Identification of quantitative trait loci and candidate genes for primary metabolite content in strawberry fruit

Improvement of nutritional and organoleptic quality of fruits is a key goal in current strawberry breeding programs.The ratio of sugars to acids is a determinant factor contributing to fruit liking,although different sugars and acids contribute in varying degrees to this complex trait.A segregating F1 population of 95 individuals,previously characterized for several fruit quality characters,was used to map during 2 years quantitative trait loci(QTL)for 50 primary metabolites,L-ascorbic acid(L-AA)and other related traits such as soluble solid content(SSC),titratable acidity(TA),and pH.A total of 133 mQTL were detected above the established thresholds for 44 traits.Only 12.9%of QTL were detected in the 2 years,suggesting a large environmental influence on primary metabolite content.An objective of this study was the identification of key metabolites that were associated to the overall variation in SSC and acidity.As it was observed in previous studies,a number of QTL controlling several metabolites and traits were co-located in homoeology group V(HG V).mQTL controlling a large variance in raffinose,sucrose,succinic acid,and L-AA were detected in approximate the same chromosomal regions of different homoeologous linkage groups belonging to HG V.Candidate genes for selected mQTL are proposed based on their co-localization,on the predicted function,and their differential gene expression among contrasting F1 progeny lines.RNA-seq analysis from progeny lines contrasting in L-AA content detected 826 differentially expressed genes and identified Mannose-6-phosphate isomerase,FaM6PI1,as a candidate gene contributing to natural variation in ascorbic acid in strawberry fruit.

Down-regulation of Fra a 1.02 in strawberry fruits causes transcriptomic and metabolic changes compatible with an altered defense response

The strawberry Fra a 1 proteins belong to the class 10 Pathogenesis-Related(PR-10)superfamily.In strawberry,a large number of members have been identified,but only a limited number is expressed in the fruits.In this organ,Fra a 1.01 and Fra a 1.02 are the most abundant Fra proteins in the green and red fruits,respectively,however,their function remains unknown.To know the function of Fra a 1.02 we have generated transgenic lines that silence this gene,and performed metabolomics,RNA-Seq,and hormonal assays.Previous studies associated Fra a 1.02 to strawberry fruit color,but the analysis of anthocyanins in the ripe fruits showed no diminution in their content in the silenced lines.Gene ontology(GO)analysis of the genes differentially expressed indicated that oxidation/reduction was the most represented biological process.Redox state was not apparently altered since no changes were found in ascorbic acid and glutathione(GSH)reduced/oxidized ratio,but GSH content was reduced in the silenced fruits.In addition,a number of glutathione-S-transferases(GST)were down-regulated as result of Fra a 1.02-silencing.Another highly represented GO category was transport which included a number of ABC and MATE transporters.Among the regulatory genes differentially expressed WRKY33.1 and WRKY33.2 were down-regulated,which had previously been assigned a role in strawberry plant defense.A reduced expression of the VQ23 gene and a diminished content of the hormones JA,SA,and IAA were also found.These data might indicate that Fra a 1.02 participates in the defense against pathogens in the ripe strawberry fruits.

The Strawberry Fruit Fra a Allergen Functions in Flavonoid Biosynthesis

The strawberry Fra a I allergen is a homolog of the major birch pollen allergen Bet v 1. It is synthesized by red ripe fruits of Fragaria x ananassa while white fruits of a mutant genotype, which is known to be tolerated by individuals affected by allergy, are devoid of it. Proteomic analyses have shown that Fra a 1 is down-regulated in the tolerated white- fruited genotype along with enzymes of the anthocyanin pigment pathway. In this study, we report the spatial and tem- poral expression of three Fra a genes that encode different isoforms, and the transient RNAi-mediated silencing of the Fra a genes in strawberry fruits of the red-fruited cultivar Elsanta with an ihpRNA construct. As a consequence of reduced levels of Fra a mRNAs, fruits were obtained that produced significantly decreased levels of anthocyanins and upstream metabolites. This effect is consistent with the parallel down-regulation of the phenylalanine ammonia lyase （FaPAL） and to a lesser extent of the chalcone synthase （FaCHS） transcript levels also found in these fruits. In naturally occurring white- fruited genotypes of F. chiloensis and F. vesca, Fra a transcript levels are higher than those of the red-fruited varieties, likely to compensate for the low expression levels of FaPAL and FaCHS in these mutant genotypes. The results demonstrate that Fra a expression is directly linked to flavonoid biosynthesis and show that the Fra a allergen has an essential biological function in pigment formation in strawberry fruit.

Auxin biosynthesis by the YUCCA6 flavin monooxygenase gene in woodland strawberry

Auxin has been regarded as the main signal molecule coordinating the growth and ripening of fruits in strawberry, the reference genomic system for Rosaceae. The mechanisms regulating auxin biosynthesis in strawberry are largely elusive. Recently, we demonstrated that two YUCCA genes are involved in flower and fruit development in cultivated strawberry. Here, we show that the woodland strawberry （Fragaria vesca L.） genome harbors nine loci for YUCCA genes and eight of them encode functional proteins. Transcription pattern in different plant organs was different for all eight FvYUCs. Functionality of the FvYUC6 gene was studied in transgenic strawberry over- expressing FvYUC6, which showed typical high-auxin pheno- types. Overexpression of FvYUC6 also delayed flowering and led to complete male sterility in F. vesta. Additionally, specific repression of FvYUC6 expression by RNA interference signifi- cantly inhibited vegetative growth and reduced plant fertility. The development of leaves, roots, flowers, and fruits was greatly affected in FvYUC6-repressed plants. Expression of a subset of auxin-responsive genes was well correlated with the changes of FvYUC6 transcript levels and free indole-3-acetic acid levels in transgenic strawberry. These observations are consistent with an important role of FvYUC6 in auxin synthesis, and support a main role of the gene product in vegetative and reproductive development in woodland strawberry.