Correlation of Fruit Texture Development and Softening with Cell Wall Degradation and Related Gene Expression in Gala Apples
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Abstract:
Changes in cell wall composition, enzyme activity, and gene expression during development and softening of Gala apples was investigated to identify correlations. The results indicated that the content and composition of cell wall material (CWM) showed a dynamic trend during development. Covalent soluble pectin (CSP) showed the highest content among the three pectins assessed and cellulose content was always significantly higher than that of hemicellulose. Firmness correlated significantly with CSP and hemicellulose content. Additionally, changes were noted in the activity and gene expression of cell wall enzymes during this stage, showing that cell wall degradation was involved in the physiological process of fruit development. After harvest, CSP content decreased rapidly with softening, water soluble pectin (WSP) content began to increase, while cellulose and hemicellulose contents decreased, all of which, correlated significantly with firmness loss. Among the cell wall enzymes, β-galactosidase (β-Gal) activity and MdGal gene expression increased most rapidly, followed by that of α-Arabinofuranosidase (α-Af) and PG polygalacturonase (PG), while pectin methylesterase (PME) activity and MdPME increased at a later stage. Moreover, correlation with firmness and cell wall components was stronger for β-Gal and α-Af than that for PG and PME. PG activity correlated strongly with CSP and hemicellulose content, while PME activity showed the least correlation with changes in cell wall components. This indicated that cell wall metabolism had a close relationship with texture softening of ‘Gala’ apple, and both, β-Gal and α-Af may play relatively more important roles in fruit softening.
