Starchy roots of the cassava plant are the world's fourth most important tropical crop, providing the majority of calories for 500 million people. However, if it's not processed properly, the plant packs a poisonous punch. Now, geneticists have developed a potential boon for the health of African subsistence farmers who rely on the crop: transgenic plants with roots practically free of cyanide-forming chemicals.
The roots and leaves of fresh cassava are loaded with a chemical called linamarin, which wards off herbivores. But human enzymes convert linamarin to cyanide in doses that can be lethal. Crushing, soaking, rinsing, and baking the root neutralizes the chemical and renders cassava safe for human consumption. However, the lengthy process often isn't done properly. This can lead to chronic cyanide poisoning and ultimately swelling of the thyroid gland and nerve-damaging disorders.
Perhaps these ill-effects could be prevented if there were less cyanide to process, pondered Richard Sayre and Dimuth Siritunga of Ohio State University in Columbus. The pair disrupted two leaf-specific genes that produce enzymes active in the first step of making linamarin. This yielded transgenic cassavas with 60% to 94% less linamarin in their leaves. Even better, the roots contained as little as 1% of original linamarin levels.
This combination of results is ideal, because the plants have enough linamarin in their leaves to deter pests, but not so much that it will be transported to the roots, Sayre says. Preliminary tests indicate that transgenic plants are somewhat stunted for a month if not fertilized with soil ammonia. After that, they appear normal, the team reports in an upcoming edition of the journal Planta.
Low-level cyanide poisoning is a problem in some regions, like Africa, where cassava is often poorly processed, agrees plant geneticist Wilhelm Gruissem of the Institute of Plant Sciences in Zurich, Switzerland. These transgenic plants are a "useful cassava prototype for further studies," he says, although field tests are required to make sure they'll reliably have less linamarin.