Plants emit chemical distress signals when attacked by chewing insects.
These “911 calls,” as entomologist Esther Ngumbi calls them, alert other insects that a dinner or a nice place to lay their eggs is available nearby. If predatory or parasitic insects detect the right signal, they swoop in like saviors to get a meal or lay their eggs in the body of herbivorous insects.
A new study explores the factors that contribute to the chemical signaling ability of corn plants, comparing how different varieties of corn respond to herbivory in the presence or absence of a soil bacterium known to promote plant health.
Ngumbi, a professor of entomology at the University of Illinois at Urbana-Champaign, led the research with the U. of I. Angela Kent and Ph.D. candidate Sierra Raglin, who is the first author of the article. The findings are reported in the journal Frontiers in Microbiology.
Research into plant-microbe interactions is an area of growing interest for farmers hoping to maximize crop productivity and resilience, Kent said.
“The plant microbiome holds great promise for improving the sustainability of agricultural production,” she said. “Our research aims to treat the microbiome and its sustainability functions as a plant trait.”
The researchers looked at a specific bacterium.
“The bacteria we tested, Bacillus altitudinus, has many plant growth-promoting characteristics,” Raglin said. “It is also known to help plants tolerate stresses such as drought or heavy metals in the soil.”
This microbe and other soil microbes have been found to influence gene expression in plant tissues and stimulate immune responses, she said.
“We wanted to see if this microbe also influenced the release of volatile compounds from plants,” she said.
The researchers grew six varieties of corn in living or sterile soil in a greenhouse experiment. Some maize seeds were inoculated with the bacillus before planting. After four weeks, the scientists subjected the corn plants to corn earworm larvae, allowing the caterpillars to feed for 24 hours before collecting and analyzing the volatile compounds produced by the plants.
The analysis revealed that the six maize varieties tested by the team reacted differently to nibbling insects. Some varieties produced more or a greater diversity of volatile compounds than others. The presence or absence of the bacteria seemed to make little difference in these responses.
Only one type of corn seemed to respond to the soil microbe by increasing its overall production of volatile compounds. More research is needed to determine if this improves its defenses, the researchers said.
The soil bacterium had another effect: it promoted plant growth when corn was grown under sterile conditions, the team found. But that condition would not be relevant for field-grown corn, the researchers said.
The results are preliminary and more work needs to be done to unravel the complex relationships, Raglin said.
“Even in agriculture, the environment is complex,” she says.
The soil bacterium the team tested is just one part of an interconnected web of microbes, and many other outdoor conditions can influence how plants react.
“We don’t want to overstate the role that any factor contributes,” Ngumbi said. “Every contributor to this interaction – the plant, the microbes, the soil – has a role to play. Care should be taken to ensure that you understand each partner’s contribution.