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New Phytol. 2019 Jan;221(2):960-975. doi: 10.1111/nph.15396. Epub 2018 Aug 31.

Salicylic acid activates poplar defense against the biotrophic rust fungus Melampsora larici-populina via increased biosynthesis of catechin and proanthocyanidins.

Author information

1
Department of Biochemistry, Max Planck Institute for Chemical Ecology, Hans-Knöll-Straße 8, 07745, Jena, Germany.
2
School of Forestry and Natural Resources, Department of Genetics, Department of Plant Biology, University of Georgia, Athens, GA, 30602, USA.
3
Key Laboratory of Forest Genetics and Biotechnology, Co-Innovation Center for Sustainable Forestry in Southern China, College of Forestry, Nanjing Forestry University, Nanjing, Jiangsu, 210037, China.
4
Department of Zoology and Entomology, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Private Bag X20, Pretoria, 0028, South Africa.

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.

KEYWORDS:

Populus nigra ; benzothiadiazole (BTH); chemical defense; condensed tannins; drought stress; flavan-3-ols; phytohormones

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