γ-Aminobutyric acid (GABA) affected ABA and ethylene metabolic genes and signal components in salt-treated poplar, indicating its potential role in signal pathways of ABA and ethylene during salt stress. GABA is a small signalling molecule that accumulates rapidly in plants exposed to various stresses. However, the relationship between GABA and other signalling molecules, such as hormones, remains unclear. Here, in the poplar woody plant under 200-mM NaCl conditions, the application of low (0.25 mM) and high (10 mM) exogenous GABA, compared to 0 mM, affected the accumulation of hydrogen peroxide and hormones, including ABA and ethylene, in different manners. Transcriptomic analysis demonstrated that 1025 differentially expressed genes (DEGs; |log2Ratio| ≥ 1.5) were widely affected by exogenous GABA under salt stress. A clustering analysis revealed that GABA could rescue or promote the effects of salt stress on gene expression. Among them, 146 genes involved in six hormone-signalling pathways were enriched, including 22 ABA- and 50 ethylene-related genes. Quantitative expression of selected genes involved in hormone-related pathways showed that ABA metabolic genes (ABAG, ABAH2, and ABAH4), ethylene biosynthetic genes (ACO1, ACO2, ACO5, ACOH1, ACS1, and ACS7) and receptor genes (PYL1, PYL2, PYL4, and PYL6) were regulated by exogenous GABA, even at a 0.1 mM level. The production of ABA was negatively correlated with ABAH expression levels at different GABA concentrations. The increase of endogenous GABA, resulting from inhibitor (succinyl phosphonate) of α-ketoglutarate dehydrogenase, affected the PYLs levels. Thus, GABA may be involved in ABA- and ethylene-signalling pathways. Our data provide a better understanding of GABA's roles in the plant responses to environmental stresses.

γ-Aminobutyric acid (GABA) influences plant growth, but little is known about how this metabolite regulates adventitious root (AR) development. Here, we investigate the effects of GABA on ARs using poplar lines overexpressing glutamate decarboxilase 2 (GAD2) and by treating poplar stem cuttings with exogenous GABA or vigabatrin (VGB; a specific GABA transaminase inhibitor). Endogenous GABA accumulation not only inhibited AR growth, but it also suppressed or delayed AR formation. Anatomical observations revealed that the GABA and VGB treatments resulted in a 1 d delay in the formation of AR primordia and the appearance of ARs. This delay coincided with changes in primary metabolism, including transient increases in hexose and amino acid levels. GABA-dependent changes in the expression of genes related to hormone synthesis and signalling, as well as analysis of hormone levels revealed that ethylene-dependent pathways were decreased at the earliest stage of AR formation. In contrast, auxin and abscisic acid were increased at 1–5 d as well as GA4 over a 5 d period of AR formation. These results demonstrate that GABA plays a crucial role in AR development. Evidence is presented demonstrating that GABA can interact with hormone-related pathways as well as carbon/nitrogen metabolism. These findings also elucidate the functions of GABA in plant development.