Biochemical and molecular characterization of A- and B-type starch granules in wheat (Triticum aestivum L.) endosperm
Mature wheat endosperm contain A- (diameter > 10 [mu]m) and B-type (diameter < 10 [mu]m) starch granules, showing a bimodal granule size-distribution. Because the two starch granule types differ significantly in chemical composition and functional properties, wheat cultivars containing predominantly A- or B-type granules would be very useful for industry. Breeding such cultivars may be accelerated by molecular and biochemical characterization of A- and B-type granules. Thus, the objective of this study was to analyze proteins and genes that may be related to the biosynthesis of the two granule types. Microsieving and centrifugal sedimentation through aqueous solutions of sucrose, maltose or Percoll were used to separate A- and B-type granules. Homogeneous A- and B-type granule populations were obtained by centrifugation through two Percoll solutions (70% and 100%). Starch granule-bound proteins, SGP-140 and SGP-145 were found to be preferentially associated with A-type granules in developing and mature wheat endosperm. Both SGP-140 and SGP-145 were different variants of SBEIc, a 152 kD isoform of wheat starch branching enzyme, and localized to the endosperm starch granules. The preferential association of SGP-140 and SGP-145 with A-type granules was related to the growth of wheat A-type granules, and the occurrence of the bimodal starch granule size-distribution in barley, rye and triticale. Starch synthase (SS) I has been implicated in the size determination of A-type granules in barley. Biochemical and molecular characterization of wheat A- and B-type granules makes it necessary to clone and characterize wheat SSI (wSSI). In developing wheat endosperm wSSI was present at similar levels in starch granules throughout development, while in soluble fractions, wSSI was not detected at five DPA, and had highest concentrations from 10 to 15 DPA. Based on single kernels, wSSI had similar concentrations in soluble fractions from 15 to 25 DPA, and more wSSI was distributed in starch granules than in soluble fractions. wSSI contributed approximately 65% of the total soluble SS activity in wheat endosperm. Two full-length cDNA clones encoding wSSI were isolated from a wheat cDNA library, and confirmed to produce active SS. wSsI transcript was detected in developing wheat endosperm and other tissues.