Parasitic Weed Research Articles

Evaluation of Sorghum Genotypes for Striga (S. hermonthica) Tolerance

Striga infestation and transmission, and the adverse impact of Striga on crop yield can essentially be diminished through selection of resistant genotypes. The study was carried out to screen sorghum genotypes for Striga hermonthica (Striga) tolerance based on their physiological responses to the parasitic effects of this weed. Seventy-five grain sorghum [Sorghum bicolor (L.) Moench] genotypes were subjected to three levels (0 mg, 2.5 mg and 5 mg/pot) of Striga at Rattary Arnold Research Institute in Zimbabwe. One way analysis of variance results show that there were significant difference among genotypes on plant height, chlorophyll content, leaf number, field weight, grain weight, panicle height for Striga*genotype interaction for all the 75 sorghum genotypes (P<0.001). The correlation matrix show that fresh panicle weight and grain dry weight showed a very high correlation (0.948) at P<0.05. Panicle height and fresh panicle weight are highly correlated (0.736) at P<0.05. Similarly, panicle height and grain dry weight were also highly correlated (0.718) at P<0.05. The heatmap analysis shows that 45%, 31% and 24% of the cultivars exhibited high, medium and low plant heights respectively. Chlorophyll content showed that 80% and 20% of the cultivars showed medium and low amounts, respectively. All the 75 genotypes recorded low leaf numbers when compared to the control experiment. It was observed that 25%, 63% and 12% for the genotypes produced high, medium and low fresh panicle weight and dry grain weight values respectively. The panicle size for most (87%) of the genotypes was medium while 13% of the cultivars showed very small sized. Principle component analysis using the scree plot Eigen values shows that the first factor contributes 58% of the cumulative variation. Two principal axes (F1 and F2) were selected, which explained about 74.45% of the total variation. Neighbour-joining hierarchical clustering analysis led to the formation of five groups for Striga*genotype interaction. It can be concluded that Striga hermonthica affects negatively crop morpho-physiologaical aspects such as plant height, chlorophyll content and leaf number as well as yield determining components such as field weight, panicle height which ultimately reduce the yield of sorghum. The existence of high variability in the response to Striga hermonthica infestation in the 75 sorghum genotypes gives the possibility to breeding interventions to improve tolerance to this parasitic weed.

Stereoselective Synthesis and Biological Profile of All Stereoisomers of Lactam Analogues of Strigolactones GR24 and GR18

AbstractStrigolactones (SLs) are signaling molecules involved in plant development and governing interactions with soil microorganisms in the rhizosphere as well as the germination of parasitic weeds. Developing their use in Crop Protection is a promising approach to a sustainable agriculture by mitigating biotic and abiotic stresses. Recently, a new class of lactam analogues of SL has emerged, namely strigolactams, displaying outstanding potency to induce the germination of parasitic weed O. cumana as well as enhanced chemical and soil stability. Herein, we describe the stereoselective synthesis of GR24 and GR18 lactams harnessing the chemistry of chiral keteniminium (KI) salts, in particular the unprecedented reactivity of chloro‐substituted KI, supported by DFT calculations. We disclose subsequently the biological activity on corn of the 32 stereopure strigolactams prepared, highlighting the crucial influence of stereochemistry and lactam substitution, rationalized by docking analyses. Finally, we performed stability studies in soil, which reveal that stereoisomers display very different half‐lives, reflecting the significant impact of stereochemistry on degradation kinetics.

Developing Drought and Weed Resistant Super-Sorghum

Future food security is one of the key global challenges facing society. Climate change presents significant threats to our ability to produce staple food crops – particularly in regions already vulnerable to droughts. Dr Kahiu Ngugi and his research team from the University of Nairobi and other institutions in Kenya investigated numerous varieties of sorghum – one of the world’s most important cereal crops. Their aim was to find new genes that would allow the crop to withstand both drought and a common parasitic weed.

Recent advances in the regulation of root parasitic weed damage by strigolactone-related chemicals.

