Resumen de Genetic analysis of fruit flavor and aroma volatile compounds in wild strawberry
Strawberry, belonging to Fragaria genus, Rosaceae family, is the most commonly consumed berry fruit crop worldwide, with a production of around 9.2 million tons during 2017. Although traditionally breeding programs have been focused on improving agronomic traits, fruit quality has become a main goal recently. Fruit flavor is the main factor responsible for the fruit quality that is a direct factor attracting customers. The diploid strawberry (Fragaria vesca) serves as an important model plant for cultivated strawberry and Rosaceae family, due to its perennial life cycle, small genome, short generation time, a simple and efficient genetic transformation system and abundant genetic resources. The main goal of this work was to study the genetic basis of fruit flavor in diploid strawberry using a isogenic lines collection (NIL) developed by a cross between the recurrent parent F. vesca and the donor parent F. bucharica. Firstly, we improved the previously developed NIL collection with 10 new lines added. Finally, this population consists of 49 lines with overlapping introgressions covering 94.8% of background of donor parental line. It is a highly relevant genetic tool for mapping a variety of traits for diploid strawberry. Fruit flavor traits, including pH, citric acid and oBrix of ripe fruits of the collection were statistically analyzed for mapping as quantitative traits loci (QTL). One stable QTL was mapped for increasing pH and two major QTL for decreasing pH. Two major QTL were mapped for decreasing citric acid and one QTL for increasing oBrix. Due to locating many major QTL for aroma volatile compounds in LG5, we deeply studied the aroma compounds in the new lines harboring introgressions in LG5. Seventeen key volatile compounds were identified and five QTL were mapped. The QTL for methyl 2-aminobenzoate and myrtenyl acetate were located in the same region LG5:20-35cM. The QTL for decreasing methyl butanoate content was located in LG5:11-20cM. Two QTL for green volatile compounds (Z)-3-hexenyl acetate and (E)-2-hexenyl acetate were located in LG5:50-76cM.
In order to investigate the major genes controlling aroma volatile compounds accumulation, eleven genes were selected as important candidate genes to analyze the transcription level in fully ripe strawberry fruits of F. vesca and NILs harboring F. bucharica alleles. Finally, three genes in LG5:0-35cM and three genes in LG5:50-76cM were verified to present significant differences in expression between the recurrent parent F. vesca and the NILs harboring F. bucharica alleles in these LG5 regions. The gene FvH4_5g29270 encoding a 3Z-2E-enal isomerase was selected as candidate gene for green leaf volatile compounds in LG5:50-76cM. For verifying the function of gene FvH4_5g29270, the gene overexpression vector was constructed and transiently expressed in F. vesca and two NILs with the F. bucharica allele of FvH4_5g29270 via agrobacterium mediated transfection. However, the green volatile compounds content results were unexpected, mainly due to the low transformation efficiency. Attending to the important fruit appearance traits, the fruit color was analyzed with eight color parameters for three harvests. Orange fruit color was observed in some NILs and was mapped in LG5:35-39cM. Eighteen candidate genes in this region were analyzed for transcript expression level in fully ripen fruits from the recurrent parent F. vesca and NILs harboring the introgression containing LG5:35-39. The gene FvH4_5g14770 encoding light-harvesting complex chlorophyll A-B binding protein was selected as a good candidate gene since it showed significantly higher transcript level in orange colored fruits than in red fruits that can be related to chlorophyll content reduction. All these results provided fundamental basis for strawberry aroma and color breeding.
