Novel Sources of Witchweed (Striga) Resistance from Wild Sorghum Accessions

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

BACKGROUND AND AIMS

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 has not been well studied.

METHODS

The effects of improved macro- and micronutrient supply on host plant performance under striga-free and infected conditions were investigated in glasshouse 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 micronutrient quantities, (2) 25 % macro- and 100 % micronutrients, (3) 100 % macro- and 25 % micronutrients, or (4) 100 % macro- and micronutrients.

KEY RESULTS

Photosynthesis rates of striga-infected plants of the sensitive genotype increased with improved nutrition (from 12.2 to 22.1 μmol m-2 s-1) but remained below striga-free levels (34.9-38.8 μmol m-2 s-1). For the tolerant genotypes, increased macronutrient 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 the tolerant Ochuti genotype, incurred at low nutrient supply, were reduced to 5 % by improved nutrient supply.

CONCLUSIONS

Increasing macronutrient supply reduces the impact of striga on host plants but can only restore losses when applied to genotypes with a tolerant background.

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.

Resolving intergenotypic Striga resistance in sorghum

Genetic underpinnings of host-pathogen interactions in the parasitic plant Striga hermonthica, a root parasitic plant that ravages cereals in sub-Saharan Africa, are unclear. We performed a comparative transcriptome study on five genotypes of sorghum exhibiting diverse resistance responses to S. hermonthica using weighted gene co-expression network analysis (WGCNA). We found that S. hermonthica elicits both basal and effector-triggered immunity-like a bona fide pathogen. The resistance response was genotype specific. Some resistance responses followed the salicylic acid-dependent signaling pathway for systemic acquired resistance characterized by cell wall reinforcements, lignification, and callose deposition, while in others the WRKY-dependent signaling pathway was activated, leading to a hypersensitive response. In some genotypes, both modes of resistance were activated, while in others either mode dominated the resistance response. Cell wall-based resistance was common to all sorghum genotypes but strongest in IS2814, while a hypersensitive response was specific to N13, IS9830, and IS41724. WGCNA further allowed for pinpointing of S. hermonthica resistance causative genes in sorghum, including glucan synthase-like 10 gene, a pathogenesis-related thaumatin-like family gene, and a phosphoinositide phosphatase gene. Such candidate genes will form a good basis for subsequent functional validation and possibly future resistance breeding.

Phenotypic and histological analyses on the resistance of melon to Phelipanche aegyptiaca

Melon (Cucumis melo L.) is an economically important crop in Xinjiang, China, but its production is constrained by the parasitic plant Phelipanche aegyptiaca that attaches to the roots of many crops and causes severe stunting and loss of yield. Rhizotron, pot, and field experiments were employed to evaluate the resistance of 27 melon cultivars to P. aegyptiaca. Then, the resistant and susceptible cultivars were inoculated with P. aegyptiaca from six populations to assess their resistance stability and broad spectrum. Further microscopic and histological analyses were used to clarify the resistance phenotypes and histological structure. The results showed that Huangpi 9818 and KR1326 were more resistant to P. aegyptiaca compared to other cultivars in the rhizotron, pot, and field experiments. In addition, compared to the susceptible cultivar K1076, Huangpi 9818 and KR1326 showed broad-spectrum resistance to six P. aegyptiaca populations. These two resistant cultivars had lower P. aegyptiaca biomass and fewer and smaller P. aegyptiaca attachments on their roots compared to susceptible cultivar K1076. KR1326 (resistant) and K1076 (susceptible) were selected to further study resistance phenotypes and mechanisms. Germination-inducing activity of root exudates and microscopic analysis showed that the resistance in KR1326 was not related to low induction of P. aegyptiaca germination. The tubercles of parasite on KR1326 were observed slightly brown at 14 days after inoculation (DAI), the necrosis and arrest of parasite development occurred at 23 DAI. Histological analysis of necrosis tubercles showed that the endophyte of parasite had reached host central cylinder, connected with host xylem, and accumulation of secretions and callose were detected in neighbouring cells. We concluded that KR1326 is an important melon cultivar for P. aegyptiaca resistance that could be used to expand the genetic basis of cultivated muskmelon for resistance to the parasite.

