Abstract
Lettuce is an important UK horticultural crop with a field gate value > £82m (Defra 2008) Although a proportion of the crop is still marketed as ‘whole head’ an increasing amount is processed into ‘ready to eat’ salad packs which significantly increases the retail value of the crop. However, the fresh salads supply chain faces many challenges. The lettuce crop in the field has high level pesticide applications aimed at controlling diseases (primarily downy mildew and botrytis) and insect pests (mainly aphids) (Defra 2003). It also receives a relatively high rate of fertilizer application, particularly nitrogen, this can cause problems for UK lettuce growers in meeting EU limits for nitrate content of lettuce heads and also has environmental implications in terms of growing lettuce crops in nitrate vulnerable zones and the embedded energy input into growing lettuce crops. Post harvest, a major concern faced by retailers and consumers is the limited shelf life of processed lettuce due to post harvest discolouration at the cut surfaces which can cause up to 30% losses. In addition there have been several documented cases where processed salads have been identified as the source of human pathogenic bacteria in a number of food poisoning outbreaks. There are therefore a number of targets for lettuce crop improvement programmes which would underpin Defra’s aim of developing of a more sustainable food supply chain in terms of: • Energy in agriculture and food • Waste reduction in the food chain • Quality foods for healthy eating • Integrated farming systems There are two basic approaches to understanding the genetic control of important key traits in crops. The ‘traditional’ approach (now termed ‘forward genetics’) involves identify plant lines which differ for the trait and then carrying out an experimental programme of crossing and analysis of the resultant segregating populations to identify the genetic factors determining the trait. The second more recent approach (known as ‘reverse genetics’) is to exploit genetic and genomic information from more basic studies in plants such as arabidopsis to identify genes which are thought likely to influence the trait in question and then to search for variants of the gene(s) and determine whether this variation results in changes in the trait. Warwick HRI is well placed to carry out forward genetics approaches to improving traits for a more sustainable lettuce supply chain and we have carried out a number of such studies with Defra and industry funding. However, there are many traits of interest for the lettuce supply chain for which a reverse genetics approach would be of value including reducing crop waste (e.g. shelf life, post harvest discolouration, pest and disease resistance), traits for sustainable production (e.g. water and nutrient use efficiency) and food safety (e.g. adhesion of bacteria to leaf surfaces). We are now developing the ability to carry out reverse genetic approaches which will allow us to respond more quickly to the demands created by stakeholders in the supply chain (e.g. policy makers, breeding companies, retailers and consumers) in response to climate change, changes in policy and the need to produce safe and healthy food. TILLING (Targetted Local Lesions IN Genomes) offers a route for carrying out reverse genetics (Comai and Henikoff, 2006) without the need for extensive genomic information. A TILLING approach requires two strategic resources; a TILLING population of seed derived from individual plants which have been treated with a mutagen to produce gene changes and a set of DNA samples produced from the individual parent plants of the seed population for molecular screening to look for changes in a ‘target’ gene. Lettuce is particularly suited for the development of a TILLING population because it is self fertile and an individual lettuce plant can produce up to 60gm of seed (= approx 60,000 seeds). Currently there is no publicly available TILLING population in lettuce. We are currently producing a TILLING seed population based on the cv Saladin which is one of the parents of the W HRI mapping population and the complementary mapping population developed by UC Davis The current project has the aim of completing the production of the Saladin TILLING population and producing the complementary DNA pools for use as a strategic resource in screening for sequence variation in ‘genes of interest’. Defra (2004) Pesticide usage survey report no 195 - Outdoor field vegetable crops in Great Britain 2003 http://www.csl.gov.uk/newsAndResources/resourceLibrary/articles/puskm/outdoorveg2003.pdf Defra (2008) Basic Horticultural statistics 2008 https://statistics.defra.gov.uk/esg/publications/bhs/2008/default.asp
United Kingdom