AUTHOR=Bai Qiuxian , Duan Bingbing , Ma Jianchao , Fen Yannan , Sun Shujiao , Long Qiming , Lv Jiaojiao , Wan Dongshi 

TITLE=Coexpression of PalbHLH1 and PalMYB90 Genes From Populus alba Enhances Pathogen Resistance in Poplar by Increasing the Flavonoid Content

JOURNAL=Frontiers in Plant Science

VOLUME=10

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.01772

DOI=10.3389/fpls.2019.01772

ISSN=1664-462X

ABSTRACT=<p>Secondary metabolites of the flavonoid pathway participate in plant defense, and bHLH and MYB transcription factors regulate the synthesis of these metabolites. Here, we define the regulatory mechanisms in response to pathogens. Two transcription factors from <italic>Populus alba</italic> var. <italic>pyramidalis</italic>, <italic>PalbHLH1</italic> and <italic>PalMYB90</italic>, were overexpressed together in poplar, and transcriptome analysis revealed differences in response to pathogen infection. The transgenic plants showed elevated levels of several key flavonoid pathway components: total phenols, proanthocyanidins (PAs), and anthocyanins and intermediates quercetin and kaempferol. Furthermore, <italic>PalbHLH1</italic> and <italic>PalMYB90</italic> overexpression in poplar enhanced antioxidase activities and H<sub>2</sub>O<sub>2</sub> release and also increased resistance to <italic>Botrytis cinerea</italic> and <italic>Dothiorella gregaria</italic> infection. Gene expression profile analysis showed most genes involved in the flavonoid biosynthesis pathway or antioxidant response to be upregulated in <italic>MYB90</italic>/<italic>bHLH1</italic>-OE poplar, but significant differential expression occurred in response to pathogen infection. Specifically, expression of <italic>PalF3H</italic> (flavanone 3-hydroxylase), <italic>PalDFR</italic> (dihydroflavonol 4-seductase), <italic>PalANS</italic> (anthocyanin synthase), and <italic>PalANR</italic> (anthocyanin reductase), which function in initial, middle, and final steps of anthocyanin and PA biosynthesis, respectively, was significantly upregulated in <italic>D. gregaria</italic>-infected <italic>MYB90</italic>/<italic>bHLH1</italic>-OE poplar. Our results highlight that PalbHLH1 and PalMYB90 function as transcriptional activators of flavonoid pathway secondary-metabolite synthesis genes, with differential mechanisms in response to bacterial or fungal infection.</p>