FRUIT SOFTENING IN POME FRUIT - THE ROLE OF ACS GENES

Abstract

Excessive fruit softening is considered undesirable in apple and Asian pears, and can lead to lower sensory value for firmness, juiciness, crispness and crunchiness, as well as increased mealiness and reduced consumer acceptability. It is widely believed that the onset of ethylene production during fruit ripening drives the key cellular structural changes that lead to softening. The 1-aminocyclopropane-1-carboxylic acid synthase (ACS) enzyme encoded by a highly divergent multi-gene family plays an important role in the biosynthesis of this hormone in many fruit crops. This study was designed to determine the relationships of internal ethylene concentration (IEC), ACS genes and fruit softening in apple and pear. In apple, 68 cultivars and selections were screened with markers for the Md-ACS1 gene, and in pear, 51 accessions were screened with cleaved amplified polymorphic sequence (CAPS) markers derived from PpACS1 and PpACS2 genes. Fruit IEC and firmness were measured at harvest and at 2-week intervals during cold-air storage at 0-4°C. In the apple study, there was a strong effect of Md-ACS1 gene on IEC during fruit ripening. The Md-ACS1-2/2 allelotype had a slower rate of postharvest IEC and flesh softening compared with Md-ACS1-1/1 and -1/2 allelotypes. In pear, the epistatic relationship between PpACS1 and PpACS2 genes produced 'AB/Ab', 'aB' and 'ab' genotypes associated with high, medium and low ethylene production, respectively. PpACS1 and PpACS2 genotypes were related to IEC after harvest in P. pyrifolia but neither IEC nor PpACS1 and PpACS2 genotype had a significant relationship with postharvest fruit firmness in either P. communis or P. pyrifolia. In both pome fruits, harvest firmness appeared to be a better predictor of fruit softening through storage, contributing 57 and 46% of the total variation in post-storage firmness in apple and pear, respectively. Breeding for high harvest firmness could be another strategy to produce cultivars with high firmness after storage. Can be produced in pome fruit.