Bountiful harvests of corn and other staple crops depend on a mysterious phenomenon known as hybrid vigor. When highly inbred varieties are crossed, their offspring become taller, more hardy and produce more grain. The researchers now report that this energy is somehow influenced by microbes in the soil, possibly through the plant's immune system.
“This is a really interesting discovery,” says Giles Oldroyd, a plant geneticist at the University of Cambridge who was not involved in the study. "I'm surprised they've studied it so far."
Charles Darwin was one of the first researchers to describe the viability of hybrids. In the early 20th century, biologists began to apply this effect in agriculture, creating inbred parental lines that produce hybrid seeds. By the 1940s, nearly every farmer in the United States was planting hybrid corn and yields had increased.
Geneticists have proposed several theories about the reasons for the viability of hybrids, but no definitive explanation has emerged.
Maggie Wagner, University of Kansas plant geneticist Lawrence, and her colleagues wondered if microbes might be involved. Tiny organisms can have a big impact on plants. For example, the leaves and roots are often colonized by communities of beneficial bacteria and fungi that help protect the plant from disease-causing microbes. Certain crops, such as soybeans and other legumes, contain microbes that feed them with nitrogen, an essential plant nutrient that farmers must otherwise supply with fertilizer.
Last year, Wagner and his colleagues found an interesting clue in a field study. They found that the leaves and roots of hybrid maize have different microbial communities from those found on inbred maize varieties.
When winter came and the fields fell fallow, Wagner tried to replicate the discovery with a laboratory experiment. The researchers planted the seeds in bags with a soil-like substance that has been sterilized to kill all germs. They then added a simple community of soil bacteria - seven strains known to colonize corn roots - in some bags, leaving others sterile. When microbes were present, the hybrids grew better than the inbred variety, as expected, with roots weighing 20% more. However, to their surprise, hybrid and inbred corn plants grew about the same in sterile soil, they report this month in the Proceedings of the National Academy of Sciences. The weight of their roots and shoots was practically the same.
The finding was confirmed when scientists repeated the experiment, adding a full complement of soil microbes to some of the sterilized bags. “The results are compelling,” says Oldroyd.
Output? “Something about being a hybrid makes the plant interact differently with microbes,” says Wagner. Based on some of the results, the team believes that microbes slow down the growth of inbreds rather than giving them much of a boost.
It is possible that the immune system of inbreds is overreacting to benign microbes, which jeopardizes their growth. (The experiment did not include any pathogens.) Alternatively, hybrid plants may better defend against weak pathogens in the soil. “We have a lot of work planned to develop this idea,” says Wagner.
Oldroyd says the results highlight the need for breeders to align crop genetics with the microbial communities they live with. The findings highlight the importance of understanding the role of soil microbes in improving agricultural productivity and sustainability, adds Korne Peters, a plant biologist at the University of Utrecht. "This is promising."