Open Access Research article

Down-regulation of POLYGALACTURONASE1 alters firmness, tensile strength and water loss in apple (Malus x domestica) fruit

Ross G Atkinson1, Paul W Sutherland1, Sarah L Johnston1, Kularajathevan Gunaseelan1, Ian C Hallett1, Deepali Mitra1, David A Brummell2, Roswitha Schröder1, Jason W Johnston1 and Robert J Schaffer13*

Author Affiliations

1 The New Zealand Institute for Plant & Food Research Limited (PFR), Mount Albert Research Centre, Private Bag 92169, Auckland 1142, New Zealand

2 PFR, Food Industry Science Centre, Private Bag 11600, Palmerston North, 4442, New Zealand

3 The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand

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BMC Plant Biology 2012, 12:129  doi:10.1186/1471-2229-12-129

Published: 2 August 2012

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.

Keywords:
Apple (Malus x domestica); Fruit softening; Intercellular adhesion; Polygalacturonase; Texture; Pectin