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Authors: Lin Du, Xingjia Li, Yimin Ding, Dengke Ma, Chunxin Yu and Liusheng Duan.
Journal of Agricultural and Food Chemistry (2024)
Abstract: "The pernicious parasitism exhibited by root parasitic weeds such as Orobanche and Striga poses substantial peril to agricultural productivity and global food security. This deleterious phenomenon hinges upon the targeted induction of the signaling molecule strigolactones (SLs). Consequently, the identification of prospective SL antagonists holds significant promise in the realm of mitigating the infection of these pernicious weeds. In this study, we synthesized and characterized D12 based on a potent SL antagonist KK094. In vivo assay results demonstrated that D12 remarkably impedes the germination of Phelipanche aegyptiaca and Striga asiatica seeds, while also alleviating the inhibitory consequence of the SL analogue GR24 on hypocotyl elongation in Arabidopsis thaliana. The docking study and ITC assay indicated that D12 can interact strongly with the SL receptor protein, which may interfere with the binding of SL to the receptor protein as a result. In addition, the results of crop safety assessment tests showed that D12 had no adverse effects on rice seed germination and seedling growth and development. The outcomes obtained from the present study suggested that D12 exhibited promise as a prospective antagonist of SL receptors, thereby displaying substantial efficacy in impeding the seed germination process of root parasitic weeds, providing a promising basis for rational design and development of further Striga-specific herbicides."
Author: Gwendolyn K. Kirschner.
The Plant Journal (2024)
Excerpts: "Even though it is known that treatment with the abscisic acid (ABA) biosynthesis inhibitor fluridone (FL) shortens the seed-conditioning period and increases the responsiveness to germination stimulants (Jamil et al., 2023), it is not known which role ABA plays in Striga germination. For the highlighted publication, Muhammad Jamil, Jian You (Eric) Wang, Yagiz Alagoz and colleagues, under the lead of Salim Al-Babili, addressed the question of how Striga hermonthica seed dormancy and germination are correlated with endogenous ABA levels (Jamil et al., 2024)."
"Additionally, the authors found that placing germinated Striga seeds next to host roots promoted root growth and increased the number of lateral roots growing towards the seeds. Therefore, ABA plays a role not only in conditioning and germination endogenously but also in exudates to manipulate surrounding seeds and the host root architecture (Figure 1b). The increased number of host lateral roots growing towards the ABA released by the germinated Striga seeds might increase the chances of Striga attachment to the host root system."
"...(Figure 1c). In comparison with untreated pots, FL treatment led to a reduction in Striga emergence and healthier host plants, and treatment with FL and the strigolactone analogue showed a similar result. This suggests that ABA inhibitors can function as suicidal germination agents, because they can break seed dormancy even before conditioning."
Authors: Muhammad Jamil, Yagiz Alagoz, Jian You Wang, Guan-Ting Erica Chen, Lamis Berqdar, Najeh M. Kharbatia, Juan C. Moreno, Hendrik N. J. Kuijer and Salim Al-Babili.
The Plant Journal (2024)
Significance Statement: The root parasitic plant Striga hermonthica is a severe threat to cereal's yield, endangering global food security. Herein, we uncover a new role of known plant hormone abscisic acid as a rhizospheric signal released by germinated Striga seeds, allowing them to better compete with surrounding un-conditioned seeds and facilitating host infestation. Our findings can help in developing strategies to control this parasite and mitigate its negative impact on food supply and income of smallholder farmers.
Abstract: "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."
Authors: Chihiro Miura, Yuki Furui, Tatsuki Yamamoto, Yuri Kanno, Masaya Honjo, Katsushi Yamaguchi, Kenji Suetsugu, Takahiro Yagame, Mitsunori Seo, Shuji Shigenobu, Masahide Yamato and Hironori Kaminaka.
