【Objective】The intricate interactions between introduced plant growth-promoting rhizobacteria (PGPR) and the indigenous microbial community are pivotal determinants of inoculation efficacy and plant health. While the direct mechanisms of PGPR are well-studied, the role of resident microbes in modulating their activity remains less understood. This study focuses on Sphingomonas sp. HJY, a known growth-promoting bacterium for rice, aims to elucidate how specific indigenous bacteria influence its plant growth-promoting capacity and to uncover the fundamental mechanisms behind this interaction. Clarifying this synergy is crucial for advancing our knowledge of rhizosphere ecology and optimizing microbial inoculation strategies. 【Method】A batch of bacteria was isolated from rice rhizosphere soil, and indigenous bacterial helpers that promoted the growth of HJY were screened by both co-culture assay and supernatant-culture assay. The bacterium with the most pronounced promotion effect was selected as a representative and identified by full?length 16S rRNA gene sequencing. To elucidate the mechanism of facilitation, the selected helper strain was cultured under iron-rich and iron-limited conditions. Its siderophore production was quantified using the Chrome Azurol S (CAS) assay. Subsequently, the impact of its filter-sterilized supernatant from both iron conditions on HJY growth was compared. Finally, a pot experiment with natural soil was conducted to assess the influence of the indigenous bacterial helper on the ability of HJY to promote rice growth. 【Result】The results showed that the indigenous bacterium labeled B35 exhibited the most significant promotion effect on HJY growth. The biomass of HJY was increased by 71.2% and 95.4% by B35 cells and its sterile supernatant, respectively, with the latter being significantly more effective. This strain was identified as Bordetella petrii. Under iron?limited conditions, the siderophore secretion by B35 was significantly higher than under iron?sufficient conditions. Moreover, the iron?limited supernatant enhanced the growth?promoting effect on HJY by 13.89% compared with the iron?sufficient supernatant, indicating that siderophores secreted by B35 were one of the main factors promoting HJY growth. Soil inoculation experiments demonstrated that, at 18 days, the plant height, fresh weight, chlorophyll content, and root length of rice treated with HJY + B35 sterile supernatant were increased by 5.9%, 11.6%, 12.7%, and 21.2%, respectively, compared with those treated with HJY alone. In contrast, the corresponding indices for rice treated with HJY + B35 cells were increased by 4.4%, 7.6%, 5.1%, and 8.0% compared with HJY alone. These results indicate that both B35 and its sterile supernatant could enhance the growth?promoting effect of HJY on rice, with the supernatant treatment showing a superior effect to the bacterial cell treatment. Inoculation with B35 alone or its supernatant alone had no significant effect on rice growth. 【Conclusion】 In conclusion, this study demonstrates that the indigenous bacterium Bordetella petrii B35 enhances the plant growth-promoting activity of Sphingomonas sp. HJY through a facilitative interaction driven largely by siderophore production. This interaction significantly improves HJY's efficacy in promoting rice growth in natural soil, with the direct application of the helper's metabolites yielding the strongest effect. These findings provide novel insights into the complex network of rhizosphere microbial interactions, highlighting how targeted exploitation of specific facilitative relationships—such as cross-feeding via siderophores—can be a crucial strategy for optimizing the performance of PGPR inoculants. This study, therefore, offers a theoretical foundation for designing more effective and stable synthetic microbial consortia for sustainable agriculture.