Harnessing plant resistance against Striga spp. parasitism in major cereal crops for enhanced crop production and food security in Sub-Saharan Africa: a review

Given their long-lasting seed viability, 15–20-year lifespan and their high seed production levels, a significant impact of parasitic plant Striga spp. on African food production is inevitable. Over the last decades, climate change has increasingly favoured the adaptability, spread and virulence of major Striga species, S. hermonthica and S. asiatica, across arable land in Sub-Saharan Africa (SSA). These parasitic weeds are causing important yield losses on several staple food crops and endangering food and nutritional security in many SSA countries. Losses caused by Striga spp. are amplified by low soil fertility and recurrent droughts. The impact of Striga parasitism has been characterized through different phenotypic and genotypic traits assessment of their host plants. Among all control strategies, host-plant resistance remains the most pro-poor, easy-to-adopt, sustainable and eco-friendly control strategy against Striga parasitism. This review highlights the impact of Striga parasitism on food security in SSA and reports recent results related to the genetic basis of different agronomic, pheno-physiological and biochemical traits associated with the resistance to Striga in major African cereal food crops.

Root cell types as an interface for biotic interactions

Root responses to environmental stresses show a high level of cell type and developmental stage specificity. Interactions with beneficial and pathogenic organisms - including microbes and parasites - elicit a set of transcriptional responses unique to each root cell type, often dependent on their differentiation state. Localized changes to the cell wall and to the integrity of root cell types can serve as a physical barrier for a range of pests. Conversely, certain microorganisms weaken existing barriers within root cell types. Interactions with microorganisms vary between roots of different developmental origins and cellular architectures. Here we provide an overview of the molecular, architectural, and structural properties of root cell types crucial to both maintaining beneficial interactions and protecting from pathogens.

Expanding Gene-Editing Potential in Crop Improvement with Pangenomes

Pangenomes aim to represent the complete repertoire of the genome diversity present within a species or cohort of species, capturing the genomic structural variance between individuals. This genomic information coupled with phenotypic data can be applied to identify genes and alleles involved with abiotic stress tolerance, disease resistance, and other desirable traits. The characterisation of novel structural variants from pangenomes can support genome editing approaches such as Clustered Regularly Interspaced Short Palindromic Repeats and CRISPR associated protein Cas (CRISPR-Cas), providing functional information on gene sequences and new target sites in variant-specific genes with increased efficiency. This review discusses the application of pangenomes in genome editing and crop improvement, focusing on the potential of pangenomes to accurately identify target genes for CRISPR-Cas editing of plant genomes while avoiding adverse off-target effects. We consider the limitations of applying CRISPR-Cas editing with pangenome references and potential solutions to overcome these limitations.

GWAS provides biological insights into mechanisms of the parasitic plant (Striga) resistance in sorghum

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.

How to resist parasitic plants: pre- and post-attachment strategies

The lifecycle of parasitic plants can be divided into pre-attachment and post-attachment phases that equate to free living and parasitic stages. Similarly, plant resistance to parasitic plants can be defined as pre-attachment and post-attachment resistance. Parasitic plants rely on host cues for successful host invasion. During pre-attachment resistance, changes in the composition of host signals can disrupt parasitic plant development and ultimately host invasion. Recent studies have only now begun to elucidate the genetic elements in the host that promote pre-attachment resistance. In comparison, new research points to post-attachment resistance using the common molecular mechanisms utilized by the plant immune system during plant-pathogen interactions. In kind, parasitic plants secrete proteinaceous and RNA-based effectors post-attachment to subvert the host immune system.

Genotypic Variation in Cultivated and Wild Sorghum Genotypes in Response to Striga hermonthica Infestation

