Identifying a plant cell barrier to breeding more nutritious crops
What if we could grow plants that are larger and also have a higher nutritional content? Scientists at Michigan State University have identified a protein that could be a major barrier to growing such plants.
Proteins perform most, if not all, of the functions of life: they promote growth, repair body tissues or build muscles. If proteins are like words, amino acids are letters. Our bodies use about 20 amino acids, in various combinations or spellings, to make different proteins.
Our bodies produce some amino acids, but there are nine essential amino acids that we and other animals cannot make. We get them from foods, such as meats, dairy products and, ultimately, plants.
For decades, scientists have been trying to increase the amino acid content of crops by intensifying their production systems, but they always come up against the same problem: crops get sick. Scientists do not know why plants suffer from the abundance of these amino acids.
The new study suggests that the target of the protein rapamycin, or TOR, is a major obstacle. The work is published in eLife.
"The TOR protein is a master regulator of metabolism in plant cells," said Pengfei Cao, a post-doc in Federica Brandizzi's laboratory. "It detects variables such as nutrient availability, energy levels, growth indices and so on. The TOR protein uses this information to control cell growth and metabolic functions."
When TOR detects an adequate amount of nutrients, it promotes growth. There's a catch: the TOR protein is so powerful in controlling many biosynthetic processes and cell structures that it can cause problems if not properly regulated.
It turns out that TOR judges nutrient availability through a sample of three amino acids. If you give a lot to the plant, TOR assumes that the nutrients are abundant and switches to an overdrive mode. In reality, nutrient availability may not be adequate.
Such an overactive TOR could change the cell structure, to the detriment of the plant's health.
Another function of TOR is to tinker with small cell filaments, called actin.
The actin filaments form the "skeleton" of the plant cell that supports the cell's endomembrane system. The endomembrane system builds many of the building blocks of the cell," Cao said. "These filaments also help to determine the shape of the cell and we find that too much TOR will lead to higher protein production and larger cell size."
"But the cell shapes are abnormal. For example, the root cells are not able to completely form the hairs in the root so that they can absorb water," Cao said.
In other words, the result is an unfortunate plant that grows at a slower rate.
"When scientists have tried to stimulate the production of amino acids in crops, the problem is not that there are too many amino acids," Cao said. "Maybe these cultures are getting sick because of the side effects on the tiny structures inside their cells. Once we understand the dynamics that make plants sick, we can try again to overproduce amino acids in a balanced and healthy way."
Cao believes that the interdisciplinary nature of the work made this breakthrough possible.
"We're working with the cellular structures of plants," says Cao. "Our people in the latest lab are studying biochemical pathways. Had we worked on this project separately, we wouldn't have the expertise to examine where defects appear."
