Yabuki Yui, Ohashi Miwa, Imagawa Fumi, Ishiyama Keiki, Beier Marcel Pascal, Konishi Noriyuki, Umetsu-Ohashi Toshiko, Hayakawa Toshihiko, Yamaya Tomoyuki, Kojima Soichi
Graduate School of Agricultural Science, Tohoku University, 468-1 Aoba, Aramaki, Sendai, 9800845, Japan.
Rice (N Y). 2017 Dec;10(1):3. doi: 10.1186/s12284-017-0143-8. Epub 2017 Jan 25.
Asparagine is one of the most dominant organic nitrogen compounds in phloem and xylem sap in a wide range of plant species. Asparaginase (ASNase; EC, 3.5.1.1) catabolizes asparagine into aspartate and ammonium; therefore, it is suggested to play a key role in asparagine metabolism within legume sink organs. However, the metabolic fate of asparagine in source and sink organs during rice seed production remains to be elucidated. Therefore, the main objective of this study is to investigate the asparagine metabolism in a temporal and spatial manner during rice seed production.
For this purpose, the expression of genes involved in asparagine catabolism, such as asparaginase1 (OsASNase1) and 2 (OsASNase2), were quantitatively measured, and contents of asparagine, aspartate and ammonium ions were determined in sink and source organs during spikelet ripening. Quantitative real-time PCR and in situ localization studies determined that OsASNase2 is expressed in the dorsal vascular bundles and nucellar projection of developing grains, as well as in mesophyll and phloem companion cells of senescent flag leaves. Amino acid measurements revealed that the aspartate concentration is higher than asparagine in both source and sink organs.
This work suggests that asparaginase dependent asparagine catabolism occurred not only in sink but also in source organs.
天冬酰胺是多种植物韧皮部和木质部汁液中最主要的有机氮化合物之一。天冬酰胺酶（ASNase；EC 3.5.1.1）将天冬酰胺分解为天冬氨酸和铵；因此，它被认为在豆科植物库器官的天冬酰胺代谢中起关键作用。然而，水稻种子生产过程中源器官和库器官中天冬酰胺的代谢命运仍有待阐明。因此，本研究的主要目的是以时间和空间的方式研究水稻种子生产过程中的天冬酰胺代谢。
为此，定量测定了参与天冬酰胺分解代谢的基因（如天冬酰胺酶1（OsASNase1）和2（OsASNase2））的表达，并在小穗成熟过程中测定了库器官和源器官中天冬酰胺、天冬氨酸和铵离子的含量。定量实时PCR和原位定位研究确定，OsASNase2在发育中籽粒的背侧维管束和珠心突起中表达，也在衰老旗叶的叶肉和韧皮部伴胞中表达。氨基酸测量结果显示，源器官和库器官中的天冬氨酸浓度均高于天冬酰胺。
这项研究表明，依赖天冬酰胺酶的天冬酰胺分解代谢不仅发生在库器官中，也发生在源器官中。
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