Novel Sources of Witchweed (Striga) Resistance from Wild Sorghum Accessions

Wild sorghum accessions grow as weedy plants and have shown remarkable immunity to Striga. The parasitic plant Striga is also known as witchweed and can destroy a crop with up to a 100% yield loss. It is estimated that over 60% of farmland under cultivation in sub-Saharan Africa is infested with one or more species of Striga, which impacts over 300 million farmers in over 25 countries with yield losses of over seven billion dollars. The lifecycle of Striga involves elevated fecundity, with each Striga flower spike producing over 50,000 seeds that remain viable in the soil for up to 14 years. Striga also has a remarkable ability to intimately link its life cycle to that of a host, such as when germination of Striga seeds and attachment to the host only occur in response to chemical cues (strigolatone) contained in the host and in some cases, non-host root exudates. To manage Striga, smallholder farmers in sub-Saharan Africa have fought back with different control methods aimed at reducing Striga seed density in the soil, including reducing the amount of Striga-seed-contaminated crop seed supplies, hand weeding, crop rotation, and the use of "trap crops" that induce germination of the parasite but are not hosts. However, these methods have been extensively encouraged for many years, but crop losses and the host range of Striga have continued to increase, which underscores the need for a sustainable Striga management strategy. An integrated approach that greatly exploits natural resistance is the ideal strategy, but sources of Striga resistance are limited and are often overcome by the parasite. Therefore, additional sources of Striga resistance need to be found for introduction to farming systems and to promote long-term resistance. Wild sorghum accessions have been identified as a potential source of Striga resistance. These accessions grow as weeds on uncultivated land and have shown remarkable immunity to Striga infestation. The researchers hypothesized that Striga-resistant sorghum was likely to be found in northeastern Africa, where the greatest diversity of both wild and cultivated sorghum is found, and because this area is the natural range of the Striga parasite. The researchers tested the resistance response of seven accessions of wild sorghum of the aethiopicum, drummondii, and arundinaceum races against N13, a cultivated Striga-resistant landrace. The laboratory experiments revealed that three wild sorghum accessions (WSA-1, WSE-1, and WSA-2) had significantly higher resistance than N13. These accessions had the lowest Striga biomass and the fewest and smallest Striga attached to them. Further microscopic and histological analysis of attached Striga haustorium showed that wild sorghum accessions hindered the ingression of Striga haustorium into the host endodermis. In one of the resistant accessions (WSE-1), host and parasite interaction led to the accumulation of large amounts of secondary metabolites that formed a dark coloration at the interphase. Field experiments confirmed the laboratory screening experiments, and the same accessions were found to have resistance against Striga.