Getting it back on our plate: a genetic tool for flavour enhancement in tomato

A group of scientists and plant breeders from China, USA, Spain, and Israel united efforts in a large-scale project to improve flavour in modern varieties (including elite inbreds and hybrids) of tomato. Consumers were also involved in this study providing the researchers with relevant information on flavour preferences.

In this study published in Science (2017), the researchers initially quantified tens of flavour-associated chemical compounds across 398 modern varieties, heritage varieties and crop wild relatives of tomato. Chemicals included sugars, acids and volatiles that were likely to contribute to flavour. From the information given by the consumer panel, the researchers managed to identify 33 chemicals that correlated with consumer preferences and 37 that correlated with flavour intensity. These chemicals were selected for genetic mapping. Therefore, Tieman et al. (2017) provides a genetic tool (a set of genetic markers associated with flavour) which can now be used as a reference roadmap for reintroduction of desirable genetic variants from heritage varieties into modern varieties through molecular breeding.

Modern commercial varieties are characterised by low levels of genetic diversity in their DNA and consequently, by a reduction in agronomic variability (including flavour) due to the intense selection pressure they suffered during their many cycles of breeding.

Flavour has not been a focus for improvement in plant breeding. Tieman et al. (2017) explained that flavour is much harder to characterize than yield, disease resistance, firmness or other more common targets in plant breeding. In this study, the researchers had to overcome problems specific to flavour characterization such as being highly affected by the environment and being a complex feature shaped by multiple chemical interactions. Flavour was also difficult to quantify as each chemical contributing to flavour exists at picomolar to nanomolar concentrations in fruits, which detection depended on the use of technically challenging chemical assays.

Among the many flavour associated genetic sites identified and mapped during this study, Lin5 was considered one of genes most likely to increase sugar content, upon reintroduction. Having detected a negative correlation between fruit size and sugar content, the researchers suggested that selection for bigger size in fruits during domestication had swept away Lin5, which was however, still maintained in unselected crop wild relatives.

Similarly, the presence of two carotenoid-derived volatiles (MHO and geranylacetone) could also be linked to the history of domestication and breeding selection in tomato. MHO (6-Methyl-5-hepten-2-one), derived from oxidative cleavage of lycopene, is proportional to the level of lycopene (colour molecule). The content of MHO, which is positively associated with flavour, is an example of how selection for colour has affected the genetic composition of the crop. Breeders, having traditionally selected towards deep-red fruits, which could in parallel caused positive selection towards higher MHO levels. On the other hand, geranylacetone, which is not linked to colour but is still positively associated with flavour, was either gradually selected against or randomly swept out, during human selection, with modern varieties showing a loss of desirable genetic variants for this compound.

In terms of future breeding targets, the authors point out that sometimes there is a trade-off between improvement of flavour and the maintenance of other desirable features. For example, sugars can only be increased at the expense of fruit size (which might be a relatively minor problem to most consumers). Alternatively, however, genetic variants for geranylacetone, which significantly enhance the perception of sweetness, could be reintroduced without affecting fruit size.

The idea of going back in time and being able to feel the taste of the “original” tomato, the one our ancestors tasted, is fascinating and attractive to consumers industry. Maybe parents would benefit the greatest from not having to spend so much energy in getting the children to eat their fruit and veg everyday, as these would taste much better!

Science partners in this study included the Chinese Academy of Agricultural Sciences (Agricultural Genomics Institute and the Institute of Vegetables and Flowers), the Plant Innovation Centre of The University of Florida (USA), RAPiD genomics (USA), the Instituto de Biologia Molecular e Celular de Plantas of The University of Valencia (SPAIN), the Faculty of Agriculture from The Hebrew University of Jerusalem (Israel), and the School of Forest Resources and Conservation of The University of Florida (USA).

This study is available at https://www.ncbi.nlm.nih.gov/pubmed/28126817. For more information, please contact the corresponding authors Sanwen Huang (huangsanwen@caas.cn) and Harry Klee (hjklee@ufl.edu).