Decoding plants' metal-transporting systems could help make food safer

The Bond Life Sciences Center's newest faculty member has a goal of breeding plants that will take up metals essential for good health, but resist accumulation of toxic metals.

Plant molecular biologist David Mendoza-Cozatl specializes in studying the mechanisms plants use to move metals, but in the process he is uncovering how plants carry other molecules from leaves to seeds. His basic research on these transport mechanisms could be of service to a wide variety of plant breeders, whether they are trying to make sweet corn sweeter or soybean seeds more oily.

Mendoza's first priority, understanding the movement of metals, has its own potential rewards.

"Contamination of food by heavy metals has been associated with cancer, bone disease, Alzheimer's, and other neurological disorders," said Mendoza. Deadly metals like cadmium and mercury can be drawn up through plant roots and passed along the food chain. Many soils are laced with industrial pollutants, and sometimes people growing food plants in such soils are not aware of this contamination.

"Many places all over the world are heavily contaminated and people still grow plants there; they have to grow plants for food," Mendoza said. Thus, creating food plants that do not accumulate toxic metals can protect human health, he said.

Conversely, "we would also like to eventually engineer plants that can selectively accumulate good metals," such as zinc and copper, which promote health, Mendoza said. "Even though they could be grown in soil that may contain a mixture (of metals), they would be able to select only beneficial metals."

Another possible outcome from Mendoza's research is plants bred to draw in quantities of toxic metals; these could be used to extract pollution from contaminated soil and to concentrate the undesirable material into a harvest that could be removed.

The first step in engineering such plants is to find the genes that control metal uptake and to understand the transport mechanisms, Mendoza said.

Phloem transport system not well understood

Plants have two primary systems for moving materials. Xylem, a fibrous woody structure occupying much of a plant's stem core, primarily transports water and basic compounds, such as salts. Phloem - a thin band often surrounding the xylem - transports more complex molecules, such as sugars, fats, hormones, and metals. As a specialist in the movement of metals, Mendoza scrutinizes the phloem transport system.

"Phloem does something that xylem cannot do; phloem moves molecules from leaves to other leaves and into seeds," Mendoza said. "There's a lot of information now about how plants take metals from soil into the roots and move them from roots to leaves," he said, but "very little is known about what happens to the metals once they reach the leaves: from leaves to other leaves, from leaves to roots, and from leaves to seeds." The cornerstone of his research is about seed nutrition. "You put things into the seeds through the phloem. It's the main pathway to load the seeds with nutrients, or metals, or toxins," he said.

"The identity of most of the phloem transporters is just not known," Mendoza continued. A transporter is a protein that assists with moving materials around; each type of transporter is created in response to an instruction from a specific gene.

Some phloem transporters have been explored, such as the ones for sucrose, but no comprehensive inventory exists for the other kinds of transporters. "I came up with the idea of creating this inventory of phloem transporters, which nobody had tried before," he said. That idea brought Mendoza to Mizzou, and implementing it is now his job. One-by-one, he is identifying each of the transporters present throughout the phloem system.

Mendoza's search was motivated by his interest in transporters that move metals, "but in the long term it (the inventory) is going to be good for any molecule that you want to move around the plant." This research could be useful to a host of other plant scientists, not just those who study metals. For example, anyone who is trying to increase the antioxidant content in one kind of seed, or the concentration of essential amino acids in another variety, would likely be interested in knowing how these substances are moved through the phloem and into seeds.

When the inventory is complete, Mendoza expects to be able to say to other scientists, "tell me which molecule you want in seeds, and I'll tell you which transporter to use," he said.

At this time, Mendoza has identified nearly 50 specific phloem transporters. "I don't know the specific function of each one yet; I need to test what they do," he said. "For example, do they transport wax, or peptides, or amino acids?" By the end of the year, he expects to complete the first inventory and to identify each of the different metal transporters, he said.

David Mendoza-Cozatl began duties as an assistant professor in MU's Division of Plant Sciences in September. He was previously a post-doctoral researcher at the University of California at San Diego. His doctorate degree is in biochemistry from Universidad Nacional Autonóma de México, in Mexico City.

