Striga Hermonthica Research Articles

Gibberellin action‐based strategies show promise in the control of Striga—A major threat to global agriculture

Societal Impact StatementThe root‐parasitic plant Striga hermonthica can cause severe crop failure, leading to significant agricultural and economic losses in Africa. Despite the promising target offered by the Striga seed germination mechanism for developing control methods, effective control methods have yet to be established. For seed germination, Striga must undergo an incubation under warm and humid conditions, a process termed conditioning, which allows it to germinate. Here, we report that the plant hormone gibberellin stimulates the conditioning process in Striga seeds. Our findings suggest that the quantitative control of gibberellin in plants is a promising approach for controlling Striga infestations and may help alleviate substantial threats to food security.Summary The root‐parasitic plant S. hermonthica is a serious agricultural threat in Africa. Although a number of control methods have been employed to combat Striga, there is still a need to develop more effective control methods. Striga seeds germinate only in the presence of host plants, owing to the need for host‐released stimulants, primarily strigolactone. To detect signals from the host, Striga seeds must be exposed to a specific warm, dark, and moist environment, which is known as conditioning. Gibberellin directly promotes germination in many nonparasitic plants, but Striga seeds are insensitive to exogenous gibberellin. Consequently, the role of gibberellin in the germination of Striga seeds is unclear. We demonstrated that the effect of the gibberellin biosynthesis inhibitor paclobutrazol was higher in seeds during the conditioning, compared with those already conditioned, suggesting that gibberellin primarily regulates conditioning rather than germination. The content of an active form gibberellin, gibberellin A4 (GA4) exhibited an increase throughout the conditioning period, and we quantified the elevated expression levels of some gibberellin biosynthesis genes. The GA4 treatment shortened the conditioning period required for strigolactone‐induced seed germination. We also discovered that gibberellin upregulated ShACO1, the gene responsible for ethylene synthesis, during conditioning, and that the inhibition of GR24‐induced germination by the ethylene biosynthesis inhibitor α‐aminoisobutyric acid was gradually alleviated by GA4. Our findings demonstrate that gibberellin is critical for conditioning and germination processes but acts as a minor stimulant in Striga seeds. This study demonstrates the potential of quantitative endogenous gibberellin control for Striga management, providing valuable insights for alleviating food security issues.

Association mapping identifies stable loci containing novel genes for developmental and reproductive traits in sorghum.

Key message We mapped 11 sorghum traits, identified 33 candidate genes, and found a grain yield gene (GID1) that regulates seed development and a grass-specific tillering gene (DUF1618) transferred to Striga hermonthica.

Bacteria as potential biocontrol agents for managing purple witchweed (Striga hermonthica) in grain sorghum

Abstract Purple witchweed [Striga hermonthica (Delile) Benth.], a highly destructive parasitic weed, poses a significant threat to sorghum [Sorghum bicolor (L.) Moench] cultivation. This hemiparasitic plant intrudes its root system into the host plant, leading to substantial yield losses, particularly in susceptible genotypes. In the pursuit of eco-friendly solutions, the biocontrol approach has gained attention as a potential management strategy for Striga. In this study, 13 bacterial strains belonging to the genera Bacillus, Gluconobacter, Pseudomonas, and Streptomyces were investigated in vitro for their efficiency in controlling the early-stage development of Striga. Among the tested strains, Streptomyces morookaensis NRRL B-12429 demonstrated significant inhibition of Striga seed germination and radicle elongation at 54.36% and 61.84%, respectively, when applied to preconditioned seeds with a synthetic germination stimulant. The effect of S. morookaensis on the inhibition of Striga seed germination was more pronounced in the presence of the host plant, sorghum, at 62.35%. However, biopriming of sorghum seeds with S. morookaensis did not enhance the inhibitory effects on Striga seed germination but resulted in a greater reduction in radicle elongation at 74.64% compared with non-primed seeds. Additionally, the biopriming with S. morookaensis promoted the growth of shoots and roots of germinating sorghum, regardless of the presence of Striga seeds. These findings highlight the potential of S. morookaensis strain NRRL B-12429 as a viable candidate for biocontrol agent applications in sorghum cultivation. Further exploration and investigation of its biocontrol capabilities can provide valuable insights for sustainable management practices against Striga infestations.

