Cell Death Regulator BI-1 Emerges as Key Switch in Plant Immunity Against Oomycete Pathogens

Scientists studying how plants fend off one of agriculture’s most destructive groups of pathogens have uncovered a molecular switch that sits at the crossroads of cellular stress and immune execution. In a study published in Stress Biology, researchers led by Yujing Fang and Xiaoyu Qiang at Northwest A&F University in China report that the endoplasmic reticulum-localized protein Bax Inhibitor-1, or BI-1, is a critical executor of resistance against the oomycete pathogen Phytophthora parasitica in Arabidopsis thaliana. Their work also reveals how this protective function is quietly undermined by a susceptibility factor known as RTP1, which physically destabilizes BI-1 and dampens the defensive cell death responses that would otherwise slow the invader down.

The findings matter because Phytophthora species, which include the agent of potato late blight, continue to devastate crops worldwide. Classical resistance genes often lose effectiveness as pathogens rapidly evolve new virulence, prompting plant scientists to look instead at susceptibility genes, host factors that pathogens exploit to establish infection. Disrupting these genes can deliver durable, broad-spectrum resistance. RTP1, which encodes a protein embedded in the endoplasmic reticulum membrane, was previously identified as just such a susceptibility factor: mutant plants lacking RTP1 show enhanced resistance to multiple biotrophic pathogens, accompanied by faster cell death, stronger oxidative bursts, and elevated expression of defense genes. What remained unknown was which downstream molecules actually carry out the cell death and immune signaling that RTP1 normally restrains.

The answer, according to the new study, is BI-1, a conserved regulator of programmed cell death that resides in the endoplasmic reticulum. When the researchers inoculated wild-type and rtp1 mutant seedlings with P. parasitica zoospores and tracked gene expression in roots over the first 24 hours of infection, they found that BI-1 transcript levels rose in both genotypes, but the induction was far stronger in the rtp1 mutants, particularly at 3, 6, and 12 hours post-inoculation. This timing coincides with the pathogen’s early biotrophic colonization phase, during which it penetrates root cells, forms appressoria, and develops haustorium-like structures. The data suggested that RTP1 normally acts to suppress BI-1 expression precisely when the plant most needs it.

To connect BI-1 to the accelerated cell death seen in rtp1 plants, the team used a fluorescein diacetate staining assay, which measures cell viability by fluorescence intensity. As expected, infected rtp1 roots showed significantly higher cell death rates than wild-type Col-0 roots. Strikingly, when the researchers generated rtp1 bi-1 double mutants, the excessive cell death largely disappeared. This genetic epistasis experiment demonstrated that BI-1 is essential for the cell death phenotype triggered by loss of RTP1, placing BI-1 downstream of the susceptibility factor in the cell death regulatory hierarchy. It echoes earlier work from the same group showing that the vacuolar processing enzyme gamma-VPE is similarly required for rtp1-mediated cell death, hinting that RTP1 coordinates parallel branches of cell death signaling located in different cellular compartments.

The importance of BI-1 extended well beyond cell death into full-blown immunity. Microscopic examination of roots infected with a GFP-tagged P. parasitica strain revealed less pathogen colonization in rtp1 mutants than in wild type, but the rtp1 bi-1 double mutants were colonized more heavily than rtp1 plants alone. Quantitative PCR measurements of pathogen biomass confirmed the pattern: the reduced fungal-like pathogen load characteristic of rtp1 mutants was substantially restored in the double mutants between 3 and 12 hours post-inoculation. At the later necrotrophic stage, seven days after inoculation, more than 60 percent of rtp1 seedlings remained healthy while over 83 percent of wild-type plants were heavily infected; the double mutants lost much of this protection, showing significantly elevated seedling death rates.

The molecular signature of the immune response told the same story. In infected rtp1 plants, the ER stress-responsive immune genes WRKY33, CBP60g, and MYB51, along with the defense marker PR1, are strongly induced. In the rtp1 bi-1 double mutants, this induction was markedly reduced. The team also tested the oxidative burst triggered by flg22, a conserved fragment of bacterial flagellin that activates pattern-triggered immunity. While rtp1 mutants mounted a stronger transient reactive oxygen species burst than wild type, the double mutants showed an essentially abolished response in a luminol-based chemiluminescence assay. Together, these results establish BI-1 as an integral node linking RTP1 to both immune gene expression and ROS production.

