Molecular Tagging and Selection for Sugar Type in Carrot Roots Using Co-dominant, PCR-based Markers

Carrot 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 that harbor 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.