In stark contrast to its external image of pretty pink flowers, the parasitic plant Striga — also known as witchweed — brutally attaches itself to crops and destroys them.
The deadly weed accounts for agricultural damage in excess of ￥1 trillion JPY ($9.5 billion USD) per year in Africa, triggering fears of a food crisis.
There is currently no effective way of wiping out Striga. However, it looks likely that Japanese technology will come to the rescue.
Leeches Off Roots, Steals Nutrients
Striga is found in tropical and subtropical semi-arid regions in Africa and South Asia. Its tiny seeds, which are about 0.2 millimeters in size, spread in the wind. As it grows, it leeches off the roots of crops such as corn. The weed steals nutrients and water, and ultimately kills off the crop. It is impossible to eliminate Striga in the soil because the seeds are so tiny. Its seeds can remain dormant for several decades, while staying resistant to both heat and weedkiller.
In Africa, 50 million hectares of arable land — about 1.3 times the size of Japan — is blighted by Striga. And this threatens to be the nail in the coffin concerning Africa’s serious food problem.
Striga has never been found in Japan. However, the chance of it appearing in the future cannot be completely ruled out. Therefore, as scientists tackle the current Striga issue in Africa, they are also looking into domestic countermeasures, just in case the weed spreads to Japan.
Blocking Enzymes That Act as Glue
Nagoya University and RIKEN have conducted detailed examinations of what happens when parasitic plants latch onto crops. The scientists had to use Phtheirospermum japonicum, a parasitic plant found in Japan that is similar to Striga, because it is forbidden to bring witchweed into the country.
Crop roots that are hosts of parasitic plants release a substance when their cell walls oxidize.
In the examination, the Phtheirospermum japonicum reacted to the released substance. Subsequently, enzymes that dissolve cell walls were secreted from the roots. The Phtheirospermum japonicum bonded with the host crop, creating a passage for stealing nutrients and water. The scientists discovered that the secreted enzymes, necessary for parasitic activity, acted as a kind of glue.
According to the Nagoya University and RIKEN team, Striga possesses the same gene that produces those “glue-like” enzymes, and its parasitic properties are thought to be the same. Therefore, if a chemical that blocks enzyme activity can be sent to agricultural areas in Africa, it might be possible to prevent parasitic damage caused by Striga.
“We won’t be able to capitalize on this discovery straight away, but we’ve shown that it is possible to control parasitic plants,” said Assistant Professor Michitaka Notaguchi of Nagoya University.
Suicide Germination in Africa
A separate team at Nagoya University looked at the germination of Striga seeds. Seeds that were spread across the soil were dormant, and remained so for several decades until host plants nearby began to sprout.
Once the host plants began to grow, they released plant hormones from their roots, which caused the Striga seeds to bud. After that, the Striga attached itself to the roots and began to act as a parasite. However, the team noticed that if the Striga could not start parasitic activity within about four days after germination, it would die.
The team decided to make some Striga seeds geminate without any host plants nearby – effectively forcing the Striga to “commit suicide.” The scientists developed artificial hormones to make the seeds bud, and then placed the seeds in a flowerpot under strict conditions.
They observed that the Striga eventually died without any host plants nearby. After that, they successfully grew corn in the same area without any issues. The team concluded that this “suicide-style” germination helped to prevent parasitic activity by the Striga seeds.
It only costs about ￥40 JPY (about $0.38 USD) to produce enough artificial plant hormone to cover a one-hectare area — which is very cheap. The team also confirmed that the hormone does not have any adverse effects on other plants or useful bacteria in the soil.
As follow-up, the team is keen on going to Africa in 2021 in order to conduct further experiments.
“Joint international research is difficult right now due to COVID-19, but we are keen to try out artificial hormone experiments in Africa, and contribute toward solving the food issue there as soon as we can,” said Associate Professor Yuichiro Tsuchiya.
(Find access to the article in its original Japanese here.)
Author: Juichiro Ito, Sankei Shimbun Science Department