谷氨酰胺是谷氨酰胺合成酶(GS)和谷氨酸合成酶(GOGAT)催化铵态氮(NH4+)同化的产物。NH4+偏好型水稻的生长依赖于根系NH4+同化和谷氨酰胺的根冠分配;然而，人们对谷氨酰胺在根中的贮藏机制知之甚少。
通过转录组和反向遗传学分析，我们发现在NH4+充足的条件下，水稻(Oryza sativa L.)氨基酸转运体样6 (OsATL6)蛋白向根液泡输出谷氨酰胺。OsATL6与OsGS1;2和OsNADH-GOGAT1在充足的nh4cl喂养的野生型(WT)根中表达，谷氨酰胺处理诱导。我们生成了两个独立的Tos17逆转录转座子插入突变体，显示OsATL6表达减少，以确定OsATL6的功能。与缺乏Tos17插入的分离株相比，OsATL6基因下调的突变株在NH4+充足条件下根系中谷氨酰胺含量较低，但木质部汁液中谷氨酰胺浓度较高，且枝条生长较好。红色荧光蛋白融合单体OsATL6在洋葱表皮细胞的瞬时表达证实了OsATL6的液泡定位。
当OsATL6在非洲爪蟾卵母细胞中表达时，谷氨酰胺从细胞流出到酸性溶液中增加。在充足的NH4+供应下，OsATL6在GS1、2和NADH-GOGAT1积累达到峰值之前，在位于凯氏带附近的厚壁组织和中柱鞘细胞中短暂积累，从而阻塞了外胞质通道，并在WT根的维管薄壁细胞中积累。
这些发现表明，OsATL6暂时储存了过量的谷氨酰胺，由NH4+同化产生，在根液泡可以转运到茎。
Glutamine is a product of ammonium (NH4+) assimilation catalyzed by glutamine synthetase (GS) and glutamate synthase (GOGAT). The growth of NH4+-preferring paddy rice depends on root NH4+ assimilation and subsequent root-to-shoot allocation of glutamine; however, little is known about the mechanism of glutamine storage in roots. Here, using transcriptome and reverse genetics analyses, we show that the rice (Oryza sativa L.) amino acid transporter-like 6 (OsATL6) protein exports glutamine to the root vacuoles under NH4+ replete conditions. OsATL6 was expressed along with OsGS1;2 and OsNADH-GOGAT1 in sufficient NH4Cl-fed wild-type (WT) roots and induced by glutamine treatment. We generated two independent Tos17 retrotransposon insertion mutants showing reduced OsATL6 expression to determine the function of OsATL6. Compared with segregants lacking the Tos17 insertion, OsATL6 knockdown mutant seedlings exhibited lower root glutamine content but higher glutamine concentration in the xylem sap and greater shoot growth under NH4+ replete conditions. The transient expression of monomeric red fluorescent protein-fused OsATL6 in onion epidermal cells confirmed the tonoplast localization of OsATL6. When OsATL6 was expressed in Xenopus laevis oocytes, glutamine efflux from the cell into the acidic bath solution increased. Under sufficient NH4+ supply, OsATL6 transiently accumulated in sclerenchyma and pericycle cells, which are located adjacent to the Casparian strip, thus obstructing the apoplastic solute path, and in vascular parenchyma cells of WT roots before the peak accumulation of GS1;2 and NADH-GOGAT1 occurred. These findings suggest that OsATL6 temporarily stores excess glutamine, produced by NH4+ assimilation, in root vacuoles before it can be translocated to shoot.
https://onlinelibrary.wiley.com/doi/epdf/10.1111/tpj.15403
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