Loss-of-Function Mutations in the Fruit Softening Gene POLYGALACTURONASE1 Doubled Fruit Firmness in Strawberry
Wildtype fruit of cultivated strawberry (Fragaria x ananassa) are typically soft and highly perishable when fully ripe. The development of firm-fruited cultivars by phenotypic selection has greatly increased shelf life, decreased post-harvest perishability, and driven the expansion of strawberry production worldwide. Hypotheses for the firm-fruited phenotype include mutations affecting the expression of genes encoding polygalacturonases that soften fruit by degrading cell wall pectins. Here we show that loss-of-function mutations in the fruit softening gene POLYGALACTURONASE1 (PG1-6A1) double fruit firmness in strawberry. PG1-6A1 was one of three tandemly duplicated polygalacturonase genes found to be in linkage disequilibrium with a quantitative trait locus affecting fruit firmness on chromosome 6A. PG1-6A1 was strongly expressed in soft-fruited (wildtype) homozygotes and weakly expressed in firm-fruited (mutant) homozygotes. Genome-wide association, quantitative trait transcript, DNA sequence, and expression-QTL analyses identified genetic variants in linkage disequilibrium with PG1-6A1 that were positively correlated with fruit firmness and negatively correlated with PG1-6A1 expression. An Enhancer/Suppressor-mutator (En/Spm) transposable element insertion was discovered upstream of PG1-6A1 in mutant homozygotes that we hypothesize transcriptionally downegulates the expression of PG1-6A1. The PG1-6A1 locus was incompletely dominant and explained 26-76% of the genetic variance for fruit firmness among phenotypically diverse individuals. Additional loci are hypothesized to underlie the missing heritability. Highly accurate, codominant genotyping assays were developed for modifying fruit firmness by marker-assisted selection of the En/Spm insertion and SNPs associated with the PG1-6A1 locus.