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

非蛋白质氨基酸4-氨基丁酸（GABA）是一种重要的代谢产物和信号物质，在植物生命中具有多种多样的作用。GABA的初级产生与多种谷氨酸脱羧酶（GAD）相关。通过GABA代谢途径的碳通量（即，GABA分流）通常在非生物和生物胁迫下增加，但各种GAD同工型的贡献仍不清楚。大多数证据表明，植物GADs是胞质的，不像细菌和哺乳动物GADs，它们是由Ca 2 +/钙调蛋白，以及pH值调节。因此，在胞质H+或Ca 2+胁迫诱导的增加，可以导致植物GABA积累。相比之下，哺乳动物GAD 65/67可以磷酸化，驻留在不同的亚细胞位置（胞质溶胶与膜），并对磷酸化反应相反。例如，GAD 65中苏氨酸95的磷酸化刺激活性，而其N-末端区域中多个丝氨酸残基的磷酸化被提议调节突触囊泡膜中GAD 65功能的未知方面。值得注意的是，拟南芥GAD 1/4在其N端结构域中具有相同的5个Ser残基，AtGAD 1/2/4具有苏氨酸93 -94双联体;这些残基的磷酸化可能会潜在地改变它们的位置/活性，并且膜缔合可以促进GABA和GABA转运蛋白/GABA激活的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）的供应，分别增加了多个丝氨酸残基的N-末端区域的单个GADs的磷酸化。此外，我们的研究表明，AtGAD 4的表达，而不是AtGAD 1和AtGAD 2，是迅速诱导拟南芥中的盐度，这是之前或伴随着几个转录因子的表达。拟议的研究将比较盐度与Pi再供应下GAD磷酸化的看似相反的作用，并研究候选转录因子在GAD 4诱导中的潜在参与。

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.