Mapping and analysis of genetic loci controlling quality traits in broccoli

Increased consumption of fruit and vegetables has been emphasised as essential to improve the health of the nation. Broccoli is the most popular green vegetable consumed in the UK and is recognised as being rich in nutrients and an excellent source of vitamins and minerals. Broccoli and other Brassica vegetables also contain valuable compounds that have been shown to have a positive effect on human health. Green vegetables such as broccoli are a problem to suppliers and consumers because of their short shelf life, which is exhibited by yellowing and wilting. In addition, loss of valuable nutrients and flavour begin as soon as the crop is harvested and this quality loss may not necessarily be linked to any visible criteria. In this project we aim to address this problem, first by identifying genetic markers that will be useful to plant breeders to create an improved broccoli crop. Secondly, by a detailed analysis of the molecular changes taking place in the lines with differing shelf life characteristics which will allow us to understand the many events that are taking place. This will improve our knowledge of the control of these events and indicate more specific targets for future improvements. We have identified two lines of broccoli that show very different shelf life characteristics. These have been crossed to create a population that can be used to identify the chromosomal loci linked to useful quality attributes such as yellowing time or good nutrient content. Genetic markers linked to useful quality traits will be identified and these will assist in the breeding of improved broccoli lines. Syngenta, the industrial partner in this project, will use the genetic markers identified to introduce chromosomal regions linked to good quality into their elite breeding lines to create new, improved crop varieties. Rapid advances in molecular techniques have allowed plant scientists to obtain a holistic view of molecular changes that are occurring in plants, during development or in response to stress. We have been using such techniques in the study of Arabidopsis leaf senescence and have identified many genes and pathways that are involved in regulating the degradative events that occur in a leaf as it yellows. In this project, we extend this to the analysis of a practical problem, the examination of the molecular events that take place during post harvest senescence in a crop.. We have shown close similarities between broccoli floret senescence and Arabidopsis leaf senescence and we have significant prior knowledge and extensive genetic and molecular resources for broccoli that we can apply to the analysis. The close relationship of broccoli to Arabidopsis allows direct comparison with the data on gene expression and metabolite composition that is already available from Arabidopsis studies. The model plant Arabidopsis has been fully sequenced so we know where all the genes are in this organism. The sequence of brassica is ongoing and is likely to be completed in the lifetime of this project. This sequence information, together with the close similarities between Brassica and Arabidopsis, may allow us to identify likely candidate genes that control the traits that we are studying. This will be important for more targeted improvement approaches in the future.

In this project we bring post genomic tools to the study of quality traits. Broccoli provides an exemplar for the integration of genetics, genomics and metabolomics in the analysis of a real life quality issue. A unique doubled haploid mapping population, generated from two broccoli parents that show very different times to yellowing after harvest, will be used to identify QTL linked to a number of quality traits such as yellowing, turgour loss and nutrient content and stability. Genetic makers linked to these quality traits will be identified for crop improvement purposes. The collaboration with Syngenta enables the delivery pipeline for genetic loci linked to valuable quality traits directly into a commercial Brassica breeding program and this will be initiated during the lifetime of the project. Syngenta broccoli breeders will utilise the results obtained in year 1 and 2 to initiate introgression of QTL of value into their elite breeding lines. The molecular component of the genetic variation in shelf life will be characterised by gene expression (microarray) and metabolome (sugars, lipids amino acids etc) analysis. In addition, levels of key nutrients such as vitamins, flavonoids and glucosinolates will be assessed both at harvest and following storage. This will result in a massively increased molecular understanding of the control of shelf life and nutrient quality in broccoli post harvest and any links between the two. This is key for providing a product with consistent high quality to the consumer and will be applicable to other brassica crops and other green vegetables. This detailed analysis, combining transcript and metabolite assessment, has not been previously carried out on senescing plant material and will be highly informative to fundamental plant senescence studies.