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Breaking the rules: how witchweed wreaks havoc on sub-Saharan crops

Lumba Lab research sheds light on germination mechanisms of the parasitic plant
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Researchers at U of T have uncovered the germination process of witchweed, or striga hermonthica, a parasitic plant that plagues farmers in sub-Saharan Africa. COURTESY OF THE INTERNATIONAL INSTITUTE OF TROPICAL AGRICULTURE/FLICKR
Researchers at U of T have uncovered the germination process of witchweed, or striga hermonthica, a parasitic plant that plagues farmers in sub-Saharan Africa. COURTESY OF THE INTERNATIONAL INSTITUTE OF TROPICAL AGRICULTURE/FLICKR

“We asked, ‘How does it break the rules?’ ” mused Michael Bunsick, a graduate student in the Department of Cell & Systems Biology at the University of Toronto. “By figuring out how it breaks the rules, we actually learned what the rules are.”

Bunsick was talking about Striga hermonthica, a parasitic plant commonly known as witchweed that is prevalent in sub-Saharan Africa.

The parasitic plant latches onto the roots of crops like rice, sorghum, maize, millet, and sugar cane, draining their resources and eventually killing them. African witchweed infestation rates in subsistence crops can reach up to 70 per cent, costing up to $10 billion in revenue every year in devastating crop losses. 

As a major contributor to poverty and malnutrition in sub-Saharan Africa, witchweed needs to be stopped, but its prevalence and mysterious biological pathways make it difficult to effectively eradicate.

U of T Professor Shelley Lumba and her lab published a new study on May 25 in Nature Plants that reveals witchweed’s unexpected germination machinery.

This research “is crucial to figuring out ways to eradicate or at least minimize infestations” Lumba said in an interview with The Varsity. She added that the study’s findings can help develop strategies to kill witchweed without damaging crops in the same plot of land.

Rule-breaking germination mechanisms

Germination is the process by which a seed or spore sprouts, usually after a period of dormancy. Germination is often triggered by environmental factors, such as the availability of water and nutrients. For a parasitic plant like witchweed, that means having access to host plants so they can take nutrients from them.

Bunsick, Lumba, and 10 other authors found that, in contrast to most non-parasitic plants, witchweed circumvents the canonical gibberellic acid (GA)-dependent germination pathway. GA is a plant hormone that is involved in many biological functions but is especially important for stimulating the seed germination process.

However, witchweed is a rule-breaker when it comes to germination. Since the plant is an obligate parasite — meaning it requires a host to live — “it has to make sure that it germinates with a host nearby,” Lumba said. “It’s really a matter of life and death.”

Instead of relying on GA signaling to germinate, witchweed seeds lie dormant in fields until crops begin to grow. At this point, the crops begin releasing ‘strigolactones’ — small molecules that are mostly used to attract symbiotic fungi to help the crops absorb essential nutrients.

Unfortunately for the crops, witchweed hijacks this symbiotic plant-fungi relationship for its own sinister purposes. It’s been previously found that witchweed has elegantly co-opted another biological pathway used to sense smoke-derived chemicals — known as the HTL pathway — to perceive these crop-exuded strigolactones. The HTL pathway does not rely on the GA hormone.

Finding the right pathway

Two questions motivated Lumba and her colleagues’ work. How do witchweed HTL receptors perceive strigolactones and initiate a process that kickstarts the germination pathway on its own, without the requirement of GA? And why is there even a preference for one hormone over the other?

Answering these questions is no small feat. In addition to being troublesome in the agricultural world, witchweed is especially difficult to study in the lab. As an invasive and potentially damaging species, it is nearly impossible to get permission to grow witchweed in an academic setting, and even if clearance is granted, witchweed’s host plant requirement poses problems for executing experiments.

So, instead of growing witchweed itself, Lumba and her lab took the model plant Arabidopsis thaliana and inserted genes of interest from witchweed through a process called transformation. The team created transgenic Arabidopsis lines, which expressed the same HTL receptors that witchweed plants use to detect strigolactones.

Arabidopsis is known to have an HTL pathway of its own. After knocking out a gene involved in this pathway, the team found that its transgenic Arabidopsis was unable to germinate in response to strigolactones. This implies that witchweed HTL receptors function through the same mechanisms as the Arabidopsis HTL pathway.

“It was kind of unexpected that this was the pathway involved,” said Lumba, “we didn’t think that the parasite did things pretty much like the way normal plants do it, but they did.”

This finding was a real ‘aha’ moment for the team, because “that told us… this is the right pathway,” Lumba exclaimed. “You can’t mess around with that… There’s no other explanation other than that we have the right pathway.”

The study further confirmed the hypothesis that witchweed operates through its own unique germination pathway.

Since Arabidopsis is a free-living plant, it normally uses GA to initiate germination. In order to figure out if perception of strigolactones was independently triggering germination, the team had to eliminate the naturally occurring GA in Arabidopsis

They observed that even with GA inhibition in Arabidopsis expressing witchweed HTL receptors, detection of strigolactones was enough to initiate germination. 

It was the final piece of the puzzle.

“It’s like you completely replaced one hormone with another,” Lumba said. “I know that in basic research you don’t get those moments very often.”

Growing with the project

Bunsick, now a second-year master’s student, has been working on this project since his second year of undergrad.

After four years of researching, “it’s like I kind of grew with the project,” he remarked. 

It was a real team effort to produce this exciting result — everyone from undergraduate students to postdoctoral fellows contributed invaluably to this project. It’s a good reminder that science is a collaborative project where the end result requires every individual’s contribution. 

Coming up with which questions to ask and what experiments to conduct is always tricky, especially when working with a relatively unknown biological system like that of witchweed.

“We had lots of coffees at Second Cup in Sid Smith, thinking about these models,” said Lumba. There were “lots of disagreements,” Bunsick joked. “Productive disagreements.”

Bunswick explained that those disagreements often served to clarify and distill their questions because, “when you can see what the two opposing views are, it’s very easy to design an experiment where you can differentiate between them.”

There is still much to be done in witchweed research to address unanswered questions about other elements that contribute to the complex germination system.

Overall, this research tells a compelling story about the previously unknown mechanism regulating witchweed germination. Not only do the findings contribute to the basic biological understanding of this — and other — parasitic species, they have the potential to accelerate efforts for mitigating witchweed infestations in the real world.

Lumba and her team are excited about the implications of their results. “Our focus has always been that if we learn more about the germination of Striga, then we can learn more about ways to try to combat Striga.” Ultimately, this may help combat crop loss, poverty, and malnutrition in sub-Saharan Africa.