Laboratory of Plant Biotechnology, Université libre de Bruxelles, 12 rue des Profs Jeener et Brachet, Gosselies 6041, Belgium.
Biotechnology of Natural Products, Technische Universität München, 85354 Freising, Germany.
Tree Physiol. 2022 May 9;42(5):1084-1099. doi: 10.1093/treephys/tpab161.
Reactive species (RS) causing oxidative stress are unavoidable by-products of various plant metabolic processes, such as photosynthesis, respiration or photorespiration. In leaves, flavonoids scavenge RS 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 with 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. The 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 not consistently evidenced with either 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.
活性氧（RS）是各种植物代谢过程（如光合作用、呼吸作用或光呼吸）不可避免的副产物。在叶片中，类黄酮清除光合作用过程中产生的 RS，保护植物细胞免受有害的氧化损伤。因此，它们的生物合成和积累在细胞水平受到严格的调控。糖基化已成为各种特殊代谢物（如类黄酮）内稳态的重要生化反应。本文对编码 UDP-糖基转移酶的 Populus tremula x P. alba（杨树）UGT72A2 进行了功能表征，该酶定位于叶绿体中。与野生型相比，UGT72A2 表达降低的转基因杨树系表现出生长和叶片氧化损伤减少的特征，表现在坏死、谷胱甘肽和脂质过氧化产物含量增加、可溶性过氧化物酶活性和 NADPH/NADP+ 比值降低等方面。它们还表现出较低的酚类、花青素和总类黄酮含量，但较高的原花青素含量。启动子分析显示存在参与光形态建成、叶绿体生物发生和类黄酮生物合成的顺式元件。UGT72A2 受杨树 MYB119 的调控，该转录因子已知可调节类黄酮生物合成途径。系统发育分析和分子对接表明 UGT72A2 可对类黄酮进行糖基化；然而，无论是在体外实验中，还是在过表达或下调 UGT72A2 的转基因杨树叶片中分析糖基化产物，都没有一致地证明实际的底物。本文提供了一些元素，强调了在杨树叶片中，类黄酮糖基化对抵抗氧化应激的重要性，并提出了关于该生化反应与氧化还原稳态系统调节之间联系的新问题。