A powerful molecular marker to detect mutations at sorghum LOW GERMINATION STIMULANT 1.

The parasitic weed Striga (Striga hermonthica) limits productivity of sorghum (Sorghum bicolor) and other cereals in sub-Saharan Africa and elsewhere. Improved host plant genetics is an effective control method but verified loci contributing to Striga resistance are limited. LOW GERMINATION STIMULANT 1 remains the only known sorghum locus affecting resistance to Striga. Functional loss (lgs1) alleles at this locus result in low Striga germination stimulant activity. We developed a robust polymerase chain reaction (PCR)-based LGS1 marker that detects all known natural lgs1 alleles. We have successfully used this marker to improve Striga resistance in our sorghum breeding program. To check its utility among diverse sets of germplasm, we genotyped 406 lines of the sorghum association panel (SAP) with the marker and phenotyped them for Striga germination stimulant activity. The SAP contains 23 lines (6%) with lgs1 mutations that involve a complete loss of this gene. Three previously described deletion alleles (lgs1-1, lgs1-2, and lgs1-3) ranging from 28.5 to 34kbp are present among SAP members with a new one, lgs1-6, missing nearly 50kbp relative to the reference genome. All 23 members of the SAP carrying lgs1 alleles had low Striga germination stimulant activity. The smaller previously described intragenic deletion mutations lgs1-4 and lgs1-5 are not present in the SAP. The LGS1 marker is useful for both detecting sources of lgs1 and introgressing Striga resistance into new genetic backgrounds.

Genome-wide association analysis of grain yield and Striga hermonthica and S. asiatica resistance in tropical and sub-tropical maize populations

BackgroundGenetic improvement for Striga hermonthica (Sh) and S. asiatica (Sa) resistance is the most economical and effective control method to enhance the productivity of maize and other major cereal crops. Hence, identification of quantitative trait loci (QTL) associated with Striga resistance and economic traits will guide the pace and precision of resistance breeding in maize. The objective of this study was to undertake a genome-wide association analysis of grain yield and Sh and Sa resistance among tropical and sub-tropical maize populations to identify putative genetic markers and genes for resistance breeding. 126 maize genotypes were evaluated under controlled environment conditions using artificial infestation of Sh and Sa. The test genotypes were profiled for grain yield (GY), Striga emergence counts at 8 (SEC8) and 10 (SEC10) weeks after planting, and Striga damage rate scores at 8 (SDR8) and 10 (SDR10) weeks after planting. Population structure analysis and genome-wide association mapping were undertaken based on 16,000 single nucleotide polymorphism (SNP) markers.ResultsA linkage disequilibrium (LD) analysis in 798,675 marker pairs revealed that 21.52% of pairs were in significant linkage (P < 0.001). Across the chromosomes, the LD between SNPs decayed below a critical level (r2 = 0.1) at a map distance of 0.19 Mbp. The genome-wide association study identified 50 significant loci associated with Sh resistance and 22 significant loci linked to Sa resistance, corresponding to 39 and 19 candidate genes, respectively.ConclusionThe study found non-significant QTL associated with dual resistance to the two examined Striga species Some of the detected genes reportedly conditioned insect and pathogen resistance, plant cell development, variable senescence, and pollen fertility. The markers detected in the present study for Sa resistance were reported for the first time. The gene Zm00001eb219710 was pleiotropic, and conditioned GY and SEC10, while Zm00001eb165170 affected SDR8 and SDR10, and Zm00001eb112030 conditioned SDR8 and SDR10 associated with Sh resistance. The candidate genes may facilitate simultaneous selection for Sh and Sa resistance and grain yield in maize after further validation and introgression in breeding pipelines. Overall, we recommend breeding maize specifically for resistance to each Striga species using germplasm adapted to the endemic region of each parasite.

Dominant-negative KAI2d paralogs putatively attenuate strigolactone responses in root parasitic plants.

