Allelic structures of UDP-glucose:limonoid glucosyltransferase affect limonoid bitterness in Citrus unshiu and C. sinensis

To develop a molecular indicator for determining the level of limonoid bitterness in Satsuma mandarin (Citrus unshiu) and navel orange (C. sinensis), the genetic background of the transcription of the limonoid glucosyltransferase gene was characterized in relation to the accumulation of non-bitter limonoid glucosides. Two types of cDNA clones (CitLGT-1 and CitLGT-2) encoding limonoid glucosyltransferase were isolated and characterized. The relationships between the gene expression patterns of CitLGTs and the non-bitter limonoid glucoside accumulation as well as the allelic structures of CitLGTs were determined. Southern blot analysis and segregation analysis of the progenies between Satsuma mandarin and tangor (C. unshiu x C. sinensis) indicated that CitLGT-1 and CitLGT-2 are transcribed from the alleles in a single locus. The genotype of the CitLGT locus in navel orange is homozygous for CitLGT-1 (CitLGT-1/CitLGT-1), whereas that in Satsuma mandarin is heterozygous for CitLGT-1 and CitLGT-2 (CitLGT-1/CitLGT-2). The levels of non-bitter limonoid glucosides in navel orange fruit were quite low at the early- to mid-developmental stage due to a defective CitLGT-2, whereas both non-bitter limonoid glucoside content and CitLGT-2 expression in Satsumamandarin fruit were high throughout fruit development. These results indicate that the expression of CitLGT-2 is a prerequisite for the accumulation of non-bitter limonoid glucosides at the early- to mid-developmental stage of fruit. The allelism of the two CitLGTs, that is, whether the CitLGT-2 is present or not, is a useful molecular indicator for predetermining the levels of accumulation of non-bitter limonoid glucosides at the early- to mid-developmental stages of Satsuma mandarin and navel orange fruits.