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Plant Health Week - Association genetics exploration in Lactuca sativa pinking discoloration which is independent of browning

Yao Lu discusses her PhD research on lettuce pinking:

Lettuce (Lactuca sativa L.), has become the fourth most popular vegetable in the market since the turn of the century and its commercial value ranks relatively high compared with other vegetables produced in the UK. It is mainly growing with leaves consumption in mind, but sometimes it is also grown for the stem and seeds. Many lettuce varieties commonly have short stems at the beginning of cultivation, but the stem and branches stretch during flowering and can display large heads, but this can differ in shape and color.

Lettuce with stretched stem

Different varieties of lettuce, including green iceberg, green cos and red varieties

As a leafy vegetable, lettuce put on the market for consumption is mostly packing as ‘read-to-eat’. For fresh cut salads, however, are relatively perishable and will turn to discoloration which has been reported as a big issue about limiting shelf life on food market. This loss on lettuce quality can pose a significantly economic lose from producer, retailer to customers with up to 30% loss of total crop production among processing and harvesting and around 35% waste after purchasing, and the reason for causing this loss is basically due to mechanical operation. The processing technologies, mainly refer to cutting and/or slicing on lettuce, will shorten the shelf life conspicuously by leading to the discoloration or inducing the microbial reaction boom. For minimally processed salad packs, it normally takes a few days to observe the discoloration on the cut leaf edges and it could be damaged by a range of internal and external factors hence leading to a subsequence of discoloration caused by pigments in lettuce.

Wounding of lettuce tissues induces a number of physiological reactions like ethylene production, respiration increase, oxidative browning and/or secondary metabolites formed. Unfortunately, there are not too much research on distinguishing the browning and pinking, but pinking, actually, is an independent phenomenon occurring on some lettuce varieties.

Symptoms of lettuce pinking and lettuce browning

Narrowly speaking, we think the pathway between browning and pinking is different. There are two reactions that Caffeic acid (CA) is involved, the action with quinic which is the natural acid compound from plants, they can release a few colorless o-diphenols. The other activity is under the participating of polyphenol oxidase and O2, and produce caffeic acid o-quinones (CAQ) that presents as a pink colour. These o-quinones could transfer the coloured-o-quinones back to colourless-o-diphenols. Depends on the speed between colour and colourless transferring, the lettuce will show significant symptom of obvious pink or brown, in other words, the reduction of reaction and its production between CA to chlorogenic acid will lead to the accumulation of CA to CAQ and thus produce more pink compounds.

Caffeic acid pathway for pinking and browning coloured compounds

Unfortunately, it is not clear the factors which trigger pinking and browning within lettuce, so the aim of this project is to identify the regulators of these pathways in order to determine how such discoloration could be prevented.

On the base of carrying out a backcrossing experiment between the lines showing maximal discoloration within the Saladin x Iceberg mapping population previously used to identify QTL associated with pinking and browning phenotypes, the parents of the population on this project were choosing from the lines which were showing consistently high and low levels of pinking only from a field trail with a set of 94 F7 recombinant inbred lettuce lines which are previously demonstrated variation in the development of pinking and browning discolouration.

For targeting the genes which might regulate the pink pathway, we analysed the transcriptome of pinking and browning RNA sequence and picked the genes that only performed as high and low pinking. And finally identified 8 genes to study the differences between the transcriptome of high pinking and low pinking lines.

By assessing RNA-seq , these 8 genes will be able to identify the expression differences of the putative regulators, and finally carrying out knockout experiments by using CRISPR to verify phenotype associated with the most likely regulator.

Sat 14 May 2022, 12:13 | Tags: Study, Research, Plant Health Week