A new method of constructing artificial plant chromosomes from small rings of naturally occurring plant DNA can be used to transport multiple genes in one fell swoop into embryonic plants.

Autonomous (independently replicating) chromosomes are generated in yeast (yeast artificial chromosomes) and human fibrosarcoma cells (human artificial chromosomes) by introducing purified DNA fragments that nucleate a kinetochore, replicate, and segregate to daughter cells. These autonomous minichromosomes are convenient for manipulating and delivering DNA segments containing multiple genes.

By contrast, the commercial production of transgenic crops relies on methods that integrate just one or a few genes into host chromosomes. That involves extensive screening to identify insertions with the desired expression level, copy number, structure, and genomic location; and long breeding programmes to produce varieties that carry multiple transgenes.

At the present time, just to add a single gene, plant scientists create hundreds of transgenic plants in which the new gene is randomly inserted into a plant chromosome. Then they screen the recipients to find the few that might be suitable for commercial use. If they want to add two genes, they create twice as many new plants, screen for single-gene successes, then cross breed them to get both new genes. That is a slow and laborious process.

With the new method, some of these extended procedures can be by-passed by inserting complete automonous mini-chromosomes in which the genetic environment of all the genes is already characterised. In this way genes for several traits can be combined onto a single DNA fragment with the genes themselves arranged in a defined sequence and context for more consistent gene expression. It also provides an independent genetic linkage linkage group that can be rapidly introgressed into various germplasms.

"This appears be the tool that agricultural scientists, and farmers, have long dreamed of," said Daphne Preuss, PhD, professor of molecular genetics and cell biology at the University of Chicago and chief scientific officer and president of Chromatin, Inc., the makers of the the mini chromosomes. It would then be possible to insert a stack of dozen or more genes together, knowing beforehand where each one was in relation to others.

"This technology could be used to increase the hardiness, yield and nutritional content of crops," she said. "It could improve the production of ethanol or other biofuels. It could enable plants to make complex biochemicals, such as medicines, at very little expense, cutting one or two years out of any new transgenic project," said Dr. Preuss, "You get a better product faster, which saves time, reduces costs, and frees up resources."

Sources:

1. Transgenics transformed. University of Chicago Medical Center (19.10.07) (http://www.agbios.com/main.php?action=ShowNewsItem&id=8921)

2. Carlson SR, Rudgers GW, Zieler H, Mach JM, Luo S, Grunden E, Krol C, Copenhaver GP, Preuss D. (19.10.07). Meiotic transmission of an in vitro-assembled autonomous maize minichromosome. PloS Genetics, 3(10), e179 [Epub ahead of print] (http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=17953486)

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