Maize's starch pathway found limited

September 23, 2002

In the first look at the molecular diversity of the starch pathway in maize, research at North Carolina State University has found that - in contrast to the high amount of diversity in many of the maize genes previously studied - there is a general dearth of diversity in this particular pathway.

That's important, says Dr. Ed Buckler, U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) researcher, assistant professor of genetics at NC State and one of the study's lead researchers, because molecular diversity essentially provides scientists and plant breeders the raw materials to make the crop better.

"Starch is the main product of maize, and is one of the pathways we want to change the most," Buckler says. "People want to use corn for sweeteners, ethanol production and processed food needs. But some of the genes in the starch pathway cannot be manipulated any more by normal breeding."

Buckler and colleagues at NC State and the University of California, Irvine, publish their findings in the Oct. 1 issue of Proceedings of the National Academy of Sciences. The online version of the paper was released on Sept. 20.

In an interesting side note to the research on diversity in maize's starch pathway, the team also conclusively identified the single nucleotide - or structural unit of a nucleic acid - responsible for the production of sweet corn in the United States. Previous research by Dr. Martha James at Iowa State University had narrowed the possibilities down to two nucleotides, according to Buckler. Sweet corn was one of the first mutations discovered in the field of genetics; that discovery occurred about 100 years ago, Buckler says.

"Currently, the identification of the U.S. sweet corn mutation is of historical and basic research interest, but in the future it could help lead to a sweet corn with a good balance of sweetness, creaminess and germination ability," Buckler said.

Buckler says limited diversity in starch and perhaps other, yet-to-be-studied maize pathways make it harder for plant breeders to increase yields of the popular crop. Therefore, to further increase yields, diversity of these important pathways must also be increased.

He adds that there are essentially three ways to solve the problem of low diversity in maize's starch pathway: crossing maize with pollen from its wild relative, teosinte; searching for and extracting important genetic material from Latin or South American maize; or using transgenics, or genetic engineering.

Each possibility's rewards come with risks, however. Teosinte's yield is not very high, so crossing it with maize would not be immediately useful; searching for diversity in "foreign" maize may not yield the necessary genetic diversity to improve U.S. maize; and genetic engineering is often met with resistance, especially from consumers.

In the paper, Buckler and his colleagues suggest an alternative. "One efficient method may be to take alleles, or genetic variants, from selected genomic regions or genes in teosinte, which has lots of diversity, and incorporate them into maize," Buckler says. This type of work has been done with the tomato and has yielded positive results, he adds.
-end-
Buckler's research is supported by the National Science Foundation and the USDA-ARS.

-kulikowski -

Note to editors: An abstract of the paper follows.
"Genetic diversity and selection in the maize starch pathway"
Authors: Edward S. Buckler IV, Sherry R. Whitt, and Larissa M. Wilson, North Carolina State University; and Maud I. Tenaillon and
Brandon S. Gaut, University of California, Irvine
Published: Oct. 1, 2002 in Proceedings of the National Academy of Sciences

Abstract: Maize is both phenotypically and genetically diverse. Sequence studies generally confirm the extensive genetic variability in modern maize is consistent with a lack of selection. For more than 6,000 years, Native Americans and modern breeders have exploited the tremendous genetic diversity of maize (Zea mays ssp. mays) to create the highest yielding grain crop in the world. Nonetheless, some loci have relatively low levels of genetic variation, particularly loci that have been the target of artificial selection, like c1 and tb1. However, there is limited information on how selection may affect an agronomically important pathway for any crop. These pathways may retain the signature of artificial selection and may lack genetic variation in contrast to the rest of the genome. To evaluate the impact of selection across an agronomically important pathway, we surveyed nucleotide diversity at six major genes involved in starch metabolism and found unusually low genetic diversity and strong evidence of selection. Low diversity in these critical genes suggests that a paradigm shift may be required for future maize breeding. Rather than relying solely on the diversity within maize or on transgenics, future maize breeding would perhaps benefit from the incorporation of alleles from maize's wild relatives.

North Carolina State University

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