Researchers have pinpointed an essential protein crucial for the production of steroidal glycoalkaloids and saponins in Solanum plants, revealing for the first time the ecological significance of these steroidal saponins in defending against insects.
A team from the Max Planck Institute for Chemical Ecology has discovered that the protein GAME15 plays a vital role in the biosynthesis of steroidal glycoalkaloids and saponins in Solanum species. Their findings, published in Science, indicate that this protein is necessary for synthesizing these natural products from cholesterol. Black nightshade (Solanum nigrum) plants lacking the ability to produce GAME15 and thus the steroidal saponins showed increased vulnerability to pests like leafhoppers and Colorado potato beetles. This research could pave the way for creating high-quality steroidal compounds for medical purposes and aid in developing targeted pest control strategies in agriculture.
The production pathway for specific steroidal compounds in nightshade plants (including potatoes, tomatoes, and eggplants) begins with cholesterol. Numerous studies have focused on the enzymes that generate steroidal glycoalkaloids. While the genes responsible for creating the frameworks of these specialized metabolites are known, researchers have not yet successfully replicated these compounds in other plant species. The ‘Specialized Steroid Metabolism in Plants’ project group, within the Department of Natural Product Biosynthesis and led by Prashant Sonawane (now Assistant Professor at the University of Missouri), aimed to uncover the missing element in this process.
Prashant Sonawane explains, “Our research was focused on identifying a crucial component of the biosynthesis pathway that had previously eluded researchers and understanding its role within that pathway. We also aimed to see if we could recreate the biosynthetic pathway once we pinpointed the missing component. Additionally, we sought to learn more about the ecological significance of steroidal saponins in these plants.”
GAME15 — a significant but previously unknown factor in steroidal compound production in Solanum
The research team chose black nightshade (Solanum nigrum) for their studies because this plant produces a variety of steroidal compounds in different parts, all originating from cholesterol. In the leaves, the principal steroidal metabolite is a saponin known as uttroside B, while in the berries, the notable compounds include steroidal glycoalkaloids like α-solasonine, α-solamargine, and malonyl-solamargine. The enzymes GAME6, GAME8, and GAME11 contribute to the production of both types of compounds and are present in both leaves and berries. The team employed confocal microscopy to identify the cellular locations of these enzymes. Through biochemical and molecular analyses, they discovered a gene that codes for the protein GAME15, which, despite being part of a family of cellulose synthase-like proteins, does not contribute to cellulose production but is crucial for steroidal compound biosynthesis.
Marianna Boccia, the study’s lead author, elaborates, “Our experiments confirmed that GAME15 interacts with the enzymes GAME6, GAME8, and GAME11, which are responsible for the early stages of cholesterol hydroxylation, leading to the pivotal furostanol-aglycone (16,22,26-trihydroxycholesterol). This represents a crucial junction between the synthesis of saponins and glycoalkaloids. When we knocked out the GAME15 gene in Solanum nigrum, we observed that these plants were unable to synthesize steroidal glycoalkaloids and saponins.”
Medical potential of steroidal compounds from Solanum
Steroidal saponins and glycoalkaloids are a group of compounds with significant medical potential. Recent research indicates that certain saponins can effectively treat liver cancer. Additionally, steroidal glycoalkaloids exhibit anti-cancer properties along with antimicrobial and anti-inflammatory effects. “With the identification of GAME15, we were able to recreate the metabolic pathway for steroidal compounds in heterologous hosts, such as Nicotiana benthamiana, all the way to producing furostanol, a precursor for steroidal saponins, and solasodine, a direct precursor for glycoalkaloids,” Prashant Sonawane remarks. This process, known as ‘pharming,’ merges ‘pharmaceutical’ and ‘farming,’ involving genetically modified plants with an integrated biosynthetic pathway designed for large-scale, cost-effective pharmaceutical production. The findings thus highlight new avenues for enhanced steroid-based compound production.
Discovery of the ecological significance of saponins in insect defense
While steroidal glycoalkaloids are recognized as vital plant defenses due to their toxicity in solanaceous plants, including potatoes, tomatoes, and eggplants, the ecological role of steroidal saponins in black nightshade had been previously unexplored. The greenhouse team observed that GAME15 knockout plants, unable to produce saponins, were more vulnerable to insect herbivores compared to wild-type plants. This prompted further ecological tests, including feeding trials with two natural pests of Solanum.
Marianna Boccia explains, “In our initial experiment, we offered two herbivores, the leafhopper Empoasca decipiens and the Colorado potato beetle Leptinotarsa decemlineata, a choice between leaves from wild-type plants (which produce steroidal saponins) and leaves from GAME15 knockout plants (which lack these saponins). After a week, we observed that both herbivore species primarily consumed the knockout leaves, showing a clear preference over the wild-type leaves. In a follow-up experiment, we conducted a ‘forced-feeding’ bioassay with Colorado potato beetles, where individual beetles were placed on detached leaves from either plant type. Within six hours, beetles eagerly fed on knockout leaves devoid of steroidal saponins while largely avoiding wild-type leaves, appearing to prefer starvation over consuming them,” providing initial evidence of the defense role of steroidal saponins in plants. This disparity in compound distribution may indicate a specialization aimed at protecting various plant tissues, with leaves being more prone to herbivore attacks while berries are at risk from pathogens.
Sarah O’Connor, Director of the Department of Natural Product Biosynthesis and senior author of the study, states, “Our discoveries illustrate how Solanaceae plants have adapted a cellulose-synthase-like protein, initially involved in cellulose production, to assume a critical role necessary for synthesizing compounds geared towards defending against pathogens. This finding opens up new possibilities for engineering crops with enhanced pest resistance and the development of significant steroid-based compounds for combating cancer and other diseases.”
