Starch debranching enzyme (R-enzyme or pullulanase) was purified to homogeneity from developing endosperm of rice (Oryza sativa L. cv. Fujihikari) using a variety of high-performance liquid chromatography columns, and characterized. A cDNA clone encoding the full length of the rice endosperm debranching enzyme was isolated and its nucleotide sequence was determined. The cDNA contains an open reading frame of 2958 bp. The mature debranching enzyme of rice appears to be composed of 912 amino acids with a predicted relative molecular mass (Mr) of 102,069 Da, similar in size to its Mr of about 100,000 Da estimated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The amino acid sequence of rice debranching enzyme is substantially similar to that of bacterial pullulanase, while it bears little similarity to that of bacterial isoamylase or to glycogen debranching enzymes from human muscle and rabbit muscle. Southern blot analyses strongly suggest that the debranching enzyme gene is present as a single copy in the rice genome. Analysis by restriction fragment length polymorphism with a probe including the 3'-untranslated region of cDNA for rice debranching enzyme confirmed that the debranching enzyme gene is located on chromosome 4.

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Summary Sesame (Sesamum indicum) seeds contain a large number of lignans, phenylpropanoid‐related plant specialized metabolites. (+)‐Sesamin and (+)‐sesamolin are major hydrophobic lignans, whereas (+)‐sesaminol primarily accumulates as a water‐soluble sesaminol triglucoside (STG) with a sugar chain branched via β1→2 and β1→6‐O‐glucosidic linkages [i.e. (+)‐sesaminol 2‐O‐β‐d‐glucosyl‐(1→2)‐O‐β‐d‐glucoside‐(1→6)‐O‐β‐d‐glucoside]. We previously reported that the 2‐O‐glucosylation of (+)‐sesaminol aglycon and β1→6‐O‐glucosylation of (+)‐sesaminol 2‐O‐β‐d‐glucoside (SMG) are mediated by UDP‐sugar‐dependent glucosyltransferases (UGT), UGT71A9 and UGT94D1, respectively. Here we identified a distinct UGT, UGT94AG1, that specifically catalyzes the β1→2‐O‐glucosylation of SMG and (+)‐sesaminol 2‐O‐β‐d‐glucosyl‐(1→6)‐O‐β‐d‐glucoside [termed SDG(β1→6)]. UGT94AG1 was phylogenetically related to glycoside‐specific glycosyltransferases (GGTs) and co‐ordinately expressed with UGT71A9 and UGT94D1 in the seeds. The role of UGT94AG1 in STG biosynthesis was further confirmed by identification of a STG‐deficient sesame mutant that predominantly accumulates SDG(β1→6) due to a destructive insertion in the coding sequence of UGT94AG1. We also identified UGT94AA2 as an alternative UGT potentially involved in sugar–sugar β1→6‐O‐glucosylation, in addition to UGT94D1, during STG biosynthesis. Yeast two‐hybrid assays showed that UGT71A9, UGT94AG1, and UGT94AA2 were found to interact with a membrane‐associated P450 enzyme, CYP81Q1 (piperitol/sesamin synthase), suggesting that these UGTs are components of a membrane‐bound metabolon for STG biosynthesis. A comparison of kinetic parameters of these UGTs further suggested that the main β‐O‐glucosylation sequence of STG biosynthesis is β1→2‐O‐glucosylation of SMG by UGT94AG1 followed by UGT94AA2‐mediated β1→6‐O‐glucosylation. These findings together establish the complete biosynthetic pathway of STG and shed light on the evolvability of regio‐selectivity of sequential glucosylations catalyzed by GGTs.

Aphanomyces root rot, caused by Aphanomyces cochlioides Drechs., is one of the most serious diseases of sugar beet (Beta vulgaris L.). Identification and characterization of resistance genes is a major task in sugar beet breeding. To ensure the effectiveness of marker-assisted screening for Aphanomyces root rot resistance, genetic analysis of mature plants' phenotypic and molecular markers' segregation was carried out. At a highly infested field site, some 187 F(2) and 66 F(3) individuals, derived from a cross between lines 'NK-310mm-O' (highly resistant) and 'NK-184mm-O' (susceptible), were tested, over two seasons, for their level of resistance to Aphanomyces root rot. This resistance was classified into six categories according to the extent and intensity of whole plant symptoms. Simultaneously, two selected RAPD and 159 'NK-310mm-O'-coupled AFLP were used in the construction of a linkage map of 695.7 cM. Each of nine resultant linkage groups was successfully anchored to one of nine sugar beet chromosomes by incorporating 16 STS markers. Combining data for phenotype and molecular marker segregation, a single QTL was identified on chromosome III. This QTL explained 20% of the variance in F(2) population (in the year 2002) and 65% in F(3) lines (2003), indicating that this QTL plays a major role in the Aphanomyces root rot resistance. This is the first report of the genetic mapping of resistance to Aphanomyces-caused diseases in sugar beet.