LIFE SCIENCE

Cucurbitacin “transport brigade” such as “melon protection”


Courtesy of respondents

In the interaction between plants and the environment, there is a large category of compounds up to 20,000 species that play an important role – plant triterpenoids, which are also an important source of drugs, health care products and cosmetics.

Elucidating the synthesis, regulation, and transport mechanisms of plant triterpenoids will not only provide molecular targets for crop quality and resistance breeding, but also provide a prerequisite for the use of synthetic biology techniques to develop the commercial value of these plant natural products.

On August 1, 2022, Nature Plants published a paper jointly completed online by Yunnan Normal University, the Institute of Agricultural Genomics of the Chinese Academy of Agricultural Sciences, South China Agricultural University, Capital Normal University, Nanjing Agricultural University, and MIT. The study reveals for the first time the molecular mechanism of the transport of cucurbitacin, a triterpenoid compound in the melon crop of the Cucurbitaceae family, and elucidates a new mechanism by which cucurbita regulates the interaction mode of rhizosphere flora to improve plant disease resistance.

“Rare” cucurbitacin

Shang Yi, co-corresponding author of the paper and a professor at the Institute of Potato Science of Yunnan Normal University, told China Science News that cucurbitacin is a triterpenoid compound unique to plants of the cucurbitaceae family, which has an unpleasant bitter taste. The accumulation of cucurbitacin in edible parts of vegetables or fruits will seriously affect the quality and economic benefits of their goods.

On the other hand, the bitter taste of cucurbitacin has a good insect resistance effect on plants, which can help plants resist the attack of pests.

Studies have shown that cucurbitacin has liver protection, anti-inflammatory, anti-cancer and other effects on the human body. From the ancient medical book “Compendium of Materia Medica” to modern medicine, cucurbitacin has been used by humans as a potential drug to treat tumors, liver diseases and so on. “However, due to the difficulty of obtaining cucurbitacin raw materials, it restricts its wide application.” Shang Yi said.

In the previous study, the team has conducted systematic research on the biosynthesis, regulation, domestication and structural diversity of cucurbitacin.

They used omics big data to find three cucurbitamol synthetic gene clusters in cucurbitaceae crops – cucumber, melon, watermelon, containing 3 triterpene cyclases, 21 P450 oxidases and 3 acetyltransferases; It was also found that 6 transcription factors that directly regulate cucurbitacin synthesis gene clusters, among which mutations occurring on the Bt promoter of the fruit cucurbitacin regulator gene, are the key to the coordinated domestication of fruit bitterness traits in melon crops of cucurbitaceae.

Shang Yi said that cucumber breeders have used the above relevant findings to cooperate in cultivating high-quality new varieties that are “bitter in leaves and insects, and the fruit is not bitter”, which solves the problem of easy and bitter production of cucumbers in south China. “The analysis of the synthesis mechanism of cucurbitacin provides a prerequisite for the efficient synthesis of cucurbitacin using synthetic biology techniques.” Shang Yi said.

After figuring out the synthesis mechanism of cucurbitacin, they wanted to understand how cucurbitacin was transported in the plant.

“The main synthetic part of cucurbitacin in melons and watermelons is the root, while the cucurbitacin in cucumbers is synthesized in leaves. Cucurbitacin will be present in the fruits of all three crops. Exactly, how is cucurbitacin transported from these synthetic organs to stored organs? Shang Yi said.

Is it waste or proactive defense?

“The distribution of metabolites in plants depends not only on their synthesis and regulation, but also on their transport between plant tissues and within cells.” Ma Yongshuo, co-corresponding author of the paper and a postdoctoral fellow at the Institute of Agricultural Genomics of the Chinese Academy of Agricultural Sciences, told China Science News that compared with the synthesis and regulation research, the research on the transport mechanism of metabolites has progressed slowly, which is a difficult point in the field of plant metabolism research.

At present, there are few studies on the mechanism of transport of plant triterpenoids, and the related transporters have not been reported. So their research began at the root of the cucurbitain synthesis site of melon and watermelon.

Zhong yang, a co-first author and a doctoral student at the Institute of Agricultural Genomics of the Chinese Academy of Agricultural Sciences, introduced that they used hydroponics to find that melon and watermelon roots can secrete cucurbitacin into the nutrient solution, and then a large amount of cucurbitacin was also detected in the rhizosphere soil of soil planting materials.

