Recent advances in targeted fungicides and immune elicitors for rice blast management

Blast disease, caused by the fungus Magnaporthe oryzae, wreaks havoc on crops worldwide, resulting in an approximate loss of 20%–30% of the annual rice yield (He et al., 2020He M. Su J. Xu Y. Chen J. Chern M. Lei M. Qi T. Wang Z. Ryder L.S. Tang B. et al.Discovery of broad-spectrum fungicides that block septin-dependent infection processes of pathogenic fungi.Nat. Microbiol. 2020; 5: 1565-1575https://doi.org/10.1038/s41564-020-00790-yCrossref PubMed Scopus (48) Google Scholar; Liu et al., 2024Liu M. Wang F. He B. Hu J. Dai Y. Chen W. Yi M. Zhang H. Ye Y. Cui Z. et al.Targeting Magnaporthe oryzae effector MoErs1 and host papain-like protease OsRD21 interaction to combat rice blast.Nat. Plants. 2024; 10: 618-632https://doi.org/10.1038/s41477-024-01642-xCrossref Scopus (6) Google Scholar). M. oryzae heads the top 10 list of the most devastating phytopathogens and serves as a model system for investigating fungal diseases in plants, thus offering insights relevant to disease control in important agricultural crops. Effective fungicides play a pivotal role in blast management, substantially bolstering food security within modern agriculture. However, there is a significant shortage of targeted fungicides tailored for management of blast disease, in addition to tricyclazole, iprobenfos, tebuconazole, trifloxystrobin, and pydiflumetofen, which target melanin, choline, ergosterol, or ATP biosynthesis (Tleuova et al., 2020Tleuova A.B. Wielogorska E. Talluri V. Stepanek F. Elliott C.T. Grigoriev D.O. Recent advances and remaining barriers to producing novel formulations of fungicides for safe and sustainable agriculture.J. Control. Release. 2020; 326: 468-481https://doi.org/10.1016/j.jconrel.2020.07.035Crossref Scopus (94) Google Scholar). Recent studies have made notable advances in investigating the cell biology of appressorium-mediated infection and characterizing the specific proteins involved in interactions between M. oryzae and rice plants, propelling the discovery of new classes of pathobiology-based targeted fungicides and novel immune elicitors against blast. The appressorium, a specialized infection structure formed by M. oryzae, facilitates the efficient penetration and invasion of host plant tissues. An outstanding study conducted by He et al. found that very-long-chain fatty acids (VLCFAs) are essential for the subcellular organization of septin GTPases during appressorium-mediated host infection in M. oryzae (He et al., 2020He M. Su J. Xu Y. Chen J. Chern M. Lei M. Qi T. Wang Z. Ryder L.S. Tang B. et al.Discovery of broad-spectrum fungicides that block septin-dependent infection processes of pathogenic fungi.Nat. Microbiol. 2020; 5: 1565-1575https://doi.org/10.1038/s41564-020-00790-yCrossref PubMed Scopus (48) Google Scholar). The absence of VLCFAs hinders septin assembly, thus impeding host penetration. Intriguingly, the herbicide metazachlor, a known inhibitor of the VLCFA biosynthesis enzyme Elo1, suppresses blast disease and shows broad-spectrum fungicidal activity against various fungal pathogens without harming host plants (Figure 1A) (He et al., 2020He M. Su J. Xu Y. Chen J. Chern M. Lei M. Qi T. Wang Z. Ryder L.S. Tang B. et al.Discovery of broad-spectrum fungicides that block septin-dependent infection processes of pathogenic fungi.Nat. Microbiol. 2020; 5: 1565-1575https://doi.org/10.1038/s41564-020-00790-yCrossref PubMed Scopus (48) Google Scholar). Another analogous case in the pursuit of new fungicides against blast disease targeted a critical phosphatide phosphatase, Pah1, which is involved in the production of diacylglycerol to regulate triglyceride synthesis, cellular signaling, and appressorium development (Zhao et al., 2024aZhao J. Chen Y. Ding Z. Zhou Y. Bi R. Qin Z. Yang L. Sun P. Sun Q. Chen G. et al.Identification of propranolol and derivatives that are chemical inhibitors of phosphatidate phosphatase as potential broad-spectrum fungicides.Plant Commun. 