American Scientist

Wild maize looks nothing like modern corn’s tall canes and plump ears nestled among long leaves. But these related plants have deep—yet surprisingly mobile—roots in ancient history, with undomesticated relatives that showed up as far back as 65,000 years ago. Ancient maize migrated independently from lowland to highland regions in Central Mexico, where it developed some unique genetic traits specific to survival at higher elevations, such as increased tolerance to high ultraviolet radiation, low temperatures, and low phosphorus soils. These adaptations can make the plant flower earlier, or help its cells be more resistant to cold.

Modern corn, a cornerstone crop of today’s food production, was first domesticated in Mexico approximately 10,000 years ago. Although it may seem to be more successful than its wild relatives, the genetic secrets hidden in the cells of ancestral varieties, commonly known as teosintes, allow them to survive in conditions that are hostile to subtropical maize varieties. Biochemist Rubén Rellán-Álvarez of North Carolina State University is trying to spread those genetic benefits by cross-breeding a highland teosinte called mexicana with highland maize.

Similar to the way wolf genes are still found in domesticated dogs, genetic traits of mexicana are evident in cultivated highland maize. Furthermore, the genes responsible for these traits can also be found in the modern maize varieties grown in northern latitudes, including in the United States. Understanding the genetic origins of these adaptive traits and what triggers the expression of the genes responsible for them may help plant geneticists develop corn that requires less fertilizer and is resilient to the effects of climate change.

Rellán-Álvarez’s team compared different environmental factors with the genetic, molecular, and physiological differences between ancestral highland and lowland maize varieties, to identify the genes responsible for the adaptive mutations to stressful environments. From this comparison, the researchers began to see some interesting patterns emerge, particularly in the gene they identified as HPC1, which controls phospholipid metabolism. Phospholipids are structural molecules in cell membranes responsible for signaling specific metabolic behaviors, such as early flowering in climates with fewer warm days.

Once the team—working with colleagues from multiple universities and agencies—identified what Rellán-Álvarez calls the “interesting regions” in the ancestral genome that conferred a fitness advantage to the highland maize varieties, they could explore whether those traits were conserved in modern maize from crossbreeding with teosinte mexicana. It took multiple crosses with different maize varieties, mixing and scrambling genomes, to create mapping populations that tease out those connections. The crosses were then grown across contrasting environments to see if their behavior matched the traits they were bred for, and to determine how those traits correlated with other factors such as yield. Overall, the team found clear signals of local adaptation, with maize varieties tending to perform better in the environment where they were historically developed. Moreover, they found specific regions of the genome that contribute to this local adaptation, including the HPC1 gene. Genetic variation in this gene was strongly correlated with yield and fitness performance on an environment-dependent basis. For example, variants of the gene from maize varieties from highland regions contributed positively to higher fitness in these highland regions.

The long-term goal of this careful analysis of crossbreeding between ancient maize and modern maize is to help breeders identify beneficial genetic variation that may not be present in modern maize and that can help farmers grow corn with less fertilizer and in increasingly challenging environmental conditions. By mapping the genes that carry fitness traits developed over millennia of natural selection, plant geneticists can use gene-editing techniques to integrate these qualities into maize to make it climate-resistant.

A similar approach has already been embraced for other traits, such as shorter stalks that are more resistant to wind damage, through both traditional breeding and gene editing—an industry-preferred method when a trait is controlled by one or a few genes. By incorporating certain highland alleles, a more cold-tolerant variety could allow farmers in the Midwest to plant in early spring. This timing could give seedlings a nutrient advantage as the soil warms and microbes begin converting fertilizer to nitrogen, a resource that is often lost before current crops are established enough to need it.

Maize’s journey from an unruly grass bearing only a handful of kernels to the expansive fields of majestic stalks growing today continues to intrigue scientists still tapping into the abilities of those ancient genes to improve modern varieties. Corn’s wild relatives hold clues to mitigating other agricultural needs, such as disease resistance, flood tolerance, and nutrient conservation. There is even a rare perennial variety.

“Perennials do very interesting things with their nitrogen metabolism,” Rellán-Álvarez notes. “At the end of the season, they will recycle that nitrogen back into the roots. And that's a trait that we're interested in understanding a little bit better and figuring out ways of incorporating in future maize varieties that allow us to use nitrogen more efficiently and reduce the impact of corn agriculture in greenhouse emissions”

What will corn look like in another 65,000 years? Rellán-Álvarez is still studying the past to find out. “What were the [genetic] regions of these wild relatives that were conserved? Are these regions associated with yield, disease resistance, nutrient efficiencies, tolerance to drought? We think that by looking back and understanding those processes, we can project into the future of modern maize.”