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BACKGROUND: Sorghum yields in sub-Saharan Africa (SSA) are greatly reduced by parasitic plants of the genus Striga (witchweed). Vast global sorghum genetic diversity collections, as well as the availability of modern sequencing technologies, can be potentially harnessed to effectively manage the parasite. RESULTS: We used laboratory assays - rhizotrons to screen a global sorghum diversity panel to identify new sources of resistance to Striga; determine mechanisms of resistance, and elucidate genetic loci underlying the resistance using genome-wide association studies (GWAS). New Striga resistant sorghum determined by the number, size and biomass of parasite attachments were identified. Resistance was by; i) mechanical barriers that blocked parasite entry, ii) elicitation of a hypersensitive reaction that interfered with parasite development, and iii) the inability of the parasite to develop vascular connections with hosts. Resistance genes underpinning the resistance corresponded with the resistance mechanisms and included pleiotropic drug resistance proteins that transport resistance molecules; xylanase inhibitors involved in cell wall fortification and hormonal regulators of resistance response, Ethylene Response Factors. CONCLUSIONS: Our findings are of fundamental importance to developing durable and broad-spectrum resistance against Striga and have far-reaching applications in many SSA countries where Striga threatens the livelihoods of millions of smallholder farmers that rely on sorghum as a food staple.

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Invasive holoparasitic plants of the genus Cuscuta (dodder) threaten African ecosystems due to their rapid spread and attack on various host plant species. Most Cuscuta species cannot photosynthesize and hence rely on host plants for nourishment. After attachment through a peg-like organ called a haustorium, the parasites deprive hosts of water and nutrients, which negatively affects host growth and development. Despite their rapid spread in Africa, dodders have attracted limited research attention, although data on their taxonomy, host range, and epidemiology are critical for their management. Here, we combine taxonomy and phylogenetics to reveal the presence of field dodder (Cuscuta campestris) and C. kilimanjari (both either naturalized or endemic to East Africa), in addition to the introduction of the giant dodder (C. reflexa), a south Asian species, in continental Africa. These parasites have a wide host range, parasitizing species across 13 angiosperm orders. We evaluated the possibility of C. reflexa to expand this host range to tea (Camelia sinensis), coffee (Coffea arabica), and mango (Mangifera indica), crops of economic importance to Africa, for which haustorial formation and vascular-bundle connections in all three crops revealed successful parasitism. However, only mango mounted a successful postattachment resistance response. Furthermore, species distribution models predicted high habitat suitability for Cuscuta spp. across major tea- and coffee-growing regions of Eastern Africa, suggesting an imminent risk to these crops. Our findings provide relevant insights into a poorly understood threat to biodiversity and economic wellbeing in Eastern Africa, and provide critical information to guide development of management strategies to avert Cuscuta spp. spread.

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Aflatoxins are carcinogenic chemical metabolites produced by Aspergillus spp. of the section Flavi. In Kenya, Aspergillus flavus is the most prevalent and has been associated with several acute and chronic aflatoxin outbreaks in the past. In this study, we evaluated the occurrence of A. flavus in soils from two agro-ecological regions with contrasting climatic conditions, aflatoxin contamination histories and cropping systems. Aspergillus spp. were first isolated from soils before the identification and determination of their aflatoxigenicity. Further, we determined the occurrence of Pseudomonas and Bacillus spp. in soils from the two regions. These bacterial species have long been associated with biological control of several plant pathogens including Aspergillus spp. Our results show that A. flavus occurred widely and produced comparatively higher total aflatoxin levels in all (100%) study sites from the eastern to the western regions of Kenya. For the western region, A. flavus was detected in 4 locations (66.7%) that were previously under maize cultivation with the isolates showing low aflatoxigenicity. A. flavus was not isolated from soils under sugarcane cultivation. Distribution of the two bacterial species varied across the regions but we detected a weak relationship between occurrence of bacterial species and A. flavus. We discuss these findings in the context of the influence of climate, microbial profiles, cropping systems and applicability in the deployment of biological control remedies against aflatoxin contamination.

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Rice grains can be attacked by a range of pathogens, including Aspergillus species, which can cause the accumulation of aflatoxins and represent a serious threat to the consumers. Aflatoxins are secondary metabolites synthesized by Aspergillus species and naturally occur in various foodstuffs. In this study, we sought to analyze the prevalence of aflatoxin-producing Aspergillus spp. in rice grains currently sold in Kenyan local markets. We analyzed a total of 98 samples randomly collected and primarily analyzed to observe moisture content and fungal growth. We then isolated Aspergillus species, characterized them morphologically and using the Internal transcribed spacer (ITS) primers. Finally, we screened them for aflatoxin-producing isolates targeting Norsolorinic Acid (nor-1) and Versicolorin (ver-1) specific genes involved in aflatoxin biosynthesis. We observed that all tested samples were contaminated. The highest prevalence of Aspergillus species and aflatoxigenic fungal species, had values of 66% and 36.4% for nor-1 and ver-1, respectively. In total, 66% of all isolates were confirmed to be aflatoxin producers. The occurrence of high contamination levels of Aspergillus species points to the possibility of production of aflatoxins in rice grains. This work provides a baseline for future studies on the occurrence of mycotoxigenic fungal species in rice grains being sold in local markets and strategies to control these aflatoxigenic strains at pre- and post-harvest levels.

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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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KEY MESSAGE: We report success of host-induced gene silencing in downregulation of aflatoxin biosynthesis in Aspergillus flavus infecting maize transformed with a hairpin construct targeting transcription factor aflR. Infestation of crops by aflatoxin-producing fungi results in economic losses as well as negative human and animal health effects. Currently, the control strategies against aflatoxin accumulation are not effective to the small holder farming systems in Africa and this has led to widespread aflatoxin exposure especially in rural populations of sub-Saharan Africa that rely on maize as a staple food crop. A recent strategy called host-induced gene silencing holds great potential for developing aflatoxin-resistant plant germplasm for the African context where farmers are unable to make further investments other than access to the germplasm. We transformed maize with a hairpin construct targeting the aflatoxin biosynthesis transcription factor aflR. The developed transgenic maize were challenged with an aflatoxigenic Aspergillus flavus strain from Eastern Kenya, a region endemic to aflatoxin outbreaks. Our results indicated that aflR was downregulated in A. flavus colonizing transgenic maize. Further, maize kernels from transgenic plants accumulated significantly lower levels of aflatoxins (14-fold) than those from wild type plants. Interestingly, we observed that our silencing cassette caused stunting and reduced kernel placement in the transgenic maize. This could have been due to "off-target" silencing of unintended genes in transformed plants by aflR siRNAs. Overall, this work indicates that host-induced gene silencing has potential in developing aflatoxin-resistant germplasm.

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