Plant Physiology (2023)
Abstract: "Orchids parasitically depend on external nutrients from mycorrhizal fungi for seed germination. Previous findings suggest that orchids utilize a genetic system of mutualistic arbuscular mycorrhizal (AM) symbiosis, in which the plant hormone gibberellin (GA) negatively affects fungal colonization and development, to establish parasitic symbiosis. Although GA generally promotes seed germination in photosynthetic plants, previous studies have reported low sensitivity of GA in seed germination of mycoheterotrophic orchids where mycorrhizal symbiosis occurs concurrently. To elucidate the connecting mechanisms of orchid seed germination and mycorrhizal symbiosis at the molecular level, we investigated the effect of GA on a hyacinth orchid (Bletilla striata) seed germination and mycorrhizal symbiosis using asymbiotic and symbiotic germination methods. Additionally, we compared the transcriptome profiles between asymbiotically and symbiotically germinated seeds. Exogenous GA negatively affected seed germination and fungal colonization, and endogenous bioactive GA was actively converted to the inactive form during seed germination. Transcriptome analysis showed that B. striata shared many of the induced genes between asymbiotically and symbiotically germinated seeds, including GA metabolism- and signaling-related genes and AM-specific marker homologs. Our study suggests that orchids have evolved in a manner that they do not use bioactive GA as a positive regulator of seed germination and instead auto-activate the mycorrhizal symbiosis pathway through GA inactivation to accept the fungal partner immediately during seed germination."
Author: Takatoshi Wakabayashi, Megumi Nakayama, Yurie Kitano, Masato Homma, Kenji Miura, Hirosato Takikawa, Masaharu Mizutani and Yukihiro Sugimoto.
Plants People Planet (2024)
Societal Impact Statement: "Parasitic witchweeds (Striga species) pose a serious threat to food security in Africa, attacking cereal grains and legumes. Chemicals released from the host roots that initiate the life cycle of Striga are known as germination stimulants, predominantly strigolactones (SLs). Strigol, the first identified SL, was isolated from the root exudates of cotton (Gossypium hirsutum), a false host of Striga, over 50 years ago. The identification of strigol synthase in cotton establishes the complete biosynthesis pathway of this emblematic SL. This discovery has the potential to advance our understanding of SL-mediated rhizosphere interactions and enhance cotton's effectiveness as a trap crop."
Authors: Muhammad Jamil, Jian You Wang, Lamis Berqdar, Yagiz Alagoz, Ahmed Behisi and Salim Al-Babili.
Weed Research (2023)
Abstract: "The witchweed Striga hermonthica, an obligate and noxious root–parasitic plant, remains a persistent threat to cereal production and poses a great challenge to smallholder farmers in Sub-Saharan Africa. Inducing suicidal germination of Striga seeds by applying strigolactone analogs is a promising strategy to deplete the Striga seed bank of infested soils. Nevertheless, there is a need to increase the efficiency and improve the applicability of this strategy, which may be achieved by testing further Striga germination stimulants. Herein, we explored the potential of cytokinins in inducing Striga seed germination. We investigated their activity as a suicidal germination agent along with fluridone that inhibits carotenoid biosynthesis and hence reduces abscisic acid formation and facilitates breaking of seed dormancy. Under lab conditions, application of fluridone (at 100 μM) or cytokinins (at 100 μM) showed 19% and 63% Striga germination, respectively, while combining different cytokinins with fluridone led to above 93% germination of treated seeds, with thidiazuron as the most active cytokinin. Using rice (cv IAC-165), we also show that co-application of fluridone and thidiazuron to Striga infested pots before planting the host led to up to 86%–100% reduction in Striga emergence. In summary, cytokinins, particularly thidiazuron, could be useful suicidal agents to induce Striga seed germination and ultimately deplete the seedbank in Striga-infested regions."
Authors: Jia Xin Yap and Yuichiro Tsuchiya.
Plant and Cell Physiology (2023)
Abstract: "Dormant seeds of a root parasitic plant Striga hermonthica sense strigolactones from host plants as environmental cues for germination. This process is mediated by diversified member of strigolactone receptors encoded by HYPOSENSITIVE TO LIGHT/KARRIKIN INSENSITIVE2 genes. It is known that warm and moist treatment of seed conditioning gradually turns dormant Striga seeds competent to respond strigolactones, while the mechanism behind it has been poorly understood. In this report, we show that plant hormone gibberellins increase the strigolactone-competence by up-regulating mRNA expressions of the major strigolactone receptors during the conditioning period. This idea was supported by poor germination phenotype when gibberellin biosynthesis is depleted by paclobutrazol during conditioning. Moreover, live-imaging with fluorogenic strigolactone-mimic, yoshimulactone green W, revealed that paclobutrazol treatment during conditioning caused aberrant dynamics of strigolactone perception after germination. These observations revealed an indirect role of gibberellins to the seed germination in Striga, which is contrastive to their roles as dominant germination stimulating hormone in non-parasitic plants. We propose a model of how the role of gibberellins become indirect during the evolution of parasitism in plants. Our work also highlight the potential role for gibberellins in field application, for instance, elevating the sensitivity of the seeds towards strigolactones in the current suicidal germination approach to alleviate the agricultural threats caused by this parasite in Africa."
