Entomology professor Esther Ngumbi is examining how flooding impacts crop plants and the insects that feed on them. In her recent review, she emphasizes that flooding is often neglected in research aimed at enhancing plant climate resilience.
The sound of greenhouse fans nearly drowns out Esther Ngumbi’s voice as she gives me a tour of her rooftop lab in Morrill Hall. The benches are crowded with tomato plants, which don’t appear healthy. Half of them are submerged in water-filled bins. Their leaves are turning yellow and wilting. A few of the fading tomato plants have managed to flower, and I spot one or two small tomatoes on a couple of thin plants.
The tomatoes are facing another challenge as well. Small baggies have been attached to their stems, and inside these bags, fat green caterpillars are munching on the tomato leaves.
Professor Ngumbi is driven by curiosity and has set up this experiment to explore her many questions. She is intentionally flooding the tomatoes to simulate what might happen in farmers’ fields under flooded conditions, a situation that is becoming increasingly common due to climate change.
“In nature, plants experience a variety of stressors during flooding,” Ngumbi explains. “When tomatoes are flooded, they become weak, making them more appealing to insects that prefer struggling plants. We are studying how these plants cope with the combined challenges of flooding and being eaten by herbivores.”
This leads to the presence of the caterpillars, which are the larvae of Manduca sexta, known as the tobacco hornworm. These caterpillars are feeding on one of the heirloom tomato types used in Ngumbi’s research: Cherokee purple and striped German.
In the greenhouse, half of the tomato plants are kept above water, allowing researchers to directly compare the stressed plants with those grown under more typical conditions. But that’s not all that’s being investigated here.
“We’re also examining the microbes,” Ngumbi notes. “Our goal is to see how the microbial community shifts under flooded conditions.”
A major aspect of Ngumbi’s research involves understanding how soil microbes impact plant health and productivity. She takes a special interest in mycorrhizal fungi, which form beneficial relationships with plant roots, providing vital nutrients like nitrogen in exchange for glucose from the plants.
All tomato plants are rooted in soil sourced from an Illinois farm, but half have been treated with mulch from a local farmer who has crafted a special mix to promote mycorrhizal fungi growth in the soil. Ngumbi aims to determine if this treatment enhances the plants’ defenses against the voracious caterpillars.
To assess the plants’ defenses, Ngumbi’s team collects gas samples emitted by the plants and analyzes them for volatile organic compounds—the chemicals that plants release to deter insect herbivores.
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Years later, Ngumbi publishes findings from her laboratory experiments. She discovers that the two types of tomatoes show different gene expressions and varying volatile compounds even before any treatment. When flooded, their chemical profiles changed significantly compared to those grown in regular conditions. The presence of herbivores influenced the volatile compound production, but flooding had a stronger effect.
Research is still ongoing, and Ngumbi’s focus on flooding effects has deepened. In a new review article in the journal Trends in Plant Research, she details numerous changes that occur when plants are submerged in water for extended periods.
“Flooding differs from other climate-related stressors because it deprives plants of oxygen, which is crucial for their growth and development,” Ngumbi states. Flooding disrupts plant metabolism, energy production, and photosynthesis. It can eradicate beneficial bacteria while encouraging harmful microbes in the soil and weaken the plants’ defenses against diseases and pests like the tobacco hornworm.
Ngumbi also cautions that increased flooding poses a threat to years of research aimed at enhancing plant resilience to climate change. Flooding could hinder initiatives to improve soil quality and microbial health, which are essential for crops to withstand other stressors like heat and drought. Additionally, flooding may undermine advancements achieved through genetic engineering or plant breeding.
With the anticipated rise in flooding frequency and intensity—estimated at about 7% for each 1° C increase in global temperatures—Ngumbi stresses the need for scientists to account for flooding impacts to ensure that significant advancements in developing climate-resilient crops are preserved.
Ngumbi is associated with the Carl R. Woese Institute for Genomic Biology at the University of Illinois.
