Friday, July 1, 2022

A Souped-Up Gene Promoter Stops Heat from Sapping Plant Defenses

The immune system of plants relies on the hormone salicylic acid, which helps fine-tune their defenses against infections and insect infestations. But at warm temperatures, plants turn off their salicylic acid production. New research from HHMI Investigators reveals why and uses genetic engineering to boost immune function during warm spells.
Credit: Lesley Warren Design Group, ON, Canada

Plants’ immune defenses falter during heat waves, rendering them more vulnerable to insects and pathogens under climate change. HHMI scientists have now figured out why high temperatures knock out a key defense system and they’ve come up with a strategy that bolsters plant immunity.

Plants feeling the heat face risks beyond wilting. During heat waves, plants’ defenses falter, rendering them more vulnerable to infection and infestation. This is especially worrisome as climate change is making heat waves more frequent and intense.

Sheng Yang He
Duke University
Plant Sciences Microbiology
“Plants actually have a very powerful innate immune system that explains why they’ve survived so long on Earth,” says plant scientist Sheng Yang He, who is a Howard Hughes Medical Institute (HHMI) Investigator at Duke University. “But now we know that this immune system may not function so well in a hot climate, especially for many cool-weather crops. Continued warming of the climate may exacerbate this reduction of innate immunity and increase diseases and insect infestations in the future.”

He’s team has unearthed new clues to why heat saps plants’ immunity. That allowed them to find a genetic solution to keep a key plant defense system online during warm spells, the researchers report June 29, 2022, in Nature.

Plants’ immune function requires the hormone salicylic acid, which helps coordinate which defenses plants raise or lower. But sweltering plants throttle back on their production of salicylic acid, and researchers haven’t known why.

He and his colleagues grew Arabidopsis, a model plant and cousin to cruciferous veggies, allowing the plants to flourish at pleasant temperatures for four weeks before cranking up the heat for a few days. Then they infected plants with a bacterial pathogen and observed that the warm plants had far lower salicylic acid levels than infected plants that escaped the heat wave. The plants’ gene expression clued the team in to regulatory genes that might be behind the plants lackluster production of salicylic acid.

To figure out which gene was the salicylic acid kingpin, the team spent many years growing various transgenic or mutant plants. Each expressed a candidate gene hacked to keep it on during hot temperatures. Eventually He’s team landed on a gene called CBP60g, that was surprisingly far upstream from salicylic acid production. But the team still did not know why it went down with heat.

“Plants actually have a very powerful innate immune system that explains why they’ve survived so long on Earth.”
Sheng Yang He, HHMI Investigator at Duke University

John D. MacMicking
Yale University
Immunology Microbiology
So He and his colleagues sifted through the gene’s known transcription factors and coactivators, exhausting every possibility. None were found to be involved in temperature-sensitivity of CBP60g expression. “We were about to give up,” He says. But then, immunologist John MacMicking published a paper last year in Nature that provided a new clue to the puzzle. MacMicking, an HHMI Investigator at Yale University, and his colleagues reported a novel protein condensate, a bead of concentrated proteins formed through a process called phase separation, involved in regulating CBP60g’s expression and producing salicylic acid during infection. Consequently, He and MacMicking started a collaboration to investigate CBP60g’s role in lapsing immunity under warm temperatures.

This bubble of proteins sits at CBP60g’s promoter region and seems to be essential for transcription, perhaps because it concentrates molecules needed to kick off the process, He says. Such phase-separated structures, which are like organelles and yet self-assemble without a lipid bilayer membrane, have been studied in animals for over a decade. But they’ve only been more closely examined in plants during the last few years, MacMicking says. “It’s going to be a very exciting time for plant biologists,” MacMicking notes. Protein condensates may be involved with germination, flowering, and growth. And it seems like they can form, perform their tasks, and then disassemble. We now need to revisit our notion of a cell’s composition, he says.

He’s team saw that their infected plants developed fewer protein bubbles at high temperatures. It’s still not clear exactly how these condensates are sensitive to heat during transcription, so He’s team found a workaround to jumpstart salicylic acid production.

Xinnian Dong
Duke University
Molecular Biology Plant Sciences
The researchers grew more transgenic plants, this time replacing CBP60g’s promoter with a switch that wasn’t temperature sensitive. That left the salicylic acid system always on. And they found that the elevated immune response came with a tradeoff: these plants sacrificed growth and made smaller leaves.

Farmers don’t want puny plants, so the team next turned to a promoter developed in 2017 by Duke University plant scientist Xinnian Dong, who is also an HHMI Investigator. Dong’s souped-up switch only turns on under infection. That allowed the Arabidopsis to have its salicylic acid defense at the ready, but deploy it only when under attack. This strategy also worked with the crop plant rapeseed.

“It is very promising that plants can be engineered to be preserve immunity under warmer conditions,” says Jian Hua, a plant biologist at Cornell University who was not part of this work. Understanding how other environmental factors affect plant immunity can help researchers develop more resilient plants, she says. “This work is an excellent example of fundamental research translating into an impactful solution for coping with climate change.”

He's collaborators are currently testing the plants with the tweaked genes in the field. All plants have CBP60g, so He and his colleagues want figure out how to edit plant genes with CRISPR to tweak plants’ endogenous CBP60g genes. Ultimately, the goal, which will take a community of plant scientists, is to give plants an edge as new threats, from increasing droughts to new pathogens, crop up.

Source/Credit: Howard Hughes Medical Institute