2024; 5100679https://doi.org/10.1016/j.xplc.2023.100679Abstract Full Text Full Text PDF Scopus (6) Google Scholar). Propranolol, a known inhibitor of Pah1, showed efficient antifungal activity toward M. oryzae. The authors then developed and synthesized a series of propranolol derivatives using computer-aided drug design and chemical modification to further enhance its antifungal efficacy (Figure 1A). One such derivative displayed a 16-fold increase in antifungal activity and mitigated rice blast and Fusarium head blight under field conditions (Zhao et al., 2024aZhao J. Chen Y. Ding Z. Zhou Y. Bi R. Qin Z. Yang L. Sun P. Sun Q. Chen G. et al.Identification of propranolol and derivatives that are chemical inhibitors of phosphatidate phosphatase as potential broad-spectrum fungicides.Plant Commun. 2024; 5100679https://doi.org/10.1016/j.xplc.2023.100679Abstract Full Text Full Text PDF Scopus (6) Google Scholar). Both studies paved the way for the design of new targeted fungicides through an innovative drug repurposing strategy. In addition to the targets Elo1 and Pah1 mentioned above, Mps1 (mitogen-activated protein kinase 1), initially identified in 1998, is crucial for appressorium-mediated host penetration by M. oryzae. Two seminal reports in 2023 highlighted Mps1 as a prime target for blast fungus management and control (Kong et al., 2023Kong Z. Zhang X. Zhou F. Tang L. Chen Y. Li S. Zhang X. Kuai L. Su W. Cui W. et al.Structure-aided identification of an inhibitor targets Mps1 for the management of plant-pathogenic fungi.mBio. 2023; 14e0288322https://doi.org/10.1128/mbio.02883-22Crossref Scopus (7) Google Scholar; Li et al., 2023Li R. Bi R. Cai H. Zhao J. Sun P. Xu W. Zhou Y. Yang W. Zheng L. Chen X.L. et al.Melatonin functions as a broad-spectrum antifungal by targeting a conserved pathogen protein kinase.J. Pineal Res. 2023; 74e12839https://doi.org/10.1111/jpi.12839Crossref Scopus (16) Google Scholar). The first report focused on identifying an inhibitor, A378-0, through high-throughput affinity screening against a DNA-encoded compound library. An innovative finding revealed that A378-0 binds strongly to Mps1 and its orthologs and blocks their function, thereby inhibiting fungal invasion and pathogenic growth of not only M. oryzae but also Fusarium oxysporum (Figure 1B) (Kong et al., 2023Kong Z. Zhang X. Zhou F. Tang L. Chen Y. Li S. Zhang X. Kuai L. Su W. Cui W. et al.Structure-aided identification of an inhibitor targets Mps1 for the management of plant-pathogenic fungi.mBio. 2023; 14e0288322https://doi.org/10.1128/mbio.02883-22Crossref Scopus (7) Google Scholar). Meanwhile, Li et al. found that melatonin inhibits fungal growth, conidial germination, and appressorium formation, effectively limiting blast disease establishment and/or severity (Li et al., 2023Li R. Bi R. Cai H. Zhao J. Sun P. Xu W. Zhou Y. Yang W. Zheng L. Chen X.L. et al.Melatonin functions as a broad-spectrum antifungal by targeting a conserved pathogen protein kinase.J. Pineal Res. 2023; 74e12839https://doi.org/10.1111/jpi.12839Crossref Scopus (16) Google Scholar). Their research, supported by genetic and biochemical evidence, suggests that melatonin acts through Mps1 and readily enters fungal cells to disrupt the activation of this important mitogen-activated protein kinase. The authors also synthesized melatonin derivatives by chemical modification, and one variant, tert-butylcarbonyl melatonin, showed 25-fold greater effectiveness in fungicidal activity (Figure 1B). These studies introduce novel kinase inhibitors and strategies for the development of highly effective and broad-spectrum fungicides, integrating various approaches such as affinity screening, chemical modification, and structural assays to target key signaling pathways essential for fungal pathogenesis. Rapid advances in platforms for protein structure prediction, including I-TASSER, have enabled effective strategies for the discovery of targeted fungicides through receptor-based virtual screening (Zhao et al., 2024aZhao J. Chen Y. Ding Z. Zhou Y. Bi R. Qin Z. Yang L. Sun P. Sun Q. Chen G. et al.Identification of propranolol and derivatives that are chemical inhibitors of phosphatidate phosphatase as potential broad-spectrum fungicides.Plant Commun. 