“The root is the main part of the synthesis of cucurbitacin, and the synthesis of triterpenoid metabolites such as cucurbitacin by plants consumes a lot of energy, but after synthesis, it is excreted into the soil, which seems to be a resource ‘waste’.” Ma Yongshuo said that in recent years, researchers have found that secondary metabolites secreted by plant roots play an important role in selectively shaping the rhizosphere microbiome, and are also a research hotspot in the field of plant metabolism, but the mechanism of secondary metabolites entering the soil has not been analyzed.

The rhizosphere microbiome, known as the plant’s “second genome,” can affect processes such as plant root growth and development, root resistance to living and abiotic organisms, and root-to-nutrient uptake. Therefore, they took cucurbitacin excretin from watermelon as the starting point for research, trying to analyze how roots secrete secondary metabolites and regulate the composition of rhizosphere microorganisms, thereby improving plant resistance.

Wang Xiaohan, co-first author of the paper and a doctoral student at the School of Life Sciences of Capital Normal University, introduced that the study found that melons and watermelons can drain cucurbidin B and cucurbitacin E from the root to the soil. Using omics big data, they identified a MATE transporter gene (CmMATE1/ClMATE1) from the melon and watermelon genomes. This gene is not only distributed in clusters with the synthesis gene of cucurbitacin, but is directly regulated by the previously discovered cucurbitacin regulators; The protein produced by this gene transports cucurbitacin B and cucurbitacin E.

Melon secretes cucurbitacin B from roots to improve disease resistance model. Courtesy of respondents

Using this gene-edited mutant, it was further demonstrated that the transporter was involved in the transport of cucurbitain in plants, and that the transport process was regulated by the biosynthesis of cucurbitacin. “When the function of the MATE transporter gene is inactivated, in order to avoid the excessive accumulation of cucurbitacin synthesized in the cell, the cell will weaken the synthesis of cucurbitacin by reducing the expression level of the synthesized gene, thereby maintaining the cucurbitacin at a low level; Conversely, when the transporter gene is overexpressed, a large amount of cucurbitacin is excreted, which further induces high expression of the synthetic gene. Ma Yongshuo explained.

Lays the foundation for the biosynthesis of cucurbitacin

So, how does cucurbitacin secrete into the soil through the roots, and how to improve plant resistance?

Xun Weibing, co-first author of the paper and associate professor of the College of Resources and Environment of Nanjing Agricultural University, introduced that in order to answer why cucurbitacin B can enhance plant disease resistance, they first determined that cucurbitacin B has no direct antagonism activity against fusarium, a soil-borne blight pathogen.

“We speculate that cucurbitacin B should indirectly inhibit the abundance of fusarium rhizosphere by regulating the interaction patterns of rhizosphere beneficial bacteria.” Xun Weibing said that through the analysis of rhizosphere microbial communities and metagenomic sequencing data, combined with in vitro biochemical experiments, it was found that the root secretion of cucurbita B can be used as a carbon source to promote the growth of Enterobacter rhizobia, and Enterobacter can promote the enrichment of Bacillus, a plant probiotic bacterium that can effectively antagonize Fusarium, which can effectively antagonize Fusarium, thus establishing an effective defense system for melon against soil-borne pathogens, Fusarium.

“Cucurbitacin is first synthesized in the cytoplasm, then captured by MATE transporters anchored on the cytoplasmic membrane, transported exocytos, and finally secreted into the soil for a role.” Shang Yi concluded.

It is worth mentioning that they also found the transport molecular mechanism of cucurbitacin in cucumbers. In cucumbers, cucurbitacin is synthesized in the leaves, and its transport also relies on the MATE transporter protein, which stores cucurbitacin in vacuoles. The vacuole is the organelle of plants that store various harmful substances, which is also a mechanism for plant self-protection.

Shang Yi said that breeders can use transporters as breeding molecular targets to breed new varieties of high-quality disease resistance that can secrete high concentrations of cucurbitacin. At the same time, this study will help to improve the application prospect of cucurbitacin in integrated pest management.

Although cucurbitacin has important medicinal value, it is difficult to obtain raw materials. The discovery of its transporter will accelerate the biological study of cucurbitacin synthesis, and achieve or accelerate the directional extracellular transport of cucurbitacin through transporters, thereby reducing its burden or toxicity on cell growth, which is conducive to improving cucurbitacin production. (Source: China Science Daily Li Chen)

Related paper information:https://doi.org/10.1038/s41477-022-01201-2



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