Abstract
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 Populus alba var. pyramidalis, PalbHLH1 and PalMYB90, 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, PalbHLH1 and PalMYB90 overexpression in poplar enhanced antioxidase activities and H2O2 release and also increased resistance to Botrytis cinerea and Dothiorella gregaria infection. Gene expression profile analysis showed most genes involved in the flavonoid biosynthesis pathway or antioxidant response to be upregulated in MYB90/bHLH1-OE poplar, but significant differential expression occurred in response to pathogen infection. Specifically, expression of PalF3H (flavanone 3-hydroxylase), PalDFR (dihydroflavonol 4-seductase), PalANS (anthocyanin synthase), and PalANR (anthocyanin reductase), which function in initial, middle, and final steps of anthocyanin and PA biosynthesis, respectively, was significantly upregulated in D. gregaria-infected MYB90/bHLH1-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.
Highlights
The Populus species comprises some of the most valuable woody plants in the world due to their rapid growth, easy propagation, adaptation, and wide application in forestation (Polle et al, 2013)
To investigate the potential mechanisms of the transcription factors PalbHLH1 and PalMYB90 involved in the poplar response to pathogens, we cloned the PalbHLH1 and PalMYB90 genes from the P. alba var. pyramidalis genome (Ma et al, 2018) (Table S1 in Datasheet 1) and aligned them with homologous genes from P. euphratica (Ma et al, 2013), P. trichocarpa (Tuskan et al, 2006), and A. majus
The flavonoid content in MYB90/bHLH1-OE5 was 0.2- to 0.5-fold higher than that in MYB90/bHLH1-OE2. These results indicate that overexpression of PalbHLH1 and PalMYB90 enhances the accumulation of secondary metabolites in the flavonoid pathway
Summary
The Populus species comprises some of the most valuable woody plants in the world due to their rapid growth, easy propagation, adaptation, and wide application in forestation (Polle et al, 2013). Secondary metabolic products play significant roles in plant resistance to pathogens Among these metabolites, flavonoids, such as anthocyanins and condensed tannins ( called proanthocyanidins, PAs), accumulate in the seed coat, leaf, stem, bark, and root in response to biotic (Lorenc-Kukuła et al, 2005; Zhang et al, 2013) and abiotic (Dixon et al, 2005; Paolocci et al, 2007) stresses. To uncover the resistance mechanism of flavonoids, their biosynthetic pathways and regulation should be clarified
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