2024; 5100679https://doi.org/10.1016/j.xplc.2023.100679Abstract Full Text Full Text PDF Scopus (6) Google Scholar; Wu et al., 2024Wu X.Y. Dong B. Zhu X.M. Cai Y.Y. Li L. Lu J.P. Yu B. Cheng J.L. Xu F. Bao J.D. et al.SP-141 targets Trs85 to inhibit rice blast fungus infection and functions as a potential broad-spectrum antifungal agent.Plant Commun. 2024; 5100724https://doi.org/10.1016/j.xplc.2023.100724Abstract Full Text Full Text PDF Scopus (3) Google Scholar). A recent study focused on Trs85, a crucial subunit of the transport protein particle III complex that is pivotal for appressorium-mediated M. oryzae infection (Wu et al., 2024Wu X.Y. Dong B. Zhu X.M. Cai Y.Y. Li L. Lu J.P. Yu B. Cheng J.L. Xu F. Bao J.D. et al.SP-141 targets Trs85 to inhibit rice blast fungus infection and functions as a potential broad-spectrum antifungal agent.Plant Commun. 2024; 5100724https://doi.org/10.1016/j.xplc.2023.100724Abstract Full Text Full Text PDF Scopus (3) Google Scholar). Trs85 modulates autophagy during appressorium formation via Ypt1, a GTPase, and harbors a key conserved amphipathic α helix associated with pathogenicity of the blast fungus. Computer-aided screening leveraging this α helix led to the discovery of SP-141, a lead compound that disrupts the Trs85–Ypt1 interaction and autophagy. SP-141, a novel antifungal agent, demonstrates significant potential for M. oryzae control and shows promise as a broad-spectrum antifungal compound (Figure 1C). In addition, the novel isopropanolamine inhibitor VS-10 with unique mechanisms was discovered by virtual screening against the trehalose-6-phosphate synthase MoTps1; it displayed remarkable fungicidal activity against M. oryzae after chemical modification (Jiang et al., 2023Jiang Z. Shi D. Chen Y. Li H. Wang J. Lv X. Zi Y. Wang D. Xu Z. Huang J. et al.Discovery of novel isopropanolamine inhibitors against MoTPS1 as potential fungicides with unique mechanisms.Eur. J. Med. Chem. 2023; 260115755https://doi.org/10.1016/j.ejmech.2023.115755Crossref Scopus (3) Google Scholar). These findings underscore the power of computer-aided virtual screening in expediting the discovery of novel signaling interfaces and targeted fungicides. Ligand-based drug design is an alternative to receptor-based virtual screening for the development of novel targeted fungicides. During rice infection, M. oryzae secretes diverse effectors to manipulate host cell processes and weaken the host defense response. Notably, Liu et al. recently found that a unique effector, Ers1, highly specific to M. oryzae and absent in other fungal species, suppresses the immune-related papain-like cysteine protease OsRD21 in rice (Liu et al., 2024Liu M. Wang F. He B. Hu J. Dai Y. Chen W. Yi M. Zhang H. Ye Y. Cui Z. et al.Targeting Magnaporthe oryzae effector MoErs1 and host papain-like protease OsRD21 interaction to combat rice blast.Nat. Plants. 2024; 10: 618-632https://doi.org/10.1038/s41477-024-01642-xCrossref Scopus (6) Google Scholar). By selecting diphenyl ether ester as an Ers1 inhibitor core, using Sybyl-x-2.0 for diaryl ether–Ers1 docking, and synthesizing various derivatives with hydroxyl and ester modifications, a new compound, FY21001, was designed (Figure 1D). FY21001 selectively binds to and inhibits Ers1, thus interrupting its interaction with OsRD21, and has proven to be highly effective for rice blast control in field trials (Liu et al., 2024Liu M. Wang F. He B. Hu J. Dai Y. Chen W. Yi M. Zhang H. Ye Y. Cui Z. et al.Targeting Magnaporthe oryzae effector MoErs1 and host papain-like protease OsRD21 interaction to combat rice blast.Nat. Plants. 