A temporal and spatial contribution of asparaginase to asparagine catabolism during development of rice grains

Yui Yabuki,Marcel Pascal Beier,Toshiko Umetsu-Ohashi,Toshihiko Hayakawa,Keiki Ishiyama,Miwa Ohashi,Soichi Kojima,Noriyuki Konishi,Tomoyuki Yamaya,Fumi Imagawa

doi:10.1186/s12284-017-0143-8

Abstract

BackgroundAsparagine 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.ResultsFor 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.ConclusionThis work suggests that asparaginase dependent asparagine catabolism occurred not only in sink but also in source organs.

Highlights

Asparagine is one of the most dominant organic nitrogen compounds in phloem and xylem sap in a wide range of plant species
Food production is dependent on nitrogen translocation through the phloem in plants, since most grain nitrogen is remobilized from source organs during senescence in crops (Szpak 2014)
Asn and Asp concentration became almost equal at 35 days after flowering (DAF)

Summary

Introduction

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; it is suggested to play a key role in asparagine metabolism within legume sink organs. The metabolic fate of asparagine in source and sink organs during rice seed production remains to be elucidated. Food production is dependent on nitrogen translocation through the phloem in plants, since most grain nitrogen is remobilized from source organs during senescence in crops (Szpak 2014). Asn is one of the most dominant organic nitrogen compounds for translocation, as well as Gln, in many plant species (Lea and Ireland 1999; Lea et al 2007). Genes encoding ASNase, has been cloned from several plant species like Lupinus arboreus (Lough et al 1992), Lupinus angustifolius (Dickson et al 1992), Arabidopsis, thaliana (Hejazi et al 2002), Lupinus luteus (Borek et al 2004), scot pine (Canas et al 2007), soybean (Cho et al 2007), and more recently Lotus japonicas (Credali et al 2011)

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