Molecular Cloning and Characterization of Apricot Fruit Polyphenol Oxidase

A reverse transcriptase-polymerase chain reaction experiment was done to synthesize a homologous polyphenol oxidase (PPO) probe from apricot (Prunus armeniaca var Bergeron) fruit. This probe was further used to isolate a full-length PPO cDNA, PA-PPO (accession no. AF020786), from an immature-green fruit cDNA library. PA-PPO is 2070 bp long and contains a single open reading frame encoding a PPO precursor peptide of 597 amino acids with a calculated molecular mass of 67.1 kD and an isoelectric point of 6.84. The mature protein has a predicted molecular mass of 56.2 kD and an isoelectric point of 5.84. PA-PPO belongs to a multigene family. The gene is highly expressed in young, immature-green fruit and is turned off early in the ripening process. The ratio of PPO protein to total proteins per fruit apparently remains stable regardless of the stage of development, whereas PPO specific activity peaks at the breaker stage. These results suggest that, in addition to a transcriptional control of PPO expression, other regulation factors such as translational and posttranslational controls also occur.

Changes in Cell Wall Composition during Ripening of Grape Berries1

Cell walls were isolated from the mesocarp of grape (Vitis vinifera L.) berries at developmental stages from before veraison through to the final ripe berry. Fluorescence and light microscopy of intact berries revealed no measurable change in cell wall thickness as the mesocarp cells expanded in the ripening fruit. Isolated walls were analyzed for their protein contents and amino acid compositions, and for changes in the composition and solubility of constituent polysaccharides during development. Increases in protein content after veraison were accompanied by an approximate 3-fold increase in hydroxyproline content. The type I arabinogalactan content of the pectic polysaccharides decreased from approximately 20 mol % of total wall polysaccharides to about 4 mol % of wall polysaccharides during berry development. Galacturonan content increased from 26 to 41 mol % of wall polysaccharides, and the galacturonan appeared to become more soluble as ripening progressed. After an initial decrease in the degree of esterification of pectic polysaccharides, no further changes were observed nor were there large variations in cellulose (30–35 mol % of wall polysaccharides) or xyloglucan (approximately 10 mol % of wall polysaccharides) contents. Overall, the results indicate that no major changes in cell wall polysaccharide composition occurred during softening of ripening grape berries, but that significant modification of specific polysaccharide components were observed, together with large changes in protein composition.