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HYPOSENSITIVE TO LIGHT (HTL) and DWARF14 (D14) mediate the perception of karrikin and strigolactone, which stimulates germination of the parasitic weed Striga . However, their role in parasitic seeds is poorly understood, and the basis for their differing responsiveness remains unclear. Here, we show that Striga hermonthica HTL proteins (ShHTLs) in ‘conserved’ and ‘intermediate’ clades are able to bind karrikin. The ‘divergent’ clade is able to hydrolyze strigolactone. Unexpectedly, we find that ShD14 is also capable of hydrolyzing strigolactone. Through comparative analysis of ShHTLs and ShD14 crystal structures, we provide insights into the basis for their selectivity. Moreover, we show that both ShD14 and divergent clade ShHTLs, but not conserved and intermediate clade ShHTLs, can interact with the putative downstream signaling component ShMAX2 in the presence of the synthetic strigolactone, rac -GR24. These findings provide insight into how strigolactone is perceived and how ligand specificity is determined. HTL and D14 receptors perceive the structurally similar signaling compounds karrikin and strigolactone. Here, the authors show that ShD14 and a divergent clade of ShHTLs from Strigae capable of recognizing strigolact are capable of recognizing strigolactone and provide structural insights into the evolution of ligand specificity.

Sugarcane is a major industrial crop highly susceptible to parasitic weed ( Striga spp. ), causing a 38% reduction in cane yield due to a longer lag phase of 20–40 days, and wider spacing. Herbicides with a longer retention and slow-release nature could allow Striga seeds to germinate and be killed before attaching to the host. Therefore, a graphene oxide based nanoformulation loaded with atrazine was synthesized and evaluated under controlled and field conditions for its release kinetics, Striga control efficiency (SCE), and cane yield for two years (2018–2019) at two locations. In-vitro assays on release kinetics showed that the release rate of active ingredient (a.i.) from the nanocomposite loaded with atrazine (NCA) was slower (64.5%) than conventional atrazine (82.1%) on the 30th day in water. Similarly, cumulative release percentage of a.i. with NCA was 4.4% compared to atrazine (16.2%) at the initial 0–3 days in soil. Further, field evaluation (deep application in 12 cm furrows) of NCA at 1.25 kg a.i./ha at 95 days after planting (DAP) found superior in delaying Striga emergence by 18–20 days over atrazine. Furthermore, NCA recorded the highest efficacy (∼ 21%) across two locations owing to reduced Striga density (66.7–68.2%) and dry weight (39.3–48.9%). Consequently, NCA at 95 DAP produced higher cane (30.6–31.0%) and sugar (30.7–36.7%) yields. Therefore, carbon-based graphene oxide with a greater surface area and low production cost would offer an environmentally benign and alternative option in controlling Striga before its haustorium attaches to sugarcane roots. This formulation represents a novel direction for developing herbicides with enhanced performance and reduced environmental impact.

Strigolactones are naturally occurring signaling molecules that affect plant development, fungi-plant interactions, and parasitic plant infestations. We characterized the function of 11 strigolactone receptors from the parasitic plant Striga hermonthica using chemical and structural biology. We found a clade of polyspecific receptors, including one that is sensitive to picomolar concentrations of strigolactone. A crystal structure of a highly sensitive strigolactone receptor from Striga revealed a larger binding pocket than that of the Arabidopsis receptor, which could explain the increased range of strigolactone sensitivity. Thus, the sensitivity of Striga to strigolactones from host plants is driven by receptor sensitivity. By expressing strigolactone receptors in Arabidopsis, we developed a bioassay that can be used to identify chemicals and crops with altered strigolactone levels.

