Submitted to: Molecular Breeding
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/23/2004
Publication Date: 1/10/2005
Citation: Yau, Y.Y., Santos, K., Simon, P.W. 2005. Molecular tagging and selection for sugar-type in carrot roots using co-dominant, PCR-based markers. Molecular Breeding. 16:1-10 Interpretive Summary: Sugars are very important in influencing the flavor of both raw and processed carrots. There is a wide range of variation in carrot sugars that varies as a function of both genetic background and growing environment, but in general the genetic influence is more important than environment. In some earlier research we discovered Rs, a gene of carrot that has a major influence on sugar content. Normally we need to grow carrots at least 90 days to measure sugar content. In this current research we developed a quick lab method to allow us to predict sugar content 100% of the time in one-week-old carrots. This method uses our knowledge of the DNA sequence of the Rs gene to make this accurate prediction. We tested the efficacy of this lab method and predicted sugar content in hundreds of plants. This research has immediate application in carrot breeding companies, where they can quickly and accurately evaluate sugars in breeding populations. Since the Rs gene is a well-defined genetic marker, breeding companies and public sector carrot researchers are also interested in using the marker we developed as a general probe to track DNA variation in cultivated and wild carrots. This probe has application in testing seed purity, following the lineages of carrots, generating carrot genetic maps, and following the evolution of the carrot genome.
Technical Abstract: Carrots storage roots accumulate free sugars. The type of sugar accumulated is conditioned by the Rs locus so that typical carrot roots (Rs/-) accumulate predominantly glucose and fructose while rs/rs plants accumulate predominantly sucrose. We recently have found rs/rs plants in one inbred line harbors a naturally occurring insertion sequence of 2.5 kb integrated into the first intron region of acid soluble invertase isozyme II. Using these facts, three primers were designed to differentiate Rs/Rs, Rs/rs and rs/rs carrot plants with simple PCR amplification. Co-dominant, PCR-based markers for acid soluble invertase isozyme II allowed genotyping of the Rs locus in 1-week-old carrot seedlings whereas mature carrot roots were needed to make this evaluation previously, and homozygous dominant plants could not be differentiated from heterozygotes without lengthy progeny testing. Marker-assisted evaluation and selection of carrot root sugar type were exercised in segregating families of diverse background and complete accuracy in predicting sugar type was realized in subsequent generations to further confirm that acid soluble invertase isozyme II conditions the Rs locus. These PCR-based markers will be useful in carrot breeding programs screening for this trait in segregating populations, for studying the distribution and origins of this trait in domestic and wild carrots, and for identifying seed mixtures as low as 10% Rs/- or 1% rs/rs.