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

Abstract

Sorghum is a major food staple in sub-Saharan Africa (SSA), but its production is constrained by the parasitic plant Striga that attaches to the roots of many cereals crops and causes severe stunting and loss of yield. Away from cultivated farmland, wild sorghum accessions grow as weedy plants and have shown remarkable immunity to Striga. We sought to determine the extent of the resistance to Striga in wild sorghum plants. Our screening strategy involved controlled laboratory assays of rhizotrons, where we artificially infected sorghum with Striga, as well as field experiments at three sites, where we grew sorghum with a natural Striga infestation. We tested the resistance response of seven accessions of wild sorghum of the aethiopicum, drummondii, and arundinaceum races against N13, which is a cultivated Striga resistant landrace. The susceptible control was farmer-preferred variety, Ochuti. From the laboratory experiments, we found three wild sorghum accessions (WSA-1, WSE-1, and WSA-2) that 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 in that these same accessions were found to have resistance against Striga. In the field, wild sorghum had low Area under the Striga Number Progressive curve (AUSNPC), which measures emergence of Striga from a host over time. We concluded that wild sorghum accessions are an important reservoir for Striga resistance that could be used to expand the genetic basis of cultivated sorghum for resistance to the parasite.

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“…Such specificity may be related to differences in quality, identity and/or quantity of the haustorium factor. The observed differential response is consistent with a previous report by Mbuvi et al (2017) who reported that the potential number of haustoria is a product of the concentration and/or quality of haustoria inducing factor and the parasite's individual ability to respond. The host specificity and variability of S. hermonthica populations might affect the efficiency of introducing new resistant cultivars as a control measure against the parasitic weed.…”

GWAS Provides Biological Insights into Mechanisms of the Parasitic Plant (Striga) Resistance in Sorghum

“…Our ndings underscore the need to continuously screen hosts for Striga resistance as a rmed by successful identi cation of new Striga resistant sorghum genotypes. The resistance by the genotypes, as determined by the number, size and biomass of Striga seedlings attached, is comparable to what has been reported in previous work that used rhizotrons [12,17]. For example, we found an average of 57 attachments on N13 compared to a mean of 56 reported by [12] and 75 [18].…”

Genetic variation in Sorghum bicolor strigolactones and their role in resistance against Striga hermonthica

“…5). The differential response in N13 could possibly be attributed to mechanical resistance, as reported by Grenier et al (2007) and Mbuvi et al (2017). In addition, high proportions of sorgomol in some genotypes, such as Wad Fahel, did not necessarily result in higher germination-stimulating activity, although it was associated with a higher Striga infection level in the pot experiment (Fig.…”