Scientists Have Found New Nuclear Effectors of Rice Blast Fungus: What Does This Have to do with Disease Management?
By. Won Gyeom Yang
“Rice Infected by M. Oryzae”, https://guardian.ng/features/agro-care/how-to-contain-rice-blast-disease-in-farms/.
Magnaporthe oryzae (M. oryzae) is a fungus responsible for the Rice Blast disease in rice plants (Oryza Sativa). Rice Blast threatens rice production on a global scale, claiming 10-30% of annual production around the world (2, 3). As a result, M. oryzae is studied almost ubiquitously around the world with numerous researchers trying to find ways to combat this pathogen.
In fact, in 2020 Seongbeom Kim and others took a deep dive into the molecular mechanisms behind M. oryzae’s interaction with rice host plants. In the study, they were able to specify two effectors of M. oryzae– MoHTR1 and MoHTR2–that acted like transcription repressors (2). However, to fully appreciate the transformative influence the research can have on disease management requires an understanding of plant immunity and the ways in which M. oryzae attempts to penetrate rice’s fortifications and rice attempts to dispel M. oryzae.
Brief Review of Plant Immunity and Plant-Pathogen Interaction
The moment MO lands on the rice plant, it triggers the rice cell’s Pattern-triggered Immunity (PTI) (1). The rice detects Pathogen-associated Molecular Patterns associated with M. Oryzae such as Chitin (fungal cell wall component) via its Pattern Recognition Receptors (PRRs) on the cell’s surface (1). Upon the activation of PTI, the rice plant produces various responses, including thickening of the cell wall, stomata closure, and secretion of anti-pathogenic enzymes such as chitinases, that enhance its fortifications (1).
When M. oryzae faces the PTI responses. It releases effector proteins that can neutralize the immune responses. For instance, M. oryzae secretes proteins like MoCel12A and MoCel12B that degrade hemicellulose to create entry points on the plant cell wall. It also produces lectins that protect its cell wall from chitinases and launches effectors such as MoHTR1 and MoHTR 2 (highlighted in the research by Kim’s group) into the plant nucleus to manipulate gene expression, creating an environment conducive to pathogenesis (1).
Additionally, against the effectors, rice activates its Effector-triggered Immunity (ETI) that counteracts the effectors’ activity. reactivation of PTI responses that were suppressed by effectors and induction of Hypersensitive Response (HR) are two main ways ETI responds to pathogens (1). HR is defined as rapid localized and programmed cell death around the point of pathogen entry. HR is effective against biotrophic and hemibiotrophic pathogens that require living host cells for development (5). MO is a hemibiotroph that initially exhibits biotrophic behavior and later switches to a necrotrophic lifestyle (consuming dead cell matter for growth) (2). As a result, if rice initiates its HR in the early stages of infection, the conditions of attacked areas become inhospitable for MO, leading to the end of MO’s aggression. Although HR is less effective against necrotrophic pathogens, it is still the strongest immune response a plant can create. It is a suicide, but a sacrifice for a greater good as the entire rice plant can live for another day by losing a couple of cells.
Beyond HR, M. oryzae constantly evolves to prevent the rice from recognizing its presence and rice constantly evolves to circumvent M. oryzae’s effort to disguise itself by strengthening its immunity.
Returning to the Kim Group’s Research
In the research, the group found that MoHTR1 and MoHTR2 were nuclear effectors directly acting on the rice DNA (2). The group first added Green Fluorescent Proteins (GFP) and Red Fluorescent Proteins (RFP) sequences under the MoHTR1 and 2 promoters to create mutants that can be located inside the host cell via confocal microscopy (2). The GFP and RFP tagged effectors were spotted inside the rice host cell nucleus, confirming MoHTR1 and 2’s function as nuclear effectors (2). Then, the group used a protein binding microarray to identify numerous sites on rice DNA to which MoHTR1 and 2 bound. From this, they found two rice genes, OsMYB4 and OsWRKY45, to be affected by MoHTR1 and 2. Then to determine the influence of the effectors on rice transcription, the Luciferase (LUC) gene was inserted under OsMYB4 and OsWRK45 promoters of a transgenic rice, which was infected with mutant M. oryzae overexpressing MoHTR1 and 2 (2). The researchers found a decreased LUC expression in transgenic rice infected by mutant M. oryzae, which suggested that MoHTR1 and 2 orchestrated a reduction in the expression of OsMYB4 and OsWRK45 (2).
This form of discovery made by Kim’s group allows for a new method of disease management because besides identifying MoHTR1 and 2 and transcriptional repressors, the group discovered even a single nucleotide change at effector binding element (EBE) of rice DNA can significantly reduce the binding affinity of the effector to the gene, which can suppress the activity of the effector (2). Thus a modification of the EBE DNA sequence can mask the plant genes from the effectors thereby enabling the rice to retain its immunity despite the presence of pathogen effectors.
Once scientists identify the way an effector acts on the plant DNA and locate the EBE, the corresponding DNA sequence of the EBE can be modified to make it undetectable by the effector proteins (4). This shields the gene from the effector, helping the plant to retain its immune response despite the presence of pathogen effectors.
The Future of Plant Resistance and Disease Management
Exploiting the relationship between plants and pathogen effectors can pioneer novel approaches to controlling plant diseases. In contrast to traditional approaches with chemical pesticides, editing the gene of the rice plant to make it difficult for the effectors to manipulate the host cell environment to negate pathogenesis can be a refreshing way to control pests.
Although there is much work ahead to fully understand how effectors engage the plant genes, new understandings of pathogenic effectors and plant immune responses are introduced to the scientific community every day; there is a lot to do, but much is awaiting to be discovered. Perhaps one day, it may be possible to create resistant crops that rely solely on gene editing targeted to nullify effectors without relying on the help of chemical pesticides that threaten our environment.
References
1. Zhang, Shiyi et al. “Action Mechanisms of Effectors in Plant-Pathogen Interaction.”
International journal of molecular sciences vol. 23,12 6758, 17 Jun. 2022, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9224169/.
2. Kim, Seongbeom, et al. "Two nuclear effectors of the rice blast fungus modulate host
immunity via transcriptional reprogramming." Nature Communications 11.1 (2020): 5840, 17 November 2020, https://www.nature.com/articles/s41467-020-19624-w#citeas.
3. UCIPM. “Rice Blast.” University of California Agriculture and Natural Resources,
https://ipm.ucanr.edu/agriculture/rice/rice-blast/
4. Zhang, Shiyi, et al. "Action mechanisms of effectors in plant-pathogen interaction."
International Journal of Molecular Sciences 23.12, 26 August 2014, https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.13015.
5. Balint-Kurti, Peter. “The Plant Hypersensitive Response: Concepts, Control and
Consequences.” Molecular Plant Pathology vol. 20,8 (2019), 15 July 2019, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6640183/.
