The Curious Coloration of Eggplants

I love eggplant. Although common supermarket varieties are dark purple, or a pricier stripy lavender, the fruits may also be white or green. Standard chlorophyll provides the green, and anthocyanin pigments add purple, red, and even essence of blue.

A new study from researchers at South China Agricultural University, the Hunan Vegetable Research Institute, and the University of Kentucky identifies the odd genetics behind the curious colors of eggplants. Their report appears in Horticulture Research.

Three Types of Genetic Information
The new study reveals a form of genetic information beyond the sequences of the bases A, C, T, and G: patterns of short repeats. This alternate DNA language forms a subtext encoded in a repetition pattern. It lies behind more than sixty human conditions, including Huntington’s disease, a form of ALS, fragile X syndrome, frontotemporal dementia, and several types of ataxia.

Yet a third form of genetic information is gene expression – that is, which genes are turned on or off under specific circumstances. Gene expression arises from the patterns of binding of three types of small molecules to a gene’s DNA sequence: acetyls, methyls, and phosphates.

To Sun or Not to Sun
Most eggplants are photosensitive – they require sunlight to survive through the chemical reactions of photosynthesis. But the rare nonphotosensitive eggplants maintain their purpleness even in shade or short daylight hours.

In contrast, most eggplants are photosensitive. Typically, exposure to sunlight turns on production of the anthocyanin pigment that imparts the characteristic dark purple, nearly black color. In these plants, prolonged shady conditions result in a pale or splotchy fruit – an important consideration for growers and gardeners.

In contrast, nonphotosensitive eggplants seemingly ignore the lack of sunlight, producing purple fruits even in the shade. The ability comes from a “stutter“ of short repeats in the sequence of a gene called SMMYB113. The variation affects the beginning part of the gene that encodes the promoter, which is an “on” switch. The stutter, a stretch of 725 DNA base pairs repeated four times, enables the plant to crank out dark anthocyanin pigments even if the sun isn’t shining. The researchers call it “reshap(ing) visible traits by rewiring gene expression.”

“Our findings offer new insights into the molecular mechanisms controlling fruit color in eggplant and lay the groundwork for the development of molecular markers to facilitate breeding for nonphotosensitive and other fruit color variants,” the researchers write.

Perhaps the color-controlling DNA repeat can be bred into other crop species, enabling a fine-tuned control over the colors of specific plants and widening growing conditions.