Co-application of γ-aminobutyric acid and Bacillus zanthoxyli HN90 is associated with improved vegetative traits and altered rhizosphere microbial profiles in rice under a fixed reduced-nitrogen regime.

Rice vegetative traits and rhizosphere microbial profiles were evaluated after co-application of γ-aminobutyric acid (GABA) and Bacillus zanthoxyli HN90 under a fixed reduced-nitrogen regime. In controlled glasshouse pot experiments in which all treatments in the second pot trial were maintained at 80% of the conventional N rate, co-application of 5 mM GABA and the plant-growth-promoting rhizobacterium Bacillus zanthoxyli HN90 was associated with a shorter, sturdier phenotype, with 29.5% greater stem diameter and 36.9% higher SPAD at maturity relative to the reduced-N control. These vegetative traits may be relevant to lodging resistance, but lodging resistance was not directly measured in the present study. Compared with the reduced-N control, the co-application coincided with shifts in rhizosphere community composition across development, including higher relative abundance of Azospirillum and Geobacter and lower relative abundance of Fusarium. Bulk rhizosphere activities of urease, alkaline phosphatase, catalase, and sucrase were also 23-93% higher under HN90G than under the reduced-N control. FAPROTAX-based annotation suggested treatment-associated shifts in the relative representation of taxa assigned to N-cycling- and methane-related categories at maturity; however, these taxonomy-based predictions were not directly validated by functional assays. Overall, under the tested reduced-N background, GABA-HN90 co-application coincided with altered vegetative traits, shifts in rhizosphere microbial profiles, and higher bulk rhizosphere enzyme activities. The study was limited to one reduced-N treatment in a controlled pot system and did not include a full-N control, direct physiological measurements, functional validation, or strain tracking, so broader implications for nitrogen-use efficiency and field performance remain unresolved.