Striga species are parasitic weeds that seriously constrain the productivity of food staples, including cereals and legumes, in Sub-Saharan Africa and Asia. In eastern and central Africa, Striga spp. infest as much as 40 million hectares of smallholder farmland causing total crop failure during severe infestation. As the molecular mechanisms underlying resistance are yet to be elucidated, we undertook a comparative metabolome study using the Striga-resistant rice (Oryza sativa) cultivar 'Nipponbare' and the susceptible cultivar 'Koshihikari'. We found that a number of metabolites accumulated preferentially in the Striga-resistant cultivar upon Striga hermonthica infection. Most apparent was increased deposition of lignin, a phenylpropanoid polymer mainly composed of 𝑝-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) aromatic units, around the site of interaction in Nipponbare. The increased deposition of lignin was accompanied by induction of the expression of corresponding enzymeencoding genes in the phenylpropanoid pathway. In addition, perturbing normal lignin composition by knocking down or overexpressing the genes that regulate lignin composition, i.e. 𝑝-COUMARATE 3-HYDROXYLASE or FERULATE 5-HYDROXYLASE, enhanced susceptibility of Nipponbare to S. hermonthica infection. These results demonstrate that enhanced lignin deposition and maintenance of the structural integrity of lignin polymers deposited at the infection site are crucial for postattachment resistance against S. hermonthica.

Host–parasite coevolution can maintain high levels of genetic diversity in traits involved in species interactions. In many systems, host traits exploited by parasites are constrained by use in other functions, leading to complex selective pressures across space and time. Here, we study genome-wide variation in the staple crop Sorghum bicolor (L.) Moench and its association with the parasitic weed Striga hermonthica (Delile) Benth., a major constraint to food security in Africa. We hypothesize that geographic selection mosaics across gradients of parasite occurrence maintain genetic diversity in sorghum landrace resistance. Suggesting a role in local adaptation to parasite pressure, multiple independent loss-of-function alleles at sorghum LOW GERMINATION STIMULANT 1 (LGS1) are broadly distributed among African landraces and geographically associated with S. hermonthica occurrence. However, low frequency of these alleles within S. hermonthica-prone regions and their absence elsewhere implicate potential trade-offs restricting their fixation. LGS1 is thought to cause resistance by changing stereochemistry of strigolactones, hormones that control plant architecture and below-ground signaling to mycorrhizae and are required to stimulate parasite germination. Consistent with trade-offs, we find signatures of balancing selection surrounding LGS1 and other candidates from analysis of genome-wide associations with parasite distribution. Experiments with CRISPR–Cas9-edited sorghum further indicate that the benefit of LGS1-mediated resistance strongly depends on parasite genotype and abiotic environment and comes at the cost of reduced photosystem gene expression. Our study demonstrates long-term maintenance of diversity in host resistance genes across smallholder agroecosystems, providing a valuable comparison to both industrial farming systems and natural communities.

The genus Striga , also called “witchweed”, is a member of the family Orobanchaceae, which is a major family of root-parasitic plants. Striga can lead to the formation of seed stocks in the soil and to explosive expansion with enormous seed production and stability once the crops they parasitize are cultivated. Understanding the molecular mechanism underlying the communication between Striga and their host plants through natural seed germination stimulants, “strigolactones (SLs)”, is required to develop the technology for Striga control. This review outlines recent findings on the SL perception mechanism, which have been accumulated in Striga hermonthica by the similarity of the protein components that regulate SL signaling in nonparasitic model plants, including Arabidopsis and rice. HTL/KAI2 homologs were identified as SL receptors in the process of Striga seed germination. Recently, this molecular basis has further promoted the development of various types of SL agonists/antagonists as seed germination stimulants or inhibitors. Such chemical compounds are also useful to elucidate the dynamic behavior of SL receptors and the regulation of SL signaling.

Striga hermonthica infestation causes significant losses of maize yield in the Nigerian savannas and several technologies have been developed and promoted to control Striga in maize. However, since no single technology has been found to be effective against Striga, integrated management is needed to achieve satisfactory and sustainable Striga control. Both on-station and on-farm trials were undertaken from 2013 to 2015 in Bauchi and Kano States of Nigeria to evaluate the performance of integrated Striga control technologies. In the on-station trials, a soybean–maize rotation did not suppress Striga in maize in either location. However, nitrogen application suppressed and reduced Striga infection, except in Bauchi in 2014. The soybean–maize rotation accompanied by N application reduced Striga damage in both locations. On farmers’ fields, rotating soybean with maize significantly reduced Striga infection. At the same time, the use of maize varieties with a combined tolerance to drought and resistance to Striga parasitism also increased maize grain yield on farmers’ fields, probably due to three factors: a reduction in Striga infection, reduced effects of a mid-season moisture deficit, and increased uptake of nutrients from the soil. We concluded that the use of Striga-resistant maize varieties in combination with the application of N fertilizer and rotation with soybean could increase the productivity of maize in Striga-infested fields in the Nigerian savannas.