Many root parasitic plants in the Orobanchaceae use host-derived strigolactones as germination cues. This adaptation facilitates attachment to a host and is particularly important for the success of obligate parasitic weeds that cause substantial crop losses globally. Parasite seeds sense strigolactones through "divergent" KARRIKIN INSENSITIVE2 (KAI2d)/HYPOSENSITIVE TO LIGHT (HTL) α/β-hydrolases that have undergone substantial duplication and diversification in Orobanchaceae genomes. After germination, chemotropic growth of parasite roots toward a strigolactone source also occurs in some species. We investigated which of the seven KAI2d genes found in a facultative hemiparasite, Phtheirospermum japonicum, may enable chemotropic responses to strigolactones. To do so, we developed a triple mutant Nbd14a,b kai2i line of Nicotiana benthamiana in which strigolactone-induced degradation of SMAX1, an immediate downstream target of KAI2 signaling, is disrupted. In combination with a transiently expressed, ratiometric reporter of SMAX1 protein abundance, this mutant forms a system for the functional analysis of parasite KAI2d proteins in a plant cellular context. Using this system, we unexpectedly found three PjKAI2d proteins that do not trigger SMAX1 degradation in the presence of strigolactones. Instead, these PjKAI2d inhibit the perception of low strigolactone concentrations by strigolactone-responsive PjKAI2d in a dominant-negative manner that depends upon an active catalytic triad. Similar dominant-negative KAI2d paralogs were identified in an obligate hemiparasitic weed, Striga hermonthica. These proteins suggest a mechanism for attenuating strigolactone signaling in parasites, which might be used to enhance the perception of shallow strigolactone gradients during root growth toward a host or to restrict germination responses to specific strigolactones.

Evaluating Mechanisms of Soil Microbiome Suppression of Striga Infection in Sorghum.

The root parasitic weed Striga hermonthica has a devastating effect on sorghum and other cereal crops in Sub-Saharan Africa. Available Striga management strategies are rarely sufficient or not widely accessible or affordable. Identification of soil- or plant-associated microorganisms that interfere in the Striga infection cycle holds potential for development of complementary biological control measures. Such inoculants should be preferably based on microbes native to the regions of their application. We developed a method to assess microbiome-based soil suppressiveness to Striga with a minimal amount of field-collected soil. We previously used this method to identify the mechanisms of microbe-mediated suppression of Striga infection and to test individual microbial strains. Here, we present protocols to assess the functional potential of the soil microbiome and individual bacterial taxa that adversely affect Striga parasitism in sorghum via three major known suppression mechanisms. These methods can be further extended to other Striga hosts and other root parasitic weeds. Key features • This protocol provides a detailed description of the methods used in Kawa et al. [1]. • This protocol is optimized to assess soil suppressiveness to Striga infection by using natural field-collected soil and the same soil sterilized by gamma-radiation. • This protocol is optimized to test bacterial (and not fungal) isolates. • This protocol can be easily extended to other host-parasite-microbiome systems.

The Evaluation of Debranone Series Strigolactone Agonists for Germination Stimulants in Orobanche Species.

Strigolactones (SLs) are plant hormones that regulate shoot branching. In addition, SLs act as compounds that stimulate the germination of root parasitic weeds, such as Striga spp. and Orobanche spp., which cause significant damage to agriculture worldwide. Thus, SL agonists have the potential to induce suicidal germination, thereby reducing the seed banks of root parasitic weeds in the soil. Particularly, phenoxyfuranone-type SL agonists, known as debranones, exhibit SL-like activity in rice and Striga hermonthica. However, little is known about their effects on Orobanche spp. In this study, we evaluated the germination-inducing activity of debranones against Orobanche minor. Analysis of structure-activity relationships revealed that debranones with electron-withdrawing substituents at the 2,4- or 2,6-position strongly induced the germination of Orobanche minor. Lastly, biological assays indicated that 5-(2-fluoro-4-nitrophenoxy)-3-methylfuran-2(5H)-one (test compound 61) induced germination to a comparable or even stronger extent than GR24, a well-known synthetic SL. Altogether, our data allowed us to infer that this enhanced activity was due to the recognition of compound 61 by the SLs receptor, KAI 2d, in Orobanche minor.

Genomic prediction of the performance of tropical doubled haploid maize lines under artificial Striga hermonthica (Del.) Benth. infestation.

