Gregg Howe, a University Distinguished Professor in the Michigan State University Department of Biochemistry and Molecular Biology (BMB), has spent his career learning how plants defend themselves from insects.

He uses Arabidopsis, a common plant for genetic research, and tomato to study the molecular basis for how they deploy the hormone jasmonate. The hormone is activated when the plant is wounded by a pest and prompts the repairing of damaged tissue, as well as the delivery of other predator-repelling defense compounds. Recently Howe has started examining rising temperatures and their ramifications on the plants.

Incorporating temperature into his work began with his involvement in the MSU Plant Resilience Institute (PRI), which supports collaborative projects aimed at developing hardier plants for a changing climate. As global temperatures increase due to climate change, Howe said plants and insects are behaving differently.

“Through the PRI, we’re looking at how commercially important plants are dealing with climate change, and one of the more obvious problems is higher temperatures,” Howe said. “We’re seeing some surprising behaviors from plants and insects that give us insight into what growers may be dealing with in the future.”

In a study published in PNAS in January 2020, Howe and a postdoctoral researcher in his lab, Nathan Havko, found a correlation between more aggressive insect feeding and elevated temperature. The project was a partnership with Thomas Sharkey, also a University Distinguished Professor in BMB and a faculty member in the PRI, who is an expert in photosynthesis, and his graduate student, Alan McClain.

Howe and Sharkey explored immune system function of tomato plants under two temperature scenarios (82 degrees Fahrenheit and a higher temperature of 100 degrees Fahrenheit to simulate a heat wave) for several days, both under hornworm caterpillar predation.

The team found the tomato plant at the elevated temperature was more susceptible to damage for two reasons. First, the hornworm caterpillar’s metabolism sped up and caused them to develop an insatiable appetite. Simultaneously, the stomata — small pores on leaves that allow the plant to cool itself — stayed closed and warmed the plant, in addition to hindering photosynthesis.

Howe said the tomatoes ramped up jasmonate production, but the increase was ineffective against the caterpillar.

“Plants are well adapted to dealing with insects and with temperature changes, but if the two happen at the same time we’ve shown that it can pose a real problem,” Sharkey said. “The plant can’t cool itself or efficiently produce food through photosynthesis, so the insect is doing more damage than the plant can repair.”

With support from the PRI, Sharkey has also investigated how different varieties of dry beans tolerate heat. Dry beans are a vital crop for Michigan agriculture, as the state ranks second in the U.S. in production.

Sharkey said some dry beans are notoriously vulnerable to heat. Pinpointing what allows tolerant varieties to thrive and sensitive varieties to flounder will give breeders an edge, Sharkey said.

He and a postdoctoral researcher, James Santiago, found that in a sensitive variety rising temperatures adversely affected pollen development and transportation of sugars, which plants rely on for growth. Meanwhile, in the more thermotolerant dry beans, sugar transport remained the same at an elevated temperature. The activity of an enzyme involved in carbon metabolism increased in the floral tissues, which helped the plant cope with high-temperature stress.

“The temperature goes up and yield declines dramatically before there is any issue with photosynthesis, which was surprising to us, and it turned out the problem is with fertilization,” Sharkey said. “If we can tell breeders specifically what traits to look for or to avoid, they can use marker-assisted breeding or genetic engineering to develop the best-suited varieties for the changing conditions.”

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