Pesticides can protect crops from hydrophobic pollutants

Researchers have revealed that commercial pesticides can be applied to crops in the Cucurbitaceae family to decrease their accumulation of hydrophobic pollutants (*1), thereby improving crop safety. The research group consisted of FUJITA Kentaro (1st year Ph.D. student) of Kobe University’s Graduate School of Agricultural Science, Academic Researcher YOSHIHARA Ryouhei (now an assistant professor at Saitama University) and Associate Professor INUI Hideyuki of Kobe University’s Biosignal Research Center, Senior Research Scientist KONDOH Yasumitsu, Technical Staff HONDA Kaori and Group Director OSADA Hiroyuki of RIKEN, and Lead Researcher HAGA Yuki and Senior Scientist MATSUMURA Chisato of Hyogo Prefectural Institute of Environmental Sciences.

The team developed two approaches to control the functions of plant proteins related to the transport of hydrophobic pollutants.

These findings will lead to these new functions of pesticides being utilized in agriculture, enabling safer crops to be produced.

These results were published online in the international scientific journal ‘Science of the Total Environment’ on June 23 and in ‘Environmental Pollution’ on July 18.

Main Points

• Crops in the Cucurbitaceae family can accumulate hydrophobic pollutants (such as dioxins) in their fruits from contaminated soil. Major latex-like proteins (MLPs) (*2) play a key role in transporting hydrophobic pollutants to their fruits.
• Approach 1: Treatment with a pesticide that suppresses the expression of MLP gene decreases the concentrations of hydrophobic pollutants in the xylem sap.
• Approach 2: Treatment with a pesticide that binds to MLPs inhibits the binding of the proteins to hydrophobic pollutants. Thus, the concentrations of hydrophobic pollutants in the xylem sap that were transported via MLPs were reduced.
• It was shown that pesticides could provide a simple and low-cost solution to the production of safer crops.
• This study revealed, for the first time in the world, a new way that pesticides can be used in agriculture, which is different from current methods.

Research Background

Hydrophobic pollutants include dioxins, the insecticide dieldrin, and endocrine disruptors. These pollutants are highly toxic, and their manufacture and use are now prohibited. However, these substances were used in large quantities up until they were banned, causing widespread environmental pollution that also affects agricultural land.

The Cucurbitaceae family includes crops such as cucumbers and squashes. Members of this family are different from other plant species in that they accumulate high concentrations of hydrophobic pollutants in their fruits. Associate Professor Inui et al. previously discovered that major latex-like proteins (MLPs) in the Cucurbitaceae family play a key role in this accumulation. MLPs bind to hydrophobic pollutants taken up from the soil by the roots of the plant. The Cucurbitaceae family then accumulate hydrophobic pollutants in the leaves and fruits via the sap in the stems (Figure 1). Consequently, MLPs are a major factor that causes crop contamination in the Cucurbitaceae family.

When crops are found to have accumulated hydrophobic pollutants above the residual limit, all crops grown on the same land are unable to be sold, resulting in great economic losses for the producer. For this reason, much research has been conducted into ways to remove hydrophobic pollutants from agricultural soil, however a cost-effective and efficient method has yet to be found. Therefore, there is an urgent need to develop technology for the cultivation of safer crops on arable land contaminated by hydrophobic pollutants.

Research Methodology

This study focused on pesticides that have been confirmed to be safe. The researchers tried two approaches designed to suppress the accumulation of hydrophobic pollutants via MLPs: utilizing a ‘pesticide which suppresses MLP gene expression’ and a ‘MLP-binding pesticide’. They aimed to produce safer cucurbitaceous crops.

Approach 1 (published in ‘Science of the Total Environment’)

The application of a pesticide that suppresses MLP gene expression reduces the concentration of MLPs in the roots and xylem sap (*3). This suggested that the concentrations of hydrophobic pollutants accumulated in the fruits via MLP would also be reduced.

