Genome-Wide Association Studies for Striga asiatica Resistance in Tropical Maize

Striga asiatica L. is a parasitic weed in cereal crops including maize leading to tremendous yield losses up to 100% under severe infestation. The available S. asiatica control methods include cultural control options such as uprooting and burning the Striga plants before they flower, field sanitation, crop rotation, intercropping, organic matter usage, improved fallows, and application of herbicides. Resource limitation among smallholder farmers renders almost all of the control methods impossible. Development and use of Striga resistant genotypes are seen as the most feasible management option. Marker identification formulates tools that are faster, cheaper, and easier to utilise in breeding for S. asiatica resistance which has low heritability. The objective of this study was to identify single nucleotide polymorphism (SNP) markers for Striga resistance using the genome-wide association study (GWAS). Genotyping by sequencing was done on tropical maize inbred lines followed by their evaluation for Striga resistance. Analysis of variance showed significant ( p < 0.05 ) variation among evaluated genotypes for Striga resistance traits such as germination distance, germination percentage, haustoria root attachments, total Striga plants emerged, total biomass, and growth rate. There were also significant differences ( p < 0.05 ) for cobs, leaves, stems, and roots weight. The broad sense heritability was fairly high (up to 61%) for most traits. The means for derived traits on stress tolerance indices were subjected to a t -test, and significant differences ( p < 0.05 ) were found for leaves, stem, roots, shoots, and total biomass. The Manhattan plots from GWAS showed the presence of three SNP markers on chromosome numbers 5, 6, and 7 for total Striga plants emerged. The identified markers for resistance to S. asiatica should be validated and utilised to breed for Striga resistance in tropical maize.

Identification of QTLs Controlling Resistance/Tolerance to Striga hermonthica in an Extra-Early Maturing Yellow Maize Population

Striga hermonthica parasitism is a major constraint to maize production in sub-Saharan Africa with yield losses reaching 100% under severe infestation. The application of marker-assisted selection is highly promising for accelerating breeding for Striga resistance/tolerance in maize but requires the identification of quantitative trait loci (QTLs) linked to Striga resistance/tolerance traits. In the present study, 194 F2:3 families of TZEEI 79 × TZdEEI 11 were screened at two Striga-endemic locations in Nigeria, to identify QTLs associated with S. hermonthica resistance/tolerance and underlying putative candidate genes. A genetic map was constructed using 1139 filtered DArTseq markers distributed across the 10 maize chromosomes, covering 2016 cM, with mean genetic distance of 1.70 cM. Twelve minor and major QTLs were identified for four Striga resistance/tolerance adaptive traits, explaining 19.4%, 34.9%, 14.2% and 3.2% of observed phenotypic variation for grain yield, ears per plant, Striga damage and emerged Striga plants, respectively. The QTLs were found to be linked to candidate genes which may be associated with plant defense mechanisms in S. hermonthica infested environments. The results of this study provide insights into the genetic architecture of S. hermonthica resistance/tolerance indicator traits which could be employed for marker-assisted selection to accelerate efficient transfer host plant resistance genes to susceptible genotypes.

Induced Resistance to Striga hermonthica in Sorghum by Gamma Irradiation

Striga species affect the potential productivity of cereals in sub-saharian Africa due to the lack of durable Striga-resistance in host crops. This study aimed at inducing new source of resistance in sorghum using gamma irradiation. Dry seeds of three Sorghum varieties; Grinkan, ICV1049 and Sariaso14 were gamma-irradiated with 200 Gy, 300 Gy, 400 Gy and 500 Gy. Screening strategies involved a 2-year field and greenhouse experiments, where mutant Sorghum families, their parents and resistant control were artificially infected with Striga hermonthica seeds. Field screenings revealedinduced genetic variability among them, forty families significantly reduced the number of emerged Striga plants or showed good Sorghum grain yield performance despite the infection by S. hermonthica ecotype from Burkina Faso. The induced putative resistant mutants were identified across the the four applied irradiation doses. Greenhouse experiment confirmed Striga resistance in seven mutant Sorghum families leading to no emergence of Burkina&rsquo;s S. hermonthica ecotype along with high resistance index (RI) and low Striga damage score. Among them, two mutants SA38M5 and IC47M5 withstood S. hermonthica ecotype from Sudan and S. asiatica ecotype from Madagascar. The induced mutants will be evaluated for release to farmers for commercial production. Further studies are ongoing on confirmed mutants to highlight their Striga resistance mechanisms and explore the potential of pyramiding different mechanism to produce durable resistance to S. hermonthica in sorghum.

Integrated management of Striga hermonthica and S. asiatica in sorghum: A review

Potential yield of sorghum [Sorghum biocolor (L.) Moench] in the semi-arid agro-ecologies of East Africa is curtailed by several biotic, abiotic and socio-economic constraints. Striga is one of the major biotic constraints that causes up to 90% yield losses in sorghum in the region. In these regions Striga hermonthica and S. asiatica, are widely distributed, and severely affecting sorghum production and productivity. Several Striga management strategies are available that can be integrated to synergistically combat the weed. The use of resistant sorghum genotypes that are compatible with Fusarium oxysporum f.sp. strigae (FOS), a biocontrol agent of Striga, together with host plant resistance could promote integrated Striga management (ISM). This strategy is yet to be explored in most SSA countries where sorghum serves as a staple food crop for millions of households. This review discusses the management options available to control S. hermonthica and S. asiatica in sorghum. Breeding sorghum for Striga resistance and compatibility to FOS are highlighted as key components of integrated Striga management.

Modern Breeding Approaches for durable Resistance against the parasitic Plant Striga

Crop losses caused by parasitic plants of the genus Striga pose a great danger to the livelihoods of millions of smallholder farmers in Africa. The parasite attaches to host crops and siphons nutrients leading to severe retardation and crop death. Controlling Striga is difficult because of the parasite’s ability to produce large amounts of seeds that can remain dormant in the soil for decades – only germinating in response to chemical cues (strigolactones) from the host. In recent years, breeding crops for host-based resistance has been prioritized. However, such programs have not taken into account Striga’s ability to overcome host resistance. As a result, introduced resistance fails because of increased Striga virulence (infection severity). This article reviews technologies for a new paradigm in Striga resistance breeding that incorporates host resistance breeding with well-informed knowledge of parasite resistance in order to ensure durability of resistance. KEY WORDS: STRIGA, HOST BASED RESISTANCE, GENOME WIDE ASSOCIATION MAPPING, RNA SEQUENCING