Abstract
<jats:p>Legume-rhizobium symbiosis is constrained by carbon allocation and nutrient exchanges between the host plant and symbiotic bacteroids, emphasizing the necessity of biostimulant-based strategies to improve symbiotic efficiency. Studies suggest that plants provide carbon substrates (mainly TCA cycle intermediates) and nitrogen assimilation precursors to support bacteroid metabolism and nitrogen fixation. The GABA shunt represents a conserved bypass, linking the GS/GOGAT cycle, TCA cycle, and broader nitrogen metabolism. In the present study, the metabolic phenotypes upon rhizospheric application of exogenous γ-aminobutyric acid (GABA) and succinate were investigated in pea. Application of exogenous GABA and succinate primarily altered the morpho-biochemical properties and root nodule establishment of the plant. While GABA supplementation reduced the root length and increased stomatal density, succinate treatment alone inhibited nodule organogenesis with disrupted symbiosome zoning. Metabolic rewirings associated with these phenotypic changes begin at the root nodule compartment, where exogenous GABA altered endogenous tricarboxylic acid (TCA) cycle pools, leading to higher turnover of TCA cycle intermediates, including succinate, fumarate, and malate, as well as elevated levels of sugars (sucrose, glucose, and fructose) and polyols (pinitol, mannitol, and myo-inositol). Concurrently, the plants accumulated substantially higher levels of asparagine (+3.9 log10-fold higher), accompanied by altered shoot protein content and enhanced elemental nitrogen content (+0.8-1%). Collectively, these findings demonstrate the potential of GABA as a biostimulant capable of altering the carbon-nitrogen dynamics and enhancing symbiotic functioning in pea root nodules, whereas succinate alters the process of nodule formation.</jats:p>