Authors: Ritsuko Mizobuchi, Kazuhiko Sugimoto, Seiya Tsushima, Shuichi Fukuoka, Chikako Tsuiki, Masaki Endo, Masafumi Mikami, Hiroaki Saika and Hiroyuki Sato.
Scientific Reports (2023)
Abstract: "Burkholderia glumae causes bacterial seedling rot (BSR) of rice and is a threat to a consistent food supply. When previously screening for resistance against B. glumae in the resistant cultivar Nona Bokra (NB) versus the susceptible cultivar Koshihikari (KO), we detected a gene, Resistance to Burkholderia glumae 1 (RBG1), at a quantitative trait locus (QTL). Here, we found that RBG1 encodes a MAPKKK gene whose product phosphorylates OsMKK3. We also found that the kinase encoded by the RBG1 resistant (RBG1res) allele in NB presented higher activity than did that encoded by the RBG1 susceptible (RBG1sus) allele in KO. RBG1res and RBG1sus differ by three single-nucleotide polymorphisms (SNPs), and the G390T substitution is essential for kinase activity. Abscisic acid (ABA) treatment of inoculated seedlings of RBG1res-NIL (a near-isogenic line (NIL) expressing RBG1res in the KO genetic background) decreased BSR resistance, indicating that RBG1res conferred resistance to B. glumae through negative regulation of ABA. The results of further inoculation assays showed that RBG1res-NIL was also resistant to Burkholderia plantarii. Our findings suggest that RBG1res contributes to resistance to these bacterial pathogens at the seed germination stage via a unique mechanism."
Authors: C. Li, L. Dong, J. Durairaj, J.-C. Guan, M. Yoshimura, P. Quinodoz, R. Horber, K. Gaus, J. Li, Y. B. Setotaw, J. Qi, H. De Groote, Y. Wang, B. Thiombiano, K. Floková, A. Walmsley, T. V. Charnikhova, A. Chojnacka, S. Correia de Lemos, Y. Ding, D. Skibbe, K. Hermann, C. Screpanti, A. De Mesmaeker, E. A. Schmelz, A. Menkir, M. Medema, A. D. J. Van Dijk, J. Wu, K. E. Koch and H. J. Bouwmeester.
Science (2023)
One-sentence summary: Elucidation of maize strigolactone biosynthetic pathway has the potential for controlling the parasitic witchweed Striga.
Editor's view: Diversity reveals infection resistance - Parasitic witchweed (Striga) reduces the yield of maize grown in infected fields. Strigolactones from maize roots encourage Striga germination. Li et al. analyzed the natural variation in types of strigolactones exuded from maize roots. Maize genotypes that produced mainly zealactol suffered less Striga infection than those that produced mainly zealactone. A single cytochrome P450 catalyzes several of the oxidative steps in strigolactone biosynthesis, including conversion of precursors to either zealactol or zealactone. —PJH
Abstract: "Maize (Zea mays) is a major staple crop in Africa, where its yield and the livelihood of millions are compromised by the parasitic witchweed Striga. Germination of Striga is induced by strigolactones exuded from maize roots into the rhizosphere. In a maize germplasm collection, we identified two strigolactones, zealactol and zealactonoic acid, which stimulate less Striga germination than the major maize strigolactone, zealactone. We then showed that a single cytochrome P450, ZmCYP706C37, catalyzes a series of oxidative steps in the maize-strigolactone biosynthetic pathway. Reduction in activity of this enzyme and two others involved in the pathway, ZmMAX1b and ZmCLAMT1, can change strigolactone composition and reduce Striga germination and infection. These results offer prospects for breeding Striga-resistant maize."
Authors: Da-Wei Wang and Zhen Xi.
Advanced Agrochem (2022)
Highlights: • Recent advancements in SL agonists/antagonists are briefly summarized. • The potential agricultural applications of SL agonists/antagonists are introduced. • The perspectives on applying of SL agonists/antagonists for sustainable agriculture are briefly discussed.
