Broccoli (Brassica oleracea var. italica) is a widely consumed vegetable, it is known as a ‘functional food’ as it contains high levels of vitamins, minerals, glucosinolates and flavonoids. However, the marketability of broccoli is limited by a very short shelf life of ~4 days. Broccoli heads are composed of florets containing hundreds of immature flower buds. The sepals are rich in chlorophyll, giving broccoli its characteristic green appearance. However, during storage the sepals become susceptible to rapid yellowing. In broccoli, post harvest senescence in induced as a result of harvesting which causes deterioration and quality loss. Yellowing and wilting cause broccoli heads to become unmarketable and rapid decline in nutrients results in a product with reduced nutritional value. This causes high levels of waste for growers, retailers and consumers.
By utilising the natural variation in shelf life, displayed by a doubled haploid (DH) population developed at WHRI, the genetics of quality in broccoli can be addressed. A linkage map has been constructed that spans 947.7 cM, covering ~100% of the Brassica oleracea genome. It incorporates a combination of 134 SSR and AFLP markers. This in conjunction with shelf life data has lead to the identification of QTLs associated with shelf life and other phenotypic traits. Most recently, vitamin C levels at harvest have been quantified from individuals in the DH population, using a HPLC approach. Findings show a high level of natural variation is present within the mapping population, with vitamin C levels ranging from 50.48-170.10 (mg/100g) Ffw-1. At present 2 QTLs have been mapped for vitamin C content at harvest. Vitamin C levels during storage will also be determined. This will allow for the identification of QTLs associated with both vitamin C content and stability.
A bioinformatics approach is also being used to BLAST search for sequence homologies, to known vitamin C biosynthesis genes (VTC genes) from Arabidopsis thaliana within GSS & EST collections. Primers will be designed to target aligned VTC gene sequences to identify orthologs in broccoli. Sequence polymorphisms between the parental lines will be used to map the orthologs, providing a marker for vitamin C biosynthesis. If the markers fall within mapped vitamin C QTLs, this would suggest that the VTC gene orthologs are the likely candidate genes underlying the QTL.
The identification of QTLs associated with shelf life and vitamin C levels will provide a useful tool for marker-assisted selection (MAS) to improve of nutritional and quality traits in broccoli.
A study by Pink et al found that doubled haploid (DH) lines derived from a commercial varitey of broccoli called 'Marathon' differed in their shelf life phenotypes even though they contained identical genetic material. This lead to the hypothesis that shelf life and time taken to yellow (days) is controlled largely by genotype, and that natural variation can be utilised as a tool to identify regions in the B. oleracea genome controlling these traits.
Therefore to study the genetic effects on broccoli quality a doubled hapolid (DH) population was created at HRI by a previous PhD student (Mathas, 2004). The DH population is derived from a cross between a 'good' shelf life performer (Mar34) and a 'poor' shelf life performer (GD33). The F1 progeny from this cross underwent microspore culture in vitro to double the chromosomes. The production of a DH mapping population provides a genetically fixed resource for the production of genetic maps and the mapping of quantitive trait loci (QTL).