Young umbrella acacia trees in Africa survive severe drought by putting their natural processes into overdrive when water is in short supply, prioritizing continued growth over water conservation, new research shows.
The study is the first genome-scale analysis of any African acacias and focuses on the umbrella acacia, an iconic feature of the African savanna.
Researchers compared the genetic response to drought stress of the umbrella acacia (Vachellia tortilis) and one of its hundreds of relatives, the splendid thorn acacia (Vachellia robusta) more commonly found in wetter regions of East Africa.
Results showed that once water becomes scarce, the umbrella acacia continues its conversion of carbon dioxide and water from sunlight into nutrients through photosynthesis and uses up all the water it can access.
"You would expect most plants, if they're being water stressed, will shut down, but at the early stage of drought stress, umbrella acacias ramp up - they go for broke," said senior author James Pease, associate professor of evolution, ecology and organismal biology at The Ohio State University.
"The splendid thorn acacia tends to be more of a water saver - holding on to water, not growing a lot. Umbrella acacia does the opposite - it tries to grow more and do more photosynthesis and capture more carbon that it's going to stockpile," he said. "Once water's not going to come for a while, it lets the above-ground biomass die and waits for water to try again the next season."
The research was published recently in The Plant Journal.
Umbrella acacias provide a staple food for giraffes, are sources of a global wood economy and the common food additive gum arabic, and are part of the legume family - all reasons to understand how genetics shape their drought tolerance at the cellular level, researchers say.
"They have to grow in these hyper-arid conditions that are really difficult for a large woody plant to grow in. They're being eaten by giraffes, they're being knocked over by elephants. They have to compete with the grasses. The grasses catch fire. So there's this whole set of pressures on them," Pease said.
"Drought stress and climate habitat shifts are not a unique problem to African acacias. But there are very few genomic studies of tropical trees and how water stress impacts them."
Seedlings of umbrella and splendid thorn acacias were grown in the lab and watered for three months, after which they were divided into two conditions: continued normal watering or complete shutoff of water - the onset of drought. Researchers collected leaves on a weekly basis and selected samples for genomic analysis representing an early drought phase, the middle of a downward slope in tree health, and severe drought.
To compare each species' response to the drought stress, the team sequenced their transcriptomes - the collection of RNA readouts of DNA instructions that indicate gene activity, and related protein changes, across the genome.
The model system represents the time of life when the trees are most at risk of dying.
"This early seedling establishment phase is when a lot of them either make it or don't based on their habits of how well they can acquire energy and water," Pease said.
The researchers believe that umbrella acacias maintain their pattern of intense nutrient collection and above-ground biomass decline for years, developing a huge root mass in the process.
"If you dig up a little acacia seedling, it has a tree's worth of roots. And once it gets the right combination of water and nutrients, it has the rootstock to support a full tree and it will transition to that," he said.
"This is the same strategy of grasses. They keep maintaining that root and will wait for water and try again - you can see that in lawns that dry out. It's really interesting to us because that's what grasses do, as opposed to most herbaceous plants and other trees."
In contrast, the study showed that the splendid thorn acacia behaved in a more expected way for a tree under drought stress: investing in water conservation and cellular function maintenance while riding out the drought.
The transcriptome analysis showed the trees used similar genetic systems to regulate photosynthesis and maintain biological stability during drought stress - but the two species activated these systems with different sets of genes and on differing time scales, said first author Ellen Weinheimer, who worked on the study as a biology graduate student at Wake Forest University, where Pease was a faculty member until 2024.
The analytical method also revealed these genetic differences in drought response were not driven by genetic mutations, the sequence changes occurring over time that evolutionary scientists have historically tracked.
"You don't necessarily see gene sequences and gene expression changing together," said Weinheimer, now a postdoctoral associate at the Yale School of Medicine. "The genes that are differentially expressed in response to drought don't necessarily have sequence changes, which shows that those two mechanisms are largely independent of each other."
Tracking gene expression alongside sequence changes in plants, animals and other systems is a focus of Pease's lab.
"We're layering how gene expression levels are changing among different species," he said. "And over evolutionary time, we're finding expression as important as the mutations, in that a mutation in one gene could affect the expression of another gene. We're learning very different things than we would if we just looked at the mutations."
This research was supported by the U.S. National Science Foundation.
Additional co-authors were Scott Cory, Nicholas Kortessis and T. Michael Anderson of Wake Forest.
