Objectives This study explored the effects of inoculation with arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) on drought tolerance in sweet potato, with a focus on their roles in modulating the structure and diversity of rhizosphere fungal communities under drought stress. The aim was to provide a theoretical foundation for developing effective microbial inoculants that enhance stress resistance and yield in crops like sweet potatoes, particularly in arid regions.

Methods A pot experiment was carried out using the sweet potato cultivar “Jin gan shu No. 9”. Four soil moisture levels were established: 70%−80% (well-watered, W0), 50%−60% (mild drought, W1), 30%−40% (moderate drought, W2), and 10%−20% (severe drought, W3) of field capacity. Under each moisture regime, treatments included single inoculations with Bacillus subtilis (BS), Bacillus mucilaginosus (BM), Funneliformis mosseae (FM) , and a non-inoculated control (CK). BS and BM are PGPR, whereas FM is an AMF species. Rhizosphere fungal communities, as well as the photosynthetic characteristics and osmotic substances in sweet potatoes were investigated.

Results Inoculation with AMF and PGPR significantly improved photosynthetic performance and osmotic regulation substance content of sweet potatoes under drought conditions. Compared with CK, FM inoculation significantly increased relative chlorophyll content (SPAD) and water use efficiency (WUE) at 40, 80, and 120 days after transplanting under W1 and W2 conditions. Net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) all reached their peak with FM inoculation under W1 conditions, specifically, Pn increased by 36.04%、98.65% and 58.74%, Tr increased by 119.70%、163.26% and 160.91% and Gs rose by 106.70%、148.70% and 72.83% at these three time periods. Additionally, FM inoculation led to a significant elevation in intercellular CO₂ (Ci) concentration at 40 and 80 days after transplanting, although a decrease in Ci concentration was observed at 120 days post-transplanting under W1 conditions. The soluble sugar content was highest under W2 with BM inoculation at all three time points, while the proline content peaked with FM inoculation. Furthermore, inoculation with BS under W1 increased the soluble protein content and reduced the malondialdehyde content. High-throughput sequencing analysis showed that inoculation with FM significantly increased the Ace and Chao1 indices for both the W1 and W2 treatments. Inoculation with BS notably enhanced the Shannon index under the W2 treatment, although it did not significantly affect the Simpson index. The dominant fungal phylum observed were Ascomycota (19.66%−73.91%), Chytridiomycota (18.37%−69.55%), and Basidiomycota (6.52%−15.86%). At the genus level, the most dominant genera included Alternaria (14.21%−75.84%), Cladosporium (8.34%−37.18%), and Fusarium (5.91%−22.67%). The abundance values of Ascomycota, Basidiomycota, and the three dominant genera were highest inoculation with BS under W1 conditions, while Chytridiomycota had the highest abundance under FM inoculation. However, inoculation with AMF and PGPR reduced the diversity and abundance of the fungal communities under the W3 conditions.

Conclusions The dominant rhizosphere fungal phyla in sweet potato were Ascomycota, Chytridiomycota, and Basidiomycota, while the dominant fungal genera are Alternaria, Cladosporium, and Fusarium. Under mild and moderate drought conditions, inoculation with Funneliformis mosseae (FM) and Bacillus subtilis (BS) increased the relative abundance of these dominant fungal phyla and genera. Additionally, FM enhanced photosynthesis and water use efficiency in sweet potato leaves, whereas BS primarily promoted the accumulation of osmoregulatory substances and alleviated membrane lipid peroxidation. Under severe drought conditions, inoculation with FM still effectively maintained osmotic balance and reduced oxidative damage, ultimately promoting the growth and development of sweet potatoes and enhancing their drought resistance. Therefore, both arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) hold promise for the development and application of microbial agents aimed at improving stress resistance and yield.