Abstract
BackgroundOur previous transcriptomic analysis revealed that downregulation of nitrogen and carbon metabolism in the basal portions of the shoots inhibited cytosolic glutamine synthetase1;2 (GS1;2), which severely reduced rice tiller number. In the present study, we used rice mutants lacking GS1;2 (gs1;2 mutants) to determine the contribution of carbon metabolism to tiller growth.ResultsMetabolomic analysis indicated the effects of carbon metabolism disorder such as reductions in the levels of sugar metabolites (e.g., sucrose and glucose 6-phosphate) in the shoot basal portions of the gs1;2 mutant seedlings. Decrease in sucrose caused by the lack of GS1;2 was successfully restored to the wild-type levels by introducing OsGS1;2 cDNA into the mutants. In the basal portions of the shoots, the lack of GS1;2 caused low expression of cytosolic fructose 1,6-bisphosphatase2 (OscFBP2), which is a key cytosolic sucrose synthesis enzyme; it is especially important in the phloem companion cells of the nodal vascular anastomoses. NH4+ supply upregulated OscFBP2 expression in the shoot basal portions of the wild type but not in those of the gs1;2 mutants. Rice mutants lacking cFBPase2 presented with ~ 30% reduction in total cFBPase activity in the basal portions of their shoots. These mutants displayed reductions in sucrose levels of the basal portions of their shoots but not in their leaf blades. They also had relatively lower tiller numbers at the early growth stage.ConclusionsMetabolomic analysis revealed that the lack of GS1;2 reduced sucrose metabolism in the basal portions of the shoots. Our results indicated that sucrose reduction was caused by the downregulation of OscFBP2 expression in the basal portions of the gs1;2 mutant shoots. The reduction in sucrose content caused by the lack of cFBPase2 resulted in lower tiller number at the early growth stage. Therefore, adequate sucrose supply via cFBPase2 may be necessary for tiller growth in the basal portions of rice shoots.
Highlights
IntroductionRice plant tiller number is a critical agronomic trait
There were no significant changes in the levels of maltose, glucose, fructose, trehalose, fructose-6-phosphate, or other sugars (Fig. 1)
As reported previously (Ohashi et al 2017), reductions in glutamine and asparagine were observed in the basal portions of the gs1;2 mutant shoots (Fig. 1)
Summary
Rice plant tiller number is a critical agronomic trait. It defines grain yield and is influenced by nitrogen and carbon availability (Mae 1997; Sakamoto and Matsuoka 2008; Liu et al 2011). The axillary buds of the tillers emerge from the basal portions of the shoots. Inorganic NH4+ is the major nitrogen source in paddy fields. It is assimilated by the coupled reactions of glutamine synthetase (GS) (which catalyzes an ATP-dependent conversion of glutamate to glutamine) and glutamate synthase (Yamaya and Kusano 2014). The evidence from previous study showed that the three GS1 isoenzymes (GS1;1, GS1;2 and GS1;3) in rice have distinct functions (Yamaya and Kusano 2014).
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