Softening rate of the Chilean strawberry (Fragaria chiloensis) fruit reflects the expression of polygalacturonase and pectate lyase genes

The Chilean strawberry (Fragaria chiloensis) is noted for its good fruit quality characters and is one of the wild parents of the commercial strawberry (Fragaria × ananassa). However, the fruit of the Chilean strawberry shows a higher level of softening than that of F. × ananassa, a factor that can affect its shelf-life and reduce its keeping quality. Fruit of both species were collected at different developmental and ripening times, and classified into four stages according to weight and color: stages 1 and 2 correspond to small, unripe and hard fruit, while stages 3 and 4 correspond to color-breaker and ripening fruit. Two full-length cDNAs were isolated from F. chiloensis, FcPG1 and FcPL1 encoding polygalacturonase (PG) and pectate lyase (PL) proteins, respectively. In F. chiloensis a considerable increase in PG transcripts was observed between stages 2 and 3, while a lower expression level was observed in F. × ananassa in the same developing stages. The expression pattern of PL in F. chiloensis revealed a gradual increment in the transcript level between stages 2 and 4, with a higher increment between stages 2 and 3 in F. × ananassa than in F. chiloensis. The increase in expression of both genes correlated well with the decrease in firmness observed in the fruit. The faster softening observed in F. chiloensis may result from the earlier and higher level expression of PG. In addition, Southern blot analyses showed that PG and PL genes belong to a multigene family in both species. This study highlights the participation of PG and PL during softening of F. chiloensis fruit, and gives an indication of target genes that can be used to help strawberry breeding programmes.