Root parasitic weeds such as Striga spp. and Orobanche spp. dramatically reduce the yields of important agricultural crops and cause economic losses of over billions of US dollars worldwide. One reason for the damage by root parasitic weeds is that they germinate after specifically recognizing the host cues, strigolactones (SLs). SLs were identified ˃50 years ago as germination stimulants for root parasitic weeds, and various studies have been conducted to control parasitic weeds using SLs and related chemicals. Recently, biochemical and molecular biological approaches have revealed the SL biosynthesis and SL receptors; using these findings, various SL-related chemicals have been developed. This review summarizes recent research on SLs and their related chemicals for controlling root parasitic weeds.

Phenotypic Diversity in Pre- and Post-Attachment Resistance to Striga hermonthica in a Core Collection of Rice Germplasms.

In sub-Saharan Africa, upland rice cultivation is expanding into rainfed areas endemic to the root parasitic weed Striga hermonthica. We evaluated the Striga resistance of 69 accessions from the World Rice Core Collection (WRC) to estimate the phenotypic diversity within the Oryza sativa species. Pre-attachment resistance was screened based on the germination-inducing activities of the root exudates, while post-attachment resistance was screened through rhizotron evaluation. The 69 WRC accessions showed a wide variation in both pre- and post-attachment resistance. Root exudates of one accession induced 0.04% germination, and those of some accessions displayed >80% germination. In the evaluation of post-attachment resistance, the successful parasitism percentages ranged from 1.3% to 60.7%. The results of these resistance evaluations were subjected to cluster analysis, which recognized five groups: group I of 27 accessions, with high pre- and post-attachment resistance; group II of 12 accessions, with high post-attachment resistance but moderate pre-attachment resistance; group III of 4 accessions, with low pre-attachment resistance; group IV of 13 accessions, with low post-attachment resistance; and group V of 13 accessions, with low pre- and post-attachment resistance. The wide variation found in the WRC accessions will help to elucidate the genetic factors underpinning pre- and post-attachment resistance.

Genetic and agronomic approaches to control Orobanche and Phelipanche spp. parasitic weeds in vegetables and legumes

Genetic and agronomic approaches to control Orobanche and Phelipanche spp. parasitic weeds in vegetables and legumes

Generation mean analysis of Striga hermonthica resistance in pearl millet (Pennisetum glaucum [L.] R. Br.)

ABSTRACT Striga hermonthica [Del.] Benth. (Sh) is a noxious parasitic weed causing substantial yield loss in sub-Saharan Africa’s pearl millet. The objective of this study was to determine the gene action and inheritance of Sh resistance in newly developed pearl millet populations to guide selection and genetic advancement. Bi-parental crosses were derived from pairs of pearl millet lines by contrasting reactions to Striga infestations. The two sets of parental lines, F1s, F2s, and backcrosses, were evaluated using a randomized complete block design with three replications. Data on the number of Striga counted at 60 and 80 days after planting were collected. The analysis of the variance showed significant (P < 0.001) differences among the generations across sets for Sh parameters. Duplicate gene action controlled the inheritance of the number of emerged Sh. Unique F2 individuals with Sh resistance were selected from the two sets for genetic advancement through recurrent selection methods for pearl millet variety development by integrating desirable agronomic and farmer-preferred traits.

Fusarium diversity associated with the Sorghum-Striga interaction in Ethiopia.

