Responses of wheat root system to in vitro inoculation with Azospirillum brasilense strains

Context Wheat is a globally important cereal crop, and enhancing its root system through microbial interactions is a promising approach to improve nutrient uptake and reduce dependence on chemical fertilizers. Aim The objective of this study was to evaluate the effects of two Azospirillum brasilense strains, AbV5/6 (commercial) and HM053 (ammonia-excreting mutant), on the root morphology and architecture of wheat genotypes under in vitro conditions. Methods Eight commercial wheat cultivars were assessed in vitro for root morphological responses, including biomass, length, area, and perimeter, to bacterial inoculation and nitrogen availability. Key results Two contrasting wheat genotypes, G5 (responsive) and G6 (non-responsive), were selected for a subsequent experiment comparing the effects of the two bacterial strains. Results revealed genotype-dependent responses to A. brasilense, with G5 exhibiting enhanced root growth traits under AbV5/6 inoculation, including a 11% increase in root length and a 10% increase in root area compared to the control. In contrast, G6 exhibited reductions in root traits following inoculation. Both strains colonized root tissues and formed endophytic biofilms near xylem vessels, as confirmed by scanning electron microscopy, but only AbV5/6 significantly increased root perimeter in G5. Conclusions Overall, root area and perimeter were more reliable indicators of bacterial responsiveness than biomass or total nitrogen content. Strain AbV5/6 was able to significantly increase root area and perimeter in wheat cultivars. Implications This study underscores the importance of genotype–strain compatibility in plant–microbe interactions and indicates the potential of AbV5/6 to promote wheat root development, particularly in genotypes with favorable traits.