Abstract: "Strigolactones (SLs) are a new class of phytohormones that not only act as signalling molecules with external rhizospheric fungus and stimulate the seed germination of root parasite weeds, but also regulate many aspects of plant development, such as regulating root and shoot architecture. These intriguing properties, which in turn, make SLs great potency for applications in agriculture. In this review, we highlight the recent advancements in applying SLs and their agonists or antagonists for agricultural applications, including controlling root parasite weeds, improving plant nutrient availability, and regulating abiotic stress responses. Although many SL agonists or antagonists have been synthesized since the first isolation of SL, strigol, in 1966, there is still no commercial application of this class of molecule. We also present an overview of the possible challenges for the practical application of SL agonists or antagonists."
Authors: Rakesh Srivastava, Rajesh Bajpai, Zeba Khan, Surendra Pratap Singh, Rajesh Mehrotra and Neeraj Kumar Dubey.
Indian Journal of Experimental Biology (2022)
Abstract: "The strigolactones (SLs) are plants hormones that have multiple functions in architecture and development. The roles of SLs in shoot branching and stem secondary growth of autotrophic plants are established. SL is also involved in the interaction between root parasitic plants and their host plants. SLs are exudates by the root of the host plant in search of a fungal partner for symbiotic association, while parasitic plants utilize this facility to detect the host root. The first formed tubercle of Phelipanche, whose germinations are driven by host-derived SLs, exudates parasitic derived SLs (PSLs) and could encourages germination of the adjacent parasitic seeds, resulting in parasite cluster formation. The existence of aboveground spikes in clusters suggests an intriguing approach for increasing parasite population by amplifying PSLs, which result in massive parasitic seed germination. PSLs probably have a role in the increased branching of Broomrapes opposing the host plant, resulting in the parasites' clustered appearance aboveground. This review highlights the distinct roles of SLs and PSLs, and their potential role in host-parasitic interaction."
Authors: Satoshi Ogawa and Ken Shirasu.
Plant Signaling & Behavior (2022)
Behavior: "Root parasitic plants in the family Orobanchaceae, such as Striga and Orobanche spp., infest major crops worldwide, leading to a multibillion-dollar loss annually. Host-derived strigolactones (SLs), recognized by a group of α/β hydrolase receptors (KAI2d) in these parasites, are important determinants for germinating root parasitic plants near the roots of host plants. Phtheirospermum japonicum, a facultative hemiparasitic Orobanchaceae plant, can germinate and grow in the presence or absence of the host and can also exhibit root chemotropism to host-derived SLs that are perceived via KAI2d. However, the importance of SLs in P. japonicum germination remains unclear. In this study, we found that germination of P. japonicum was suppressed in the absence of nitrate ions and that germination of P. japonicum was promoted by exogenous strigol, an SL, under such conditions. We propose a model in which P. japonicum may select either independent living or parasitism in response to ambient nitrogen conditions and host presence."
Authors: Yuchao Chen, Yi Kuang, Liyang Shi, Xing Wang, Haoyu Fu, Shengxiang Yang, Diego A. Sampietro, Luqi Huang and Yuan Yuan.
Frontiers in Plant Science (2021)
Abstract: "Orobanche and Striga are parasitic weeds extremely well adapted to the life cycle of their host plants. They cannot be eliminated by conventional weed control methods. Suicidal germination induced by strigolactones (SLs) analogs is an option to control these weeds. Here, we reported two new halogenated (+)-GR24 analogs, named 7-bromo-GR24 (7BrGR24) and 7-fluoro-GR24 (7FGR24), which were synthesized using commercially available materials following simple steps. Both compounds strongly promoted seed germination of Orobanche cumana. Their EC50 values of 2.3±0.28×10−8M (7BrGR24) and 0.97±0.29×10−8M (7FGR24) were 3- and 5-fold lower, respectively, than those of (+)-GR24 and rac-GR24 (EC50=5.1±1.32–5.3±1.44×10−8; p<0.05). The 7FGR24 was the strongest seed germination promoter tested, with a stimulation percentage of 62.0±9.1% at 1.0×10−8M and 90.9±3.8% at 1.0×10−6M. It showed higher binding affinity (IC50=0.189±0.012μM) for the SL receptor ShHTL7 than (+)-GR24 (IC50=0.248±0.032μM), rac-GR24 (IC50=0.319±0.032μM), and 7BrGR24 (IC50=0.521±0.087μM). Molecular docking experiments indicated that the binding affinity of both halogenated analogs to the strigolactone receptor OsD14 was similar to that of (+)-GR24. Our results indicate that 7FGR24 is a promising agent for the control of parasitic weeds."