Sorghum production is seriously threatened by the root parasitic weeds (RPWs) Striga hermonthica and Striga asiatica in sub-Saharan Africa. Research has shown that Striga control depends on eliminating its seed reserves in soil. Several species of the genus Fusarium (Nectriaceae, Hypocreales), which have been isolated from diseased Striga plants have proven to be highly pathogenic to all developmental stages of these RPWs. In the present study 439 isolates of Fusarium spp. were found associated with soils from Sorghum growing fields, Sorghum rhizosphere, or as endophytes with Sorghum roots and seeds, or as endophytes of Striga stems and seeds. Based on multi-locus phylogenies of combinations of CaM, tef1, rpb1 and rpb2 alignments, and morphological characteristics, 42 species were identified, including three species that are newly described, namely F. extenuatum and F. tangerinum from Sorghum soils, and F. pentaseptatum from seed of Striga hermonthica. Using a previously published AFLP-derived marker that is specific to detect isolates of F. oxysporum f.sp. strigae, an effective soil-borne biocontrol agent against Striga, we also detected the gene in several other Fusarium species. As these isolates were all associated with the Striga/Sorghum pathosystem, the possibility of horizontal gene transfer among these fusaria will be of interest to further investigate in future. Citation: Lombard L, van Doorn R, Groenewald JZ, Tessema T, Kuramae EE, Etolo DW, Raaijmakers JM, Crous PW (2022). Fusarium diversity associated with the Sorghum-Striga interaction in Ethiopia. Fungal Systematics and Evolution 10: 177-215. doi: 10.3114/fuse.2022.10.08.

Identification of novel canonical strigolactones produced by tomato.

Canonical strigolactones (SLs), such as orobanchol, consist of a tricyclic lactone ring (ABC-ring) connected to a methylbutenolide (D-ring). Tomato plants have been reported to produce not only orobanchol but also various canonical SLs related to the orobanchol structure, including orobanchyl acetate, 7-hydroxyorobanchol isomers, 7-oxoorobanchol, and solanacol. In addition to these, structurally unidentified SL-like compounds known as didehydroorobanchol isomers (DDHs), whose molecular mass is 2 Da smaller than that of orobanchol, have been found. Although the SL biosynthetic pathway in tomato is partially characterized, structural elucidation of DDHs is required for a better understanding of the entire biosynthetic pathway. In this study, three novel canonical SLs with the same molecular mass as DDHs were identified in tomato root exudates. The first was 6,7-didehydroorobanchol, while the other two were not in the DDH category. These two SLs were designated phelipanchol and epiphelipanchol because they induced the germination of Phelipanche ramosa, a noxious root parasitic weed of tomato. We also proposed a putative biosynthetic pathway incorporating these novel SLs from orobanchol to solanacol.

Maize Rotation Combined with Streptomyces rochei D74 to Eliminate Orobanche cumana Seed Bank in the Farmland

Orobanche cumama wallr. is the sunflower root parasitic weed with special life stage in which seed germination and parasitism take place in the soil. In practice, applying microbial agents and trapping crop rotation are utilized separately, or just one of them is selected to control O. cumana. The development of the sunflower industry is severely constrained on the farmland, where there is high density of O. cumana’s seed banks. In this study, two biological control methods were combined to solve the problem of O. cumana parasitism. The bioassay experiment showed that the high concentration fermentation filtrates of Streptomyces rochei D74 could effectively inhibit the germination and growth of the germ tube of O. cumana seeds. As the concentration was increased to 3.1 mg/mL, O. cumana was almost unable to sprout. A two-year pot experiment revealed that the use of D74 agents and sunflower–maize–sunflower rotation together promoted sunflower growth, as shown by the biomass accumulation, plant height, and denser root systems. The combined method resulted in a significant decrease in the number of O. cumana parasitism, compared to one method alone. Additionally, it affected the bacterial community composition of sunflower rhizosphere, mostly leading to an increase in Streptomyces and Brevibacterium and a decrease in Arthrobacter. This experiment, combined with multiple biological control, means significantly reducing the parasitism of O. cumana, which provides an effective foundation for practical application.