First, experiments were carried out to select a pesticide that could suppress MLP gene expression from five types of pesticide used on the Cucurbitaceae family (the insecticides Guardbait, Starkle, and Diazinon and the fungicides Benlate and Daconil). Daconil was chosen because treatment showed that its active ingredient (*4) could reduce MLP gene expression (Figure 2 A.). Next, it was confirmed that concentrations of MLPs in the roots and xylem sap of zucchini grown in soil contaminated with hydrophobic pollutants were reduced by the Daconil treatment (Figure 2 B.). Furthermore, the concentrations of hydrophobic pollutants in the xylem sap decreased by 52% (Figure 2 C.).

Approach 2 (published in ‘Environmental Pollution’)

The application of MLP-binding pesticide inhibits the binding of MLPs to hydrophobic pollutants in the roots. In other words, the amount of MLPs that binds to the pollutants is reduced. It was hypothesized that this approach would decrease the concentrations of hydrophobic pollutants accumulated in the fruits via MLPs.

First of all, chemical arrays (*5) with approximately 22,000 compounds from the RIKEN NPDepo (*6) were used to identify compounds that bound to MLPs. The insecticide Colt that can be applied to crops in the Cucurbitaceae family was selected from commercial pesticides with similar structures to MLP-binding compounds. When Colt’s active ingredient was reacted with both MLPs and the hydrophobic pollutants, the concentration of MLPs bound to these pollutants decreased by 78%. In addition, the concentrations of these pollutants in the xylem sap fell by 15% after Colt treatment.

Further Research

This study revealed, for the first time in the world, that it is possible to cultivate safer crops in contaminated soil through the control of the plant’s functions. This achievement could reduce the number of cases where producers experience economic losses due to being unable to sell crops grown in contaminated soil. Furthermore, this will also provide consumers with safe produce.

A new method of utilizing pesticides has been revealed by this research. For the first time in the world, this study has revealed a novel aspect of pesticides beyond their original functions of preventing pests or weed growth. Pesticides are thought to be extremely safe because they have to pass numerous strict safety tests. Furthermore, the standardized treatment of crops with pesticides is simple and inexpensive. Therefore, it is anticipated that the method developed by this study to reduce hydrophobic pollutants using pesticides will become widespread across the globe.

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Glossary

1. Hydrophobic pollutants:

Hydrophobic pollutants are chemical substances that do not decompose easily in the environment and accumulate easily within organisms. They are highly toxic and have been shown to be carcinogenic and neurotoxic in humans. Hydrophobic pollutants include dioxins, polychlorinated biphenyls, and dieldrin. Their use and manufacture have been banned in 181 countries.

2.Major latex-like protein (MLP):

This protein is found in many species of plants, including Arabidopsis thaliana, grape and apple. Particularly in the Cucurbitaceae family, MLPs bind to hydrophobic pollutants, transporting them to the leaves and fruits, where these pollutants then accumulate. The original functions of these proteins have yet to be fully clarified.

3.Xylem sap:

Xylem sap is a fluid found in the xylem, which is part of the vascular bundle along with phloem. It transports water and nutrients absorbed from the roots to the leaves and fruits.

4. Active ingredient:

The chemical in a commercial pesticide that performs the product’s main function (eg. insecticide or fungicide). Pesticides also contain ingredients other than their active ingredient, such as spreading agents, which ensure that the active ingredient adheres to the plants or the pests. This study confirmed that active ingredients in pesticides could control MLP functions.

5.Chemical array:

An organic chemical compound is immobilized on a chip, enabling a highly efficient evaluation of the compound’s physical interactions with proteins of interest. It is difficult to immobilize varied types and complex structures of organic compounds, compared to immobilizing single-structured DNA for DNA arrays. RIKEN’s Chemical Biology Research Group has developed an immobilization method using a carbene with non-specific avidity (divalent carbon that only has 6 valence electrons and no charge) that will bind with the compound regardless of its functional group.

6. RIKEN NPDepo (Natural Product Depository):

A chemical compound library being developed by the Chemical Resource Development Research Unit of RIKEN’s Chemical Biology Research Group. In addition to collecting and storing natural compounds isolated from sources such as actinobacteria, they are also accepting deposits from researchers in order to build up a library of diverse chemical compounds.

Acknowledgements

This research was supported by funding from the Japan Crop Protection Association and a Sasakawa Scientific Research Grant (No. 2019-5004).