Cytokinins Promote Rust Susceptibility in Poplar by Suppressing Defence Signalling, Reducing Defence Accumulation and Reprogramming Primary Metabolism

ABSTRACT Cytokinins (CKs), classically recognised as plant growth regulators, are increasingly implicated in plant‐microbe interactions. Studies in herbaceous plants suggest that CKs can function as signalling molecules with dual roles in plant defence, either restricting or facilitating pathogen infection. However, their roles in tree‐pathogen interactions remain largely unexplored. Here, we investigated CK induction, metabolism, and functions in black poplar ( Populus nigra ) during infection by the rust fungus Melampsora larici‐populina , an obligate biotrophic pathogen. We found that most isoprenoid CKs accumulated rapidly in poplar leaves after rust inoculation but declined during the fungal proliferation and sporulation stages. In contrast, poplar‐specific aromatic CKs, such as topolins, were induced throughout infection. Exogenous application of the predominant isoprenoid CK trans ‐zeatin ( t Z) and poplar‐specific aromatic CK (topolins) significantly enhanced rust colonisation. Elevated CKs reduced the accumulation of antifungal flavan‐3‐ols and defence‐related hormones, including salicylic acid and jasmonates. Furthermore, increased CK levels, especially ortho ‐topolin, markedly increased free amino acid concentrations, potentially enhancing nutrient availability for the biotrophic rust. Collectively, our results reveal that poplar CKs weaken the tree's chemical defences while enhancing the accumulation of host‐derived nutrients, thereby promoting infection by the rust fungus. These findings reveal a previously unexplored role of CKs in tree‐pathogen interactions.