Ecological determinants and risk areas of Striga hermonthica infestation in western Kenya under changing climate

AbstractStriga hermonthica (Del.) Benth is a parasitic weed that is damaging major cereal crops in sub‐Saharan Africa (SSA). Although Striga is recognised as an agricultural scourge, there is limited information available indicating the extent of its growth and spread as impacted by the changing climate in Kenya. This study investigated the impact of current climate conditions and projected future (2050) climate change on the infestation of Striga hermonthica in the western Kenya region. Specifically, the study aimed to predict Striga hermonthica habitat suitability in five counties in the western Kenya region through using the maximum entropy (MaxEnt) model and bioclimatic, soil, topographic and land use, and land cover (LULC) variables. Striga hermonthica geolocations were collected and collated and ecological niche models were developed to determine the habitat suitability. The results showed that approximately 1767 km2 (10% of the total study area) is currently highly suitable for Striga hermonthica occurrence. The future projections showed a range between 2106 km2 (19% of the total study area) and 2712 km2 (53% of the total study area) at the minimum carbon (RCP 2.6) and the maximum carbon emission scenarios (RCP 8.5) respectively. Elevation, annual precipitation, LULC, temperature seasonality and soil type were determined to be the most influential ecological predictor variables for Striga hermonthica establishment. The study revealed the importance of using climate, soil, topographic and LULC variables when evaluating agricultural production constraints such as Striga's prevalence. The methodology used in this study should be tested in other Striga affected areas.

Наследование устойчивости к расе G Orobanche cumana Wallr. в F2 и ВС1 у линии подсолнечника RGM

One of the most harmful biotic factors influencing the stability of sunflower yields is a parasite weed – broomrape (Orobanche Cumana Wallr.). Among the different measures of this parasite control, the usage of genetic resistance is the most ecologic. Lines of the different origin resistant to a race G were developed in V.S. Pustovoit All-Russian Research Institute of Oil Crops. However, inheritance of a trait of resistance was not studied in all lines. Thus, the purpose of this work was studying the genetic control of resistance to a broomrape race G in a line RGM in F2 and BC1. To analyze inheritance, we used the line RGM resistant to the broomrape race G and susceptible to this race the sterile and fertile forms of a line VK 680 of VNIIMK breeding. Resistance of plants in F2 and BC1 was tested in protected ground infected with seeds of a broomrape race G using a method of earlier diagnosis by A.Ya. Panchenko. Reciprocal crosses of the line RGM and susceptible sunflower lines resulted that all obtained hybrids F1 had low middle level of broom-rape infestation. When crossing heterozygous plants F1 and a susceptible parental form of sunflower in BC1 two phenotypes were obtained: susceptible and incomplete resistant (intermediate) in the ratio of 1 : 1. This is typical for monogenic inheritance of a trait. A value x2 was equal 1.38 that certifies the hypothesis the only gen controls resistance of this line with a probability from 0.20 to 0.30. In F2 population splitting for three phenotypical classes was observed: resistant, intermediate, and susceptible in the ratio of 1 : 2 : 1. A value x2 was equal 0.58 that certifies the hypothesis the only gen controls resistance of the studied line with an incomplete dominance with a probability from 0.20 to 0.30.

Silencing of a mannitol transport gene in Phelipanche aegyptiaca by the tobacco rattle virus system reduces the parasite germination on the host root

ABSTRACT Root parasitic weed Phelipanche aegyptiaca is an obligate plant parasite that causes severe damage to host crops. Agriculture crops mainly belong to the Brassicaceae, Leguminosae, Cruciferae, and Solanaceae plant families affected by this parasitic weed, leading to the devastating loss of crop yield and economic growth. This root-specific parasitic plant is not able to complete its life cycle without a suitable host and is dependent on the host plant for nutrient uptake and germination. Therefore, selected parasitic genes of P. aegyptiaca which were known to be upregulated upon interaction with the host were chosen. These genes are essential for parasitism, and reduced activity of these genes could affect host-parasitic interaction and provide resistance to the host against these parasitic weeds. To check and examine the role of these parasitic genes which can affect the development of host resistance, we silenced selected genes in the P. aegyptiaca using the tobacco rattle virus (TRV) based virus-induced gene silencing (VIGS) method. Our results demonstrated that the total number of P. aegyptiaca parasite tubercles attached to the root of the host plant Nicotiana benthamiana was substantially decreased in all the silenced plants. However, silencing of the P. aegyptiaca MNT1 gene which encodes the mannitol transporter showed a significantly reduced number of germinated shoots and tubercles. Thus, our study indicates that the mannitol transport gene of P. aegyptiaca plays a crucial role in parasitic germination, and silencing of the PaMNT1 gene abolishes the germination of parasites on the host roots.

