In the relentless arms race between plants and their microbial assailants, immune defenses mounted by the plant cell surface stand as critical sentinels against invading pathogens. A recent breakthrough from a team led by Zhang, Wang, and Jiang uncovers a sophisticated mechanism by which the infamous plant pathogen genus Phytophthora sabotages this frontline immunity. Their study, soon to be featured in Nature Plants, reveals a conserved trypsin-like serine protease secreted by Phytophthora species that meticulously targets and cleaves the plant immune co-receptor BAK1, crippling the plant’s ability to mount an effective defense. This discovery redefines our understanding of how oomycete pathogens circumvent plant pattern-triggered immunity (PTI) and highlights an evolutionary battleground at the host-pathogen interface.
At the heart of the plant immune system lies the pattern recognition receptor (PRR) complex, which identifies conserved microbial molecules known as microbe-associated molecular patterns (MAMPs). Activation of these receptors triggers PTI, a robust defense signaling cascade that equips the plant to fight off infection. Among the PRRs, the receptor-like kinase BAK1 (BRI1-associated kinase 1) serves a pivotal role as a co-receptor partnering with multiple PRRs to transduce immune signals. Its extracellular leucine-rich repeat (LRR) domain enables BAK1 to interact with diverse ligand-bound PRRs, setting in motion intracellular phosphorylation events that amplify defense responses.
Despite BAK1’s critical role in orchestrating PTI, little was known about how pathogens might strategically disable this immune hub, especially within the apoplastic space where host and pathogen first meet. The apoplast, the extracellular matrix outside plant cells, acts as a battleground rich with defensive proteins and enzymes. Understanding how microbial effectors operate in this milieu is essential to unveiling pathogen virulence tactics. To probe this mystery, the researchers embarked on an expansive screen of Phytophthora apoplastic effectors capable of suppressing cell death induced by INF1, an elicitin recognized by plants to trigger immune responses.
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The screening effort, leveraging advanced molecular tools and plant model systems including soybean and Nicotiana benthamiana, unveiled PsTry1—a previously uncharacterized trypsin-like serine protease secreted by Phytophthora sojae. PsTry1’s striking capacity to inhibit multiple immune responses prompted deeper investigation into its mode of action. Biochemical assays demonstrated PsTry1’s affinity for BAK1, directly associating with the co-receptor’s extracellular domain and cleaving it with high specificity. This proteolytic activity severely impairs the immune receptor’s function, effectively disarming the plant’s sensor network before intracellular signaling can be propagated.
Crucially, the study elucidated that PsTry1’s immune suppression depends entirely on its enzymatic activity. Using mutagenesis to inactivate the catalytic triad of PsTry1 abolished its ability to degrade BAK1 and to suppress PTI responses. This dependency highlights proteolysis as the molecular weapon by which the pathogen disables critical immune components. Such targeted cleavage differs from more passive or indirect suppression strategies, revealing a direct and aggressive microbial counterattack deployed in the extracellular matrix.
Further structural and mutational analyses of BAK1 uncovered the significance of a single amino acid residue—Leu163—in the extracellular domain. Alanine scanning mutagenesis pinpointed this leucine as essential for recognition and cleavage by PsTry1. Substituting Leu163 prevented PsTry1 from cleaving BAK1, consequently restoring the receptor’s immune function. This finding underscores the exquisite molecular specificity mediating pathogen interference and suggests that subtle alterations in host receptors could potentially confer resistance to such proteolytic attack.
One of the most compelling revelations of this study was the high conservation of PsTry1 across diverse Phytophthora species, indicating that this protease is a widely employed virulence factor in this genus. Multiple homologues from other pathogenic Phytophthora isolates exhibited similar abilities to cleave BAK1 and suppress PTI, suggesting that this mechanism is a fundamental strategy in oomycete pathogenesis. Such conservation also raises the intriguing possibility that interrupting PsTry1 activity could provide broad-spectrum resistance against a variety of Phytophthora pathogens.
The implications of these findings extend beyond basic science, offering tangible avenues for agricultural innovation. Crop plants like soybean face significant yield losses due to Phytophthora-induced diseases, and BAK1’s vulnerability to degradation has now been exposed as a key weakness. Designing plant varieties that either modify the Leu163 site or express protease inhibitors targeting PsTry1 could help safeguard immune integrity. Additionally, elucidating the structure of PsTry1 opens opportunities for developing chemical inhibitors that can neutralize its proteolytic activity in the apoplast.
This research provides a vivid example of the molecular tug-of-war at the plant-pathogen interface, where every residue and enzymatic activity can tilt the balance between susceptibility and resistance. By revealing the precise molecular sabotage employed by Phytophthora, the study enriches our conceptual framework of plant immunity and microbial virulence. Furthermore, it sheds light on the nuanced role of the apoplast as a dynamic zone of host-pathogen communication—a space where microbial effectors must either evade or directly neutralize host defenses.
Another notable aspect is that the targeted interference with BAK1—an immune hub involved in responses to multiple MAMPs—allows the pathogen to broadly suppress recognition and signaling. This multiplicity underscores why BAK1 is such a critical node in plant defense, acting as a co-receptor for various PRRs detecting different microbial signatures. By disabling BAK1, Phytophthora not only evades detection by INF1 but also neutralizes signaling triggered by other MAMPs, effectively blinding the plant immune system.
The study also raises fascinating evolutionary questions. How have Phytophthora species conserved and optimized this protease to specialize on BAK1? Conversely, can plants evolve or engineer variants of BAK1 resistant to cleavage without compromising immune function? Answering these questions could illuminate the evolutionary pressures shaping plant immune receptors and microbial effectors in their perpetual conflict.
In conclusion, Zhang and colleagues have uncovered a previously unrecognized molecular modus operandi employed by Phytophthora pathogens—the secretion of a conserved apoplastic trypsin-like serine protease that undermines plant immunity by proteolytic cleavage of BAK1. This work not only expands our understanding of apoplastic immune suppression mechanisms but also sets the stage for developing innovative strategies to bolster crop resistance against devastating oomycete diseases. As agriculture confronts rising pathogen pressures under changing climates, such mechanistic insights are invaluable in guiding next-generation plant protection.
The elucidation of PsTry1’s role and specificity exemplifies the power of combining biochemical, genetic, and structural approaches in decoding plant-microbe interactions. It underscores the importance of scrutinizing the apoplast—a frontier often overshadowed by intracellular signaling—in the immune dialogue between plants and pathogens. With these findings, the path is now open toward harnessing molecular precision to fortify crops against pathogens that have long exploited the vulnerabilities of plant cell surface immunity.
Subject of Research: Plant Immunity and Microbial Pathogen Effector Mechanisms
Article Title: A conserved Phytophthora apoplastic trypsin-like serine protease targets the receptor-like kinase BAK1 to dampen plant immunity
Article References:
Zhang, S., Wang, L., Jiang, H. et al. A conserved Phytophthora apoplastic trypsin-like serine protease targets the receptor-like kinase BAK1 to dampen plant immunity. Nat. Plants (2025). https://doi.org/10.1038/s41477-025-02039-0
Image Credits: AI Generated
Tags: BAK1 co-receptor functionevolutionary dynamics of plant immunityimmune signaling cascade in plantsmicrobial molecular patterns recognitionNature Plants research findingsoomycete pathogen strategiespattern-triggered immunity in plantsPhytophthora protease mechanismplant immunity suppressionplant-pathogen interaction researchreceptor-like kinases in plant defenseserine protease in plant defense
