Accumulation of Catechin and Proanthocyanidins in Black Poplar Stems After Infection by Plectosphaerella populi : Hormonal Regulation, Biosynthesis and Antifungal Activity.

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.
(Copyright © 2019 Ullah, Unsicker, Reichelt, Gershenzon and Hammerbacher.)