Perspectives on the metabolism of strigolactone rhizospheric signals.

Strigolactones (SLs) are a plant hormone regulating different processes in plant development and adjusting plant's architecture to nutrition availability. Moreover, SLs are released by plants to communicate with beneficial fungi in the rhizosphere where they are, however, abused as chemical cues inducing seed germination of root parasitic weeds, e.g. Striga spp., and guiding them towards host plants in their vicinity. Based on their structure, SLs are divided into canonical and non-canonical SLs. In this perspective, we describe the metabolism of root-released SLs and SL pattern in rice max1-900 mutants, which are affected in the biosynthesis of canonical SLs, and show the accumulation of two putative non-canonical SLs, CL+30 and CL+14. Using max1-900 and SL-deficient d17 rice mutants, we further investigated the metabolism of non-canonical SLs and their possible biological roles. Our results show that the presence and further metabolism of canonical and non-canonical SLs are particularly important for their role in rhizospheric interactions, such as that with root parasitic plants. Hence, we proposed that the root-released SLs are mainly responsible for rhizospheric communications and have low impact on plant architecture, which makes targeted manipulation of root-released SLs an option for rhizospheric engineering.

Evaluation of propiconazole, a triazole-containing fungicide as an inhibitor of strigolactone production

The inhibitory effect of propiconazole on strigolactone (SL) production and Striga hermonthica emergence were evaluated using rice plants. A significant reduction in SL levels was detected in root and root exudates after rice was treated with different doses of propiconazole. Propiconazole induced second tiller bud outgrowth in rice seedlings. In accordance with the results of 4-deoxyorobanchol levels in root and root exudates, propiconazole-treated rice attenuated the degree of infestation of the root parasitic weed, S. hermonthica. Overall, these results suggest that propiconazole has potential as a plant growth regulator for agriculture and a new scaffold for developing potent inhibitors of SL production.

Crop diversification and parasitic weed abundance: a global meta-analysis

Parasitic weeds cause huge annual losses to food production globally. A small number of species from the genera Cuscuta, Orobanche, Phelipanche and Striga have proliferated across many agroecological zones. Their control is compromised due to the lack of efficacy of conventional herbicides and their rapid adaptation to new resistant crop cultivars. A broad range of studies suggest consistent reductions in parasitic weed densities owing to increased spatial (intercropping) and temporal diversity (crop rotation). However, to date, no synthesis of this body of research has been published. Here we report the results of a meta-analysis using 1525 paired observations from 67 studies across 24 countries, comparing parasitic weed density and crop yields from monocrop and more diverse cropping systems. We found both spatial and temporal crop diversification had a significant effect on parasitic weed density reduction. Furthermore, our results show effects of spatial diversification are stronger in suppressing parasitic weeds than temporal effects. Furthermore, the analysis indicates intercrops which alter both microclimate and soil chemistry (e.g. Crotalaria, Stylosanthes, Berseem clover and Desmodium) are most effective in parasitic weed management. This analysis serves to underline the viability of crop diversification as a tool to enhance food security globally.

Characterization of Conyza bonariensis Allelochemicals against Broomrape Weeds.

The study of allelopathic activity of plants and the isolation and characterization of the responsible allelochemicals can lead to the development of environment friendly alternative approaches to weed control. Conyza species are invasive weeds that use allelopathic activity as part of a successful strategy to outcompete neighboring plants. Broomrape weeds are parasitic plants that use host-induced germination and the formation of a haustorium as strategies to infect host plants. The control of broomrape infection in most affected crops is limited or non-existing. In the current study, we investigated the allelopathic activity of Conyza bonariensis organic extracts in suicidal germination and radicle growth of four broomrape species (Orobanche crenata, Orobanche cumana, Orobanche minor and Phelipanche ramosa). A bioactivity-driven fractionation of Conyza bonariensis extracts led to the identification of two germination-inducing molecules and two growth-inhibitory compounds. The germination-inducing metabolites had species-specific activity being hispidulin active on seeds of O. cumana and methyl 4-hydroxybenzoate active in P. ramosa. The growth-inhibitory metabolites (4Z)-lachnophyllum lactone and (4Z,8Z)-matricaria lactone strongly inhibited the radicle growth of all parasitic weed species studied. Some structure-activity relationships were found as result of the study herein presented.

