When temperatures rise too high, it is not only humans who suffer. Heat stress has a significant impact on the plants that people rely on for food and resources. Certain plant defences are not designed to withstand high temperatures, leaving them vulnerable to pathogens and insect pests.
"Model results for areas of Europe and North America associated with severe heat waves in Chicago in 1995 and Paris in 2003 show that future heat waves in these areas will become more intense, more frequent, and longer lasting in the second half of the twenty-first century," according to an American Association for the Advancement of Science study.
Plant pests and diseases have had a global impact, with up to 40% of crop production lost globally, according to the United Nations Food and Agriculture Organization. Every year, this costs the global economy more than $220 billion.
According to a Duke University press release, scientists believe they have identified a protein in plant cells that explains why immunity falters when temperatures rise, and they have discovered how to reverse the loss and boost plant defenses. Scientists have long known that high temperatures have an impact on a plant's ability to produce salicylic acid, a defense hormone that activates the immune system and prevents invaders from causing too much damage. However, researchers did not fully comprehend the molecular basis of this immune meltdown.
Sheng-Yang He, a Duke University biologist, and Bethany Huot, a graduate student at the time, discovered in 2017 that even short heat waves can have a significant impact on hormone defenses in Arabidopsis thaliana plants, making them more susceptible to infection by the bacterium Pseudomonas syringae.
Normally, salicylic acid levels in plant leaves increase sevenfold to prevent bacteria from spreading when pathogens attack. Plants are unable to produce enough defence hormones to fight infection when temperatures exceed 86 degrees for two days, according to the release.
"Plants get a lot more infections at warm temperatures because their basal immunity is lower," He explained. "We wanted to know how plants react to heat." And can we actually fix it so that plants can withstand heat?"
A different team discovered around the same time that phytochromes, a molecule found in plant cells, act as internal thermometers, allowing plants to sense warmer temperatures and activate growth and flowering. This prompted He to ask, "Could these same heat-sensing molecules be what's knocking down the immune system when things warm up, and be the key to restoring it?"
To put their theory to the test, the researchers infected normal and mutant plants with P. syringae bacteria that were active regardless of temperature. The plants were grown at temperatures ranging between 73 and 82 degrees. Even at high temperatures, the phytochrome mutants were unable to produce enough salicylic acid, proving the theory.
Danye Castroverde and Jonghum Kim, co-first authors, spent several years conducting similar experiments with various gene suspects. When the mutant plants became ill during warm spells, they devised a new strategy.
The researchers used next-generation sequencing to examine gene readouts in infected Arabidopsis plants grown at standard and elevated temperatures. The test discovered that the gene CBP60g was in charge of many of the genes that were suppressed at higher temperatures.
"Because the CBP60g gene acts as a master switch that controls other genes, anything that downregulates or 'turns off' CBP60g means many other genes are turned off as well — they don't make the proteins that allow a plant cell to build up salicylic acid," according to the press release.
Further research revealed that when the temperature is too high, the cellular machinery required to begin reading out genetic instructions in the CBP60g gene cannot assemble properly, resulting in the plant's immune system failing. Even under heat stress, mutant Arabidopsis plants with the CBP60g gene kept defence hormone levels high and kept bacteria at bay.
The researchers then discovered a way to manipulate heat-resistant plants to activate the CBP60g switch only when they are threatened, preventing growth stunting. This is critical for increasing plant defences without reducing crop yields. According to He, these findings could be beneficial for food supplies impacted by climate change.
According to He, in order to have an impact on future food security, the strategy used to protect immunity in Arabidopsis plants must also work in crops. Fortunately, the researchers discovered that high temperatures had a similar effect on salicylic acid defences in tomato, rapeseed, and rice. So far, follow-up experiments in rapeseed have yielded promising results, according to the release.
"We were able to strengthen the entire plant immune system at warm temperatures. If this holds true for crop plants as well, it's a huge deal because we'll have a very powerful weapon," he explained.