Striga hermonthica (Del.) Benth., a parasitic weed, causes substantial yield losses in maize production in sub-Saharan Africa. Breeding for Striga resistance in maize is constrained by limited genetic diversity for Striga resistance within the elite germplasm and phenotyping capacity under artificial Striga infestation. Genomics-enabled approaches have the potential to accelerate identification of Striga resistant lines for hybrid development. The objectives of this study were to evaluate the accuracy of genomic selection for traits associated with Striga resistance and grain yield (GY) and to predict genetic values of tested and untested doubled haploid maize lines. We genotyped 606 doubled haploid lines with 8,439 rAmpSeq markers. A training set of 116 doubled haploid lines crossed to 2 testers was phenotyped under artificial Striga infestation at 3 locations in Kenya. Heritability for Striga resistance parameters ranged from 0.38-0.65 while that for GY was 0.54. The prediction accuracies for Striga resistance-associated traits across locations, as determined by cross-validation (CV) were 0.24-0.53 for CV0 and from 0.20 to 0.37 for CV2. For GY, the prediction accuracies were 0.59 and 0.56 for CV0 and CV2, respectively. The results revealed 300 doubled haploid lines with desirable genomic estimated breeding values for reduced number of emerged Striga plants (STR) at 8, 10, and 12 weeks after planting. The genomic estimated breeding values of doubled haploid lines for Striga resistance-associated traits in the training and testing sets were similar in magnitude. These results highlight the potential application of genomic selection in breeding for Striga resistance in maize. The integration of genomic-assisted strategies and doubled haploid technology for line development coupled with forward breeding for major adaptive traits will enhance genetic gains in breeding for Striga resistance in maize.

Chromosome-scale pearl millet genomes reveal CLAMT1b as key determinant of strigolactone pattern and Striga susceptibility

The yield of pearl millet, a resilient cereal crop crucial for African food security, is severely impacted by the root parasitic weed Striga hermonthica, which requires host-released hormones, called strigolactones (SLs), for seed germination. Herein, we identify four SLs present in the Striga-susceptible line SOSAT-C88-P10 (P10) but absent in the resistant 29Aw (Aw). We generate chromosome-scale genome assemblies, including four gapless chromosomes for each line. The Striga-resistant Aw lacks a 0.7 Mb genome segment containing two putative CARLACTONOIC ACID METHYLTRANSFERASE1 (CLAMT1) genes, which may contribute to SL biosynthesis. Functional assays show that P10CLAMT1b produces the SL-biosynthesis intermediate methyl carlactonoate (MeCLA) and that MeCLA is the precursor of P10-specific SLs. Screening a diverse pearl millet panel confirms the pivotal role of the CLAMT1 section for SL diversity and Striga susceptibility. Our results reveal a reason for Striga susceptibility in pearl millet and pave the way for generating resistant lines through marker-assisted breeding or direct genetic modification.

Genetic diversity analysis of tropical and sub-tropical maize germplasm for Striga resistance and agronomic traits with SNP markers.

Striga hermonthica (Sh) and S. asiatica (Sa) are major parasitic weeds limiting cereal crop production and productivity in sub-Saharan Africa (SSA). Under severe infestation, Striga causes yield losses of up to 100%. Breeding for Striga-resistant maize varieties is the most effective and economical approach to controlling the parasite. Well-characterized and genetically differentiated maize germplasm is vital to developing inbred lines, hybrids, and synthetic varieties with Striga resistance and desirable product profiles. The objective of this study was to determine the genetic diversity of 130 tropical and sub-tropical maize inbred lines, hybrids, and open-pollinated varieties germplasm using phenotypic traits and single nucleotide polymorphism (SNP) markers to select Striga-resistant and complementary genotypes for breeding. The test genotypes were phenotyped with Sh and Sa infestations using a 13x10 alpha lattice design with two replications. Agro-morphological traits and Striga-resistance damage parameters were recorded under a controlled environment. Further, high-density Diversity Array Technology Sequencing-derived SNP markers were used to profile the test genotypes. Significant phenotypic differences (P<0.001) were detected among the assessed genotypes for the assessed traits. The SNP markers revealed mean gene diversity and polymorphic information content of 0.34 and 0.44, respectively, supporting the phenotypic variation of the test genotypes. Higher significant variation was recorded within populations (85%) than between populations using the analysis of molecular variance. The Structure analysis allocated the test genotypes into eight major clusters (K = 8) in concordance with the principal coordinate analysis (PCoA). The following genetically distant inbred lines were selected, displaying good agronomic performance and Sa and Sh resistance: CML540, TZISTR25, TZISTR1248, CLHP0303, TZISTR1174, TZSTRI113, TZDEEI50, TZSTRI115, CML539, TZISTR1015, CZL99017, CML451, CML566, CLHP0343 and CML440. Genetically diverse and complementary lines were selected among the tropical and sub-tropical maize populations that will facilitate the breeding of maize varieties with Striga resistance and market-preferred traits.

