How fruits or flowers produce particular colouration or colour patterning is a question that often has complex answers. It may also seem to be a relatively trivial question. Don’t be fooled though – Barbara McClintock famously won the 1983 Nobel Prize in Physiology or Medicine for discovering transposons while working on why maize kernels often have mosaic colour patterns. So these ‘trivial’ questions can actually tell us a lot about fundamental underpinning mechanisms in biology.
Similar to mosaic maize kernels, Green-Stripe (also known as Green Zebra) is a naturally-occurring trait in tomatoes used to produce tomato cultivars with a striking fruit colour variegation. This colour pattern is not only attractive to tomato growers and the buying public, but is also thought to be more attractive to seed-spreading bird. However, what causes this colour patterning in these tomatoes is unknown. In a recent paper available Open Access in New Phytologist, Genzhong Liu and colleagues from China investigate the genetic basis for the colour variegation of Green-Stripe tomatoes.
Using a complex series of genetic experiments, Liu and colleagues pin down the presence of the Green-Stripe trait to a single genetic locus, which they identify to be a gene called TOMATO AGAMOUS-LIKE 1 (TAGL1). TAGL1 encodes a transcription factor that is already known to be involved in the development of tomato fruits, but its involvement in the Green-Stripe trait is a new discovery. Surprisingly, tomato plants with or without the Green-Stripe trait show no difference in the coding sequence of the TAGL1 gene or its immediately surrounding regulatory regions.
The authors therefore hypothesise that the differences between plants with or without the Green-Stripe trait may be due to an epigenetic difference (a reversible heritable DNA modification) in the TAGL1 gene or its adjacent regions. The Green-Stripe trait consists of uneven fruit colouration based on green stripes and light green stripes. Liu and colleagues find that epigenetic modification of the upstream sequence of the TAGL1 gene is greater in the green stripe areas compared to the light green stripe areas. The authors then show that enhanced epigenetic modification upstream of TAGL1 down-regulates its expression in green-stripe areas of the fruit relative to light green stripe areas.
In order to identify how differential expression of TAGL1 in fruits may lead to colour variegation, Liu and colleagues investigate what other kinds of genes may also be differentially expressed in similar patterns. They find that around 90 genes involved in chloroplast development are upregulated in green stripe areas compared to light green areas. Further work directly links action of TAGL1 to downregulation of some of these chloroplast development genes. So in green stripe areas, low expression of TAGL1 due to the epigenetic modification allows expression of chloroplast development genes, resulting in the green pigmentation so characteristic of chlorophyll. In light green stripe areas, expression of TAGL1 is higher due to reduced epigenetic modification, which acts to decrease expression of the chloroplast development genes and produces a lighter green colour.
Liu and colleagues provide a nice example of linking a plant trait recognisable to a wide variety of people to a distinct event and the DNA-level, and providing an idea of what happens in-between. Heritability of plant traits has been known as a principle since the days of Gregor Mendel, but detailed knowledge of the different ways that this actually happens is much more recent to us, supported by the now wide-availability of genetic and molecular techniques. Long may it continue!