Changing the cycle: Deciphering the uptake and metabolism of sulfur in plants

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Hideki Takahashi, MSU assistant professor of biochemistry, uses GFP to observe subcellular processes in plant cells (view larger image)

A key component of crop nutrition involves assessing and enhancing soil health. According to the U.S. Department of Agriculture, in 2013, U.S. farmers spent more than $11.3 billion on fertilizers to boost soil fertility and to provide their land with lacking nutrients.

Many growers are burdened by the high cost of fertilizers and have concerns about their environmental impact. Often, nutrients that are not absorbed by crops leach into the surrounding water and soil systems, becoming a source of pollution that affects all components of that ecosystem.

Hideki Takahashi, Michigan State University (MSU) assistant professor of biochemistry, is uncovering insights into the uptake and metabolism of sulfur, an agriculturally important macronutrient. His long-term goal is to help plants be more efficient at sulfur uptake, thereby reducing farmers’ reliance on certain fertilizers. This research will also enable scientists to find efficient and sustainable ways to utilize sulfur-containing metabolites that are essential to human and plant health.

“It’s important to understand these processes and pathways because plant sulfur metabolism is an essential component of the sulfur cycle in nature; also, plants produce many useful sulfur-containing compounds,” he said. “Some of these compounds are related to stress mitigation in plants; others can help humans become healthier.”

Takahashi explained that all cells require sulfur to function optimally. Plants and animals employ sulfur as a nutrient to aid in the production of proteins, amino acids and enzymes, and to overcome disease and other stresses.

To help reduce fertilizer use in agricultural systems, Takahashi is exploring two ways to make plants more effi nt at sulfur uptake. The first requires engineering sulfate transport systems in roots.

“If we increase the number of sulfate transporters in plant roots from one to 10, the plant will be able to take up 10 times more sulfate,” he explained. “Or, if we can change the characteristics of that transporter to take up 10 times more molecules, then the plant will also take up more sulfate. This may sound basic, but it’s very challenging to accomplish.”

The second course he’s exploring involves finding a way to increase the expression of genes responsible for producing sulfate transporters, which work to move sulfate across the cell membrane. In theory, it is possible to produce more sulfate transporters if the transcription factors responsible for initiating the expression of these transporters are overexpressed.

“However, plants are really smart and clever, so there should be a feedback mechanism that says, ‘Oh — now there’s too much sulfur; don’t do this,’” he said. “If that happens, we’ll have to find a way to override this response.”

Takahashi’s second research goal — to synthesize sulfur-containing metabolites that are useful for human health — also requires complex biochemical engineering.

“To achieve this, we need to manipulate sulfur metabolic pathways in the cell,” he said. “We have to identify which enzymes are expressed in what cell compartments during sulfur metabolism. In the past 20 years, most of the discoveries that could be made about model plant species have been revealed. Because of that, we can focus on modifying specifi pathways, processes and interesting enzymes to explore the effects of those actions.”

The benefi of the sulfur-containing metabolites that he plans to synthesize depends on the plant being studied.

One genus of plants that he cites as an example is Brassica, a genus of plants in  the mustard family that includes cabbage, cauliflower, broccoli, Brussels sprouts and oil-producing rapeseed canola. Brassicas are relatives of Arabidopsis thaliana, the model plant species that Takahashi uses in his research. Research indicates that sulfur- containing metabolites from plants in this family induce detoxification enzymes, stimulate the immune system, decrease the risk of cancers, and inhibit malignant transformation and carcinogenic mutations of cancer cells in humans.

To date, Takahashi and his team have made major progress in their research eff  ts to reach their goals.

Through genetic screenings of mutant Arabidopsis, Takahashi’s team has identifi a transcription factor that activates the expression of sulfate transporters in plants.

“This piece of information is very important because this gene is an activator,” he explained. “So if we want to overexpress sulfate transporters, we can introduce this transcription factor into plants to see how sulfate transporters are overexpressed.”

Using Arabidopsis, he has also cloned and characterized many sulfate transporters. His findings uncovered which sulfate transporters are expressed in specifi cell types and membrane systems. He also identifi  several important transporters that are responsible for the uptake of sulfate in the epidermal layer of plant roots.

“Moving forward, my research will expand to see how these transporters and metabolic enzymes can change the balance of sulfur use in plant cells,” he concluded. “My major interest is in the biochemistry of sulfur metabolism, but key scientifi discoveries from this work have the potential to leverage the development of plants and compounds that will improve our quality of life in the long term.”

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