Complex horticultural quality traits in broccoli are illuminated by evaluation of the immortal BolTBDH mapping population

Authors: STANSELL, ZACHARY - Cornell University; BJORKMAN, THOMAS - Cornell University; Farnham, Mark

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2019
Publication Date: 9/18/2019
Citation: Stansell, Z., Bjorkman, T., Farnham, M.W. 2019. Complex horticultural quality traits in broccoli are illuminated by evaluation of the immortal BolTBDH mapping population. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2019.01104.
DOI: https://doi.org/10.3389/fpls.2019.01104

Interpretive Summary: Horticultural quality characteristics of mature broccoli heads have traditionally been difficult to quantify in breeding programs and the underlying genetics that result in quality heads is presumed to be complex and controlled by many genes with small effects. Moreover, improvement efforts within this crop are generally constrained by a lack of understanding of the complex genetic regulations of the many genes conferring a wide range of horticultural quality outcomes associated with plant architecture, developmental and physiological processes, and yield. In light of the limited information informing broccoli researchers, an ARS scientist at Charleston, SC collaborated with scientists at Cornell University to gain a better understanding of genes conferring quality attributes of broccoli heads. They conducted field and laboratory studies to characterize head trait variation in a unique broccoli population, to produce robust DNA markers for the population, to identify DNA sequences associated with the head traits, and to examine potential candidate genes found within the sequences. The studies identified a total of 82 DNA markers that were located on particular chromosomes and that were significantly linked with broccoli heading and other plant traits. These markers were also aligned with numerous candidate genes that affect different attributes like plant architecture, biomass, bud morphology, leaf morphology, head characteristics, and developmental processes. These results are of great interest to geneticists working to better understand the complex genetics of broccoli and they will also inform future efforts by breeders working to develop broccoli varieties that produce consistent quality heads under differing environmental conditions.

Technical Abstract: A publicly available double-haploid B. oleracea biparental mapping population BolTBDH (Chinese kale x broccoli; N = 175) segregating for 25 horticultural traits was evaluated by applying genotype-by-sequencing, QTL identification, and candidate gene analyses. The physical locations of 51 single and 31 epistatic QTL and their candidate genes were identified within six trait classes: architecture, biomass, bud morphology, leaf morphology, head quality, and phenology. We describe four single QTL explaining a cumulative 67.4% of variance in overall broccoli heading quality including the QTL OQ_9 which contains two adjacent FLC homologs on chromosome 9 (Bo9g173400 and Bo9g173370). Two heading quality QTL (OQ_4 and OQ_8) appear to interact epistatically with OQ_9. Additional heading quality QTL for flower bud size and uniformity, crown diameter, uniformity, shape, extension, and compaction are identified. Two phenology hotspots reduce days to flowering and maturity by -8.4 / -7.9 days and -7.7 /-7.4 days and contain the BoFLC homologs Bo3g024250 and the BoFLC copies identified in OQ_9. Strong candidates BoLMI1 (Bo3g002560) for serrated leaf margins and leaf apex shape, BoCCD4 (Bo3g158650), a carotenoid cleavage dioxygenase implicated in flower color, and BoAP2 (Bo1g004960) implicated in the hooked sepal horticultural defect were also identified.