Striga, or witchweed, is the main weed affecting many cereals including rice, maize, sorghum and millet. One species, Striga hermonthica, is responsible for more crop loss in Africa than any other individual species of weed. Striga is a hemi-parasitic weed; its roots latch onto the roots of its host (e.g. a crop plant such as rice) and take water and nutrients from the host plant. Muhammad Jamil and his colleagues at Wageningen University in the Netherlands have found a way to reduce germination of Striga seeds, thereby preventing crop plants from being affected in the first place.
Striga species are finely tuned in to their hosts. They only germinate in the presence of chemicals that are emitted by the roots of the host plants. Such chemicals include strigolactones, which are plant hormones derived from carotenoids (pigment molecules). Once the Striga plant has germinated, it attaches to the roots of its new host and establishes a connection between the xylems of the plants. The Striga plant then continues to develop and emerges from the ground. The most damage is done to the host before the Striga plant is even visible above the ground. This means that controlling this weed is difficult for farmers because by the time they see it in their field it has probably already done its damage to their crops. Jamil et al. have found that by reducing the amount of carotenoids produced by rice plants, fewer strigolactones are produced, and therefore fewer Striga seeds are triggered to germinate.
The Africa Rice Center in Tanzania has recently developed 18 new cultivars of high-yielding rice that can tolerate the environmental stresses of sub-Saharan Africa. These were tested to determine their resistance to Striga before it becomes attached and to find out whether this was due to how much strigolactone these cultivars produced. They found a high variation in resistance to Striga, however those cultivars that had a low production of strigolactones had a higher resistance to Striga. These results suggest that, by breeding for low strigolactone production, rice plants could be developed that are more resistant to Striga than those currently grown.
As part of this research, a way of screening plants for Striga resistance was developed. This could help to efficiently identify resistant cultivars for farmers to grow. By using such cultivars in combination with other anti-Striga strategies, farmers with Striga infestations in their fields could relatively easily increase their crop yield.
Jamil, M., Rodenburg, J., Charnikhova, T. & Bouwmeester, H.J. (2011) Pre-attachment Striga hermonthica resistance of New Rice for Africa (NERICA) cultivars based on low strigolactone production. New Phytologist 192: 964-975.
Jamil, M., Charnikhova, T., Verstappen, F., Bouwmeester, H. (2010) Carotenoid inhibitors reduce strigolactone production and Striga hermonthica
infection in rice. Arch Biochem Biophys. 504 (1): 123-31.
Toussaint, E. (2012) Grain crops with lower carotene levels are less affected by parasitic plants. http://www.wageningenuniversity.nl/UK/newsagenda/news/Ne_Striga_.htm