Introduction
Sorghum bicolor (L.) Moench is a well-adapted C4 grass that is a staple food to millions in sub-Saharan Africa (SSA). Despite its great economic value, sorghum cultivation in this region is threatened by Striga, a genus of parasitic plants that feeds from its hosts by obstructing nutrients and water through a specialized organ called haustorium (Hearne 2009). Striga also inflicts phytotoxicity on its hosts (Fujioka, Samejima, Mizutani, Okamoto & Sugimoto 2019a), severely impairing the growth and yield of infested crops (Frost, Gurney, Press & Scholes 1997). Infestation in SSA is estimated at 100 million ha (Spallek, Mutuku & Shirasu 2013) with a corresponding annual loss of USD 7 billion (Ejeta, 2007), which increases exponentially every year (Rodenburg, Demont, Zwart & Bastiaans 2016). Striga’ssuccess is attributed to its well-adapted parasitic lifestyle that includes seed fecundity, longevity, ease of dispersal, and capacity to manipulate host immunity (Runo & Kuria 2018).
For smallholder farmers in SSA, the most efficient, and sustainableStriga control method is an integrated approach that incorporates natural host resistance as a major component (Mwangangi et al.2021). Such resistance can be achieved by interfering with critical communication exchange between the host and the parasite (Jamil et al ., 2021).
At the fore of this host to Striga dialogue is the parasite’s perception of host-derived chemo attractants commonly known as strigolactones (SLs), Reviewed in (Al-Babili & Bouwmeester 2014). SLs are structurally diverse and differ in their efficiency and specificity in inducing seed germination in root parasitic plants (Guercio, Palayam & Shabek 2023). In sorghum, most genotypes emit 5-deoxystrigol as the dominant SL while a few emit orobanchol (Gobena et al. 2017; Mohemed et al. 2018). Orobanchol is a poor inducer ofStriga germination, so accordingly, sorghum genotypes that emit primarily orobanchol are termed Striga resistant. Advances in molecular genetics have now shown that sorghum genotypes that emit orobanchol have a mutation in a single recessive gene identified as theLOW GERMINATION STIMULANT 1 (LGS1 ) locus located at the tip of chromosome 5 (Satish, Gutema, Grenier, Rich & Ejeta 2012; Gobenaet al. 2017). The causative gene encodes a sulfotransferase and is missing in producers of 5-deoxystrigol (Gobena et al. 2017). Further evidence has shown that loss-of-function alleles have been subject to selection in sorghum landraces from SSA, where Strigaand sorghum co-evolved, conferring Striga resistance to some African sorghum accessions (Bellis et al. 2020). This finding was upheld by the widespread discovery of new lgs1 African sorghum genotypes harboring Striga resistance (Mallu et al. 2021).
Encouraged by these results and motivated by the potential of increasing sorghum yields in SSA through the cultivation of Striga resistant sorghum, we sought to mine for new alleles of Striga resistant sorghum. For this study, we selected a large collection of sorghum accessions, the Sorghum Accession Panel (SAP), maintained by the United States Department of Agriculture Agricultural Research Service (USDA-ARS) (https://www.ars-grin.gov/). High quality genotypic data is available for the SAP including genotyping by sequencing, single nucleotide polymorphism (SNP) markers (Morris et al. 2013) and whole genome sequencing (WGS) (Boatwright et al. 2022) making the SAP suitable for subsequent molecular breeding studies. The SAP provides a further advantage because it comprises converted tropical lines (Kleinet al. 2008) of diverse genotypes and geographic origin as well as breeding lines that are photoperiod-insensitive, elite inbred, improved cultivars, and landraces selected to capture maximum genetic and phenotypic diversity (Casa et al. 2008). We describe how the SAP, and genomics resources helped us to identify new Strigaresistant, early maturing sorghum. Incorporating this material into breeding programs can potentially alleviate losses caused byStriga parasitism and positively impact food security in smallholder farms of Africa.