Strigolactones released from plant roots trigger both seed germination of parasitic weeds such as Striga spp. and hyphal branching of the symbionts arbuscular mycorrhizal (AM) fungi. Generally, strigolactone composition in exudates is quantitatively and qualitatively different among plants, which may be involved in susceptibility and host specificity in the parasite–plant interactions. We hypothesized that difference in strigolactone composition would have a significant impact on compatibility and host specificity/preference in AM symbiosis. Strigolactones in root exudates of Striga-susceptible (Pioneer 3253) and -resistant (KST 94) maize (Zea mays) cultivars were characterized by LC-MS/MS combined with germination assay using Striga hermonthica seeds. Levels of colonization and community compositions of AM fungi in the two cultivars were investigated in field and glasshouse experiments. 5-Deoxystrigol was exuded exclusively by the susceptible cultivar, while the resistant cultivar mainly exuded sorgomol. Despite the distinctive difference in strigolactone composition, the levels of AM colonization and the community compositions were not different between the cultivars. The present study demonstrated that the difference in strigolactone composition has no appreciable impact on AM symbiosis, at least in the two maize cultivars, and further suggests that the traits involved in Striga-resistance are not necessarily accompanied by reduction in compatibility to AM fungi.

• Striga hermonthica (Striga) is an obligate hemiparasitic weed, causing severe yield losses in cereals, including rice, throughout sub‐Saharan Africa. Striga germination depends on strigolactones (germination stimulants) exuded by the host roots. The interspecific New Rice for Africa (NERICA) cultivars offer a potentially interesting gene pool for a screen for low germination‐inducing rice cultivars. • Exudates were collected from all NERICA cultivars and their parents (Oryza sativa and Oryza glaberrima) for the analysis of strigolactones. In vitro and in situ Striga germination, attachment and emergence rates were recorded for each cultivar. • NERICA 1 and CG14 produced significantly less strigolactones and showed less Striga infection than the other cultivars. NERICAs 7, 8, 11 and 14 produced the largest amounts of strigolactones and showed the most severe Striga infection. Across all the cultivars and parents, there was a positive relationship between the amount of strigolactones in the exudate and Striga germination, attachment and emergence rates. • This study shows that there is genetic variation in Striga pre‐attachment resistance in NERICA rice. Cultivars combining this pre‐attachment resistance with post‐attachment resistance (already identified) can provide a key component for durable integrated management of this noxious weed in cereal production systems in sub‐Saharan Africa.

The parasitic plant Striga hermonthica causes extensive crop losses, particularly in Africa. Strigolactone hormones can be used to initiate germination of Striga seeds when no host crop is present, which causes the nascent Striga plants to die. Unfortunately, strigolactones are also used by crop plants to establish beneficial mutualisms. Uraguchi et al. developed a hybrid molecule that can initiate Striga germination without interfering with strigolactone-dependent events in the host (see the Perspective by Bouwmeester). The compound has the potential to diversify routes toward protecting fields from Striga infestation. Science , this issue p. 1301 ; see also p. 1248 Chemical screening yields a strigolactone receptor agonist that may aid control in fields infected with a parasitic weed. The parasitic plant Striga hermonthica has been causing devastating damage to the crop production in Africa. Because Striga requires host-generated strigolactones to germinate, the identification of selective and potent strigolactone agonists could help control these noxious weeds. We developed a selective agonist, sphynolactone-7, a hybrid molecule originated from chemical screening, that contains two functional modules derived from a synthetic scaffold and a core component of strigolactones. Cooperative action of these modules in the activation of a high-affinity strigolactone receptor ShHTL7 allows sphynolactone-7 to provoke Striga germination with potency in the femtomolar range. We demonstrate that sphynolactone-7 is effective for reducing Striga parasitism without impinging on host strigolactone-related processes.