Tomatoes live in their own ‘parallel universe’ in the plant kingdom

Researchers have deciphered a surprising genetic mystery centered around the sugary substance found in what gardeners commonly refer to as “tomato tar.” This sticky, gold-black substance, which often clings to the fingers of those who have pruned tomato plants barehanded, is actually made up of acylsugars and acts as a natural flypaper for potential pests.

Getting to the root of the acylsugar tomato enigma

Robert Last, a researcher at Michigan State University (MSU), led the team that made these fascinating discoveries. The Last lab specializes in acylsugars and the tiny, hair-like structures called trichomes, where they are produced and stored.

Recently, other researchers reported finding acylsugars in tomato roots, which came as a surprise to the plant science community. The MSU team wanted to investigate how these root acylsugars functioned and where they originated from.

The researchers found that tomato plants synthesize chemically unique acylsugars in their roots and trichomes through two parallel metabolic pathways.

“What’s so remarkable about these specialized metabolites is that they’re typically synthesized in highly precise cells and tissues,” said Rachel Kerwin, a postdoctoral researcher at MSU and first author of the latest paper.

“Take for instance acylsugars. You won’t find them produced in the leaves or stems of a tomato plant. These physically sticky defense metabolites are made right in the tip of the trichomes,” she continued.

Knocking out genes to find tomato acylsugar secrets

To tackle this evolutionary enigma, the lab members collaborated with specialists at MSU’s Mass Spectrometry and Metabolomics Core and staff at the Max T. Rogers Nuclear Magnetic Resonance facility (NMR).

They compared metabolites from tomato seedlings’ roots and shoots and found noticeable differences in the chemical make-up of aboveground and belowground acylsugars, defining them as different classes of acylsugars entirely.

Robert Last offers a useful analogy to explain the geneticist’s approach to biology: “Imagine trying to figure out how a car works by breaking one component at a time. If you flatten a car’s tires and notice the engine still runs, you’ve discovered a critical fact even if you don’t know what the tires exactly do.”

By knocking out candidate genes, the researchers proved the existence of two parallel metabolic pathways in the same plant.

When genes cluster for a purpose

Jaynee Hart, a postdoctoral researcher and second author on the paper, investigated the functions of trichome and root enzymes.

She found a promising link between root enzymes and the root acylsugars, further proving the parallel metabolic pathways in a single tomato plant.

The paper also reports an unexpected twist concerning biosynthetic gene clusters (BGCs), which are collections of genes physically grouped on the chromosome and contribute to a particular metabolic pathway.

The Last lab discovered that the root-expressed acylsugar enzyme resides in the same cluster as the genes linked to trichome acylsugars in tomato plants, despite being expressed in different tissues and under different conditions.

19 million years of evolution

Kerwin delved into the evolutionary trajectory of Solanaceae species to identify when and how these two unique acylsugar pathways developed.

The researchers highlighted a moment some 19 million years ago when the enzyme responsible for trichome acylsugars was duplicated, eventually leading to the root-expressed acylsugar pathway.

“Plants have evolved to make so many amazing poisons and other biologically active compounds,” said Last. “From pharmaceuticals, to pesticides, to sunscreens, many small molecules that humans have adapted for different uses come from the arms race between plants, microbes and insects.”

These breakthroughs also emphasize the importance of natural pesticides, which defense metabolites like acylsugars ultimately represent.

Last raises the question, “If we find that these root acylsugars are effective at repelling harmful organisms, could they be bred into other nightshades, thereby helping plants grow without the need for harmful synthetic fungicides and pesticides? These are questions at the core of humanity’s pursuit of purer water, safer food and a reduced reliance on harmful synthetic chemicals.”

Acylsugars, tomato plants, and the quest for safer food

In summary, the discoveries made by the Last lab at Michigan State University reveal the fascinating world of acylsugars and the complex evolutionary history of tomato plants.

By unraveling the mysteries of tomato tar and identifying the parallel metabolic pathways responsible for acylsugar production in trichomes and roots, these researchers have opened up new avenues for exploring the potential of natural pesticides.

Their work contributes to our understanding of plant biology and raises important questions about the future of sustainable agriculture and the role that acylsugars could play in reducing our reliance on harmful synthetic chemicals.

As we continue to explore the secrets hidden within the Solanaceae family, we move closer to achieving the goal of safer food, purer water, and a greener future for all.

The full study was published in the journal Science Advances.

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