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. We conclude that SA activates
flavan-3-ol biosynthesis in poplar against rust
infection.