Though to outward appearances they could hardly be more different, both the kernels and the leaves of maize plants contain cells with identical genetic material. The cells differ from one another not because their nuclei hold different DNA, but because the genes contained in their DNA are uniquely "expressed."
So why exactly do kernel cells express kernel genes, while leaf cells express leaf genes? That's the question Karen Cone, a renowned corn geneticist and associate professor of biological sciences at MU, plans to address with the help of researchers from five other universities and a $6.6 million grant from the National Science Foundation.
Cone believes she'll find the answer by revisiting DNA's "packaging." Because DNA is itself a huge molecule, she explains, it must be wound tightly to fit inside a nucleus. This binding-up process is accomplished via proteins and other genetic materials in what geneticists call "chromatin," the nuclear package containing the genetic codes of all higher organisms.
Previous research has shown that the tightness of genes within this package affects gene expression. In some cases, Cone says, "silent genes are silent because they are packaged up tighter into the DNA complex."
What Cone and other researchers don't fully understand is exactly which maize genes are regulated in this way.
"There are other mechanisms for regulating gene expression, and we don't know which genes use chromatin packaging as the primary mechanism for turning their genes on or off and which genes use another mechanism. ... If we knew a gene was manipulated by this mechanism, we could maybe open that gene a little more to get it expressed a little more," Cone says.
That might be beneficial if, say, other studies had shown the gene to improve crop yield or drought resistance.
To study chromatin's effects, Cone will take advantage of certain pigment-producing "reporter genes." These are genes that have already been identified as chromatin-influenced, and thus are handy for benchmarking genetic activity. "What makes them really good reporters is we can see the effects," says Cone, who has studied one of the reporter genes for 15 years. "We either have purple or we don't."
Cone and the other researchers will also use reporters to identify "chromatin regulators," the genes involved in winding the DNA. This will involve blocking the expression of the genes scientists think might be the chromatin regulators in some plants, then crossing these plants with others that contain reporter genes. "We'll see what happens, if there is less or more pigment," Cone says.
One day, she adds, such work could help scientists better understand both the genetics of corn and the people who eat it. "Many multicellular organisms, including humans, package DNA just like corn does," says Cone. "We know there are genetic disorders in humans due to problems in this packaging mechanism."