The high accumulation of phloridzin makes apple (Malus domestica) unique in the plant kingdom, which suggests a vital role of its biosynthesis in physiological processes. In our previous study, silencing MdUGT88F1 (a key UDP-GLUCOSE: PHLORETIN 2’-O-GLUCOSYLTRANSFERASE gene) revealed the importance of phloridzin biosynthesis in apple development and Valsa canker resistance. Here, results from MdUGT88F1-silenced lines showed that phloridzin biosynthesis was indispensable for normal chloroplast development and photosynthetic carbon fixation by maintaining MdGLK1/2 (GOLDEN2-like1/2) expression. Interestingly, increased phloridzin biosynthesis did not affect plant (or chloroplast) development, but reduced nitrogen accumulation, leading to chlorophyll deficiency, light sensitivity, and sugar accumulation in MdUGT88F1-overexpressing apple lines. Further analysis revealed that MdUGT88F1-mediated phloridzin biosynthesis negatively regulated the cytosolic glutamine synthetase1-asparagine synthetase-asparaginase (GS1-AS-ASPG) pathway of ammonium assimilation and limited chlorophyll synthesis in apple shoots. The interference of phloridzin biosynthesis in the GS1-AS-ASPG pathway was also assumed to be associated with its limitation of the carbon skeleton of ammonium assimilation through metabolic competition with the tricarboxylic acid cycle. Taken together, our findings shed light on the role of MdUGT88F1-mediated phloridzin biosynthesis in the coordination between carbon and nitrogen accumulation in apple trees.

中文翻译：

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MdUGT88F1介导的根皮苷生物合成协调苹果中的碳和氮积累

根皮苷的高积累使苹果 (Malus domestica) 在植物界独树一帜，这表明其生物合成在生理过程中的重要作用。在我们之前的研究中，沉默 MdUGT88F1（一个关键的 UDP-GLUCOSE：PHLORETIN 2'-O-GLUCOSYLTRANSFERASE 基因）揭示了根皮苷生物合成在苹果发育和 Valsa 溃疡病抗性中的重要性。在这里，来自 MdUGT88F1 沉默品系的结果表明，通过维持 MdGLK1/2 (GOLDEN2-like1/2) 表达，根皮苷生物合成对于正常的叶绿体发育和光合碳固定是必不可少的。有趣的是，增加的根皮苷生物合成不会影响植物（或叶绿体）的发育，但会减少氮的积累，导致 MdUGT88F1 过表达苹果系中的叶绿素缺乏、光敏感性和糖积累。进一步的分析表明，MdUGT88F1 介导的根皮苷生物合成负调控了苹果芽中铵同化和限制叶绿素合成的胞质谷氨酰胺合成酶 1-天冬酰胺合成酶-天冬酰胺酶 (GS1-AS-ASPG) 途径。GS1-AS-ASPG 途径中根皮苷生物合成的干扰也被认为与其通过与三羧酸循环的代谢竞争限制铵同化的碳骨架有关。总之，我们的研究结果揭示了 MdUGT88F1 介导的根皮苷生物合成在苹果树碳和氮积累之间的协调中的作用。