomic positions, but complex traits cannot be transferred in a coordinated manner. Furthermore, the integrity of the host genome can be disturbed by transgene insertion. These limitations stimulated the development of a chromosome-based vector system suitable for transferring large genes, gene complexes, and/or multiple genes together with regulatory elements for safe, controlled, and persistent expression, avoiding rearrangements that are often linked with insertion events. Additionally, engineered chromosomes could be used to address questions concerning the function of specific chromosomal domains (e.g., centromeric regions). Chromosome engineering has been applied successfully in mammals and yeasts but has lagged in plants. Recent efforts to generate maize minichromosomes by J. Birchler and colleagues from the University of Missouri (Yu et al., 2006, 2007) open new venues for plant biotechnology and chromosome biology.