Striga hermonthica is the most important parasitic weed in sub-Saharan Africa and remains one of the most devastating biotic factors affecting sorghum production in the western regions of Kenya. Farmers have traditionally managed Striga using cultural methods, but the most effective and practical solution to poor smallholder farmers is to develop Striga-resistant varieties. This study was undertaken with the aim of identifying new sources of resistance to Striga in comparison with the conventional sources as standard checks. We evaluated 64 sorghum genotypes consisting of wild relatives, landraces, improved varieties, and fourth filial generation (F₄) progenies in both a field trial and a pot trial. Data were collected for days to 50% flowering (DTF), dry panicle weight (DPW, g), plant height (PH, cm), yield (YLD, t ha^-1), 100-grain weight (HGW, g), overall disease score (ODS), overall pest score (OPS), area under Striga number progress curve (ASNPC), maximum above-ground Striga (NS_max), and number of Striga-forming capsules (NSFC) at relevant stages. Genetic diversity and hybridity confirmation was determined using Diversity Arrays Technology sequencing (DArT-seq). Residual heterosis for HGW and NS_max was calculated as the percent increase or decrease in performance of F₄ crossover midparent (MP). The top 10 best yielding genotypes were predominantly F₄ crosses in both experiments, all of which yielded better than resistant checks, except FRAMIDA in the field trial and HAKIKA in the pot trial. Five F₄ progenies (ICSVIII IN × E36-1, LANDIWHITE × B35, B35 × E36-1, F6YQ212 × B35, and ICSVIII IN × LODOKA) recorded some of the highest HGW in both trials revealing their stability in good performance. Three genotypes (F6YQ212, GBK045827, and F6YQ212xB35) and one check (SRN39) were among the most resistant to Striga in both trials. SNPs generated from DArT-seq grouped the genotypes into three major clusters, with all resistant checks grouping in the same cluster except N13. We identified more resistant and high-yielding genotypes than the conventional checks, especially among the F₄ crosses, which should be promoted for adoption by farmers. Future studies will need to look for more diverse sources of Striga resistance and pyramid different mechanisms of resistance into farmer-preferred varieties to enhance the durability of Striga resistance in the fields of farmers.

New pre-attachment Striga resistant sorghum adapted to African agro-ecologies

BACKGROUND

Pre-attachment resistance to the parasitic plants Striga hermonthica and S. asiatica occurs in sorghum mutants designated low germination stimulant 1 (lgs1). However, only a few of these mutants have been identified and their resistance validated. Additionally, pre-attachment resistance in sorghum beyond lgs1 mutants has not been explored. We used lgs1-specific markers to identify new lgs1-like mutants in a diverse global sorghum collection. The sorghum collection was also evaluated for pre-attachment resistance against Striga using an in vitro assay that measured Striga germination activity and radicle growth.

RESULTS

From a total of 177 sorghum accessions, 60 recorded mean germination levels of below 42%, which is comparable with the previously identified lgs1-like sorghum (SRN39 and IS9830) used as controls in this study. Furthermore, 32 of these accessions recorded Striga radicle lengths comparable or lower than the controls (0.42 mm). Thirty-eight accessions contained the lgs1 mutation and although overall, lgs1 mutants had considerably reduced Striga germination, some low inducers of Striga germination were wild-type for lgs1. Germination was positively but weakly correlated with radicle length pointing to additional radicle growth inhibitory activity.

CONCLUSIONS

lgs1 mutations, alongside other mechanisms for low Striga germination stimulation, are prevalent in sorghum, and poor Striga radicle growth is suggestive of host-derived inhibition. As an outcome, our study makes available multiple Striga-resistant sorghum with adaptability to diverse agro-ecological regions in sub-Saharan Africa making immediate deployment possible. © 2021 Society of Chemical Industry.

Combining host plant defence with targeted nutrition: key to durable control of hemiparasitic Striga in cereals in sub-Saharan Africa?

Host plant defence mechanisms (resistance and tolerance) and plant nutrition are two of the most widely proposed components for the control of hemiparasitic weeds of the genus Striga in tropical cereal production systems. Neither of the two components alone is effective enough to prevent parasitism and concomitant crop losses. This review explores the potential of improved plant nutrition, being the chemical constituent of soil fertility, to fortify the expression of plant inherent resistance and tolerance against Striga. Beyond reviewing advances in parasitic plant research, we assess relevant insights from phytopathology and plant physiology in the broader sense to identify opportunities and knowledge gaps and to develop the way forward regarding research and development of combining genetics and plant nutrition for the durable control of Striga.

Current progress in Striga management

The Striga, particularly S. he rmonthica, problem has become a major threat to food security, exacerbating hunger and poverty in many African countries. A number of Striga control strategies have been proposed and tested during the past decade, however, further research efforts are still needed to provide sustainable and effective solutions to the Striga problem. In this paper, we provide an update on the recent progress and the approaches used in Striga management, and highlight emerging opportunities for developing new technologies to control this enigmatic parasite.

Breeding maize (Zea mays) for Striga resistance: Past, current and prospects in sub-saharan africa

Striga hermonthica, causes up to 100% yield loss in maize production in Sub-Saharan Africa. Developing Striga-resistant maize cultivars could be a major component of integrated Striga management strategies. This paper presents a comprehensive overview of maize breeding activities related to Striga resistance and its management. Scientific surveys have revealed that conventional breeding strategies have been used more than molecular breeding strategies in maize improvement for Striga resistance. Striga resistance genes are still under study in the International Institute for Tropical Agriculture (IITA) maize breeding programme. There is also a need to discover QTL and molecular markers associated with such genes to improve Striga resistance in maize. Marker Assistance Breeding is expected to increase maize breeding efficiency with complex traits such as resistance towards Striga because of the complex nature of the host-parasite relationship and its intersection with other environmental factors. Conventional alongside molecular tools and technical controls are promising methods to effectively assess Striga in Sub-Saharan Africa.

