Binding the Witch, Saving the Crops

There’s an old superstition that says a bit of salt will protect against witches. Unfortunately for farmers in sub-Saharan Africa, it takes more than just salt to protect against witchweed, a parasitic plant that can decimate entire crops. Witchweed — known as Striga to biologists — earned its menacing moniker by leeching off valuable crops like maize, cowpea and sorghum, draining them of vital nutrients and causing healthy-looking plants to suddenly yellow and wither, as if they’d been “bewitched.” Now, researchers at the University of Toronto have discovered chemical compounds and genes that may help to someday end the Striga curse by halting the plant’s growth before it can even begin.

Similar to humans, plants generate hormones that regulate their growth. One of the more recently identified plant hormones is strigolactone, which is responsible for inhibiting shoot branching — the outgrowth of shoots from existing buds — while positively influencing the symbiotic relationship between plants and fungi crucial for the development of most plant species. However, strigolactone, which gained its name from Striga, has another important function for the seeds of parasitic plants: It acts as a cue for dormant seeds to germinate and infect crops.

Many researchers who work on parasitic plants have now started seeking methods to control strigolactone, explained Peter McCourt, a cell and systems biologist at the University of Toronto. To that end, McCourt and his research team screened various chemicals and genes that might do the trick. They found compounds that alter strigolactone levels in mouse-ear cresses — small flowering plants related to cabbage — in addition to mutants of the plant that produce less strigolactone than normal. Their study was published in early September in Nature Chemical Biology.

With Striga infesting some 123 million acres of land in sub-Saharan Africa, according to the International Institute of Tropical Agriculture in Nigeria, researchers hope that farmers might someday be able to apply chemicals or selectively breed plants to reduce the ability of their crops to germinate Striga.

McCourt pointed out that more research is needed to determine if reduced levels of strigolactone will adversely affect crop growth. There is at least “a ten-year window from the results we have now to someone actually using it in the field,” he said.

But Fen Beed, a plant pathologist with the agriculture institute’s research station in Uganda, doubts the discovery will yield a stand-alone solution to Striga infestation. “The only way of tackling such a vigorous weed,” he wrote in an e-mail, “is through an integrated approach” where other control methods — such as rotating crops, or coating crop seeds with protective fungi or herbicides — are incorporated alongside genetic resistances.

However, Beed is still hopeful that the new research will pay off. “Any steps that reduce the levels of infestation of Striga … will significantly improve food security” in sub-Saharan Africa, he said.