Abstract
Inorganic ammonium ions are assimilated by a coupled reaction of glutamine synthetase and glutamate synthase (GOGAT). In rice, three genes encoding either ferredoxin (Fd)-GOGAT, NADH-GOGAT1, or NADH-GOGAT2, have been identified. OsNADH-GOGAT2, a newly identified gene, was expressed mainly in fully expanded leaf blades and leaf sheaths. Although the distinct expression profile to OsNADH-GOGAT1, which is mainly detected in root tips, developing leaf blades, and grains, was shown in our previous studies, physiological role of NADH-GOGAT2 is not yet known. Here, we isolated retrotransposon mediated-knockout mutants lacking OsNADH-GOGAT2. In rice grown under paddy field conditions, disruption of the OsNADH-GOGAT2 gene caused a remarkable decrease in spikelet number per panicle associated with a reductions in yield and whole plant biomass, when compared with wild-type (WT) plants. The total nitrogen contents in the senescing leaf blade of the mutants were approximately a half of the WT plants. Expression of this gene was mainly detected in phloem companion cells and phloem parenchyma cells associated with large vascular bundles in fully expanded leaf blades, when the promoter region fused with a β-glucuronidase gene was introduced into the WT rice. These results suggest that the NADH-GOGAT2 is important in the process of glutamine generation in senescing leaves for the remobilization of leaf nitrogen through phloem to the panicle during natural senescence. These results also indicate that other GOGATs, i.e., NADH-GOGAT1 and ferredoxin-GOGAT are not able to compensate the function of NADH-GOGAT2.
Highlights
In rice (Oryza sativa L.) plants, approximately 80% of the total nitrogen in the panicle arises from remobilization through the phloem from senescing organs (Mae and Ohira, 1981)
It is only known that the OsNADH-GS is coupled with a glutamate synthase (GOGAT) 2 gene is mainly expressed in fully expanded leaf blades and leaf sheaths at vegetative stage (Tabuchi et al, 2007)
We have successively extended our fundamental understanding of the physiological function of the newly identified NADH-GOGAT2 in rice
Summary
In rice (Oryza sativa L.) plants, approximately 80% of the total nitrogen in the panicle arises from remobilization through the phloem from senescing organs (Mae and Ohira, 1981). Synthesis of glutamine is the essential step for nitrogen remobilization in rice, as asparagine is synthesized from glutamine (Ireland and Lea, 1999). NADH-GOGAT is coded for by two genes, OsNADH-GOGAT 1 and OsNADH-GOGAT 2, while one gene codes for Fd-GOGAT (Tabuchi et al, 2007). The major function of GS2 and FdGOGAT is in photorespiratory nitrogen metabolisms (Lea and Miflin, 2003). The three GS1 and two NADH-GOGAT species appear to be more important for the normal growth and development (Yamaya and Oaks, 2004), since a mutant lacking either gene for GS2 or Fd-GOGAT was able to grow normally under non-photorespiratory conditions (Kendall et al, 1986; Wallsgrove et al, 1987)
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