by Robert Sanders
Berkeley and Carnegie Institution scientists have found a mutation in plants that makes the tap root accumulate large amounts of oils, proteins and starch. The discovery could lead to genetically engineered plants that store commercially useful substances in an enlarged root.
The finding also could make possible the creation of more nutritious root crops with a better balance of oil, protein and starch. Most root crops in the Third World, such as cassava and taro, are a source of starch only and often a poor source, and when eaten as a dietary staple can lead to deficiency diseases.
The mutation, called pickle because it creates a root that looks like a pickle, was discovered independently by two teams, who reported their findings in a joint paper in the July 4 issue of Science.
The mutation was found in the experimental plant Arabadopsis thaliana, a member of the mustard family commonly used for genetic experiments.
Once the teams find and clone the gene involved in the mutation, any scientist will easily be able to track down the analogous gene in other plants. Eventually they should be able to engineer plants that store oils or proteins in an enlarged tap root. Most enlarged tap roots, ranging from turnips and radishes to the sweet potato, contain mainly starch or sugar and almost no oil or protein.
"Before we can exploit this finding we need to study the developmental biology and genetic defects of the mutant," cautions Z. Renee Sung, professor of plant and microbial biology and leader of one of the teams. "What is the genetic program that allowed the activation of perhaps hundreds of genes telling the root cells to devote all their energy to synthesis and storage of these oil, protein and starch products?"
"One of my goals is to make what I call an 'oil potato'-a highly productive plant that produces a commercially useful oil," says Christopher Somerville, director of the Department of Plant Biology of the Carnegie Institution of Washington in Stanford, and leader of the second team. "This is a real step in that direction." Somerville has already created plants with seeds containing a type of plastic, and other plants whose seeds contain a precursor to nylon.
Aside from commercial or agricultural applications, Sung thinks that mutations in the pickle gene may have been the first step in the evolution of plants with large storage roots, including widespread root crops like the sweet potato, yam, cassava, carrot, turnip and radish. Perhaps the original line evolved a big root as a result of an initial mutation in the pickle gene," Sung says.
The two teams both chanced on the mutation independently-Somerville and post-doctoral fellow Joe Ogas while looking for Arabadopsis mutants with altered patterns of cell division; Sung and research associate Jin-Chen Cheng while looking for starch-producing mutants.
When Ogas and Somerville saw Sung's poster at a scientific meeting two years ago, they suggested a collaboration on the research.
Sung has studied Arabadopsis mutants for nearly eight years as a way to discover the genes responsible for maturation of the plant, in particular the development of plant roots, flowers and seeds.
When she and Cheng found a mutation four years ago that produced plants with an abnormal tap root that stored abnormal amounts of starch, they dubbed it starchy.
Somerville's team, long interested in the genes controlling oil composition in plants, found a mutation in the same gene that caused Arabadopsis to store oil in its tap root, and dubbed it pickle.
Both eventually adopted pickle as the name of the mutations and the gene itself.
Normally plants produce a primary or tap root that grows downward and lateral roots that grow outward, all of which provide nutrients and water-plus support-for the plant. The tap root is normally not a storage organ, and is enlarged in only a few plants, such as the radish, to store starch.
Root crops are generally not good sources of starch, however, and are poor sources of protein and oil.
The pickle mutation creates tap roots rich in all three, Sung says, and in fact mimics what happens in seeds, which typically are the major structures accumulating and storing proteins and oils.
That's the reason seeds are excellent sources of these substances, and are nutritionally superior to root crops.
The scientists found, in fact, that the mutated plant fails to switch the tap root cells from their seed or embryonic program of storing protein and oil to the adult program.
"Normally after germination the plant begins to express a new set of genes that cause the seedling to mature into an adult," Somerville says. "In this mutation the cells destined to become primary root cells retain the character of embryonic cells-they fail to make the switch from embryonic to adult."
Sung also found that the root cells fail to elongate, proliferating randomly and making the tap root thicken and eventually become opaque and green like a pickle.
Some roots turn out more than 100 times larger than the plant's typical tap root.
Lateral roots, the ones that grow out parallel to the ground rather than straight into the soil, are unaffected.
"The mutation is in a gene important in the switch from embryo to adult, and it will help us pinpoint when the conversion from embryo to adult takes place in the developing plant," Sung adds.
"All this helps us answer the very fundamental question of how cells know what they are."
Somerville also found that gibberellin-a common plant hormone required for germination of the seed and important also in further growth and development after germination-plays an important part in the tap root's switch from embryo to adult.
The pickle mutation has its greatest effect when gibberellin is not present during the first 24 hours of growth.
Somerville showed that if a gibberellin inhibitor is present, nearly 100 percent of the plants develop a pickle root.
"This establishes a new role for gibberellin," Somerville says.
The research was supported by a National Science Foundation grant to Sung and a U.S. Department of Energy grant to Somerville.