Endophytic seed-associated microbial diversity and its impact on seedling growth of the Andean tree Nothofagus obliqua (Mirb.) Oerst

Symbiotic microorganisms are essential for promoting plant growth and establishment from the early stages of plant development. However, the diversity of seed-associated endophytes in native Andean trees and their role in growth promotion and seedling establishment have scarcely been studied. This study aimed to characterize the microbial diversity associated with seeds of Nothofagus obliqua (Mirb.) Oerst. Viable seeds were collected from healthy young trees in a section of the Nahuelbuta Mountains, south-central Chile. Then, they were processed to characterize total microbial diversity using a 16S rRNA gene and an internal transcribed spacer (ITS) region metabarcoding approach. The diversity of culturable bacteria was determined and tested for plant growth-promoting effects. Effects on seed germination, seedling development, and plantlet establishment were evaluated by in vivo inoculations. Seed-associated microbial diversity was dominated by Ascomycota and Proteobacteria, with Diaphorte and Pantoea being the most abundant genera. Five different strains of culturable bacteria were identified, with Rahnella aquatilis being the strain with the most traits that promote plant growth. Bioaugmentation with R. aquatilis improved seed germination, plantlet growth, and establishment of N. obliqua plantlets in the field. Specifically, bioaugmentation with R. aquatilis stimulated height (+ 52%), stem cross-sectional area (+ 89%), stomatal conductance to water vapor (+ 25%), and leaf mass area (+ 29%). These results provide evidence for the beneficial properties of seed-associated bacteria that can support the establishment of native forest tree species in the southern Andes.