Independent evidence that BI-1 acts positively in defense came from overexpression experiments. Transient expression of BI-1 in Nicotiana benthamiana leaves before inoculation produced significantly smaller infection lesions and lower pathogen biomass at 48 hours. In Arabidopsis, transgenic plants overexpressing BI-1 developed fewer water-soaked lesions on detached leaves and supported less pathogen growth in both leaves and roots, while bi-1 loss-of-function mutants were more susceptible than wild type and showed weakened induction of the same ER stress-responsive immune genes. The immune function of BI-1 appears conserved across plant-pathogen systems: overexpression enhances resistance to rice blast, silencing of the wheat ortholog increases susceptibility to stripe rust, and BI-1 cooperates with the IRE1/bZIP60 pathway to restrict viral movement in plants.

Perhaps the most mechanistically revealing result concerns how RTP1 antagonizes BI-1. The two proteins physically interact, and when the researchers co-expressed tagged versions of both in Nicotiana benthamiana, BI-1 protein accumulation dropped by roughly 57 percent at two days and 62 percent at three days after infiltration. Critically, treating the leaves with MG132, a proteasome inhibitor, failed to restore BI-1 levels, indicating that RTP1 promotes BI-1 degradation through a proteasome-independent route. Given that BI-1 interacts with the autophagy protein ATG6 and both localize to the ER, the authors propose that RTP1 may steer BI-1 toward vacuolar or autophagic degradation. Intriguingly, RTP1 was recently shown to destabilize another ER-localized immune protein, the cytochrome P450 enzyme CYP71B3, suggesting a recurring strategy in which this susceptibility factor suppresses defense by depleting ER-localized immune components.

The study also clarified how BI-1 transcription is driven during infection. The BI-1 promoter contains G-box elements recognized by basic leucine zipper transcription factors, along with canonical ER stress response elements. Induction of BI-1 upon infection was blunted in bzip28 and bzip60 single mutants and most severely attenuated in the bzip28 bzip60 double mutant, pointing to redundant roles for these two unfolded protein response transducers. Dual-luciferase reporter assays showed that the activated forms of bZIP28 and bZIP60 each activated the BI-1 promoter, with co-expression of both producing an even stronger response, and yeast one-hybrid assays confirmed direct physical binding of each transcription factor to the promoter. BI-1 thus sits under dual control: transcriptionally activated by the ER stress sensing pathway and post-translationally dismantled by RTP1.

Taken together, the work establishes the RTP1-BI-1 module as a pivotal interface connecting ER stress perception to immune execution. Because BI-1 and gamma-VPE operate in distinct compartments yet both mediate rtp1-driven immunity, RTP1 emerges as a central hub orchestrating multiple ER stress-associated cell death pathways. For crop breeders, the implications are tangible: manipulating the RTP1-BI-1 axis, or the bZIP60 and bZIP28 transcription factors that feed into it, could offer a route to durable, broad-spectrum resistance against oomycete pathogens without relying on rapidly outmaneuvered resistance genes. The authors note that future work should map the precise hierarchy of this signaling network and determine how spatially separated cell death regulators are coordinated to fine-tune plant immunity.

Subject of Research: ER stress-associated plant immunity and the RTP1-BI-1 regulatory module governing resistance to Phytophthora parasitica in Arabidopsis

Article Title: Bax inhibitor-1 confers resistance to Phytophthora parasitica and is antagonized by RTP1 in Arabidopsis

Article References: Fang, Y., Zhang, J., Gao, X., Guo, S., Xu, X., Wang, B., Zheng, Q., Shan, W., & Qiang, X. (2026). Bax inhibitor-1 confers resistance to Phytophthora parasitica and is antagonized by RTP1 in Arabidopsis. Stress Biology, 6(1), Article 46. https://doi.org/10.1007/s44154-026-00318-0

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00318-0

Keywords: plant immunity, Bax inhibitor-1, RTP1, Phytophthora parasitica, ER stress, unfolded protein response, bZIP60, bZIP28, programmed cell death, susceptibility genes, Arabidopsis, disease resistance