Radicle Growth Regulation of Root Parasitic Plants by Auxin-related Compounds.

Root parasitic plants in the Orobanchaceae, such as Striga and Orobanche, cause significant damage to crop production. The germination step of these root parasitic plants is induced by host-root-derived strigolactones. After germination, the radicles elongate toward the host and invade the host root. We have previously discovered that a simple amino acid, tryptophan (Trp), as well as its metabolite, the plant hormone indole-3-acetic acid (IAA), can inhibit radicle elongation of Orobanche minor. These results suggest that auxin plays a crucial role in the radicle elongation step in root parasitic plants. In this report, we used various auxin chemical probes to dissect the auxin function in the radicle growth of O. minor and Striga hermonthica. We found that synthetic auxins inhibited radicle elongation. In addition, auxin receptor antagonist, auxinole, rescued the inhibition of radicle growth by exogenous IAA. Moreover, a polar transport inhibitor of auxin, N-1-naphthylphthalamic acid, affected radicle bending. We also proved that exogenously applied Trp is converted into IAA in O. minor seeds, and auxinole partly rescued this radicle elongation. Taken together, our data demonstrate a pivotal role for auxin in radicle growth. Thus, manipulation of auxin function in root parasitic plants should offer a useful approach to combat these parasites.

Gene cloning and functional analysis of an iridoid oxidase gene in Rehmannia glutinosa

Rehmannia glutinosa is one of the commonly used Chinese herbal medicines, which has activities of heat-clearing,blood-cooling, Yin-nourishing, and body fluid-promoting. Iridoid glycosides are the main bioactive in R. glutinosa. Iridoid oxidase is a key rate-limiting enzyme in the biosynthetic pathway of iridoid glycosides. In this study, an iridoid oxidase gene Rg IO was screened based on the transcriptome data, followed by bioinformatics analysis, expression characteristic detection, and subcellular localization analysis. The results show that the coding region of Rg IO is 1 536 bp, with 511 amino acids encoded, and the molecular weight is about 58 258. 01. The protein sequence of Rg IO contains the conserved domains and motifs of cytochrome P450 oxidases. Rg IO has the highest sequence identities with its ortholog proteins in Striga asiatica, Striga hermonthica, and Centranthera grandiflora and has good sequence identities(77. 28%) with Catharanthus roseus Cr IO. Rg IO shows specific expression in the leaf of R. glutinosa. In response to MeJA induction, the expression of MeJA in leaves and roots after treatment increases by 3. 15 and 1. 3 times at 3 h and 6 h,respectively. The result of subcellular localization shows that Rg IO is distributed in the endoplasmic reticulum. Agrobacterium-mediated transient expression of Rg IO gene in leaves of R. glutinosa makes the content of catalpol increase by 0. 82 times compared with the transient expression of the empty vector. This study provides a key target gene for the molecular regulation and biosynthesis of catalpol in R. glutinosa and lays a foundation for revealing the complete biosynthetic pathway of catalpol.

Evaluation of granular formulated strigolactone analogs for Striga suicidal germination.

