Crops Evolved by Swapping Genetic Modules Between Cells: Study

Comparing individual cells across corn, sorghum and millet reveals evolutionary differences among these important cereal crops, according to a new study led by New York University researchers.

Corn, sorghum, and millet provide food for humans and animals around the world. Corn and sorghum are ancient relatives that evolved into two different species roughly 12 million years ago, and millet is a more distant relative.

Despite their shared ancestry, the crops have substantial differences in key traits—for instance, sorghum is far more tolerant to drought than is corn, and the plants release unique gooey substances from their roots to shape how they interact with their surrounding soil. These differences may be traced back to corn undergoing a whole genome duplication after its split from sorghum.

"The importance of these crops, their evolutionary proximity, and their functional differences present an exciting opportunity for comparing patterns of gene expression at the cell level," said

Bruno Guillotin, a postdoctoral associate in NYU's Department of Biology and the study's first author. "While these three crops are similar, how they differ from each other is important because they have traits that we may want to transfer from one to the other, such as drought tolerance."

The researchers conducted single-cell mRNA profiling of the roots of corn, sorghum, and millet, dissecting the roots to look at the cells individually and observing precisely where genes are expressed in a particular cell. They then compared the same specialized cells across the three crops.

In examining how cells have evolved and diverged in the different species, the researchers identified several trends that point to "tinkering"—or the rearrangement of existing elements—of cells over time. First, they observed that cells often trade gene expression modules, or groups of 10 or 50 genes with coordinated functions, between cell types over evolution.

"This gene module swapping has been shown in animal systems, but the data we generated is the first time it's been illustrated on a large level in plants," added Birnbaum.

This swapping of modules was demonstrated in a discovery about root slime—the gooey substance filled with nutrients that roots emit into the soil. Slime is useful for lubricating the soil so roots can pass through and can attract beneficial bacteria that protect the plant or provide hard-to-get nutrients.