QTL dynamics for fruit firmness and softening around an ethylene-dependent polygalacturonase gene in apple (Malus x domestica Borkh.)

Abstract

Apple fruit are well known for their storage life, although a wide range of flesh softening occurs among cultivars. Loss of firmness is genetically coordinated by the action of several cell wall enzymes, including polygalacturonase (PG) which depolymerizes cell wall pectin. By the analysis of 'Fuji' (Fj) and 'Mondial Gala' (MG), two apple cultivars characterized by a distinctive ripening behaviour, the involvement of Md-PG1 in the fruit softening process was confirmed to be ethylene dependent by its transcript being down-regulated by 1-methylcyclopropene treatment in MG and in the low ethylene-producing cultivar Fj. Comparing the PG sequence of MG and Fj, a single nucleotide polymorphism (SNP) was discovered. Segregation of the Md-PG1(SNP) marker within a full-sib population, obtained by crossing Fj and MG, positioned Md-PG1 in the linkage group 10 of MG, co-located with a quantitative trait locus (QTL) identified for fruit firmness in post-harvest ripening. Fruit firmness and softening analysed in different stages, from harvest to post-storage, determined a shift of the QTL from the top of this linkage group to the bottom, where Md-ACO1, a gene involved in ethylene biosynthesis in apple, is mapped. This PG-ethylene-related gene has beeen positioned in the apple genome on chromosome 10, which contains several QTLs controlling fruit firmness and softening, and the interplay among the allelotypes of the linked loci should be considered in the design of a marker-assisted selection breeding scheme for apple texture.

Fig. 1.

Ethylene evolution and firmness dynamics in Mondial Gala (a, c) and Fuji (b, d) apple fruit. For Mondial Gala, the continuous lines represent the normal ethylene and softening physiology, while the dashed lines indicate the distorted ripening in 1-MCP-treated fruit (a and c). 1-MCP was not applied to Fuji fruit, thus only normal ripening is shown. Standard errors are shown by vertical lines. Points of the ethylene production curves (Mondial Gala control/Mondial Gala 1-MCP-treated/Fuji) characterized by the same letter are not significantly different (LSD test P ≤0,05). In all graphs the days of assessment are reported on the x-axes.

Fig. 2.

Md-PG1 transcript accumulation in control and treated (1-MCP) fruit samples for Mondial Gala (MG), and non-treated Fuji (Fj) fruit. Standard errors are shown by vertical lines.

Fig. 3.

Md-PG1_SNP polymorphism in Fuji and Mondial Gala and segregation in their progeny. (a) DNA sequence particulars at the polymorphic SNP site for the parent cultivars. (b) SNP segregation revealed by electrophoresis on agarose gel. The two parental cultivars are indicated with Fj for Fuji and MG for Mondial Gala, while the seedlings are coded with numbers from 1 to 9. (This figure is available in colour at JXB online.)

Fig. 4.

Apple linkage group 10 for Mondial Gala and Fuji developed from the Fj×MG population. For Mondial Gala, two mapped functional markers based on candidate genes are shown in bold: Md-PG1 and Md-ACO1.

Fig. 5.

Frequency distributions for fruit firmness measured in the Fuji×Mondial Gala population at harvest for year 2 (a) and 3 (b), and after 2 months of cold storage for year 2 and 3, respectively (c and d).

Fig. 6.

Fruit firmness and softening QTL dynamics over linkage group 10 of Mondial Gala and Fuji. LOD profiles are plotted for Mondial Gala and Fuji with regard to the fruit firmness at harvest (a and f), fruit firmness after 2 months of cold storage (b, g), fruit softening after 2 months of cold storage (c, h), firmness after 30 d of room temperature ripening following harvest (d, i), and relative softening (e, j). Black and grey lines represent year 2 and 3, respectively. Dotted lines indicate the interval mapping computation, and the solid lines show multiple QTL mapping (MQM) profiles. The analysis of firmness and softening measured after 30 d of room temperature was carried out only in year 3. The two horizontal dashed lines at LOD 1.6 and 3.3 indicate chromosome-wide and genome-wide thresholds after a permutation test.

Fig. 6.

Fruit firmness and softening QTL dynamics over linkage group 10 of Mondial Gala and Fuji. LOD profiles are plotted for Mondial Gala and Fuji with regard to the fruit firmness at harvest (a and f), fruit firmness after 2 months of cold storage (b, g), fruit softening after 2 months of cold storage (c, h), firmness after 30 d of room temperature ripening following harvest (d, i), and relative softening (e, j). Black and grey lines represent year 2 and 3, respectively. Dotted lines indicate the interval mapping computation, and the solid lines show multiple QTL mapping (MQM) profiles. The analysis of firmness and softening measured after 30 d of room temperature was carried out only in year 3. The two horizontal dashed lines at LOD 1.6 and 3.3 indicate chromosome-wide and genome-wide thresholds after a permutation test.