Lack of Cytosolic Glutamine Synthetase1;2 Activity Reduces Nitrogen-Dependent Biosynthesis of Cytokinin Required for Axillary Bud Outgrowth in Rice Seedlings

Abstract

A mutation abolishing cytosolic glutamine synthetase1;2 (GS1;2) activity impairs assimilation of ammonium into glutamine in both roots and basal portions of shoots, and severely decreases axillary bud outgrowth (tillering) in mutant rice seedlings. Although the gs1;2 mutant phenotype is independent of strigolactone, which inhibits tillering, it also demonstrates glutamine- or related metabolite-responsive biosynthesis of cytokinin (CK), which promotes tillering. Here, we examined the connection between GS1;2 and CK biosynthesis during tillering, focusing on basal portions of the shoots as well as apical and axillary bud meristems in the gs1;2 mutant. Despite a sufficient ammonium supply, decreases in precursor CK contents and a decrease in ammonium assimilation into glutamine were observed in basal portions of mutant shoots. Reintroducing expression of OsGS1;2 cDNA driven by its own promoter restored precursor CK contents and ammonium assimilation to wild-type levels. In basal portions of the shoots, glutamine-responsive adenosine phosphate-isopentenyltransferase4 (OsIPT4), which is also predominant in rice roots, was the predominant isogene for IPT, which synthesizes CK. Cell-specific expression of OsIPT4 in phloem companion cells in nodal vascular anastomoses connected to the axillary bud vasculature also decreased in the gs1;2 mutant. Expression of CK-responsive type-A response regulator genes as local indicators of active CKs was also abolished in the axillary bud meristem of the mutant. These results suggest that the lack of GS1;2 activity decreased levels of glutamine or a related metabolite required for CK biosynthesis, causing a deficiency in active CK in the axillary bud meristem necessary for tillering.

Rice nitrate transporter OsNPF7.2 positively regulates tiller number and grain yield

“…4d ). Two of the 18 genes, OsNPF6.5 and OsGS1;2 , were reported to promote tiller bud outgrowth and the increase in rice tiller number (Hu et al 2015 ; Ohashi et al 2017 ). Expression of both OsNPF6.5 and OsGS1;2 was up-regulated in OX lines and down-regulated in Ri lines and mutant osnpf7.2 (Additional file 2 Figure S2).…”

OsCYP71D8L as a key regulator involved in growth and stress response by mediating gibberellins and cytokinins homeostasis in rice

“…Similarly, OsMCA1 is involved in the deactivation of bioactive GA, and its mutation results in GA-deficient phenotypes with shorter root and reduced height (Liu et al 2015). In rice, cytokinin is essential for the control of entire life cycle including root development, shoot meristem activity, vegetative and reproductive branching, spikelets per panicle, and grain production(Ashikari et al, 2005; Du et al, 2017; Gao et al, 2014; Gu et al, 2015; Jiang et al, 2017; Kurakawa et al, 2007; Li et al, 2013; Ohashi et al, 2017; Panda et al, 2018; Wu et al, 2017; Wu et al, 2016; Zhao et al, 2015). For examples, cytokinin receptor OsCKT1 is necessary for the regulation of root development, and Osckt1 exhibits insensitivity to cytokinin treatment with normal root growth (Ding et al, 2017).…”

Transcriptomic and physiological analyses of rice seedlings under different nitrogen supplies provide insight into the regulation involved in axillary bud outgrowth

“…Auxin inhibits axillary bud outgrowth, and mutants defective in auxin biosynthesis and transport display accelerated outgrowth of axillary buds [15,16]. However, cytokinin (CK) promotes the outgrowth of axillary buds, and CK-related rice mutants with increased or decreased CK contents [17][18][19]. In addition, mutants defective in DWARF AND LOW-TILLERING (DLT), whose product functions in the brassinosteroids (BRs) pathway, present relatively few tillers [20].…”