Synthetic minichromosomes in plants: past, present, and promise.

The status of engineered mini-chromosomes/artificial chromosomes/synthetic chromosomes in plants is summarized. Their promise is that they provide a means to accumulate foreign genes on an independent entity other than the normal chromosomes, which would facilitate stacking of novel traits in a way that would not be linked to endogenous genes and that would facilitate transfer between lines. Centromeres in plants are epigenetic, and therefore the isolation of DNA underlying centromeres and reintroduction into plant cells will not establish a functional kinetochore, which obviates this approach for in vitro assembly of plant artificial chromosomes. This issue was bypassed by using telomere-mediated chromosomal truncation to produce mini-chromosomes with little more than an endogenous centromere that could in turn be used as a foundation to build synthetic chromosomes. Site-specific recombinases and various iterations of CRISPR-Cas9 editing provide many tools for the development and re-engineering of synthetic chromosomes.