A 2.5-kb insert eliminates acid soluble invertase isozyme II transcript in carrot (Daucus carota L.) roots, causing high sucrose accumulation.

The predominant storage carbohydrates of mature carrot (Daucus carota L.) storage roots typically are the free sugars glucose and fructose. This trait is conditioned by the Rs allele. A naturally occurring recessive mutation, rs/rs, conditions a shift from these reducing sugars to sucrose. RT-PCR and sequencing revealed a unique 2.5 kb insert in the first and largest intron near the 5' end of the acid soluble invertase isozyme II gene of rs/rs carrots. This insert was not totally spliced out during mRNA processing. While the wild-type acid-soluble invertase isozyme II transcript (ca. 2 kb) was detected in Rs/Rs roots and leaves, none was observed in rs/rs roots throughout development. RT-PCR of rs/rs leaves revealed two novel transcripts (2.7 kb and 3.2 kb). A comparison of enzyme activity between the near-isogenic Rs/Rs and rs/rs carrot lines revealed very low acid-soluble invertase activity in rs/rs roots whereas neutral invertase, sucrose synthase and sucrose phosphate synthase levels were comparable. Those results and linkage analysis indicate that Rs is a candidate locus for carrot vacuolar acid-soluble invertase isozyme II. Although the 2.5 kb insert does not occur in the Rs wild-type acid-soluble invertase isozyme II allele, it does occur elsewhere in the genome of Rs/Rs plants.