Mendoza uses the plant Arabidopsis for his studies; this fast-growing weed is a "research model" of choice for plant scientists worldwide. This plant's entire genome has been sequenced, which helps scientists compare studies and build on each other's discoveries. Findings involving Arabidopsis are usually applicable to other plants. Mendoza expects to translate his findings into practical applications first with staple grains, then later with livestock forages.

For the present, Mendoza is investigating the toxics cadmium, mercury, and arsenic, plus the essential metals iron, zinc, manganese and copper. The plant transport mechanism that Mendoza studies does not apply to chromium or lead, but he said he may expand his studies to include these elements after the initial inventory of phloem transporters has been completed.

Plant breeding could take two paths

Once the genetics of phloem metal transport are understood, work can begin on breeding plants that will take up fewer toxic metals and accumulate more beneficial metals, Mendoza said.

He sees two paths to accomplish this. One is plant bioengineering, which is the genetic modification of plants. But another option takes into consideration a growing resistance to purchasing GMO seeds, particularly by buyers outside the U.S. This option uses genetic knowledge to assist natural breeding.

"Because we know which gene it is, or group of genes, we can go to natural populations - take soybean, maize, cocoa or coffee - and find which of these natural varieties have or don't have these traits that we know mediates metal uptake," Mendoza said. He believes such plants exist. "It's very likely. We just have to find them."

Once they're located, hybrid varieties can be bred using time-honored hand-pollination methods. Researchers today have an advantage over old-time plant breeders; they can test the DNA of parents or offspring to search for genes that offer desired characteristics.

These two possible paths apply to any seed production, not just those relating to metal uptake. One disadvantage of natural breeding is that it produces results more slowly than does genetic engineering.

"At some point we're going to run out of time, and run out of space to grow food," Mendoza said. "We need plants more tolerant to stress. We need plants that can do more with less water, but more heat and more contaminants. It may require a bit more than just breeding to get those plants ready."

Controlling metal uptake in plants has potential benefits

In the past, land contaminated with heavy metals, such as at Superfund sites, have been remediated by moving the contaminated soil elsewhere, which is an inefficient and expensive process, Mendoza said. He's hoping his research will lead to creation of engineered plants that can remove toxic metals, which would offer a cheaper and more efficient way clean up such sites, he said. Called "phytoremediation," this method concentrates metals from the soil into the plant. When the plant is harvested and incinerated, the metals are contained in the ash, he said.

Plants currently exist that naturally accumulate significant amounts of toxic metals. Scientists have been able to identify them by examining plants growing on mine tailings. "They look healthy, and they can tolerate high concentrations of metals, but the problem is they grow so slowly that it's not efficient for phytoremediation. So we want to identify the mechanisms of these natural hyper-accumulators and transfer them into a fast growing plant," Mendoza said.

Similar to phytoremediation is "phytomining." Even with today's high-tech mining processes, "the purification of precious metals is not particularly efficient," Mendoza said. He is working to develop plants that can concentrate metals from mine wastes, so "plants can actually rescue or recover precious metals."

A third way to utilize plants at mining sites is through "phytostabilization."

"In mine tailings, especially if there is a lot of wind, those soils are going to spread all over the place," Mendoza said. Scientists are working to "generate plants that accumulate metals, and then grow those plants in mine tailings, to protect the soil, to keep it on that site and stop the blowing and spreading." This method would be helpful on isolated sites, such as one "where remediation is not immediately needed, but you want to prevent further spread of the contaminants." A down side of this technique is "the plants may move toxic metal into the leaves," he said. "If an animal consumes the leaves, and then something consumes that animal, it goes up the food chain."

These efforts to develop plants that can remediate toxic soils and remove or stabilize metals at mine sites are certainly important, but they are secondary to Mendoza's primary focus.

"The main goal of this research is: how can we make safe food having essential metals, without having to be concerned about plants accumulating the toxic ones?" he said.