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.
We conclude that SA activates flavan-3-ol biosynthesis in poplar against rust infection.
Description:
Supplementary Material: Fig. S1 Effect of rust infection on the concentrations of the jasmonic acid (JA) precursor, cis‐OPDA, and JA catabolites in black poplar leaves over the course of infection.
Fig. S2 Effect of rust infection on the accumulation of flavan‐3‐ols, abscisic acid and salicylic acid in expanding systemic leaves of black poplar trees.
Fig. S3 Effect of rust infection on the relative expression levels of WRKY23 , WRKY70 and PR5 .
Fig. S4 Phytohormone application before rust infection did not alter the levels of other phenolic metabolites in poplar besides flavan‐3‐ols.
Fig. S5 Jasmonate concentrations in Populus nigra leaves after exogenous phytohormone application followed by rust infection.
Fig. S6 Accumulation of flavan‐3‐ols in black poplar leaves treated with the salicylic acid analog benzothiadiazole.
Fig. S7 Concentrations of jasmonates, epigallocatechin, salicinoids and phenolic acids in poplar (Populus × canadensis Robusta) leaves under mild drought stress and rust infection.
Table S1 Primer sequences used in this study.
Table S2 Levels of phenolic metabolites in black poplar leaves 1 d after exogenous hormone treatment.
Table S3 Levels of endogenous hormones in black poplar leaves 1 d after exogenous hormone treatment.
