Departamento de Bioquímica Vegetal y Biología Molecular, Facultad de Química, C/ Profesor García González, 1, 41012, Sevilla, Spain.
BMC Genomics. 2017 Oct 12;18(1):781. doi: 10.1186/s12864-017-4200-x.
Asparagine is a very important nitrogen transport and storage compound in plants due to its high nitrogen/carbon ratio and stability. Asparagine intracellular concentration depends on a balance between asparagine biosynthesis and degradation. The main enzymes involved in asparagine metabolism are asparagine synthetase (ASN), asparaginase (NSE) and serine-glyoxylate aminotransferase (SGAT). The study of the genes encoding for these enzymes in the model legume Lotus japonicus is of particular interest since it has been proposed that asparagine is the principal molecule used to transport reduced nitrogen within the plant in most temperate legumes.
A differential expression of genes encoding for several enzymes involved in asparagine metabolism was detected in L. japonicus. ASN is encoded by three genes, LjASN1 was the most highly expressed in mature leaves while LjASN2 expression was negligible and LjASN3 showed a low expression in this organ, suggesting that LjASN1 is the main gene responsible for asparagine synthesis in mature leaves. In young leaves, LjASN3 was the only ASN gene expressed although at low levels, while all the three genes encoding for NSE were highly expressed, especially LjNSE1. In nodules, LjASN2 and LjNSE2 were the most highly expressed genes, suggesting an important role for these genes in this organ. Several lines of evidence support the connection between asparagine metabolic genes and photorespiration in L. japonicus: a) a mutant plant deficient in LjNSE1 showed a dramatic decrease in the expression of the two genes encoding for SGAT; b) expression of the genes involved in asparagine metabolism is altered in a photorespiratory mutant lacking plastidic glutamine synthetase; c) a clustering analysis indicated a similar pattern of expression among several genes involved in photorespiratory and asparagine metabolism, indicating a clear link between LjASN1 and LjSGAT genes and photorespiration.
The results obtained in this paper indicate the existence of a differential expression of asparagine metabolic genes in L. japonicus and point out the crucial relevance of particular genes in different organs. Moreover, the data presented establish clear links between asparagine and photorespiratory metabolic genes in this plant.
由于其高氮/碳比和稳定性，天冬酰胺是植物中非常重要的氮运输和储存化合物。天冬酰胺细胞内浓度取决于天冬酰胺生物合成和降解之间的平衡。参与天冬酰胺代谢的主要酶是天冬酰胺合成酶（ASN）、天冬酰胺酶（NSE）和丝氨酸-乙醛酸氨基转移酶（SGAT）。研究模式豆科植物百脉根中这些酶的编码基因特别有趣，因为有人提出，在大多数温带豆科植物中，天冬酰胺是植物体内运输还原氮的主要分子。
在百脉根中检测到参与天冬酰胺代谢的几种酶的基因表达差异。ASN 由三个基因编码，LjASN1 在成熟叶片中表达量最高，而 LjASN2 的表达可忽略不计，LjASN3 在该器官中的表达水平较低，表明 LjASN1 是成熟叶片中天冬酰胺合成的主要基因。在幼叶中，只有 LjASN3 表达 ASN 基因，尽管表达水平较低，而编码 NSE 的三个基因都高度表达，尤其是 LjNSE1。在根瘤中，LjASN2 和 LjNSE2 是表达量最高的基因，表明这些基因在该器官中具有重要作用。有几条证据支持百脉根中天冬酰胺代谢基因与光呼吸之间的联系：a）LjNSE1 缺失突变体植物的 SGAT 两个编码基因表达显著下降；b）缺乏质体谷氨酰胺合酶的光呼吸突变体中天冬酰胺代谢相关基因的表达发生改变；c）聚类分析表明，参与光呼吸和天冬酰胺代谢的几个基因的表达模式相似，表明 LjASN1 和 LjSGAT 基因与光呼吸之间存在明显联系。
本文的研究结果表明，百脉根中天冬酰胺代谢基因存在差异表达，并指出了特定基因在不同器官中的关键相关性。此外，所提供的数据在该植物中建立了天冬酰胺和光呼吸代谢基因之间的明确联系。
