Scientists identify key phase for tomato heat tolerance

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By studying tomato varieties that produce fruit in exceptionally hot growing seasons, biologists at Brown University identified the growth cycle phase when tomatoes are most vulnerable to extreme heat, as well as the molecular mechanisms that make the plants more heat tolerant.

The discovery, detailed in a study in Current Biology, could inform a key strategy to protect the food supply in the face of climate instability, the researchers said. Agricultural productivity is particularly vulnerable to climate change, the study noted, and rising temperatures are predicted to reduce crop yields by 2.5% to 16% for every additional 1 degree Celsius of seasonal warming.

The scientists took some lessons from evolution to experiment with how best to speed up the adaptation process for varieties of tomato plants, explained study author Sorel V. Yimga Ouonkap, a research associate in molecular biology, cell biology and biochemistry at Brown. It would take a long time to wait for evolution to weed out the vulnerable tomato varieties like Heinz in favor of those that can handle extreme heat, a process that might also jeopardize the qualities that make vulnerable crops commercially desirable.

“We’re trying to figure out thermoregulation at a molecular and cellular level, and identify what and where we need to improve so that we can target those in commercial plant cultivars and conserve everything about them except for this one aspect that makes them vulnerable to extreme heat,” Ouonkap said. “Over time, you can start accumulating different resistance mechanisms as the growing conditions continue to change.”

Understanding thermotolerance, or the ability of a plant to withstand extreme temperatures, is a promising strategy to address climate adaptation, said study author Mark Johnson, a professor of biology at Brown.

“Imagine if you could just make a Heinz tomato more resilient to temperature stress without affecting the flavor profile or the way people experience the tomato,” Johnson said. “That would be a great advantage.”

Plant reproduction: a ripe area for research

The plant reproduction phase has been the focus of research in Johnson’s lab for many years. While the scientific literature includes studies of how heat stress affects plant growth in general, or the development of key reproductive structures, there was an absence of work that specifically examined what happens after pollen lands on the stigma during plant reproduction, Johnson said.

For Ouonkap’s thesis project, he focused on the pollen tube growth phase of the plant reproductive cycle. He studied different cultivars of tomato plants known for their ability to produce fruit in exceptionally hot growing seasons. The tomato varieties in the study were native to the Philippines, Russia and Mexico and were all grown in the Plant Environment Center at Brown.

Collaborating with scientists at the University of Arizona, Ouonkap studied how heat stress affects the ability of the pollen to grow in the flower of the tomato plant. He focused on how gene expression changes when tomato pollen produced by plants growing in optimal greenhouse conditions were exposed to high temperatures when growing in a petri dish.

The team’s partners in Arizona found that exposure to high temperature solely during the pollen tube growth phase limits fruit and seed production more significantly in tomato cultivars that were heat sensitive than those that were heat tolerant. Importantly, Ouonkap found that pollen tubes from the Tamaulipas variety of tomato, known to be tolerant to heat, have enhanced growth under high temperature. His molecular analysis of the pollen tube in these tomatoes allowed the research team to pinpoint the mechanisms that were associated with thermotolerance.

Tomatoes are an ideal organism for this kind of research, the researchers said. The ability of different varieties to adapt to a variety of extreme climates offer scientists insights into how species vary in their responses to environmental conditions. Tomatoes are also an important commercial crop in countries all over the world, from the Mediterranean to Egypt to Turkey to California—some of which are among the most vulnerable to extreme heat conditions.

With the right molecular mechanisms now identified, a next step would be determining specific techniques for enabling tomato growth in different climates. In one hypothetical scenario, scientists might develop a small molecule that could prime the pollen in the plants to be able to withstand a heat wave, Johnson explained.

“When the weather forecast showed two weeks of high temperatures during the pollen tube growth phase, the farmer would apply a product to plants that would change the gene expression so that the pollen would be resilient to heat,” he said.

While that type of manipulation is still far off in the future, the researchers said this area of research is ripe for exploration.