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Abstract

Poplar trees synthesize flavan-3-ols (catechin and proanthocyanidins) as a defense against
foliar rust fungi, but the regulation of this defense response is poorly understood. Here, we
investigated the role of hormones in regulating flavan-3-ol accumulation in poplar during rust
infection.
We profiled levels of defense hormones, signaling genes, and flavan-3-ol metabolites in
black poplar leaves at different stages of rust infection. Hormone levels were manipulated by
external sprays, genetic engineering, and drought to reveal their role in rust fungal defenses.
Levels of salicylic acid (SA), jasmonic acid, and abscisic acid increased in rust-infected leaves
and activated downstream signaling, with SA levels correlating closely with those of flavan-3-
ols. Pretreatment with the SA analog benzothiadiazole increased flavan-3-ol accumulation by
activating the MYB–bHLH–WD40 complex and reduced rust proliferation. Furthermore,
transgenic poplar lines overproducing SA exhibited higher amounts of flavan-3-ols constitutively
via the same transcriptional activation mechanism. These findings suggest a strong association
among SA, flavan-3-ol biosynthesis, and rust resistance in poplars. Abscisic acid also
promoted poplar defense against rust infection, but likely through stomatal immunity independent
of flavan-3-ols. Jasmonic acid did not confer any apparent defense responses to the
fungal pathogen.