A clubroot pathogen PBS3-like effector manipulates hormonal crosstalk to alter root morphology during colonization

O_LISalicylic acid (SA) and auxin are key regulators of plant immunity and development. The clubroot pathogen Plasmodiophora brassicae encodes PbGH3, an effector related to the GH3 family involved in phytohormone homeostasis. Although PbGH3 was proposed to conjugate auxin in vitro, its biological function in planta has remained unclear. This study aimed to determine the in vivo role of PbGH3 during host colonization. C_LIO_LIWe generated Arabidopsis thaliana and Brassica napus lines overexpressing PbGH3 and characterized their developmental phenotypes, hormone profiles, gene expression, and susceptibility to infection. Structural modeling was performed to assess PbGH3 similarity to plant GH3 proteins, and functional complementation was tested using the Arabidopsis gh3.12 mutant. C_LIO_LIThe expression of PbGH3 in Arabidopsis induced auxin-related developmental phenotypes without detectable accumulation of auxin conjugates. Instead, PbGH3 structurally and functionally resembled GH3.12/PBS3 inducing increased conjugated SA levels, reduced jasmonic acid, suppressed PIN2 expression, and increased root hair number and infection. PbGH3 complemented SA-related defects in the gh3.12 mutant. C_LIO_LIPbGH3 functions as a modulator of SA metabolism rather than an auxin-conjugating enzyme, likely competing with host GH3.12/PBS3 to constrain effective SA accumulation. This reveals a novel strategy by which P. brassicae disrupts SA-auxin homeostasis to promote host colonization and ensure disease development. C_LI PLAIN LANGUAGE SUMMARYThis study shows that the clubroot pathogen uses a protein called PbGH3 to modify the plants salicylic acid balance. This alters root traits and increases susceptibility to infection. Arabidopsis and canola plants engineered to produce PbGH3 showed similar changes, revealing that the pathogen uses this protein to disrupt hormone regulation and create conditions that support its colonization.