Isolation and Identification of an Endophytic Fungus Aspergillus sp. and Its Growth-Promoting Effects on Nymphaea candida Seedlings Through Modulation of the Rhizosphere Microbial Community

Nymphaea candida Presl is a rare and endangered waterlily species, and cultivating robust seedlings suitable for artificial propagation has become a critical issue for the conservation of this species. In this study, Aspergillus sp., an endophytic fungus isolated from the roots of N. candida, showed the capability of solubilizing phosphate and potassium and producing siderophores. The application of Aspergillus sp. significantly increased leaf length, leaf width, leaf number, and root length of N. candida seedlings by 97.83%, 131.37%, 94.12%, and 171.25%, respectively. Meanwhile, Aspergillus sp. application significantly enhanced soil organic matter content, alkali-hydrolyzable nitrogen content, sucrase activity, and peroxidase activity by 6.57%, 31.62%, 23.26%, and 7.53%, respectively. Moreover, Aspergillus sp. enriched beneficial microorganisms including Cyanobium, Aquicella, and Cryptomycota to form a more stable rhizosphere soil microenvironment. Additionally, Aspergillus sp. upregulated genes involved in photosynthesis and photosynthesis–antenna protein pathways in N. candida leaves, with the expression levels of psbA, petG, and psbH significantly increasing by 2.17, 4.48, and 0.28-fold, respectively. Therefore, the endophytic fungus Aspergillus sp. might be a reliable tool for the propagation of N. candida seedlings, which would be helpful for the conservation of this rare and endangered aquatic plant species.