UC Davis

Strawberry, Fragaria × ananassa, is a popular fruit among consumers and is one of the top 10 most important crops for the state of California. F. × ananassa is a relatively new allo-octoploid species that originated in 18th century Europe and breeders have been focused on generating new strawberry cultivars with improved size, yield, disease resistance, and overall quality ever since. A crucial fruit quality trait for consumers is flavor which is comprised of a variety of factors, predominantly the combination of sweetness, acidity, berry texture, and aroma of ripe berries. Over 300 volatile organic compounds (VOCs) have been discovered in strawberry that contribute to aroma, but the biosynthesis and regulation of such VOCs are complex and pose challenges for aroma-focused strawberry breeding. The University of California Strawberry Breeding Program, with which this research was conducted in partnership, has remained at the forefront of strawberry breeding making crucial discoveries and advances in fruit yield, disease resistance, shelf-life, and high-quality genomic resources and is keen to make similar advances in aroma traits. Thus, this thesis was conducted with the goal of cataloging and studying a wide range of strawberry species, ecotypes, and cultivars to better understand the biosynthetic players responsible for strawberry aroma and elucidate patterns between domestication.

The first chapter provides a general overview of the history of strawberry breeding and strawberry aroma research. The primary VOCs for strawberry and the known pathways, enzymes, and intermediates for each is described. Ideas for future advances for aroma-focused strawberry breeding and consumer work are also provided.

The second chapter details work conducted on the terpene synthase (TPS) family of strawberry. A total of 75 terpene synthases were identified within the most recent octoploid strawberry genome and an additional 14 were identified from de novo transcriptomes of a variety of diploid and octoploid strawberry accessions. Twenty-seven mono-, sesqui- and diterpene synthases were functionally characterized. Terpenes which have not yet been described in strawberry were detected: nine aromatic terpenes were detected in planta from a diversity panel of 96 field-grown strawberry accessions and up to 30 additional mono-, sesqui- and diterpenes with possible roles in aroma or defense were detected as products formed by the 27 characterized candidates either in vivo or in vitro. Future work can focus on further investigating this functional atlas of terpene metabolism in strawberry for terpene-focused breeding.

The third chapter details comprehensive aroma profile analysis of 98 accessions of field-grown strawberry that span the domestication history of Fragaria spp. Out of 171 volatile organic compounds detected, sixteen were found to be important for distinguishing ripe berry metabolite profiles as either one of seven domestication classification groups or as one of the 98 accessions. Trends of both commercially important fruit quality traits such as firmness, size, and sugar content as well as abundance trends for eight focal metabolites were examined that shed light on how strawberry flavor has been altered through domestication and what the next steps for aroma-focused breeding could be.