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

SUMMARY Salicylic acid (SA) and auxin are key regulators of plant immunity and development. The clubroot pathogen Plasmodiophora brassicae encodes Pb GH 3 , an effector related to the GH 3 family involved in phytohormone homeostasis. Although Pb GH 3 was proposed to conjugate auxin i n vitro , its biological function in planta has remained unclear. This study aimed to determine the in vivo role of Pb GH 3 during host colonization. We generated Arabidopsis thaliana and Brassica napus lines overexpressing Pb GH 3 and characterized their developmental phenotypes, hormone profiles, gene expression, and susceptibility to infection. Structural modeling was performed to assess Pb GH 3 similarity to plant GH 3 proteins, and functional complementation was tested using the Arabidopsis gh3 . 12 mutant. The expression of Pb GH 3 in Arabidopsis induced auxin-related developmental phenotypes without detectable accumulation of auxin conjugates. Instead, Pb GH 3 structurally and functionally resembled GH 3.12 /PBS3 inducing increased conjugated SA levels, reduced jasmonic acid, suppressed PIN 2 expression, and increased root hair number and infection. Pb GH 3 complemented SA-related defects in the gh3 . 12 mutant. Pb GH 3 functions as a modulator of SA metabolism rather than an auxin-conjugating enzyme, likely competing with host GH 3.12 /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. PLAIN LANGUAGE SUMMARY This study shows that the clubroot pathogen uses a protein called Pb GH 3 to modify the plant’s salicylic acid balance. This alters root traits and increases susceptibility to infection. Arabidopsis and canola plants engineered to produce Pb GH 3 showed similar changes, revealing that the pathogen uses this protein to disrupt hormone regulation and create conditions that support its colonization.