In plants, L-serine (Ser) biosynthesis occurs through various pathways and is highly dependent on the atmospheric CO₂ concentration, especially in C₃ species, due to the association of the Glycolate Pathway of Ser Biosynthesis (GPSB) with photorespiration. Characterization of a second plant Ser pathway, the Phosphorylated Pathway of Ser Biosynthesis (PPSB), revealed that it is at the crossroads of carbon, nitrogen, and sulphur metabolism. The PPSB comprises three sequential reactions catalysed by 3-phosphoglycerate dehydrogenase (PGDH), 3-phosphoSer aminotransferase (PSAT) and 3-phosphoSer phosphatase (PSP). PPSB was overexpressed in plants exhibiting two different modes of photosynthesis: Arabidopsis (C₃ metabolism), and maize (C₄ metabolism), under ambient (aCO₂) and elevated (eCO₂) CO₂ growth conditions. Overexpression in Arabidopsis of the PGDH1 gene alone or PGDH1, PSAT1 and PSP1 in combination increased the Ser levels but also the essential amino acids threonine (aCO₂), isoleucine, leucine, lysine, phenylalanine, threonine and methionine (eCO₂) compared to the wild-type. These increases translated into higher protein levels. Likewise, starch levels were also increased in the PPSB-overexpressing lines. In maize, PPSB-deficient lines were obtained by targeting PSP1 using Cas9 endonuclease. We concluded that the expression of PPSB in maize male gametophyte is required for viable pollen development. Maize lines overexpressing the AtPGDH1 gene only displayed higher protein levels but not starch at both aCO₂ and eCO₂ conditions, this translated into a significant rise in the nitrogen/carbon ratio. These results suggest that metabolic engineering of PPSB in crops could enhance nitrogen content, particularly under upcoming eCO₂ conditions where the activity of GPSB is limited.

中文翻译：

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丝氨酸/甘氨酸网络的代谢工程作为提高作物氮含量的手段


在植物中，L-丝氨酸 （Ser） 生物合成通过各种途径发生，并且高度依赖于大气中的 CO₂ 浓度，尤其是在 C₃ 物种中，因为 Ser 生物合成的乙醇酸盐途径 （GPSB） 与光呼吸有关。第二种植物 Ser 途径，即 Ser 生物合成磷酸化途径 （PPSB） 的表征表明，它处于碳、氮和硫代谢的十字路口。PPSB 包括由 3-磷酸甘油酸脱氢酶 （PGDH）、3-磷酸 Ser 氨基转移酶 （PSAT） 和 3-磷酸化 Ser 磷酸酶 （PSP） 催化的三个连续反应。PPSB 在表现出两种不同光合作用模式的植物中过表达：拟南芥 （C₃ 代谢） 和玉米 （C₄ 代谢），在环境 （aCO₂） 和高海拔 （eCO₂） CO₂ 生长条件下。与野生型相比，PGDH1 基因单独或 PGDH1、PSAT1 和 PSP1 在拟南芥中单独或 PGDH1、PSAT1 和 PSP1 的组合过表达增加了 Ser 水平，但也增加了必需氨基酸苏氨酸 （aCO₂）、异亮氨酸、亮氨酸、赖氨酸、苯丙氨酸、苏氨酸和蛋氨酸 （eCO₂）。这些增加转化为更高的蛋白质水平。同样，PPSB 过表达品系中的淀粉水平也增加。在玉米中，通过使用 Cas9 核酸内切酶靶向 PSP1 获得 PPSB 缺陷系。我们得出结论，PPSB 在玉米雄配子体中的表达是活花粉发育所必需的。 过表达 AtPGDH1 基因的玉米品系在 aCO₂ 和 eCO₂ 条件下仅表现出较高的蛋白质水平，而淀粉水平则没有，这转化为氮/碳比的显着增加。这些结果表明，作物中 PPSB 的代谢工程可以提高氮含量，尤其是在即将到来的 GPSB 活性有限的 eCO₂ 条件下。