Leaf necrosis resulting from down-regulation of poplar glycosyltransferase UGT72A2.
Marc BehrNathanael SpeeckaertElisabeth KurzeOriane MorelMartine PrévostAdeline MolNassirou Mahamadou AdamouMoussa BaragéJenny RenautWilfried SchwabMondher El JaziriMarie BaucherPublished in: Tree physiology (2021)
Reactive species causing oxidative stress are unavoidable by-products of various plant metabolic processes, such as photosynthesis, respiration or photorespiration. In leaves, flavonoids scavenge reactive species produced during photosynthesis and protect plant cells against deleterious oxidative damages. Their biosynthesis and accumulation are therefore under tight regulation at the cellular level. Glycosylation has emerged as an essential biochemical reaction in the homeostasis of various specialized metabolites such as flavonoids. This article provides a functional characterization of the Populus tremula x P. alba (poplar) UGT72A2 coding for a UDP-glycosyltransferase that is localized in the chloroplasts. Compared to the wild type, transgenic poplar lines with decreased expression of UGT72A2 are characterized by reduced growth and oxidative damages in leaves, as evidenced by necrosis, higher content of glutathione and lipid peroxidation products as well as diminished soluble peroxidase activity and NADPH to NADP+ ratio under standard growing conditions. They furthermore display lower pools of phenolics, anthocyanins and total flavonoids but higher proanthocyanidins content. Promoter analysis revealed the presence of cis-elements involved in photomorphogenesis, chloroplast biogenesis and flavonoid biosynthesis. UGT72A2 is regulated by the poplar MYB119, a transcription factor known to regulate the flavonoid biosynthesis pathway. Phylogenetic analysis and molecular docking suggest that UGT72A2 could glycosylate flavonoids, however the actual substrate(s) was(ere) not consistently evidenced with in vitro assays nor analyses of glycosylated products in leaves of transgenic poplar overexpressing or downregulated for UGT72A2. This article provides elements highlighting the importance of flavonoid glycosylation regarding protection against oxidative stress in poplar leaves and raises new questions about the link between this biochemical reaction and regulation of the redox homeostasis system.
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