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Authors: Jian You Wang, Guan-Ting Erica Chen, Justine Braguy and Salim Al-Babili.
Trends in Plant Science (2024)
Highlights: Strigolactones (SLs) are structurally diverse and divided into canonical and non-canonical subgroups. SLs are generally considered as plant hormones, best known for inhibiting shoot branching/tillering. SLs are rhizospheric signals important for arbuscular mycorrhizal symbiosis, which may be their ancestral function that is conserved from liverworts to flowering plants. Recent results have revealed functional specificity, indicating that canonical SLs are not the tillering/branching inhibitory hormone in rice or tomato. Increasing the content of 4-deoxyorobanchol in rice by interrupting its hydroxylation affects root, shoot, and panicle growth, suggesting that this canonical SL has hormonal functions. Reducing the levels of canonical SLs by genome editing or applying specific inhibitors is a promising strategy for reducing Striga parasitism.
Abstract: "Strigolactones (SLs) act as regulators of plant architecture as well as signals in rhizospheric communications. Reduced availability of minerals, particularly phosphorus, leads to an increase in the formation and release of SLs that enable adaptation of root and shoot architecture to nutrient limitation and, simultaneously, attract arbuscular mycorrhizal fungi (AMF) for establishing beneficial symbiosis. Based on their chemical structure, SLs are designated as either canonical or non-canonical; however, the question of whether the two classes are also distinguished in their biological functions remained largely elusive until recently. In this review we summarize the latest advances in SL biosynthesis and highlight new findings pointing to rhizospheric signaling as the major function of canonical SLs."
Authors: Zhangshuai Yang, Guanyu Liang, Chenxu Liu, Zhaohui Chu and Ning Li.
Plant Cell Reports (2024)
Key message The maize F-box protein ZmFBL41 targets abscisic acid synthase 9-cis-epoxycarotenoid dioxygenase 6 for degradation, and this regulatory module is exploited by Rhizoctonia solani to promote infection.
Abstract: "F-box proteins are crucial regulators of plant growth, development, and responses to abiotic and biotic stresses. Previous research identified the F-box gene ZmFBL41 as a negative regulator of maize (Zea mays) defenses against Rhizoctonia solani. However, the precise mechanisms by which F-box proteins mediate resistance to R. solani remain poorly understood. In this study, we show that ZmFBL41 interacts with an abscisic acid (ABA) synthase, 9-cis-epoxycarotenoid dioxygenase 6 (ZmNCED6), promoting its degradation via the ubiquitination pathway. We discovered that the ectopic overexpression of ZmNCED6 in rice (Oryza sativa) inhibited R. solani infection by activating stomatal closure, callose deposition, and jasmonic acid (JA) biosynthesis, indicating that ZmNCED6 enhances plant immunity against R. solani. Natural variation at ZmFBL41 across different maize haplotypes did not affect the ZmFBL41–ZmNCED6 interaction. These findings suggest that ZmFBL41 targets ZmNCED6 for degradation, leading to a decrease in ABA levels in maize, in turn, inhibiting ABA-mediated disease resistance pathways, such as stomatal closure, callose deposition, and JA biosynthesis, ultimately facilitating R. solani infection."
Authors: Lin Du, Jijun Yan, Chunxin Yu, Chunying Wang, Weiming Tan and Liusheng Duan.
Journal of Plant Growth Regulation (2024)
Abstract: "Strigolactone (SL) biosynthesis inhibitors have shown impressive activity in increasing shoot branching and inhibiting seed germination of the root parasitic plants Striga spp. and Orobanche spp. Herein, novel 1H-1,2,4-triazole derivatives were designed as SL biosynthesis inhibitors based on the backbone modification strategy, 33 target compounds were chemical synthesized and screened on Arabidopsis thaliana and Oryza sativa. The structure–activity relationship analysis enabled the discovery of a potential SL biosynthesis inhibitor B4 with the promising activity in increasing shoot branching, elongating taproot by inhibiting the biosynthesis of 4-deoxyorobanchol (4DO). We further found that B4-treated A. thaliana showed increased branching phenotype with the upregulated gene expression of AtMAX3 and AtMAX4. These results indicated that B4 might be a potent SL biosynthesis inhibitor and provide a unique scaffold for the development of new SL biosynthesis inhibitors."
