Abstract:
Flavan-3-ols including the monomeric catechin and the polymeric proanthocyanidins
(PAs) are abundant phenolic metabolites in poplar (Populus spp.) previously described
to protect leaves against pathogen infection. However, it is not known whether stems
are also defended in this way. Here we investigated flavan-3-ol accumulation, activity,
and the regulation of formation in black poplar (P. nigra) stems after infection by a newly
described fungal stem pathogen, Plectosphaerella populi, which forms canker-like
lesions in stems. We showed that flavan-3-ol contents increased in P. populi-infected
black poplar stems over the course of infection compared to non-infected controls.
Transcripts of leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR)
genes involved in the last steps of flavan-3-ol biosynthesis were also upregulated upon
fungal infection indicating de novo biosynthesis. Amending culture medium with catechin
and PAs reduced the mycelial growth of P. populi, suggesting that these metabolites act
as anti-pathogen defenses in poplar in vivo. Among the hormones, salicylic acid (SA) was
higher in P. populi-infected tissues compared to the non-infected controls over the course
of infection studied, while jasmonic acid (JA) and JA-isoleucine (JA-Ile) levels were higher
than controls only at the early stages of infection. Interestingly, cytokinins (CKs) were
also upregulated in P. populi-infected stems. Poplar saplings treated with CK showed
decreased levels of flavan-3-ols and SA in stems suggesting a negative association
between CK and flavan-3-ol accumulation. Taken together, the sustained upregulation of
SA in correlation with catechin and PA accumulation suggests that this is the dominant
hormone inducing the formation of antifungal flavan-3-ols during P. populi infection of
poplar stems.