Striga hermonthica, an obligate root parasitic weed, poses a significant threat to cereal production in sub-Saharan Africa. Lowering Striga seed bank in infested soils is a promising strategy to mitigate infestation levels. The dependency of Striga seed germination on strigolactones opens up the possibility of a 'suicidal germination' approach, where synthetic germination stimulants induce lethal germination in the absence of a host. Implementing this approach requires active germination stimulants with a suitable formulation for field application. Here, we describe the development of slow-releasing granular formulation of two potent germination stimulants 'Methyl Phenlactonoate 3' and 'Nijmegen-1' and the assessment of their activity under Laboratory, greenhouse, mini-field, and field conditions. Under laboratory conditions, the granular formulation of either of the two germination stimulants (1.25 mg per plate, corresponding to 0.09 mg a.i.) induced Striga seed germination at a rate of up to 43%. With 10 mg granular product (0.75 mg a.i.) per pot, we observed 77-83% reduction in Striga emergence under greenhouse pot conditions. Application of the formulated stimulants under artificially or naturally infested fields resulted in approximately 56%, 60%, and 72% reduction in Striga emergence in maize, sorghum, and millet fields in Kenya and Burkina Faso, respectively. Our findings on the newly designed granular formulation of Methyl Phenlactonoate 3 and Nijmegen-1 reveal encouraging prospects for addressing the Striga problem in Africa. These findings underscore several significant advantages of the formulated stimulants, including suitability for the African agricultural context, and, most importantly, their effectiveness in reducing Striga infection. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The soil microbiome modulates the sorghum root metabolome and cellular traits with a concomitant reduction of Striga infection

Sorghum bicolor is among the most important cereals globally and a staple crop for smallholder farmers in sub-Saharan Africa. Approximately 20% of sorghum yield is lost annually in Africa due to infestation with the root parasitic weed Striga hermonthica. Existing Striga management strategies are not singularly effective and integrated approaches are needed. Here, we demonstrate the functional potential of the soil microbiome to suppress Striga infection in sorghum. We associate this suppression with microbiome-mediated induction of root endodermal suberization and aerenchyma formation and with depletion of haustorium-inducing factors, compounds required for the initial stages of Striga infection. We further identify specific bacterial taxa that trigger the observed Striga-suppressive traits. Collectively, our study describes the importance of the soil microbiome in the early stages of root infection by Striga and pinpoints mechanisms of Striga suppression. These findings open avenues to broaden the effectiveness of integrated Striga management practices.

Macronutrient application rescues performance of tolerant sorghum genotypes when infected by the parasitic plant striga.

Infection by the hemi-parasitic plant Striga hermonthica causes severe host-plant damage and seed production losses. Increased availability of essential plant nutrients reduces infection. Whether, how and to what extent, it also reduces striga-induced host-plant damage is not well studied. Effects of improved macro- and micronutrient supply on host-plant performance under striga-free and infected conditions were investigated in greenhouse pot assays. One striga-sensitive and two striga-tolerant genotypes were compared. Plants growing in impoverished soils were supplied with (1) 25% of optimal macro- and micronutrients quantities, (2) 25% macro- and 100% micronutrients, (3) 100% macro- and 25% micronutrients, or (4) 100% of macro- and micronutrients. Photosynthesis rates of striga-infected plants of the sensitive genotype increased with improved nutrition (12.2 to 22.1 µmol/m2/s1) but remained below striga-free levels (34.9-38.8 µmol/m2/s1). For the tolerant genotypes, increased macro-nutrient supply offset striga-induced photosynthesis losses. Striga-induced relative grain losses of 100% for the sensitive genotype were reduced to 74% by increased macronutrients. Grain losses of 80% in tolerant Ochuti, incurred at low nutrient supply, were reduced to 5% by improved nutrient supply. Increasing macro-nutrient supply reduces striga impact on host-plants but can only restore losses when applied to genotypes with a tolerant background.

Laboratory Screening of Sorghum Lines for Incompatibility: A Postattachment Resistance Mechanism to the Parasitic Weed Striga hermonthica

Laboratory Screening of Sorghum Lines for Incompatibility: A Postattachment Resistance Mechanism to the Parasitic Weed Striga hermonthica

Contribution of Signaling Partner Association to Strigolactone Receptor Selectivity.