Mapping and Validation of Alectra vogelii Resistance in the Cowpea Landrace B301

Cowpea is the most important food legume in West and Central Africa and a valuable economic commodity in the region. Among the major biotic constraints to cowpea production are root parasitic weeds of which Alectra vogelii (Benth.) is of increasing importance. The cowpea landrace B301 was previously identified as a source of Alectra resistance, but neither the genes nor genomic loci conferring this resistance have been mapped. Therefore, to map and identify genetic markers linked to Alectra resistance for use in the molecular improvement of cowpea, we developed an F2 population from a cross of the susceptible variety 524B with B301. The population was phenotyped for resistance to A. vogelii and genotyped with a cowpea high density single nucleotide polymorphism (SNP) microarray. Putative resistance loci were mapped in F2 populations by categorical trait–multiple interval mapping and validated by selective genotyping. Selective genotyping indicated that the resistance loci on Vu04 (Rav1) and Vu11 (Rav2) were significantly associated with resistance (p ≤ 0.01). Using marker assisted backcrossing, the two resistance loci were introgressed independently into the susceptible 524B genetic background. Phenotyping and genotyping of the segregating backcross families delineated Rav1 to a 10 cM on chromosome 4 and Rav2 to a 6.7 cM interval in chromosome 11. These two loci are desirable for breeding Alectra resistant cowpea varieties due to their simple inheritance and ability to independently confer complete immunity to the parasite.

Identification of Structural Features of Hydrocinnamic Acid Related to Its Allelopathic Activity against the Parasitic Weed Cuscuta campestris.

Cuscuta campestris is a parasitic weed species that inflicts worldwide noxious effects in many broadleaf crops due to its capacity to withdraw nutrients and water directly from the crop vascular system using haustorial connections. Cuscuta campestris control in the majority of crops affected is non-existent, and thus, research for the development of control methods is needed. Hydrocinnamic acid occurs naturally in the rhizosphere, playing regulatory roles in plant-plant and plant-microbe communities. The toxicity of hydrocinnamic acid against C. campestris was recently identified. In the present work, a structure-activity relationship study of 21 hydrocinnamic acid analogues was performed to identify key structural features needed for its allelopathic action against the seedling growth of this parasitic plant. The findings of this study provide the first step for the design of herbicides with enhanced activity for the control of C. campestris infection.

Identification of Allelochemicals with Differential Modes of Phytotoxicity against Cuscuta campestris

Cuscuta campestris is a parasitic weed species with noxious effects in broadleaf crops worldwide. The control of Cuscuta in the majority of crops affected is limited or non-existing. We tested, for the first time, the effect of eighteen metabolites in in vitro-grown Cuscuta seedlings. We found that 2-benzoxazolinone, hydrocinnamic acid and pisatin caused the strongest inhibition of seedling growth. In addition to seedling growth, pisatin caused necrosis of the Cuscuta seedling, occurring mostly at the seedling shoot. Scopoletin and sesamol treatments caused toxicity, observed as a black staining, only at the Cuscuta root apices, while caffeic acid, ferulic acid and vanillic acid caused toxicity, observed as brown staining, in the root apices. The structure–activity relationships in four structural derivatives of 2-benzoxazolinone, and five structural derivatives of hydrocinnamic acid, were also studied. The identification of new herbicidal modes of action against Cuscuta is the first step in creating new alternatives to sustainable chemical control of parasitic weeds.