Plant science's next top models

BACKGROUND

Model organisms are at the core of life science research. Notable examples include the mouse as a model for humans, baker's yeast for eukaryotic unicellular life and simple genetics, or the enterobacteria phage λ in virology. Plant research was an exception to this rule, with researchers relying on a variety of non-model plants until the eventual adoption of Arabidopsis thaliana as primary plant model in the 1980s. This proved to be an unprecedented success, and several secondary plant models have since been established. Currently, we are experiencing another wave of expansion in the set of plant models.

SCOPE

Since the 2000s, new model plants have been established to study numerous aspects of plant biology, such as the evolution of land plants, grasses, invasive and parasitic plant life, adaptation to environmental challenges, and the development of morphological diversity. Concurrent with the establishment of new plant models, the advent of the 'omics' era in biology has led to a resurgence of the more complex non-model plants. With this review, we introduce some of the new and fascinating plant models, outline why they are interesting subjects to study, the questions they will help to answer, and the molecular tools that have been established and are available to researchers.

CONCLUSIONS

Understanding the molecular mechanisms underlying all aspects of plant biology can only be achieved with the adoption of a comprehensive set of models, each of which allows the assessment of at least one aspect of plant life. The model plants described here represent a step forward towards our goal to explore and comprehend the diversity of plant form and function. Still, several questions remain unanswered, but the constant development of novel technologies in molecular biology and bioinformatics is already paving the way for the next generation of plant models.

Witchweed’s Suicidal Germination: Can Slenderleaf Help?

The parasitic plant Striga hermonthica (Delile) Benth. is stimulated to germinate by biomolecules (strigolactones) produced in the roots of host and some non-host plants. Non-hosts induce Striga’s suicidal germination and are therefore used as trap crops. Among trap crops, the Slenderleaf legume in the genus Crotalaria (Crotalaria brevidens (L.) Benth.) and (Crotalaria orchroleuca (G.) Don.) has been popularized in African smallholder farms. However, the Striga germination efficiency of these locally grown Crotalaria varieties (landraces) is unknown. Also unclear is Crotolaria’s extent to inhibiting Striga growth, post germination. Extensive parasite penetration can expose the trap crop to secondary infections and possible phytotoxicity from Striga. We used in vitro germination assays to determine the Striga germination efficiency of 29 Crotalaria landraces. Furthermore, we determined Crotalaria’s ability to inhibit Striga attachment and growth using histological analysis. We found that: i) Crotalaria stimulated germination of Striga seeds at frequencies ranging between 15.5% and 54.5% compared to 74.2% stimulation by the synthetic strigolactone (GR24) used a positive control; ii) Crotalaria blocked Striga entry at multiple levels and did not allow growth beyond the pericycle, effectively blocking vascular connection with the non-host. Hence, Crotalaria is suitable as a trap crop in integrated Striga management.

Genetic variation in Sorghum bicolor strigolactones and their role in resistance against Striga hermonthica

Sorghum is an important food, feed, and industrial crop worldwide. Parasitic weeds of the genus Striga constitute a major constraint to sorghum production, particularly in the drier parts of the world. In this study we analysed the Striga germination stimulants, strigolactones, in the root exudates of 36 sorghum genotypes and assessed Striga germination and infection. Low germination-stimulating activity and low Striga infection correlated with the exudation of low amounts of 5-deoxystrigol and high amounts of orobanchol, whereas susceptibility to Striga and high germination-stimulating activity correlated with high concentrations of 5-deoxystrigol and low concentrations of orobanchol. Marker analysis suggested that similar genetics to those previously described for the resistant sorghum variety SRN39 and the susceptible variety Shanqui Red underlie these differences. This study shows that the strigolactone profile in the root exudate of sorghum has a large impact on the level of Striga infection. High concentrations of 5-deoxystrigol result in high infection, while high concentrations of orobanchol result in low infection. This knowledge should help to optimize the use of low germination stimulant-based resistance to Striga by the selection of sorghum genotypes with strigolactone profiles that favour normal growth and development, but reduce the risk of Striga infection.