Plants get a lot more infections at warm temperatures because their level of basal immunity is down.
Rising global temperatures are posing a threat not only to people and wildlife but plants as well. The question of why many plants succumb as the heat increases is now better understood, thanks to a team of scientists who have identified the mechanism in plant cells that regulates their immunity.
It has long been known that increased temperatures compromises plants’ ability to make a defense hormone called salicylic acid, which boosts their immune system against invaders. However, how this defense mechanism works at the molecular level wasn’t well understood.
Duke University biologist Sheng-Yang He and his team set out to find out by experimenting with a small flowering plant called Arabidopsis thaliana, a member of the mustard family that is related to such cultivated species as radish and cabbage.
The scientists realized that even brief exposure to excess heat can dramatically impact the plants’ hormonal defenses, leaving them more prone to infection by a bacterium called Pseudomonas syringae.
Under usual circumstances the levels of salicylic acid in a plant’s leaves grows by sevenfold in the face of an attack by the bacteria to stop the infection from spreading. However, when temperatures surpass 30 degress Celcius for just two days, plants lose their abailit to make enough defense hormone.
“Plants get a lot more infections at warm temperatures because their level of basal immunity is down,” the scientist explains.
He and his colleagues from Yale University, the University of California at Berkeley, and Tao Chen Huazhong Agricultural University in China began to wonder if there was a way to help plants retain their robust immune defense mechanism even as temperatures rise.
They decided to investigate by building on the work of another research team, which had found that heat-sensing molecules in plant cells called phytochromes essentially function as thermometers that sense warmer temperatures so that plants can activate their growth and flowering in the spring.
He and his colleagues created mutant plants whose phytochromes were always active regardless of the temperature, infected them with P. syringae bacteria, and grew them in warmer temperatures. What they found was that these mutants fared no better than normal plants and likewise failed to produce enough salicylic acid for self-protection.
So the scientists tried a different strategy. They compared gene activations in infected Arabidopsis plants at normal and elevated temperatures. It turned out that many of the genes that were suppressed at higher temperatures were regulated by a gene called CBP60g, which acts like a master switch that controls other genes. If CBP60g is turned off, several other genes get turned off as well.
It further transpired that the CBP60g gene doesn’t activate properly when it gets too hot, which compromises plants’ immune system. However, if the CBP60g gene is switched on permanently through genetic tinkering, plants can continue to keep their defense hormone levels up and bacteria at bay.
These findings, which are detailed in a study, could be good news for global food security as crops could be protected against heat stress as temperatures continue to rise as a result of climate change.
Already, up to 40% of food crops worldwide are lost to pests and diseases each year, costing the global economy some $300 billion, the scientists note.
It remains to be seen whether this strategy, which works in Arabidopsis plants, can be applied to other plants as well. However, the team has found that elevated temperatures also impair salicylic acid defenses in common crop plants such as tomato, rapeseed and rice.
“Follow-up experiments to restore CBP60g gene activity in rapeseed thus far are showing the same promising results. In fact, genes with similar DNA sequences are found across plants,” the scientists report in a statement on their findings.
“We were able to make the whole plant immune system more robust at warm temperatures. If this is true for crop plants as well, that’s a really big deal because then we have a very powerful weapon,” He says.