title: Down-regulation of POLYGALACTURONASE1 alters firmness, tensile strength and water loss in apple (Malus x domestica) fruit.
abstract: BACKGROUND
  While there is now a significant body of research correlating apple (Malus x domestica) fruit softening with the cell wall hydrolase ENDO-POLYGALACTURONASE1 (PG1), there is currently little knowledge of its physiological effects in planta. This study examined the effect of down regulation of PG1 expression in 'Royal Gala' apples, a cultivar that typically has high levels of PG1, and softens during fruit ripening.
  
  
  RESULTS
  PG1-suppressed 'Royal Gala' apples harvested from multiple seasons were firmer than controls after ripening, and intercellular adhesion was higher. Cell wall analyses indicated changes in yield and composition of pectin, and a higher molecular weight distribution of CDTA-soluble pectin. Structural analyses revealed more ruptured cells and free juice in pulled apart sections, suggesting improved integrity of intercellular connections and consequent cell rupture due to failure of the primary cell walls under stress. PG1-suppressed lines also had reduced expansion of cells in the hypodermis of ripe apples, resulting in more densely packed cells in this layer. This change in morphology appears to be linked with reduced transpirational water loss in the fruit.
  
  
  CONCLUSIONS
  These findings confirm PG1's role in apple fruit softening and suggests that this is achieved in part by reducing cellular adhesion. This is consistent with previous studies carried out in strawberry but not with those performed in tomato. In apple PG1 also appears to influence other fruit texture characters such as juiciness and water loss.
authors.affiliation: The New Zealand Institute for Plant & Food Research Limited (PFR), Mount Albert Research Centre, Private Bag 92169, Auckland 1142, New Zealand.
authors.forename: Ross G
authors.initials: RG
authors.lastname: Atkinson
authors.forename: Paul W
authors.initials: PW
authors.lastname: Sutherland
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Johnston
authors.forename: Kularajathevan
authors.initials: K
authors.lastname: Gunaseelan
authors.forename: Ian C
authors.initials: IC
authors.lastname: Hallett
authors.forename: Deepali
authors.initials: D
authors.lastname: Mitra
authors.forename: David A
authors.initials: DA
authors.lastname: Brummell
authors.forename: Roswitha
authors.initials: R
authors.lastname: Schröder
authors.forename: Jason W
authors.initials: JW
authors.lastname: Johnston
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Schaffer
mesh_terms: D002448:Cell Adhesion; D002473:Cell Wall / Q000378:metabolism; D003433:Crosses, Genetic; D015536:Down-Regulation / Q000235:genetics*; D005638:Fruit / Q000201:enzymology* / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D018506:Gene Expression Regulation, Plant; D027845:Malus / Q000201:enzymology* / Q000235:genetics / Q000502:physiology / Q000648:ultrastructure; D010368:Pectins / Q000378:metabolism; D018526:Plant Transpiration / Q000235:genetics; D030821:Plants, Genetically Modified; D011096:Polygalacturonase / Q000235:genetics / Q000378:metabolism; D058105:Polymerization; D012621:Seasons; D013489:Suppression, Genetic; D013718:Tensile Strength; D014574:Uronic Acids / Q000378:metabolism; D014867:Water / Q000378:metabolism*
chemical_list: D014574:Uronic Acids; C018402:traumacel; D014867:Water; D010368:Pectins; D011096:Polygalacturonase