Authors:: Shinjiro Yamaguchi, Yoshiya Seto and Junko Kyozuka.
Plant and Cell Physiology (2023)
Excerpts: "Strigolactones (SLs) are a group of carotenoid-derived signaling molecules. In 2010, the first special issue on SL was published in Plant & Cell Physiology. Since then, research on various aspects of SLs has progressed remarkably. As is often the case in scientific research, new discoveries in SL research are creating new important questions. In this special issue, four review articles highlight the recent important discoveries and discuss future perspectives in the field. In addition, seven original manuscripts report novel findings related to SLs."
"SL research has been progressing rapidly, but there is still a lot to learn about the biological roles, biosynthesis, transport, perception, signaling and evolution of SLs. For instance, we still do not know the biologically active form of SL as an endogenous hormone. And further studies are required to understand the exact mechanisms of SL perception and signaling in plants. SL receptors in microorganisms have not been clarified yet, and a KL has yet to be identified to better understand its biological functions and evolution. We hope that this special issue will attract the interest of PCP’s broad readership and encourage more researchers to make new discoveries in this active research field."
Authors: Muhammad Jamil, Yagiz Alagoz, Jian You Wang, Guan-Ting Erica Chen , Lamis Berqdar, Najeh M. Kharbatia, Juan C. Moreno, Hendrik N. J. Kuijer and Salim Al-Babili.
bioRxiv (2023)
Abstract: "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 significant 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 released by germinated parasitic seeds to provide a competitive advantage and support host infestation."
Authors: Sjors Huizinga and Harro J. Bouwmeester.
Plant and Cell Physiology (2023)
Abstract: "Root parasitic plants of the Orobanchaceae, broomrapes and witchweeds, pose a severe problem to agriculture in Europe, Asia, and especially Africa. These parasites are totally dependent on their host for survival, and therefore their germination is tightly regulated by host presence. Indeed, their seeds remain dormant in the soil until a host root is detected through compounds called germination stimulants. Strigolactones (SLs) are the most important class of germination stimulants. They play an important role in planta as a phytohormone, and, upon exudation from the root, function in the recruitment of symbiotic arbuscular mycorrhizal fungi. Plants exude mixtures of various different SLs, possibly to evade detection by these parasites and still recruit symbionts. Vice versa, parasitic plants must only respond to the SL composition that is exuded by their host, or else risk germination in the presence of non-hosts. Therefore, parasitic plants have evolved an entire clade of SL-receptors, called HTL/KAI2s, to perceive the SL cues. It has been demonstrated that these receptors each have a distinct sensitivity and specificity to the different known SLs, which possibly allows them to recognize the SL-blend characteristic of their host. In this review we will discuss the molecular basis of SL sensitivity and specificity in these parasitic plants through HTL/KAI2s, and review the evidence that these receptors contribute to host specificity of parasitic plants."
Authors: Rouhallah Sharifi, Jianping Chen, Zongtao Sun and Jian Chen.
Trends in Parasitology (2023)
Abstract: "Strigolactones from the exudates of maize root induce germination of the parasitic witchweed Striga. Recently, Li et al. characterized the biosynthesis pathway of two strigolactones, zealactol and zealactonoic acid, which induce less Striga germination than the major maize strigolactone, zealactone. This study provides a promising strategy for plant protection against parasitic witchweed.
Authors: Sylvia Mutinda, Muhammad Jamil, Jian You Wang, Lamis Berqdar, Elijah Ateka, Emily S. Bellis, Salim Al-Babili and Steven Runo.
Authorea (2023)
Abstract: "Sorghum is a food staple for millions of people in sub-Saharan Africa, but its production is greatly diminished by parasitic weeds of the Striga genus. An efficient and cost-effective way of managing Striga in smallholder farms of Africa is to deploy resistant varieties. Here, we leverage genomics and the vast genetic diversity of sorghum – evolutionarily adapted to cope with Striga parasitism in Africa – to identify new Striga resistant sorghum genotypes. We exploit a Striga resistance mechanism that hinges on essential communication molecules – strigolactones exuded by hosts to trigger parasite seed germination. We used the Sorghum Association Panel (SAP) to search for sorghum genotypes with a mutation on the LOW GERMINATION STIMULANT 1 ( LGS1) locus that make them ineffective in inducing Striga germination. Our analysis led us to identify new lgs1 sorghum genotypes which we named SAP lgs1. SAP lgs1 had the SL exudation profile of known lgs1 sorghum whose hallmark is production of the low inducer of germination, orobanchol. Laboratory and field resistance screens showed that the SAP lgs1 genotypes also exhibited remarkable resistance against Striga. By potentially reducing crop losses due to Striga parasitism, our findings have far-reaching implications for improving food security in Africa."