The parasitic plant witchweed, Striga hermonthica, results in agricultural losses of billions of dollars per year. It perceives its host via plant hormones called strigolactones, which act as germination stimulants for witchweed. Strigolactone signaling involves substrate binding to the strigolactone receptor, followed by substrate hydrolysis and a conformational change from an inactive, or open state, to an active, or closed state. In the active state, the receptor associates with a signaling partner, MAX2. Recently, it was shown that this MAX2 association process acts as a strong contributor to the uniquely high signaling activity observed in ShHTL7; however, it is unknown why ShHTL7 has enhanced MAX2 association affinity. Using an umbrella sampling molecular dynamics approach, we characterized the association processes of AtD14, ShHTL7, a mutant of ShHTL7, and ShHTL6 with MAX2 homologue OsD3. From these results, we show that ShHTL7 has an enhanced standard binding free energy of OsD3 compared to those of the other receptors. Additionally, our results suggest that the overall topology of the T2/T3 helix region is likely an important modulator of MAX2 binding. Thus, differences in MAX2 association, modulated by differences in the T2/T3 helix region, are a contributor to differences in signaling activity between different strigolactone receptors.

Screening pearl millet genotypes for resistance to Striga hermonthica and compatibility to Fusarium oxysporum f.sp. strigae

Pearl millet (Pennisetum glaucum [L.] R. Br., 2n = 2x = 14) is a nutritionally rich and climate-resilient food crop cultivated globally. It is a crucial staple crop in Burkina Faso and the dry Sahel region, encompassing Niger, Mali, and Senegal. However, the yield of pearl millet is relatively low in the region (<0.85 tons ha−1) due to Striga hermonthica (Sh) infestation, bird damage, insect pests, diseases, and low-yielding open-pollinated landrace varieties. The study aimed to screen genetically diverse pearl millet accessions for Sh resistance and compatibility to a Striga bio-control agent, Fusarium oxysporum f. sp. Strigae (FOS), to select contrasting and promising parents for resistance breeding and production. One hundred and fifty genotypes were evaluated in Sh hotspot fields in the rain-fed and greenhouse conditions using an alpha lattice design and two replications in Burkina Faso. Significant differences were recorded among the tested pearl millet genotypes for the assessed agro-morphological and Striga resistance. Days to flowering was significantly delayed in the assessed genotypes due to Sh infestation. Applying FOS on pearl millet seed significantly reduced the mean Striga number in Sh-infested conditions. The following genotypes: IP-3098, IP-6112, IP-9242, IP-10579, and IP-11358 were identified as exhibiting Sh resistance and were compatible to FOS. The pearl millet genotypes supported few to none Sh emerged plants with comparatively low values under the Striga number progress curve. The selected genotypes are useful parents to breed for Striga resistance and integrated management in Burkina Faso and related agroecologies.

Abscisic acid inhibits germination of Striga seeds and is released by them likely as a rhizospheric signal supporting host infestation.

Seeds of the root parasitic plant Striga hermonthica undergo a conditioning process under humid and warm environments before germinating in response to host-released stimulants, particularly strigolactones (SLs). The plant hormone abscisic acid (ABA) regulates different growth and developmental processes, and stress response; however, its role during Striga seed germination and early interactions with host plants is under-investigated. Here, we show that ABA inhibited Striga seed germination and that hindering its biosynthesis induced conditioning and germination in unconditioned seeds, which was significantly enhanced by treatment with the SL analog rac-GR24. However, the inhibitory effect of ABA remarkably decreased during conditioning, confirming the loss of sensitivity towards ABA in later developmental stages. ABA measurement showed a substantial reduction of its content during the early conditioning stage and a significant increase upon rac-GR24-triggered germination. We observed this increase also in released seed exudates, which was further confirmed by using the Arabidopsis ABA-reporter GUS marker line. Seed exudates of germinated seeds, containing elevated levels of ABA, impaired the germination of surrounding Striga seeds in vitro and promoted root growth of a rice host towards germinated Striga seeds. Application of ABA as a positive control caused similar effects, indicating its function in Striga/Striga and Striga/host communications. In summary, we show that ABA is an essential player during seed dormancy and germination processes in Striga and acts as a rhizospheric signal likely to support host infestation.