AUTHOR=Ullah Chhana , Unsicker Sybille B. , Reichelt Michael , Gershenzon Jonathan , Hammerbacher Almuth 

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

JOURNAL=Frontiers in Plant Science

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.01441

DOI=10.3389/fpls.2019.01441

ISSN=1664-462X

ABSTRACT=<p>Flavan-3-ols including the monomeric catechin and the polymeric proanthocyanidins (PAs) are abundant phenolic metabolites in poplar (<italic>Populus</italic> 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 (<italic>P. nigra</italic>) stems after infection by a newly described fungal stem pathogen, <italic>Plectosphaerella populi</italic>, which forms canker-like lesions in stems. We showed that flavan-3-ol contents increased in <italic>P. populi</italic>-infected black poplar stems over the course of infection compared to non-infected controls. Transcripts of leucoanthocyanidin reductase (<italic>LAR</italic>) and anthocyanidin reductase (<italic>ANR</italic>) genes involved in the last steps of flavan-3-ol biosynthesis were also upregulated upon fungal infection indicating <italic>de novo</italic> biosynthesis. Amending culture medium with catechin and PAs reduced the mycelial growth of <italic>P. populi</italic>, suggesting that these metabolites act as anti-pathogen defenses in poplar <italic>in vivo</italic>. Among the hormones, salicylic acid (SA) was higher in <italic>P. populi</italic>-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 <italic>P. populi</italic>-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 <italic>P. populi</italic> infection of poplar stems.</p>