Authors: Shinsaku Ito.
Bioscience, Biotechnology, and Biochemistry (2023)
Abstract: "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 more than 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."
Authors: Ikuo Takahashi, Hikaru Koishihara and Tadao Asami.
Journal of Pesticide Science (2022)
Abstract: 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."
Authors: Suzanne Daignan Fornier, Alexandre de Saint Germain, Pascal Retailleau, Jean-Paul Pillot, Quentin Taulera, Lucile Andna, Laurence Miesch, Soizic Rochange, Jean-Bernard Pouvreau and François-Didier Boyer.
Journal of Natural Products (2022)
Abstract: "Strigolactones (SLs) are plant hormones exuded in the rhizosphere with a signaling role for the development of arbuscular mycorrhizal (AM) fungi and as stimulants of seed germination of the parasitic weeds Orobanche, Phelipanche, and Striga, the most threatening weeds of major crops worldwide. Phelipanche ramosa is present mainly on rape, hemp, and tobacco in France. P. ramosa 2a preferentially attacks hemp, while P. ramosa 1 attacks rapeseed. The recently isolated cannalactone (14) from hemp root exudates has been characterized as a noncanonical SL that selectively stimulates the germination of P. ramosa 2a seeds in comparison with P. ramosa 1. In the present work, (−)-solanacol (5), a canonical orobanchol-type SL exuded by tobacco and tomato, was established to possess a remarkable selective germination stimulant activity for P. ramosa 2a seeds. Two cannalactone analogues, named (±)-SdL19 and (±)-SdL118, have been synthesized. They have an unsaturated acyclic carbon chain with a tertiary hydroxy group and a methyl or a cyclopropyl group instead of a cyclohexane A-ring, respectively. (±)-SdL analogues are able to selectively stimulate P. ramosa 2a, revealing that these minimal structural elements are key for this selective bioactivity. In addition, (±)-SdL19 is able to inhibit shoot branching in Pisum sativum and Arabidopsis thaliana and induces hyphal branching in the AM fungus Rhizophagus irregularis, like SLs."
Authors: Dawei Wang, Zhili Pang, Haiyang Yu, Benjamin Thiombiano, Aimee Walmsley, Shuyi Yu, Yingying Zhang, Tao Wei, Lu Liang, Jing Wang, Xin Wen, Harro J. Bouwmeester, Ruifeng Yao and Zhen Xi.
Nature Communications (2022)
Editor's view: Strigolactone agonists could potentially help control noxious weeds by promoting suicidal germination. Here the authors describe a series of small molecule agonists that stimulate germination via the Striga ShHTL7 receptor and show that stereochemistry and hydrolysis-independent signalling mediate potency.
Abstract: "The development of potent strigolactone (SL) agonists as suicidal germination inducers could be a useful strategy for controlling root parasitic weeds, but uncertainty about the SL perception mechanism impedes real progress. Here we describe small-molecule agonists that efficiently stimulate Phelipanche aegyptiaca, and Striga hermonthica, germination in concentrations as low as 10−8 to 10−17 M. We show that full efficiency of synthetic SL agonists in triggering signaling through the Striga SL receptor, ShHTL7, depends on the receptor-catalyzed hydrolytic reaction of the agonists. Additionally, we reveal that the stereochemistry of synthetic SL analogs affects the hydrolytic ability of ShHTL7 by influencing the probability of the privileged conformations of ShHTL7. Importantly, an alternative ShHTL7-mediated hydrolysis mechanism, proceeding via nucleophilic attack of the NE2 atom of H246 to the 2′C of the D-ring, is reported. Together, our findings provide insight into SL hydrolysis and structure-perception mechanisms, and potent suicide germination stimulants, which would contribute to the elimination of the noxious parasitic weeds.
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