The study, published in the journal Stress Biology, focuses on a long-standing puzzle in plant immunity: how the actin cytoskeleton, a dynamic meshwork of protein filaments just beneath the cell membrane, communicates with the signaling pathways that mobilize defense. Scientists have known for years that when a plant detects pathogen-associated molecular patterns, or PAMPs, through cell-surface receptors, the actin network rapidly reorganizes. Filament density, orientation and bundling all shift within minutes. Disrupting this network, whether chemically or genetically, typically leaves plants more vulnerable to infection. What remained unclear was which molecular players orchestrate the remodeling and how it connects to downstream immune outputs such as reactive oxygen species production.

To answer these questions, the team turned to the rice-Magnaporthe oryzae pathosystem, a classic model in which fungal spores land on the leaf surface, germinate, and build dome-shaped infection structures called appressoria. These structures generate enormous turgor pressure through glycerol accumulation, allowing the fungus to mechanically puncture the plant cuticle. The host cytoskeleton and cell wall at the infection site are critical barriers to this penetration, making them natural focal points for studying basal immunity.

The researchers first confirmed the importance of actin polymerization using latrunculin B, a macrolide compound derived from marine sponges that binds monomeric actin and prevents filament elongation. When seedlings of the japonica cultivars TP309 and Nipponbare were pretreated with the drug before spray inoculation with a virulent M. oryzae isolate, they became markedly more susceptible to blast, with higher concentrations producing stronger effects. Quantitative PCR revealed greater fungal growth in treated plants. The drug also broadly suppressed defense hormone pathways: genes in the salicylic acid pathway, including OsPAL1, OsICS1 and PR4, and genes in the jasmonic acid and ethylene pathways, including OsAOS2 and OsERF1, were all downregulated. In other words, a structurally intact actin network is not merely a passive barrier but an active prerequisite for full immune signaling.

That result pointed the investigators toward BUI1, also known as RMD or OsFH5, a class II formin protein previously famous for its role in rice development. Loss of BUI1 function causes bent uppermost internodes, dwarfism, wavy panicles and abnormal seeds, all traced to impaired cell elongation and disrupted cytoskeletal arrays. The protein carries three conserved domains: a PTEN domain that targets it to the chloroplast surface, a proline-rich FH1 domain that interacts with profilin to promote actin polymerization, and an FH2 domain that binds the growing ends of actin filaments. Notably, the team found that latrunculin B treatment downregulated BUI1 expression, hinting that the gene sits within the very regulatory circuit the drug perturbs.

Testing the bui1 mutant, derived from the moderately resistant cultivar Zhejing 22, confirmed the suspicion. After punch inoculation with M. oryzae, the mutant developed significantly larger lesions than wild type, and field trials in a blast nursery showed dramatically reduced resistance under natural infection pressure. Using a GFP-tagged fungal strain, the researchers tracked infection in real time and found that appressorium formation and maturation were both more efficient on bui1 leaf sheath cells, suggesting that BUI1-mediated actin organization actively impedes this critical early step of fungal invasion. BUI1 expression itself was upregulated upon fungal inoculation, consistent with a positive role in defense.

The most striking finding came when the team visualized actin filaments directly. Using fluorescent phalloidin staining and confocal microscopy, they showed that treatment with the PAMPs chitin or flg22 triggered a rapid, transient increase in actin filament bundling in wild-type leaf sheath cells within five minutes. In the bui1 mutant, this response was severely attenuated, and the baseline cytoskeleton appeared disorganized. During actual fungal infection, wild-type cells accumulated dense actin bundles around infection sites by twelve hours post-inoculation, while the mutant failed to mount this defense-associated remodeling. BUI1, the data showed, is essential for PAMP-triggered actin reorganization, placing a formin protein squarely within the earliest tier of the plant immune response.

Broad-spectrum resistance is the holy grail of crop protection, and BUI1 delivered on that front too. The mutant proved more susceptible not only to blast but also to bacterial blight, developing longer lesions after inoculation with the Xoo strain PXO99A, and to sheath blight when tested in CRISPR-generated knockout lines in the Nipponbare background. Conversely, plants overexpressing BUI1 under its native promoter showed enhanced bacterial blight resistance proportional to expression level. Time-course transcriptome analysis after Xoo infection revealed extensive transcriptional reprogramming in wild-type plants that was largely absent in the mutant, with key defense pathways, including phenylpropanoid biosynthesis, MAPK signaling and plant-pathogen interaction networks, failing to activate without BUI1.

Perhaps most intriguingly, BUI1 turned out to act downstream of ROD1, a calcium sensor previously shown to suppress rice immunity by activating catalase-mediated ROS scavenging, whose loss confers broad-spectrum resistance. When the researchers knocked out BUI1 in the rod1 background, the strong resistance of rod1 plants was significantly compromised against blast, bacterial blight and sheath blight alike, though the double mutants remained less susceptible than fully wild-type plants. Yeast two-hybrid assays detected no direct physical interaction between BUI1 and ROD1 or the ROD1-interacting catalase OsCatB, indicating the relationship is genetic rather than structural. DAB staining and hydrogen peroxide measurements showed that the double mutants accumulated lower ROS levels than rod1 alone, pointing to ROS homeostasis as the mechanistic bridge.

RNA sequencing after M. oryzae infection explained why. In the bui1 knockout background, genes encoding peroxidases and metallothioneins, including OsPOD, POXA, OsCatB, OsMT1d and OsMT1f, were strongly upregulated, while ROS-producing genes such as RbohA and RbohB were unchanged. The mutant exhibited higher peroxidase activity and correspondingly lower hydrogen peroxide accumulation. BUI1, it appears, maintains immune competence primarily by restraining ROS-scavenging pathways, allowing the oxidative burst that hallmark of pattern-triggered immunity to reach effective levels. The protein also contains intrinsically disordered regions predicted to mediate liquid-liquid phase separation, and truncated variants lacking these regions failed to undergo phase separation in vitro, suggesting that BUI1 may form biomolecular condensates that serve as organizational hubs integrating cytoskeletal dynamics with defense signaling. For breeders, the message is clear: a single formin gene links the cell’s structural skeleton, its oxidative chemistry and its hormone signaling into one coordinated defense program, and tuning that gene could yield rice lines that shrug off multiple diseases without yield penalties.

Subject of Research: The role of the rice formin gene BUI1 in coordinating actin cytoskeleton remodeling and reactive oxygen species homeostasis during broad-spectrum plant disease resistance.

Article Title: BUI1 coordinates actin cytoskeleton remodeling and ROS homeostasis to confer broad-spectrum disease resistance in rice

Article References: BUI1 coordinates actin cytoskeleton remodeling and ROS homeostasis to confer broad-spectrum disease resistance in rice. (n.d.). https://doi.org/10.1007/s44154-026-00321-5

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00321-5

Keywords: BUI1, rice immunity, actin cytoskeleton, ROS homeostasis, Magnaporthe oryzae, pattern-triggered immunity, formin, ROD1, broad-spectrum resistance, Xanthomonas oryzae, peroxidase, plant pathology