2024; 10: 618-632https://doi.org/10.1038/s41477-024-01642-xCrossref Scopus (6) Google Scholar). This work pioneered a ligand-driven strategy for development of fungicides targeting pathogen-specific effectors, thus offering an eco-friendly and sustainable solution for crop protection. Although targeted chemical interventions have proven effective, the enhancement of intrinsic crop resistance represents an alternative strategy. A recent exciting breakthrough revealed that exogenous application of the novel immune elicitor OsSSP1 (Oryza sativa secretory small protein 1) boosts rice resistance against blast disease. Upon sensing M. oryzae attack, OsSSP1 is secreted into the apoplast to trigger a defense response in the host plant (Zhao et al., 2024bZhao T. Ma S. Kong Z. Zhang H. Wang Y. Wang J. Liu J. Feng W. Liu T. Liu C. et al.Recognition of the inducible, secretory small protein OsSSP1 by the membrane receptor OsSSR1 and the co-receptor OsBAK1 confers rice resistance to the blast fungus.Mol. Plant. 2024; 17: 807-823https://doi.org/10.1016/j.molp.2024.04.009Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar). This response depends on an as-yet uncharacterized transmembrane protein OsSSR1 (Oryza sativa secretory small protein receptor 1) and the coreceptor OsBAK1. Such a "self-recognition process" via secreted proteins in the host could be a milestone finding that deepens our understanding of host defense strategy and fungus–host interactions. Application of exogenous OsSSP1 over 2 weeks maintained effective rice resistance to M. oryzae without affecting yield, indicating its excellent potential as a sustainable immune elicitor alternative to chemical fungicide-based control and management of rice blast. The identification of pathogenicity proteins, which are conserved among M. oryzae strains from different rice growing areas or plant pathogenic fungi and are distinct from human, animal, and other microbial proteins, is pivotal for development of fungicides targeting rice blast. Furthermore, exploring the molecular interactions and signaling pathways in the M. oryzae–rice interaction will reveal essential regulators of pathogenicity and host defense, as well as conserved binding sites for interactions between these regulators and their partners, thus identifying additional fungicide/immune elicitor targets. In addition to chemical agents, double-stranded RNAs and short peptides also show promise, with technological innovations enhancing their practical application for fungal disease control. Integration of computational methods such as virtual screening, molecular docking, and artificial intelligence tools like AlphaFold 3 and RoseTTAFold could accelerate the development of new fungicides by illustrating the structures of fungal proteins, identifying lead compounds through virtual screening, revealing the binding sites between receptors and ligands, modifying existing compounds or creating new ones, and assessing the potential environmental impact of new fungicides on non-target organisms (Peng et al., 2024Peng Z. Lu P. Yang J. AI accurately predicting the structure of biomolecular interactions.Cell Res. 2024; 0: 1-2https://doi.org/10.1038/s41422-024-00991-8Crossref Scopus (1) Google Scholar). Finally, cutting-edge methods will improve fungicide efficiency, stability, and safety while reducing overall costs and time to market and in-field applications. This research was supported by grants from the National Natural Science Foundation of China (32171944) and the Innovation Program of the Chinese Academy of Agricultural Sciences (Y2023QC22 and CAAS-CSCB-202301).