Metabolism and Functions of 4-Aminobutyrate (GABA) in Plants: Regulation of Glutamate Decarboxylases

非蛋白氨基酸4-氨基丁酸（GABA）作为一种重要的代谢物和信号在植物生命中具有多种多样的作用。GABA的初级生产与多种谷氨酸脱羧酶（GAD）有关。在非生物和生物胁迫下，碳通过GABA代谢途径（即GABA分流）的通量通常会增加，但各种GAD异构体的贡献尚不清楚。大多数证据表明，植物GADs是细胞质的，与细菌和哺乳动物的GADs不同，它们受Ca2+/钙调蛋白以及ph的调节。因此，应激诱导的细胞质H+或Ca2+的增加可以引起植物GABA的积累。相比之下，哺乳动物GAD65/67可以被磷酸化，位于不同的亚细胞位置（细胞质或膜），并对磷酸化做出相反的反应。例如，GAD65中苏氨酸95的磷酸化刺激活性，而其n端区域多个丝氨酸残基的磷酸化被认为可以调节GAD65在突触囊泡膜中功能的未知方面。值得注意的是，拟南芥GAD1/4在其N端结构域具有相同的5个丝氨酸残基，而AtGAD1/2/4具有苏氨酸93-94双链；这些残基的磷酸化可能潜在地改变它们的位置/活性，并且膜结合可能促进GABA和GABA转运体/GABA活化阴离子通道之间的相互作用。最近对玉米幼苗和拟南芥悬浮细胞的研究表明，盐度和无机磷（Pi）分别增加了单个GADs n端区域多个丝氨酸残基的磷酸化。此外，我们的研究表明，盐可以快速诱导拟南芥中AtGAD4的表达，而不是AtGAD1和AtGAD2，并且在此之前或伴随着几种转录因子的表达。本研究将比较盐度和Pi补给条件下GAD磷酸化的作用，并探讨候选转录因子在GAD4诱导中的潜在作用。

The non-proteinogenic amino acid 4-aminobutyrate (GABA) is being revealed as an important metabolite and signal with multiple and diverse roles in plant life. The primary production of GABA is associated with multiple glutamate decarboxylases (GAD). The flux of carbon through the GABA metabolic pathway (i.e., the GABA shunt) generally increases under abiotic and biotic stress, but the contributions of the various GAD isoforms remain unclear. Most evidence suggests that plant GADs are cytosolic, and unlike bacterial and mammalian GADs, they are regulated by Ca2+/calmodulin, as well as pH. Consequently, stress-induced increases in cytosolic H+ or Ca2+ can cause plant GABA to accumulate. In contrast, mammalian GAD65/67 can be phosphorylated, reside in different subcellular locations (cytosol vs membrane), and respond oppositely to phosphorylation. For example, phosphorylation of threonine95 in GAD65 stimulates activity, whereas phosphorylation of multiple serine residues in its N-terminal region is proposed to regulate unknown aspects of GAD65 function in the synaptic vesicle membrane. Notably, Arabidopsis thaliana GAD1/4 possess the same five Ser residues in their N terminal domain, and AtGAD1/2/4 possess a threonine93-94 doublet; phosphorylation of these residues could potentially modify their location/activity, and membrane association could facilitate interaction between GABA and GABA transporters/GABA-activated anion channels. Recent research with maize seedlings and Arabidopsis suspension cells demonstrated that salinity and inorganic phosphorus (Pi) supply, respectively, increases the phosphorylation of multiple serine residues in the N-terminal region of the single GADs under consideration. Furthermore, our research showed that expression of AtGAD4, but not AtGAD1 and AtGAD2, is rapidly induced in Arabidopsis by salinity, and this is preceded or accompanied by expression of several transcription factors. The proposed research will compare the seemingly contrasting roles of GAD phosphorylation under salinity versus Pi resupply, and investigate the potential involvement of candidate transcription factors in GAD4 induction.